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262

Chng 7. Cng trnh tho l ngoi thn p

Bin son: GS.TS Ng Tr Ving

PGS.TS Phm Ngc Qu

7.1 Phn loi

Cng trnh tho l ngoi thn p l nhng cng trnh tho l b tr vai p chn hoc gn p chn khi iu kin cho php.

C th phn loi cng trnh tho l ngoi thn p theo nhiu cch khc nhau.

1. Theo hnh thc c hay khng c ca van Cng trnh tho l ngoi thn p c chia ra:

a) Cng trnh tho l ngoi thn p c ca van:

Cao trnh ngng trn thp hn MNDBT t 3 n 7 mt.

u im chnh ca loi ny: gim chiu cao p, gim din tch ngp lt; Li dng mt phn dung tch hu ch lm nhim v ct l, gim chiu rng trn nc; khi c d bo l chnh xc th loi ny c an ton cao.

Nhc im ca loi ny l thng c ct nc trn v lu lng n v ln; Cng trnh ni tip v tiu nng phc tp hn; qun l, bo dng, vn hnh kh khn hn v chi ph qun l ln.

Tho l ca van thng dng nhng cng trnh t cp III tr ln.

b) Cng trnh tho l ngoi thn p khng c ca van:

Cao trnh ngng trn bng mc nc dng bnh thng. Tuy c nhc im nh b rng trn ln, mc nc l cao, din tch ngp lt ln, loi trn khng c ca van c nhiu u im nh t ng tho l, qun l vn hnh thun tin, chi ph qun l nh. Cng trnh khng c ca van thng dng vi cng trnh cp IV, V v mt s cng trnh cp III.

2. Theo hnh thc ngng trn

a) Cng trnh tho l ngoi thn p c ngng nh rng:

Loi ny thng dng khi a hnh tuyn trn rng, cao trnh t nhin ln hn cao trnh ngng trn; khng yu cu h s lu lng ln.

b) Cng trnh tho l ngoi thn p c ngng thc dng:

Dng khi a hnh khng rng hoc cn h s lu lng ln, hoc khi a cht km phi x l hoc cn h tm thi cao trnh ngng trn dn dng v tho l thi cng.

3. Theo hnh thc ni tip sau ngng trn.

a) Cng trnh tho l ngoi thn p c ni tip dc nc.

b) Cng trnh tho l ngoi thn p ni tip bc nc.

c) Cng trnh tho l ngoi thn p ni tip kt hp dc nc, bc nc.


263

4. Theo hnh thc tiu nng

a) Cng trnh tho l ngoi thn p c tiu nng y (o b, xy tng hoc b tng kt hp).

b) Cng trnh tho l ngoi thn p c tiu nng mt.

c) Cng trnh tho l ngoi thn p c tiu nng phng xa.

5. Theo hnh thc cu to

a) ng trn dc:

L loi cng trnh tho l c phng ca dng chy trn ngng trn gn nh song song vi dng chy trong sng.

b) ng trn ngang:

L loi cng trnh tho l c phng ca dng chy trn ngng trn gn nh vung gc vi dng chy trong sng.

c) Ging tho l:

L loi cng trnh tho l mt c tuyn ngng trn dng trn v dng chy qua ngng trn tp trung vo ging ng xung ng hm ngang v v h lu.

d) Xi phng tho l:

L loi cng trnh tho l c mt ct ngang kn, li dng chn khng trong ng kn tng lu lng x v t ng tho, ngt.

7.2 Nguyn tc b tr - c im s dng I. Nguyn tc b tr 1. Trit li dng a hnh (eo ni, yn nga, vai p) b tr cng trnh tho l

ngoi thn p, nhm gim khi lng o, p, gim khi lng xy lp.

2. Ch n iu kin a cht m bo n nh ca tng hng mc cng trnh, gim khi lng x l nn mng, trnh st l mi o, mi p.

3. Ca vo trn c thun, ca ra khng gy bt li cho s lm vic an ton v hiu qu ca cc hng mc cng trnh khc nh p, cng, u tu, nh my thu in.

4. m bo ni tip tt gia cng trnh tho l ngoi thn p vi b v vai p.

5. m bo thi cng c v cht lng.

6. Qun l, vn hnh, sa cha thun li; to iu kin cc hot ng dn sinh kinh t, x hi c an ton v tin li.

7. To thun li cho s b tr hi ho cm cng trnh u mi v tng m quan cho cng trnh thy li.

II. c im s dng 1. Xt v a hnh th ni c eo ni, yn nga gn cm u mi hoc ni i thoi

vai p, s dng cng trnh tho l ngoi thn p s kinh t hn cc loi khc. Nhng khi


264

a hnh cht hp vn dng c mt s hnh thc cng trnh tho l ngoi thn p nh ging tho l, xi phng tho l, ng trn ngang...

2. Cng trnh tho l ngoi thn p c b tr trn nn l tt nht, tuy nhin vn c th b tr trn nn t vi ct nc trn, lu lng tho khng ln.

3. Tuyn trn (theo phng dng chy tr v sng chnh) l thng hoc cong vi bn knh cong hp l khng gy bt li cho ch thu lc, nhng cng khng lm tng khi lng o p.

4. Dng chy trong cng trnh tho l l dng cao tc v vy cn la chn lu tc ln nht hp l v ch nhng hin tng thu lc bt li hoc phc tp.

5. V iu kin a cht ni tuyn trn i qua yu hoc v phi h thp cao trnh ngng trn tho l thi cng, cng s dng cng trnh tho l ngoi thn p. Trong trng hp ny ngng trn c th s dng loi thc dng hoc s dng ca van.

6. Nguyn tc s dng tng hp i vi cng trnh thy li c vn dng trit khi thit k, thi cng cng trnh tho l. Cng trnh tho l l mt im nhn v kin trc trong tng th u mi; l hng mc cng trnh c nh hng mnh n mi trng sinh thi, mi trng x hi, mi trng kinh t, l mt im sng ca du lch vng h.

7. Cng trnh tho l ngoi thn p c s dng nhiu cn bi v n c hin trng thi cng rng (v i khi c lp), qun l khai thc thun tin; bo dng, sa cha t gy nh hng n s lm vic bnh thng hoc an ton ca cc cng trnh khc.

7.3 ng trn dc I. iu kin s dng ng trn dc tho l l loi cng trnh tho nc kiu mt v thng gp nht. N

c dng khi khng th s dng cng trnh tho l trong thn p hoc dng n kinh t hn. c im ch yu ca n l phn ngng trn thng thng nh mi hnh thc trn khc, t eo ni (hnh 7-1) hoc mt vai p (hnh 7-2) hoc c hai vai p (hnh 73).

Loi cng trnh ny c nhng u im sau y:

1. Thi cng v qun l n gin v l cng trnh h.

2. Xy dng c trong iu kin a hnh khc nhau, c th b tr u p, st ven b hoc nhng vng eo ni khc trong lu vc, cch xa thn p.

3. Yu cu v a cht khng cao, c th xy trn nn , nn xu v c trn nn t.

4. Lu lng tho c th t hng chc m3/s n hng vn m3/s, chiu di din trn t hng chc n hng trm mt, tu theo yu cu ca cng trnh, ph thuc vo tnh hnh a cht ca nn v h lu cng trnh (do tr s lu lng ring q quyt nh). Bng 7-1 gii thiu mt s ng hm xy dng Vit Nam

5. Vic s dng hoc tng kh nng tho l ca cng trnh khng phc tp nh cng trnh ngm, an ton v d phng tho l ln, do ng trn tho l l loi cng trnh tho l an ton.

6. L loi trn h nn d quan trc; qun l khai thc thun li; bo dng sa cha i tu d; to cnh quan chung p v hng v.


265

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Bng 7-1. Mt s ng trn c xy dng Vit Nam

TT Cng trnh FLV (Km2) ThitkmaxxQ (m

3/s) BTrn (m) q (m3/sm) Ghi ch

1 H Thc B 6430 3230 30 107,7

2 H Ni Cc 535

Trn Chnh 850 24 34,4

Trn b sung 585 16 36,6

3 H Yn Lp 183 830 24 34,6

4 H Yn M 137 320 18 17,8

5 H K g 223

Trn chnh 1065 20 53,3

Trn s c 699 65 10,8

6 H Sng Rc 155 1250 27 46,3

7 H Ph Ninh 235

Trn 1 401 37 10,8

Trn 2 1053 20 52,6

Trn s c 696 24 29,0

8 H T Sng Hinh 772 6952 72 96,6

9 H T Tr An 14.600 18.448 120 153,7

10 H T Yaly 7455 17.400 90 193,3

11 H Du Ting 2700 2800 60 46,7

II. c im lm vic ng trn dc l loi cng trnh tho l c ch

lm vic n nh, an ton hn cc loi cng trnh tho l khc. Cc kt cu ca cng trnh l kt cu h nn thi cng, qun l v sa cha thun li.

Ch thu lc ca ng trn bao gm hu ht nhng vn tnh ton ca p trn, knh h v tiu nng. Do trong thit k ng trn, sau khi xc nh c chiu di trn nc Bt v ct nc trn H theo so snh kinh t - k thut, phi ln lt thit k cc thnh phn ca cng trnh theo trnh t t thng lu n h lu hp l nht.

Hnh 7-3. ng trn dc hai u p.

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268

III. Cc b phn ca ng trn 1. Knh dn v ca vo

Knh dn vo c thit k hng dng chy vo ngng trn c thun, gp phn tng h s lu lng, gim nhng bt li sinh ra ca vo. Knh thng c dc bng khng v i khi c dc ngc. Chiu di knh c th ln hay nh tu theo a hnh. Mt ct ngang ca knh c th ch nht hoc hnh thang. Trn mt bng, tuyn knh dn vo c th thng hoc cong, i xng hoc khng i xng (hnh 7-4).

Tng cnh ca vo c th s dng loi tng trng lc, tng sn, tng hp bng gch, xy hay bng b tng, b tng ct thp. Mt bng cc loi tng cnh thng lu nh hnh 7-5. Sn thng lu ca vo bng xy hoc b tng, b tng ct thp. Chiu di sn thng lu c th ly bng (35).H, vi H: ct nc thng lu ngng trn.

Tit din knh tng i ln v thu hp dn v pha ngng trn. Khi lng cng trnh knh thng ln.

Hnh 7-4. Knh dn v tuyn hng dng pha thng lu

1. Ngng trn; 2. Knh dn; 3. B knh; 4. Tng hng dng.

Hnh 7-5. Cc loi tng cnh trc ngng trn

a) Tng cnh thng; b) Tng cnh nghing nc khng trn qua; c) Tng cnh c gc xin v un cong; d) Tng cnh m rng hnh cung trn; e) Tng cnh m rng theo dng elip;

f) Tng cnh cong gm nhiu cung trn khng ng tm.

1 1

b)a) c)

18

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67

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269

2. Ngng trn

Ngng trn l mt b phn quan trng nht ca ng trn. Ngng c th dng nh rng hoc thc dng: Trn ngng c th b tr ca van hay khng ca van, c th c m tr, cu giao thng, cu cng tc, khe van, khe phai, khe ln, khp ni, thit b quan trc v.v...

Ni chung tuyn ngng trn l thng v b tr trng tuyn p chn (khi trn x l vai p). Khi iu kin a hnh, a cht thun li v qui m trn khng ln c th b tr tuyn ngng cong hoc gy khc (dng ch V, dng zch zc hoc dng hnh thang v.v....). Nguyn tc b tr sao cho dng chy vo ra c thun li; ph hp vi ch qun l, khai thc cng trnh sau ny (hnh 7-6).

Chiu rng trn nc qua tnh ton thy lc xc nh ra. Chiu di (theo chiu nc chy) ca ngng trn c xc nh tu thuc vo vic b tr cc thit b pha trn; m bo ng vin chng thm v phn b p lc y mng; ch thu lc ca dng chy trn ngng.

3. Ni tip h lu Sau ngng trn l b phn ni tip h lu bao gm knh tho (c th l dc nc hoc

bc nc); thit b tiu nng cui knh tho v c th c knh dn nc ra lng sng c, knh tho c b tr theo a hnh (thng hoc cong) gim khi lng knh tho c th l dc nc; hoc bc nc hoc phi hp c dc nc, bc nc.

a) Dc nc:

Thc cht dc nc l knh c dc ln, l loi cng trnh ni tip n gin (hnh 7-6) a)

b)

Hnh 7-6. Cc hnh thc dc nc.

a) Trn nn ; b) Trn nn t (kch thc trong hnh ghi theo m).

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Trn mt bng, tuyn dc nc c th thng hoc cong, on u dc thng l on thu hp vi gc thu hp q thng chn theo kinh nghim l 220. Kch thc v dc ca dc nc xc nh theo lu tc trung bnh cho php ca vt liu lm dc nc: [V](m/s). dc ca dc nc c th l hng s trn ton dc hoc thay i theo tng on dc cho ph hp a hnh. Cng trong trng hp sau ngng trn b tr dc nc m rng dn khuych tn dng chy cho tt. V d nh dc nc trn s 2 (h Ph Ninh) chiu rng dc t 22m u dc m rng ti 36m, trn di dc 63m vi i=0,1 (hnh 7-7).

Dng dc nc khi: dc a hnh t nhin i0 nh hn dc cho php [id]:

[id] = [ ]

RCV

2

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, (7 - 1)

trong : C: h s sdi ;

R: bn knh thu lc.

Khi i0 > [id] nu dng dc nc th cn c bin php x l v d nh s dng m nhm gia cng, hoc thay vt liu lm dc nc.

Vt liu lm dc nc l xy, b tng, b tng ct thp. Mt ct ngang l ch nht hoc hnh thang. Mt ct ngang c xc nh theo iu kin thu lc. Bn y c th dng bn hoc bn sn; lin khi hoc tch ri vi bn y tng cnh. Tng cnh hai bn dc nc c chiu cao c xc nh t mc nc ln nht trong dc c k n hm kh khi lu tc trong dc nc V>3m/s. Tng c th chn dng tng trng lc, tng sn hoc tng ta nghing trc tip ln mi o.

Hnh 7-8. S hnh ct trn x l h Du Ting (Ty Ninh)

1. Tng ngc; 2.Cu cng tc ; 3. Ca van; 4. Tm quay ;

5. Dc nc ; 6. Mi phun; 7. Mi h xi d kin.

Hnh 7-9. Ct dc trn x l H Trui (Tha Thin Hu)

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6


272

Hnh 7-8 gii thiu dc nc trn x l h Du Ting (Ty Ninh) c dc 8%, cui dc l mi phun. Dc nc trn x l H Trui (Tha Thin Hu) c i = 15%, tiu nng bng hnh thc mi phun c ngng mi lch th hin trn hnh 7-9.

b) Bc nc:

- iu kin xy dng v c im cu to: Khi knh tho nm trn nn t dc, nu lm dc nc th khng m bo n nh trt v tiu nng cui dc nc gp kh khn, nn xy dng nhiu bc nc ni tip nhau a nc t ngng trn xung h lu. Cc bc nc cn c tc dng tiu hao nng lng trong sut chiu di dng chy v b phn tiu nng cui knh tho s thit k n gin hn (hnh 7-10).

So vi dc nc, khi lng cng trnh ca bc nc ln hn do loi cng trnh ny ch c xem l hp l khi xy trn nn t a hnh t nhin c dc ln, khng thun li cho vic xy dng dc nc. Cng c trng hp, knh tho gm mt on l dc nc v mt on tip theo l nhiu bc nc.

Bc nc trong ng trn dc tho l thng l loi nhiu bc.

- Phn vo: Phn vo (tc l ngng trn vo bc nc th nht) l p trn nh rng, do dng chy vo bc ny s n nh v d ni tip. Phn vo c tng cnh (hnh 7-10) ko di v pha thng lu (11,5)H. Trng hp xy trn nn t, pha trc ngng trn cng c th thit k sn ph chng thm, pha trn c gia c chng xi l do lu tc ln gy nn. Chiu di sn ph khng ngn hn 3H.

Dng ca tng cnh v ca knh dn vo (nu c) c thit k ging nh ca ng trn ni chung.

Thng thng ngng trn ca bc nc l loi t trn, v ct nc trn khng qu ln, cho iu kin ni tip trong mi bc c d dng.

Hnh 7-10. Bc nc.

-Bc: Bc nc gm nhiu cp. Phi thit k th no cho trong mi cp u c nc nhy ngp n nh. V vy cui mi cp thng c tng tiu nng, lm vic nh mt b

8

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5

4

I - I

1

2

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3

7

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9

4

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Po/4


273

tiu nng. Trong tnh ton thu lc cng cp n loi bc nc khng c tng tiu nng mi cp, nhng thc t thit k nh vy khng kinh t, v v mt k thut cng khng c li.

Trong mt h thng bc nc, chiu di v chiu cao mi cp u bng nhau d dng cho tnh ton v thi cng, t l gia chiu di v chiu cao mi cp khng nh hn 2.

Chiu cao mi cp c tnh theo biu thc:

dNP

P 0 += (7 - 2)

trong :

P0 - Tng chnh v a hnh t u cp th nht n cp cui cng.

N - S cp.

d - chiu su b (chiu cao ngng cui bc) ca mi cp.

Chiu di ln nht ca mi cp khng nn qu 20m ( thun li cho b tr khe ln).

Mi cp gm c tng ng, bn y v tng bin. Tng ng lm vic nh mt p trn, nc t cp pha trn trn vt qua tng ng ri xung cp di. V vy, tng ng l mt loi tng trng lc, c khe ln tch khi bn y. Trong mi cp u c b tr l thot nc kch thc 10x10(cm) n 20x20(cm) bn trong c tng lc ngc gin p lc thm v lc y ni.

Bn y trong mi cp lm nhim v nh bn y ca b tiu nng, chiu dy c th tnh s b theo cng thc:

t = 0,25 0Pq (7 - 3)

Tng bin ca bc nc thng l tng trng lc.(i khi cng c th cc loi tng khc).

- Phn ra: Phn ra l cp cui cng. Cu to ca n v c bn ging cc cp gia. Nhng thun li cho tiu nng, b tiu nng cui cng lm theo dng khuch tn v trong b c th t m, hoc lm tng cui b, tu theo tnh hnh c th. Phng php tnh ton v cu to ging nh b tiu nng sau p trn.

c) Thit b tiu nng:

Cui knh tho l thit b tiu nng. C th p dng cc hnh thc: tiu nng y, tiu nng phng xa, tiu nng mt v cc thit b tiu nng ph tr.

Ni tip bc nc th c th mi bc l mt b tiu nng hoc nng lng tha dn xung c cho bc cui cng v bc cui cng c s dng nh thit b tiu nng y.

Ni tip dc nc th c th s dng cc loi thit b tiu nng khc nhau. Vi mi cng trnh c th cn cn c vo: a hnh, a cht, ct nc trn, lu lng n v, mc nc h lu, b tr tng th cm cng trnh u mi, tnh kinh t... chn hnh thc tiu nng cho hp l.

Tiu nng y l hnh thc li dng ni ma st tiu hao nng lng tha. Sau thit b tiu nng vn phi gia c tip (thng gi l sn sau th 2). Hnh thc ny dng vi trng hp ct nc thp, nn t, nn yu. Thuc v hnh thc ny c: o b, xy tng hoc


274

b tng kt hp. Ngoi ra cn p dng cch gim su sau nc nhy bng b tr m nhm, dm tiu nng; to tng phn dng khuych tn u h lu, lm y dc ngc khi mc nc h lu nh, lm y dc thun khi mc nc h lu ln. y b thng dng b tng ct thp, c b tr l thot nc, chiu dy bn y b t 0,8 1,5(m). Tng cnh b tiu nng thng m rng dn v dng xy, b tng, b tng ct thp. Sn sau th hai thng c kt cu mm, thch hp vi s bin dng ca nn.

Tiu nng mt l hnh thc tiu nng ng vi trng thi chy mt. Kinh nghim cho bit, hiu qu tiu nng mt so vi tiu nng y khng km hn nhiu, nhng chiu di sn sau ngn hn. Chiu dy ca sn sau cng nh hn. Dng hnh thc tiu nng mt khi mc nc h lu n nh v ln hn su lin hip vi su co hp; lu lng x ln nhng chnh lch mc nc thng h lu khng ln; b h lu n nh v chng xi tt.

Tuy vy tiu nng mt gy tr ngi cho thuyn b i li h lu v e do mt n nh b; d sinh nc nhy phng xa; sn sau lm vic vi ch thu lc thay i lin tc.

Khi chn chiu cao nh nht ca bc to dng mt cn tho mn cc yu cu:

- Khng c dng phun vi lu lng nh nht.

- Khng c dng hi lu vi lu lng ln nht.

- Chiu cao bc nh hn su nc h lu.

- Gc nghing nh bc q = 100150 hoc nh bc nm ngang.

- Bn knh cong nh bc R = 1025(m) vi trn c ct nc trung bnh v ln; R = 512(m) vi trn c ct nc nh.

Tiu nng phng xa l hnh thc li dng ma st vi khng kh tiu hao mt phn nng lng, phn cn li s c tiu ton bi lp m nc h lu. N c dng kh ph bin cng trnh thy li v c mc p dng cao. iu kin thc hin c hnh thc tiu nng phng xa l: nh mi phun phi cao hn mc nc ln nht h lu, chiu cao ct nc trc p ln to ra dng phun phng xa; chiu su ct nc h lu cng ln chiu su h xi khng qu ln.

Hnh thc tiu nng ny thng dng vi cng trnh c ct nc trung bnh v ln. Tuy vy hnh thc ny cng c nhng hn ch nht nh: h xi do dng phun to ra c th h mc nc h lu nh hng n cc yu cu dng nc; xung kch ca dng phun to thnh dng cun ngc hoc sng v mi p; mi phun c th xy ra kh thc; dng phun to ra sng m nh hng n giao thng v mi trng.

d) Knh dn nc ra lng sng c: c chiu di ln hay nh; mt ct n nh hay m rng dn; dc ln hay nh.. tu vo a hnh, a cht la chn. Quan h mc nc vi lu lng knh sau thit b tiu nng c xc nh qua tnh ton thu lc hay t ti liu thc o...

IV. Kh nng tho nc Kh nng tho nc qua ng trn dc c th tnh theo s chy qua p trn

ngng thc dng (6-2; chng 6) hoc qua p trn ngng nh rng. p trn nh rng l loi p c chiu rng nh ngng C theo chiu nc chy trong

phm vi:


275

(8 10) H > C > (2 3)H Nu C H th dng chy qua p ging nh qua knh h y nm ngang Nu C < 2H, h s lu lng ca p khng n nh v ln hn ca p trn nh rng

(bin thnh nh nhn).

Dng chy t knh dn qua ngng trn c th pht sinh hai xoy nc trc ngang: xoy nc mp trc ngng v pha trn ngng (trng hp P > 0). Nu ngng trn c tr bin v tr gia, ti cc mp tr cn c th pht sinh xoy nc trc ng. Nhng hin tng trn lm tng tn tht do co hp ng v ngang gy nn v lm gim kh nng tho.

p trn nh rng lm vic c th theo ch chy khng ngp hoc chy ngp. Trong trng hp chy ngp, kh nng tho s gim rt nhiu. Do i vi ngng trn ca ng trn tho l cn c nhng bin php thit k tng kh nng tho v thng thng ngng trn lm vic theo ch chy khng ngp.

C nhiu phng php tnh ton p trn nh rng:

1. Phng php ca P.K.Kixlev;

2. Phng php ca A.R. Brzinxki;

3. Phng php ca N.N. Pavlovxki;

4. Phng php ca R.R. Tsugav;

Chng ti s trnh by hai phng php thng dng ca: P.K. Kixlev v ca R.R. Tsugav.

a) Tnh ton ngng trn nh rng chy khng ngp (hnh 7-11)

- Tiu chun khng ngp:

Theo Beklonj th tiu chun khng ngp nh sau: hn < 32Ho

Theo Bahmv: hn < hpg hoc hn < P + hpg ;

Theo P.K. Kixlev: hn < (hpg + Z'') v hn < 1,25hpg ;

Theo R.R. Tsugav: hn < nHo hoc hn < (P + nHo) ,

trong : Z''- su hi phc sau ngng trn.

Z'' = g

VVV 2nnpg -

Vpg, Vn - lu tc phn gii v lu tc sau p.

hn - chiu su mc nc h lu so vi ngng trn.

hpg - chiu su phn gii, ly bng 0,66 Ho

Ho - ct nc trn.

n - h s ly bng: 0,85 0,75 P - chiu cao ngng p so vi y knh dn.


276

Hnh 7- 11. S tnh ton p trn nh rng khng ngp.

- Tnh kh nng tho nc: p trn nh rng chy khng ngp c hai ch h mc nc Zt v Zh. V tn tht ct nc trn chiu di ngng trn khng ng k nn sau ch h Zt, mt nc trn ngng coi nh nm ngang.

Theo P.K.Kixlev, su nc trn ngng trn ly bng su phn gii (chng minh t gi thit t nng dng chy nh nht):

h = hpg = 2

2

212

j+j

Ho 0,6Ho

Lu lng tnh theo biu thc

Q = mb 23

0Hg2 (7 - 4)

hoc: Q = Mb H 23

0 , (M = m g).2

Cc h s j (h s lu tc), m (h s lu lng) tra theo bng ca N.N Pavlvxki (bng 7 - 2).

Bng 7-2. Cc tr s j, m, M ca p trn nh rng

iu kin chy vo p j m M

Khng c sc cn thu lc 1 0,385 1,70

iu kin chy vo thun li 0,95 0,365 1,62

Ngng c mp vo trn cong 0,92 0,350 1,55

Ngng c mp nghing 0,88 0,335 1,48

Ngng c mp vung gc 0,85 0,320 1,42

iu kin chyvo khng thun 0,80 0,300 1,33

Cn ch rng theo kt qu th nghim, nu lm ngng trn c mp nghing, trong trng hp c knh dn vo tuyn cong, th loi mp nghing 450 c h s lu lng ln nht.

Ho

H

o

h =

h 21 h p

g 2h

Z

P

h n

z

P 1

Po

ld d d

C l1 21 2 3

O 1 2 3

O 1 2 3O

P 1Vo

O

1h h pg 2h V

P

h n

3

3

l C l

d2 d3d1

1 2

2

ZZ"


277

Theo R.R. Tsugaev, su nc trn ngng trn phi l:

h < hpg < 32Ho

Lu lng cng tnh theo biu thc (7 - 4)

Nu Wt > 4 (bH), vi:(Wt - mt ct t ca knh dn vo pha trc ngng) th v0 rt nh c th b qua, v lu lng tnh theo biu thc:

Q = mb g2 H3/2

H s m tra theo bng ca .I.Kumin, c k n co hp bn do thay i chiu rng v co hp ng do chiu cao P v dng ngng. Khi khng c co hp bn hoc khng c ngng (tc l BK = BT hoc P = 0); m tra theo (bng 7 - 3) v (bng 7- 4).

Khi co hp bn (Bt < Bk) hoc c ngng (p > 0) h s lu lng m xc nh theo biu thc: m = mh + (mb - mh) Fh + (0,385 - mb) FhFb (7 - 5)

Hoc: m = mb + (mh- mb) Fb + (0,385 - mh) FhFb (7 - 6)

y mh ly theo hng cui (bng 7 - 2) (h = ) v mb ly theo hng u (bng 7 - 3) (b = 0) tu theo iu kin co hp bn. H s m s c tnh theo biu thc (7 - 5) nu mb > mh, theo biu thc (7 - 6) nu mb < mh.

Cc gi tr Fh v Fb tnh theo cc biu thc sau y:

Fh = P2HH+

(7 - 7)

Fb =T

T

B5,25,3B

- (7 - 8)

Bng 7- 3. H s m ca p trn nh rng khi khng c co hp bn (theo .I. Kumin)

H

P a

H

P

H

45P

cotga r/H a/H

h = HP

0 1 2 2,5 0,025 0,10 0,4 0,8 1 0,025 0,1 0,2

0,2 0,366 0,377 0,382 0,382 0,372 0,375 - - - 0,371 0,376 -

0,6 0,350 0,370 0,379 0,380 0,361 0,367 0,374 - - 0,359 0,367 -

1,0 0,342 0,367 0,377 0,378 0,355 0,362 0,371 0,376 - 0,353 0,363 -

2,0 0,333 0,363 0,375 0,377 0,349 0,358 0,368 0,375 0,382 0,347 0,358 -

6,0 0,325 0,360 0,374 0,376 0,344 0,354 0,366 0,373 0,380 0,341 0,354 0,36

0,320 0,358 0,373 0,375 0,340 0,351 0,364 0,372 0,375 0,337 0,352 0,35


278

Bng 7-4. H s m khi khng c ngng (P = 0) v c co hp (theo .I.Kumin).

q

q

Bk Bt

r

BBk t

BBk

a

at

cotgq r/BT a/BT

k

t

BP =

0 1 2 3 0 0,1 0,3 0,5 0 0,05 0,1 0,2

0,0 0,320 0,35 0,353 0,350 0,320 0,342 0,354 0,360 0,320 0,340 0,345 0,350

0,2 0,324 0,352 0,355 0,352 0,324 0,345 0,356 0,362 0,324 0,343 0,348 0,352

0,4 0,330 0,356 0,358 0,356 0,330 0,349 0,359 0,364 0,330 0,347 0,351 0,356

0,6 0,340 0,361 0,363 0,361 0,340 0,354 0,363 0,368 0,340 0,354 0,357 0,361

0,5 0,355 0,369 0,370 0,369 0,355 0,365 0,371 0,373 0,355 0,364 0,366 0,369

1,0 0,385 0,385 0,385 0,385 0,385 0,385 0,385 0,385 0,385 0,385 0,385 0,385

Trong trng hp chy khng ngp, chiu su nc trn ngng trn c tnh theo biu thc (R.R.Tsugaev)

Q = jh1b )hg(H2 10 - (7 - 9)

y j c xc nh ph thuc vo h s m nu trn v tra theo bng ca .I.Kumin (bng 7 - 5).

Bng 7- 5. Quan h gia j v m.

m 0,30 0,31 0,32 0,33 0,34 0,35 0,36 0,37 0,38

j 0,943 0,950 0,956 0,963 0,970 0,976 0,983 0,990 0,996

b) Tnh ton ngng trn nh rng chy ngp (hnh 7 - 12)

Hnh 7 - 12. S tnh ton ngng trn nh rng chy ngp.

Ho

d P

h nZ

h pg

vo

h 1 v

P1

dc2 c c2

h = h

Z Z"=

Z


279

Chy ngp xy ra khi hn > 1,25hpg,

hoc: Hn > nH0 vi n = 0,85 0,75. Lu lng tnh theo biu thc:

Q = m.sn.2/3

0H.g2b , (7 - 10)

trong : sn - h s ngp, tra theo bng N.N.Pavlovxki (bng 7 - 6)

Bng 7 - 6. H s ngp sn ca p trn nh rng

hn/H0 sn hn/H0 sn hn/H0 sn

0,70 1,000 0,90 0,739 0,980 0,360

0,75 0,974 0,92 0,676 0,990 0,257

0,80 0,928 0,94 0,598 0,995 0,183

0,83 0,889 0,95 0,552 0,997 0,142

0,85 0,855 0,96 0,499 0,998 0,116

0,87 0,815 0,97 0,436 0,999 0,082

Theo quy phm C8-76 (phng php ca R.R.Tsugaev v .I.Kumin), lu lng tho c tnh theo biu thc (7 - 9), nhng h s lu tc j c hiu l jn ph thuc vo m theo s liu ca .I.Kumin (bng 7-7).

Bng 7-7. H s jn ca p trn nh rng chy ngp

m 0,30 0,31 0,32 0,33 0,34 0,35 0,36 0,37 0,38

jn 0,76-0,78 0,81 0,84 0,87 0,90 0,93 0,96 0,98 0,99

Chiu su h1 trn ngng tnh bng h1 = h2 = hn - Z". y hn bit, tr s Z" = xhpg (x xc nh theo biu , xem Quy phm tnh ton thu lc p trn).

c) Tr s co hp ngang do tr pin:

Tr s co hp ngang e do nh hng ca tr pin c th xc nh theo biu thc (7- 11).

e = BdB -

, (7 - 11)

Vi: B l chiu rng lng dn ca vo trn.

Ch : Vic s dng cc bng trn, khi knh dn dng v tng hng dng hai bn theo dng ng dng, do nh hng ca thu hp knh dn hoc tng hng dng khng ng k, nn mb = 0,385.


280

V. Tnh ton thu lc v tiu nng h lu 1. Tnh ton thu lc knh dn vo ngng trn

a) Knh dn vo ngng trn c ng vin hai b dng cong trn:

B li v b lm ca knh l hai ng cong c bn knh r1, r2 khng ng tm, v vy tit din knh dn thu hp dn v pha ngng (hnh 7-13).

Hnh 7-13. S tnh ton knh dn tuyn cong.

Tnh ton dng b knh loi ny phi m bo tho mn hai yu cu: khng c xoy nc pha do lc ly tm gy nn,tc l dng chy phi bm st b; mt khc bn knh b li khng c qa ln, v nu r1 qu ln, tuy lc ly tm ca dng chy gim, nhng khi lng o knh s ln, v do knh dn di tn tht ct nc trong knh cng tng thm.

Qua nghin cu, tm c quan h gia cc yu t hnh hc ca knh (bn knh r1, r2, chiu cao ngng P) vi cc yu t thu lc nh sau:

2

1

2r

1 )rr

1(F4,1hr

+= , (7 - 12)

trong : r1,r2 - bn knh cong ca b li v b lm.

h - chiu su dng chy ca mt ct ngng trn.

Fr - h s Frout (= )ghv 2

.

Biu thc (7 - 12) cha th dng xc nh c r1 v r2 (hai n s).

Cng trnh tho l c h s lu lng ln nht ty thuc ng hc ca knh tho,

biu th qua t s oH

p.

Vi mi tr s oH

p nht nh, nu bn knh knh dn nh, ti ven b li s sinh hin

tng tch dng, h s lu lng ca cng trnh tho l s gim, mc d khi lng cng trnh gim. Ngc li, nu bn knh knh ln, khi lng cng trnh tng ln, mt khc tn tht ct nc theo chiu di cng tng, do h s lu lng s gim.

H

Pi = 0

I - I

r2or

1r

I

I

S S O O2 1 1 2

Vo


281

Hnh 7-14. Biu quan h

=

ot

o

Hp

fBr

.

Qua nhiu th nghim tm c ng cong quan h )Hp

f(Br

ot

o = cho h s lu

lng ln nht v khi lng knh l hp l nht (hnh 7-14), trong :

P - chiu cao ngng trn.

Ho - ct nc trc ngng trn.

ro - bn knh ca trc knh (= 2rr 21 + ).

BT - chiu rng trn nc.

Quan h trn cho bit nu ng hc, biu th qua t s oH

p ln, lu tc trong knh

nh, h s Frout nh, bn knh knh s nh, ph hp vi ngha vt l, do lc ly tm ca dng chy gim, hin tng tch dng khng xy ra.

Dng biu thc (7 - 12) v tr s ro tra th (hnh 7 - 14) s gii c r1 , r2 hp l .

b) Knh dn tuyn cong dng ng dng:

Knh dn tuyn cong dng b l cung trn tuy thi cng d dng, nhng tn tht ct nc vn cn ln do dng chy trong on cong gy nn v phn b lu lng trong cc khoang p vn khng ng u.

ng vin hai b knh v tuyn ca trc knh dn l tng nht l c dng ng dng v ngng trn t trng vi ng th. Vi iu kin bin nh vy, dng chy trong knh s l dng th, khng tch khi b v vung gc vi ngng trn. Trong trng hp ny lu lng ca p trn c th gn t ti tr s trong trng hp bi ton phng v s phn b lu lng trong cc khoang p l u nht.

2. Tnh ton thu lc dc nc sau ngng trn v tiu nng cui dc

Mc ch tnh ton thu lc dc nc l tm ng mc nc cao nht; xc nh ct nc, lu tc dng chy ti nhng mt ct in hnh ng vi nhiu cp lu lng khc nhau; kim tra s xut hin v x l nhng hin tng thu lc bt li trn dc nc.

0

0,1

0,2

0,3

P/Ho

10 20 r T/Bo


282

a) i vi dc nc c chiu rng y khng i:

dc trong dc nc ln hn dc phn gii (i > ipg), do dng chy trong dc nc l dng chy xit. Tu theo chiu su dng chy ti u dc nc, ng mt nc c th l ng nc bII hoc ng nc dng cII trong vng xit.

Xt cc trng hp sau y:

- Trng hp khng c ngng trn (hnh 7 - 15a). Pha sau ngng trn l dc nc, chiu su ti u dc nc, cui ngng trn s l su phn gii hpg, do ng mt trong dc s l ng nc bII. Nu dc nc di, su ti on cui s dn dn tim cn vi ng N - N, tc l gn bng su chy u.

- Trng hp c ngng trn thp (hnh 7 - 15b). Trong trng hp ny, su co hp sau ngng hc > ho, nn trong dc c ng nc bII.

Hnh 7 - 15. S tnh ton ng

mt nc sau ngng trn.

a) Khng c ngng trn;

b) Ngng trn thp; c) Ngng trn cao.

Hnh 7 - 16. S tnh ton ng

mt nc sau ngng trn c ca.

a) on chuyn tip di;

b) on chuyn tip ngn.

- Trng hp c ngng trn cao (hnh 7 - 15c) Ngng trn c th l p trn nh rng, ngng cao nn c lung nc xung u dc, hnh thnh on co hp hc < ho, do dc c ng nc dng cII.

Nu ngng trn l p trn thc dng th iu ny l chc chn.

- Trng hp c ca van iu tit (hnh 7 -16). Nu phn vo ca dc nc lm theo s chy di ca van, v c mt on chuyn tip i = 0, tu theo su on cui chuyn tip, trn dc nc s hnh thnh ng nc bII (khi h1> ho), hoc ng nc dng cII (khi h1< ho). Khi h1 > ho, trong on ny c nc nhy.

ng mt nc trong dc nc c th tnh theo phng php cng trc tip hoc phng php s m thu lc ca B.A.Bahmtev.

* Phng php cng trc tip :

D L = Ji

-

' (7 - 13)

h h o h c h1

h o

a)

h o

h c

b)

h 1

a)

b)

c)


283

vi:

+-

+'=D

g2vh

g2h

21

1

22

2 .

i - dc ca dc nc.

+=

222

22

2

121

21

2

RCQ

RCQ

21J

Ch s "1" l ca mt ct u, ch s "2" l ca mt ct cui ca on dc nc tnh ton (hnh 7 - 17).

Vic tnh ton c thc hin tun t t on u n on cui dc nc.

Hnh 7 -17. S tnh thu lc dc nc.

* Phng php s m thu lc:

T phng trnh vi phn c bn vit cho dng chy n nh trong knh h c dc y i (hnh 7 - 17) ta c phng trnh (7 - 14):

Fr1Ji

gBQ1

KQi

dldh

3

2

2

2

--

=-

-= , (7 - 14)

trong : K = f(h) = Cw R Nu thay hm K = f(h) bng hm s m K2 Ahx, nhn thy trong mt on dng

khng u, h thay i th x cng thay i theo, nhng rt t, do c th chn mt tr s x trung bnh:

x = 2h"lgh'lgK"lgK'lg

--

t J = x

BC.yi 2

v bin i s c phng trnh ng mt nc sau y:

- Khi i > 0 : )]()()[j1()(lhi

1212210

j-j---=- (7-15)

- Khi i = 0 : )]()[()i)(j(lhi

1212k21k

k ----=- (7-16)

1

DL

i1

V1

1h

2V

2

2

2h


284

trong : oh

h=h ; h0 - su chy u.

j(h) - hm s ph thuc x, c cho trong cc gio trnh thu lc hoc s tay thu lc. ix, hk - dc v su phn gii.

y(x) - hm s ph thuc x, c cho trong cc gio trnh thu lc.

x

BC.gi

j2

kk =

b) i vi dc nc c chiu rng y thay i dn:

Trong trng hp ny c th dng phng php cng trc tip tnh ton song cn ch ti mt ct tnh ton no th ly b rng y chnh ti mt ct .

Dc nc c chiu rng y thay i dn c th c cc iu kin sau y:

- Chiu rng thay i dn nhng chiu su khng thay i, tc l ng bin c chiu cao khng i. Trng hp ny c th tnh nh trn, hoc c th dng phng php ca V..Zyurin:

( ) ( )[ ]1221

21

tb

2

bb

gcl j-j

--

=D (7 - 17)

trong : ( )hj - hm s xc nh theo bng ca ph lc cc gio trnh thu lc vi s m thu lc x=2.

,KK

0

11 = ,K

K0

22 = i

QK0 =

- Dc nc c chiu rng thay i, cn chiu su c th thay i theo quy lut ng thng, hoc ct nc lu tc thay i theo quy lut ng thng. Trong thit k dc nc, thc t t dng nhng iu kin bin phc tp nh vy, nhng c th dng phng trnh B.T. Emtxv tnh.

c) Tnh ton on thu hp u dc nc:

Hnh 7 -18. S tnh ton on thu hp u dc nc.

a) Mt bng; b) Ct theo trc;

1. Knh dn; 2. Ngng trn; 3. on thu hp; 4. Dc nc.

I

1

pgh 12pgh

i > ipg

4

2

b)

1

B1

2

I - I

3

2av2g

2B 43 BB

a)3

4

+h$= 2gav

pg2

q I


285

Dc nc c chiu rng thay i dn tuy tit kim c khi lng cng trnh, nhng thi cng trn tuyn di phc tp. Thng thit k ng trn tho l c on thu hp sau ngng trn, trc khi i vo dc nc (hnh 7-18) cn nhm khc phc mt s hin tng thu lc bt li trn dc nc.

C th thit k on thu hp u dc nc theo hai phng php sau y:

1-Phng php cng trc tip (ng dng biu thc 7-13):

Nh trn trnh by, cn phi xc nh s bin i ca b khi dc v chiu su nc mt ct u v mt ct cui on thu hp bit. Theo phng php ny, chiu rng ti u dc nc khng khng ch trc c v gc thu hp c th khng tho mn iu kin thu lc, v nu b cui qu nh, s c th pht sinh hin tng sng xin.

2- Phng php cn c v c tnh dng chy sau ngng trn v u dc nc (p trn nh rng):

Gc thu hp, theo kinh nghim ly bng 022=q v vi tr s gc ny, dng chy khng b co hp t ngt. Chiu di on thu hp t Bt n Bd khng ln ly qu ln, v do c th gi thit tn tht thu lc trong on ny l khng ng k.

Bit rng chiu su dng chy ti cui ngng trn v u dc nc (tc l cui on thu hp) u bng chiu su phn gii, nn c th thit k on thu hp th no cho chiu su dng chy ti mt ct no trong c on thu hp u bng chiu su phn gii ca dng chy ti mt ct .

T nng trong cc mt ct ti on thu hp xc nh theo biu thc

' = ,hg2v

pg

2

+a

trong : -a

g2v2

ct nc lu tc.

Bit: hpg= 32

gqa

hoc: ( )g

vhgqh

22pg

23

pg

a=

a= ,

trong : q = hpg.v .

Do : .2

hg2v pg2 =a Nh vy: ' = .h

23h

g2v

pgpg

2

=+a

Ta thy, nu ly ng mt nc trong h cha lm chun th:

- Cao trnh mt nc trong mi mt ct bng ' - =a

g2v2

' - 2

h pg , tc l thp hn mt

nc h mt on bng 2

h pg .


286

- Cao trnh y on thu hp trong mi mt ct bng ,h23

pg-' tc l thp hn mt

nc h mt on bng pgh23

.

Do trong thc t thit k, khi bit lu lng qua p trn, chiu rng trn nc, chiu rng dc nc trn mt bng, xc nh chiu di v dng on thu hp; sau khi tnh c su phn gii trong mi mt ct, ng mt nc v y on thu hp c xc nh rt n gin v bo m ch dng chy ng vi iu kin bin: su cui p trn v u dc nc l su phn gii.

Phng php nu trn tng i n gin, cho php v c ng mt nc v ng vin y theo hai ng cong xung nhng nc ngy cng su hn.

d) Tnh ton on cong trong dc nc:

Trong nhiu trng hp, rt ngn chiu di dc nc, hoc trnh cc vt chng ngi, ngi ta lm dc nc c tuyn cong.

* Tn tht ct nc:

Trong trng hp dng chy khng tch khi b cong li (r1 1,5 Bd), tn tht ct nc c nu trong cc biu thc ca A.X.Ofitxrov, I.L.Rozovxki, Buxinxki... Trong thit kt c th dng biu thc ca A.X.Ofitxrov:

ht = 0,8 xqg2

vRB 20 , (7 - 18)

trong : ht - tn tht ct nc .

xq= 2(1 - cosq).

q- gc un cong. B-chiu rng dc nc.

R- bn knh cong ca trc dc nc.

Vo- lu tc trung bnh trn trc dc nc.

Tn tht ny s c cng vo tn tht ca dng chy khng u trn dc nc.

* dc ngang ca ng mt nc:

Do nh hng ca lc ly tm, mc nc ca b li thp hn mt nc ca b lm. Mt nc trong mt ct ngang ti ch cong c dng cong vng ln v dc Ir ti im c bn knh cong l r c tnh theo biu thc:

Ir = a= tggrv2

, (7 - 19)

trong : v - l lu tc trung bnh trn im trc i qua im cn tnh.

Trong iu kin khng c tch dng v xoy nc ti b li, lu tc trn mt ct ngang phn b theo quy lut din tch.


287

v =

1

2

rr

ln.hr

QrC

= , (7 - 20)

trong : r - bn knh ti im xt.

Q - Lu lng qua mt ct.

h - chiu su trung bnh.

r1, r2 - bn knh b li v b lm.

Mc nc pha b li s cao hn pha b lm mt cao Dh tnh theo biu thc:

Dh = ==r

r

2r

rr

1

2

1

drgrvdrI

-

22

21

1

222

2

r1

r1

rr

lngh2

Q (7 - 21)

i vi dc nc c chiu rng khng ln lm (Bd < 10m), gc nghing ca mt nc tnh gn ng nh sau (hnh 7 - 19a):

lr = tga = gRv 2

, (7 - 22)

trong : v - lu tc trung bnh ti trc dc.

R - bn knh cong ca trc.

Vn tnh ton trn c ngha rt thc tin trong thit k dc nc, v t tnh c chiu cao tng bo v dc ti on cong (tng pha b lm cao hn b li Dh) v c nhng bin php gim tr s gc nghing v chng tch dng khi b li.

Hnh 7 19. Mt ct ngang dc nc ti ch cong.

a) s tnh ton; b) mt ct ngang c y nghing;

c) tng phn dng gim Dh.

c)

b)

a)

b

a

a


288

Trong xy dng dc nc thng dng cc bin php sau y:

1. Ti on cong ca dc nc, y ngang ca dc nc lm nghing mt gc b < a (hnh 7 - 19b) gim khi lng o v chng hin tng tch dng.

2. Nu Bd v lu tc trong dc ln, c th lm cc tng phn dng phn dng chy trong dc nc thnh nhiu lung. Lm nh vy s gim c nghing ca mt nc (hnh 7 - 19c)

3. Bn knh b li nn chn 5.1Br

d

l

e) Vn hm kh trong dc nc:

Trong dc nc c lu tc ln, lp khng kh gn mt dng chy s b ht vo lp nc. Cc bt kh pha trn vo lp nc trn vng mt, chuyn ng cng vi dng chy, v do chiu su dng chy s tng so vi tnh ton khi khng c hm kh. Hin tng ny lm cho hai tng bn ca dc nc phi tng hn so vi tnh ton bnh thng.

C nhiu phng php tnh n hm kh trong dc nc khc nhau:

a) Phng php ca A.L. Nitriporovits. Chiu su dng chy c hm kh tng ln, c th xem tng ng vi kt qu do tng

nhm ca dc nc v dc nc c hm kh c nhm tng ng:

n' = zn , (7 -23) trong : n - nhm thc ca dc nc;

z - H s tng nhm, ph thuc vo d dc ca dc nc, ly nh sau (bng 7-8):

Bng 7-8. H s tng nhm x

i 0,1 - 0,2 0,2 - 0,4 > 0,4

z 1,33 1,33 - 2,00 2,0 - 3,33

Trong tnh ton ng mt nc, s dng h s nhm n'

Trong trng hp h nh c th s b tnh chiu su c hm kh nh sau:

h' = Ah , (7 - 24)

trong A - h s k ti nh hng ca hm kh (bng 7 -9)

Bng 7-9. Tr s ca h s A.

v (m/s) Chiu su h (m) 5 10 15

0,25 1,10 1,30 1,60

0,40 1,05 1,13 1,35

0,60 - 1,05 1,20


289

b) Phng php ca N.B. Ixatrenko. Tr s Frout (Fr) phn gii m t bt u c hm kh c tnh theo biu thc:

Frpg = 45 14

R1

D- (7 - 25)

trong :

RD

- nhm tng i ca dc nc trong dc b tng ly bng 0,02 - 0,04; trong

dc c m nhm gia cng ly bng 0,05 - 0,10.

Nu khng k n hm kh th:

Frpg = gRv2

Lu lng khng kh b ht vo tnh theo biu thc:

14

n

k

R145Fr)

R85,0035,0(

ww

--+==a , (7 - 26)

trong : n

k

ww

- t l gia khi lng khng kh v khi lng nc.

Chiu su dng chy c hm khi tnh theo biu thc:

hk = (1 + a)h. (7 - 27) g) Vn sng trong dc nc:

Khi dng chy trong dc nc c xit ln, trong dc nc c th xy ra hin tng sng truyn t trn xung di theo chu k. Chiu cao ca sng kh ln, vt qu cao an ton ca b dc nc to nn p lc ng ln bn y; v khi sng truyn xung dc nc, s lm vic ca b tiu nng s b ri lon, v nh hng n c ch lm vic ca knh tho sau b tiu nng.

Nguyn nhn ch yu pht sinh sng trong dc nc l khi t l hb

ln, dc ca

dc nc ln, do nh hng ca nhm y dc, lp nc gn y hu nh b gi li cn lp nc pha trn b trt i vi tc ln (hnh 7 - 20).

Hnh 7-20. Sng trong dc nc.

ho


290

Theo T.K.Voynits-Txanojnxki, dng chy trong dc nc khng pht sinh hin tng sng khi:

,12Bh2x)12(2

Bh2x

Fr1

o0

2

-+--

> (7-28)

trong : Fr, w, B, h - s Froul; din tch mt ct t; chiu rng; chiu su dng chy trc vng c sng.

x- s m thu lc, ly theo B.A. Bahmtv.

0a -h s ng lng, xc nh theo A.X Obrazovxki:

)k21)(k21(

)k1.()k1(

21

22

21

o ++++

=a (7 - 29)

Cg

k1 c= ; k2 = 2k1 (1-b/c0);

C: h s szi xc nh theo biu thc ca N.N. Pavlovxki.

c= 0,36 - hng s Karman.

c0,b - chu vi t v chiu rng y ca dc.

i vi dc nc khng c sng 0a = 1,037 - 1,15.

i vi dc nc c sng 0a = 1,01 - 1,039.

Khi tng 0a phn bn phi ca bt ng thc (7 - 28) s gim rt nhanh, do trong tnh ton s b, nn ly tr s 0a nh.

Theo th nghim ca E.P. Forov, hin tng sng trong dc nc xy ra khi c cc yu t sau y:

- Khi t l hb

ln.

- Khi dc y ln hn 0,025 - 0,030.

- Ch xy ra trong dc nc mt ct ch nht hoc hnh thang.

-Do nh hng ca sng t thng lu.

- Do thay i t ngt ca dc dc dc nc.

V vy E.P. Forov ngh lm dc nc c mt ct parabn hoc hnh tam gic, a

gic gim t l hb

. Nhng nhng dc nc ny kh thi cng v s gp kh khn ln

trong vic ni tip h lu.

h) Dc nc c m nhm gia cng:

M nhm gia cng trong dc nc l cc loi m c hnh dng khc nhau, xy lin vi y dc nc hoc c vi hai b tng thm nhm, tng ma st dc ng.


291

Kt qu ca vic xy cc m nhm nh vy lm tng chiu su dng chy trong dc, gim lu tc, thm ch c th bin dng xit thnh dng m, v lm nhim v tiu nng theo dc. V vy nhng m nhm gia cng c xy trn nhng dc nc c dng xit ln, cn phi gim lu tc ph hp vi yu cu ca vt liu.

Vic tnh ton dc nc c cc m nhm gia cng ch yu l da trn yu cu khng ch lu tc, chn c loi m nhm cn thit v sau tnh c cc yu t thu lc ca dc.

F.I. Pikalov v I.Ya. Fankvits da vo kt qu nghin cu th nghim xut lin h gia cc yu t thu lc v cc yu t hnh hc ca m nhm:

k = f(a,b), (7 - 30) trong : k - nhm ring.

k = hb;h;

vRi

C1

=bs

=a=

h - chiu su ca nc trn m nhm.

s - chiu cao m nhm. b - chiu rng ca dc nc tit din ch nht.

Quan h k = f(a, b) ph thuc vo dng m nhm. Theo th nghim khong cch c li nht gia cc cnh ca m l = ds v b thay i t 1 n 12 c biu th trong cc trng hp sau y:

1. M nhm hnh dch dc kp (hnh 7 21a)

1000k = 116,1 - 6,1a - 1,2b (7 - 31)

Dng trong iu kin 12 a 5 2. M nhm dch dc n (hnh 7 - 21b)

1000k = 85,8 - 3,9a - 0,8b (7 - 32)

Dng trong iu kin 8 a 3,5 v 6 b1 3. M nhm dng dm thng c gc (hnh 7 - 21c)

1000k = 47,5 - 1,2a + 0,1b (7 - 33) 4. M nhm dng dm trn (hnh 3 -21c)

1000k = 50,5 - 3,3a + 0,2b (7 - 34) 5. M nhm dng thang t xen k (hnh 7 -21d)

1000k = 54,2 - 2,1a + 0,33b (7 - 35)

Cc trng hp 3, 4 v 5 dng trong iu kin 8 a 3 6. M nhm hnh qun c (hnh 7 - 21e)

1000k = 52,0 - 5,1a + 0,8b (7 - 36)

Dng trong iu kin 5 a 2,


292

trong : b = Ch

Nb+

s-;

N - s m trong mt hng.

M nhm (hnh 7 - 21a, b, c, d, e) c sp t theo th t hiu qu tiu nng gim dn.

7. M nhm kiu bc thang t xui dng chy (hnh 7 - 21f)

1000k = 22 + 10 a-b 67,09 (7 - 37)

8. M nhm kiu bc thang t ngc dng chy (hnh 7 - 21g)

1000k = 38 + 10 a-b 33,19 (7 - 38)

Cc trng hp 7 v 8 dng trong iu kin i = 12%

9. M nhm bn thnh tng:

%6) khi i = 1(S - 121 + 5,35k = 1000%10) khi i = 1(S - 126 + 5,39k = 000%15) khi i = 1(S - 131 + 5,59k = 1000

(7 - 39)

Hnh 7 - 21. Cc loi m nhm gia cng.

i) Tiu nng cui dc:

Cui dc thng dng cc hnh thc tiu nng y, tiu nng phng xa, ngoi ra c th dng tiu nng mt v cc thit b tiu nng ph tr.

a) Tiu nng y: Vic tnh ton c thc hin vi lu lng tiu nng (lu lng cho (hc'' - hh)max ).

Chiu su b tiu nng khi s dng hnh thc tiu nng o b vi bi ton phng c thc hin trn h 3 phng trnh (7 - 40), (7 - 41), (7 - 42)

* Phng trnh ni tip mc nc thng h lu:

hc = )hE(g.2q

c0 -j (7 - 40)

h

c)

b)

a)

h

h

g)

f)

e)

d)

d


293

Hoc: F(tc) = 2

3

0E.

q

j (7 40a)

* Phng trnh nc nhy c vit di dng xc nh su lin hip sau nc nhy.

y tnh ton vi nc nhy ti ch vi su trc nc nhy l hc trong lng dn lng tr, mt ct ch nht.

hc'' = 0,5.hc

-

a+ 1

h.gq.8.

1 3c

20 (7 - 41)

* Phng trnh hnh hc:

Hb = s.hc'' = d + hh + DZ, (7 - 42)

trong : DZ = 2b

2

2h

2

h.g.2q

h.g.2.q

-j

. (7 - 43)

(7- 42) v (7- 43) c xc nh trn c s dng chy ra khi b l dng chy ngp qua p trn rng (hnh 7- 22). Khi iu kin khng tho mn th cn iu chnh cc phng trnh ny cho thch hp.

Hnh 7- 22. S xc nh chiu su b.

Cc bc tnh ton:

Bc 1: S b ly d1 = (hc'' - hh)max

Bc 2: Tnh '0E = (E0 + d1)

trong : g2vPhE

2

1oa

++=

P1 : chiu cao cui dc so vi y knh h lu

h, v : chiu su, lu tc dng chy cui dc.

Bc 3: Tnh hc, hh, DZ Bc 4: Tnh li d theo (7- 42)

Nu d gn vi d1 th l chiu su b cn tm. Nu khc th tnh li vi d1 l d. Vic tnh ton trn c th da vo cc bng ca Agrotskin tm hc, hc''. Hoc c th s dng my tnh vi vic tnh hc, hc'' trc tip.

Chiu cao tng tiu nng (khi s dng hnh thc xy tng) da theo hnh 7 - 23.

H

P

dhDZ

L L1 2

1

hP

h,v

1 hc


294

Trong trng hp ny, y knh h lu gi nguyn v xy mt tng c chiu cao l C chn ngang dng chy.

Hnh 7- 23. S xc nh chiu cao tng tiu nng.

Chiu su mc nc trong b l hb. Yu cu hb = s.hc''. Mt khc hb = H1 + C, v vy:

C=s.hc'' - H1 (7 - 44) Vi H1 l chiu cao ct nc trn qua tng, c tnh theo s p trn thc dng

mt ct hnh thang chy ngp.

H1 = 2''c

232

n )h.(q.

g.2g.2.m.q

sa

-

s, (7 - 45)

trong : sn - h s ngp c xc nh theo 1H

hn ;

m - h s lu lng ca tng.

Thay (7-45) vo (7-44) chng ta c chiu cao tng:

2"c

23/2

n

"c )h(

q.

g2g2mq

hCs

a+

s-s= (7-46)

Sau khi tnh c C cn kim tra li dng nc nhy sau tng. Nu sau tng cn c nc nhy khng ngp th cn phi lm tng th 2, th 3... n khi sau tng c nc nhy ngp hoc cn nhc c s dng gii php tiu nng ny khng.

H s lu lng ca tng tiu nng s b c th ly m = 0,4 n 0,42.

Tnh ton tiu nng b tng kt hp (bi ton phng) da trn nguyn tc chn chiu cao tng ln nht sao cho sau tng c nc nhy ngp, sau xc nh chiu su b sao cho trong b c nc nhy ngp s = 1,05 1,1 (Hnh 7 - 24).

Trnh t tnh ton nh sau:

* Xc nh chiu cao tng Co sao cho sau tng c nc nhy ti ch:

Xc nh su co hp sau tng hc1 l su trc nc nhy ti ch c su sau nc nhy l hh

hc1 =

-

a+ 1

h.gq.8.1.

2h

3h

20h (7 - 47)

Tnh t nng mt ct trc tng so vi y h lu sau tng:

P

h,v

h

DZh

1

c

n

CH1

hhHb

H01

H

P


295

E10= hc1 + 21c

2

2

h.g.2.q

j (7 - 48)

Tnh chiu cao tng Co (ng vi sau tng nc c nc nhy ti ch)

Co = E10 - H10 (7 - 49)

Vi H10 = 3

2

g2m.q

(7 - 50)

Hnh 7 - 24. S tnh ton b tng tiu nng kt hp.

* Xc nh do theo iu kin xy ra nc nhy ti ch trong b:

T do + Co + H1 = hb = (h''c)0

do = (hc'')0 - (Co + H1) = (h''c)o -

a-

g2V.E

2b

10

Ta c: do = (h''c)0 -

a- 2

0''c

2

10 )h.(g.2qE (7 - 51)

Trong (7 - 51) mun xc nh c do phi c (h''c)0 m (h''c)0 li ph thuc vo do (qua Eo) v vy phi tnh ng dn.

* Sau khi c do, Co chng ta ly C b hn Co mt cht:

C th ly C = 0,95Co

V khi : d = s.(h''c)0 - H1 C (7 - 52) trong H1 tnh theo (7 - 45).

* Chiu di b: L

Chiu di b (b o, b do xy tng) phi di nc nhy nm gn trong b, khi hiu qu tiu nng ca b mi m bo.

Chng ta c th tnh theo nhiu cng thc khc nhau nhng u c dng chung: L = L1 + L2 , (7 - 53)

trong L1: chiu di qu hoc chiu di nc ri tnh theo cc cng thc ca thu lc.

L2 = bln vi b = 0,7 0,8.

H

P

1

h,v

P

ch LL1 2

hc1C

d00

H1 hDZ0

Hb

H01E10

h

E0E '0


296

y: Ln l chiu di nc nhy tnh theo cc cng thc ca thu lc. *Chiu di sn sau th hai (Ls)

Ls = k H.q D (m) , (7 - 54)

Trong : DH: chnh lch mc nc thng h lu (m). q: lu lng n v cui sn tiu nng (m3/sm).

K: h s ph thuc vo a cht nn lng dn.

K= 10 12 vi ct mn, ct pha.

K= 8 9 ci ct to, t c tnh dnh.

K= 6 7 vi t st cng.

Phm vi s dng ca (7 - 54) l H.q D = 1 n 9.

b) Tiu nng phng xa: Ni dung chnh ca tnh ton xc nh c chiu di phun xa v chiu su h xi, t

xc nh c hiu qu tiu nng v tm cc bin php gia c. S tnh ton mng phun xem (hnh 7 - 25).

Chiu di phun xa ca tia dng ca mng phun nm ngang tnh theo biu thc:

l = 0,45jv HP + , (7 - 55) trong : v, h - Lu tc v chiu su dng nc ti cui mng phun khi khng c hm kh.

q - h s ph thuc P ly theo (bng 7 - 10).

Bng 7 10. Tr s ca h s j.

P(m) 1 2 3 4 5

j 0,97 0,94 0,91 0,89 0,87

Chiu di phun xa ca tia dng ca mng phun nm ngang tnh theo biu thc:

l = g

h)g(P2sinvcosvcossinv 222 +++j (7 - 56)

Vn tc ri vo nc vo ca tia dng tnh theo cc biu thc sau y:

vo = ;vv2y

2x + (7 - 57)

vx = j v;vy = ).hP(g.2 +j

- Khi mi phun c dc ngc im th:

+

j++

+-j= m0

h2m

2m0

2r iH.

h5,0hPii1.H.2L (7 - 58)

trong : j - l h s lu tc ; im - dc ca mi phun ;


297

hh chiu su nc h lu ;

H0 = E0 P hh -0,5h ;

E0 nng lng ton phn ca dng chy ti mi phun ly i vi y knh h lu.

Gc nghing ca tia dng ri xung h xi tnh theo biu thc:

tgq = V

)hP(g2 + (7 - 59)

Hnh 7- 25. S tnh ton mng phun.

a) Mt ct dc trc mng; b) S tnh lung nc khuch tn.

Chiu su ca h xi c th tnh nh sau:

- Coi chiu su h xi bng chiu su b tiu nng:

dx = s''ch - hh (7-60)

trong : s = 1,05 1,1 hh - ct nc h lu.

''ch - su lin hip vi su hc ti mt ct co hp y h xi.

- Theo E.A. Zamarin, xem lung nc nhp vo khi nc ca h xi s khuch tn ra tit din t cng tng th lu tc cng gim cho ti khi bng tr s khng xi i vi t ca nn. Phng trnh ca lung nc chy loe rng trong h xi nh sau:

L = 1,4 qlg cp

o

avv

, (7 - 61)

trong : q - lu lng n v ti ch lung nc ri xung nc, ly q 10m3/s.m. vcp - lu tc cho php i vi t nn.

q

2.a

L

B

h

B+2.L

.tgjh=2.L.tgj

P

l

b)

a)h

L

L

h

dx

h

t

r


298

a = 0,7 - 0,8 - h s gim lu tc.

Khi q 100, chiu su h xi ln nht: t = h + 2Ltga; (7 - 62) Khi q > 100 , chiu su h xi ln nht: t = Lsinq,

trong :

2a - gc trung tm trung bnh ca tia dng chy loe rng xc nh theo biu thc:

avcp = )Ltg2h)(Ltg2b(Q

a+a+ (7 - 63)

-Theo M.X. Vzgo: t = K.Ag2

vpq2

+ (7 - 64)

trong : q - lu lng n v.

A - h s gim chiu su h xi do c ngm kh, ly theo (bng 7 - 11).

K - h s xi l, ly theo (bng 7 - 12).

Bng 7-11. Tr s ca h s A.

Tr s ca h s A khi lu tc v(m/s) Chiu su h(m) 5 10 15 20 25

0,2 0,70 0,64 0,62 0,61 0,60

0,5 0,88 0,71 0,66 0,63 0,62

0,7 1,00 0,90 0,70 0,66 0,64

Bng 7 - 12. Tr s ca h s K.

Tr s ca h s K khi gc q () Loi t

0 12 25 40 60 90

Rt yu (ct chy) 1,4 1,8 2,4 2,8 3,3 4,5

Chc 1,4 1,7 2,0 2,4 2,7 3,3

- Theo Z.E. Myahulap, tnh chiu su h xi trong trng hp t ri:

t = 2,4q h0

h25,0ctg175,01

sinV

5,2W

+q-

q

-

h, (7- 65)

trong : vo - lu tc ch nc ri.

hh - chiu su h lu, sau h xi.

h - h s chuyn t lu tc trung bnh sang lu tc thc, ly bng 1,5 2,0.

W - th thu lc ca t: 0

od

75,1d)(2

Wgg-g

= . (7- 66)


299

d - ng knh ht t, m nhng ht nh hn n chim 90% trong cc ht.

gd, go - trng lng n v ca t nn v ca nc: go = (1 - S); S - lng ngm kh, ly bng 0,8.

Biu thc (7 - 66) c th tnh gn ng trong trng hp nn trong xem d l ng knh trung bnh ca vin ri rc.

- Theo A.N Patrasep: t ca t khng dnh l:

25,0

90

05,0

dZ

q9,3t

= , (7- 67)

trong : q - lu lng n v.

Z0 - chnh lch ct nc c k n lu tc ti gn.

d90 - ng knh ht m trng lng ca cc ht khng ln hn n chim 90%.

- Theo P.A. Yuzixki, (hnh 7 - 26) chiu di h xi trn nn theo hng dng chy tnh theo biu thc:

l = 4,5tp + 2hpg , (7 - 68)

trong : hpg - su phn gii.

Mi thng lu ca h c dc 1: 3,0; cn h lu 1: 1,5.

Hnh 7 - 26. H xi tnh theo T.A. Yuzixki.

a) S tnh ton ; b) Hnh dng v kch thc h xi.

Cc biu thc trn cho thy rng chiu su ca h xi ph thuc rt ln vo tr s ca q. V vy ngi ta tm cch gim tr s lu lng n v cui mng phun v tng khuch tn dng nc bng cc bin php sau y (hnh 7 - 27).

1. Lm gc loe cui mng phun 80 n 120.

2. Lm tng phn dng cui mng phun, gia cc cp tng gn nhau ly loe rng trung tm l 150-180 v bn cnh l 80 60, do lm tng gc loe chung.

3. Lm cc m phng cui mng: khi lu tc 8 - 15m/s th chiu cao ca m phng khong (1,2 - 2,0)h, chiu rng khe h khong 1,5 - 2,0 ln chiu rng m phng. Cc tia dng phng ra t m phng s ngm kh nhiu hn v b khng kh lm khuch tn nhanh. M phng c dng hnh cung vi gc phng ra b = 300 - 600 (hnh 7 - 27).

4. Mng phun lm kiu hnh loe cong hai chiu.

tp

l

lh

to

va

1:5

2h

1:3

R

h1

t


300

Cc bin php nu trn u qua th nghim trn m hnh thu lc xc nh hiu qu. Nu lung nc b khuch tn nhiu, phn tn thnh trn ma ln v hu nh khng gy xi l. Ngoi ra cn dng bin php mng phun c khe h y: nc mt phn chy qua khe v mt phn phun xung h lu, cng gim c rt nhiu chiu su h xi.

Hnh 7- 27. Cc loi m phng v tng phn dng cui mng phun. a) Tng phn dng ; b) M phng; 1. Bn y dc nc; 2. Cng xn; 3. Tr ; 4. M

phng, tng phn dng; 5. Cu i li.

3. Tnh ton thu lc bc nc sau ngng trn Kh nng tho l ca bc nc tu thuc kh nng tho ca bc u tin. Ngng ca

bc u tin thng l p trn nh rng (P = 0), nn dng biu thc chy khng ngp tnh ton.

Ni dung c bn ca tnh ton thu lc bc nc l xc nh kch thc ca mi bc vi iu kin thu lc c li.

a) Bc nc khng c tng tiu nng cui bc (hnh 7 -28): Trng hp ny t gp trong thc t thit k.

Hnh 7-28. Bc nc khng c tng tiu nng.

a) Bc nc di; b) Bc nc ngn.

a)

1 4

33

2

5 5

b5:8

4

b)5

15:184

4

4

33

54

12

6444 4

P

l1 lol

l2

hc

h'

h

PG PGCo

bo

i>0 pgh

hcC

C

Cohpg

hpg

a)

b)


301

Ti ch ri ca dng nc xung bc u tin, hnh thnh mt ct co hp vi chiu su hc v trn bc ny hnh thnh ng nc dng. Nu chiu di l ca bc khng , ng nc dng c chiu su ti cui cp h < hpg (hpg - su phn gii) v t nng ca dng chy ti mt ct cui cng khng phi l nh nht. Do ng nng s tch lu li trong dng chy v dng chy c th nhy qua bc tip theo, lm h hi cng trnh. V vy chiu di l ca mi bc nc nhiu cp, khng c tng tiu nng phi cha c ng nc dng t chiu su hc n hpg v c tnh an ton:

l = l1 + l0 + l2 , (7 - 69)

trong : l1 - chiu di phng xa ca lung nc t cao P xung

(l1 = P + hpg)

l0 - chiu di ng nc dng.

l2 - chiu di an ton, ly bng 2hpg.

Chiu di ng nc dng c th tnh s b theo biu thc:

l0 = )hh75,0(i1

cpgpg

- ,

trong : ipg - dc phn gii.

Nh vy chiu di ton b mt bc c th tnh theo biu thc:

l = P + 3hpg + l0 (7 - 70)

b) Tnh ton bc nc c tng tiu nng:

Vic lm tng tiu nng c mc ch to nn nc nhy ngp trong mi cp. Nh vy ch dng chy trong mi bc s n nh, bn y mi bc c bo v tt.

Mun cho trong mi bc c nc ngp lng, t l su lin hip phi l

sng y nhsinhpht s 2

h

h Nu 2

h

h

1

2

1

2 . Trong thc t thit k, tt nht c c

65hh

1

2 -= , l iu kin kinh t, v nu t l cng ln, hiu qu tiu nng cng ln,

nhng b tiu nng phi lm su (tc l d phi ln). Thng thng ly chiu cao mi bc 3 - 5(m).

Hnh 7 29. S tnh ton bc nc.

H

Ph2

d

H1

Po/N

Z

l1 l2

h1


302

Trnh t tnh ton theo cc bc sau: (hnh 7-29)

1. Xc nh chiu cao bc P; v d ca bc trc

2. Tnh : F( t c)= 23

0

q

Ej , (7 - 71)

vi: E0 =P+ d + H* + g2

2an

H* l H khi tnh bc u tin, l H1 khi tnh cho cc bc tip theo

j : h s lu tc.

d: chiu su trc bc tnh ton (khi tnh bc u tin th d = 0).

q: lu lng n v.

v: lu tc chy vo.

P: chiu cao bc tnh t y bc trn n bc tnh ton.

T F( t c) xc nh hoc tra ra t c, t c", t c: h2 = t c".E0 ; h1=t c.E0 . 3. Tnh su nc trong bc hb vi iu kin nc chy ngp c:

s=1,05 1,1; hb=s.h2 4. Tnh H1 nh qua p trn thc dng:

H1= 2b

232

gh2q

g2mq

-

(7 - 72)

5. Tnh chiu su b: d = hb - H1 (7 - 73)

6. Chiu di bc tnh theo (7 - 53)

Ring bc cui cng tnh nh tiu nng y cui dc nc.

7.4 ng trn ngang

I. iu kin s dng Khi khng c nhng v tr thch hp lm ng trn dc, v d lc khng tm c

a hnh thin nhin loi hnh yn nga, c bit lc gp th ni dc v hp, bt khi lng thi cng, h gi thnh, m bo cho p c an ton, ta c th dng ng trn ngang (mng trn ngang). Khi ta b tr ng trn ngang theo ng ng mc dc b. ng trn ngang l loi ng trn h gm cc b phn: ngng trn, mng trn bn nm trc tip sau ngng trn v on tho nc tip theo (c th gm mt on knh, cng trnh ni tip c th l dc nc hay bc nc v on knh dn nc v lng sng c).

ng trn ngang thch hp nht i vi b l , lc n gin nht v kinh t nht. Vi nn t mm, cng c th dng loi ng trn ngang, nhng t ngng trn n on ni tip u phi gia c, v do vn u t c th ln hn.


303

II. c im lm vic c im ch yu ca ng trn ngang l sau khi chy qua ngng trn vo mng

trn, dng nc s chy theo phng vung gc, hoc gn vung gc vi ngng trn. Dng chy trong mng trn ngang l dng bin lng, chy xon c kh phc tp.

Trong iu kin a hnh dc, sn ni l , lm ng trn ngang c rt nhiu u im: c th b tr ngng trn theo ng ng mc ca sn ni. Chiu di ca ngng trn m bo tho lu lng l cn tho hon ton tu thuc vo ngi thit k; m bo khi lng o t c hp l nht; do ct nc trn ngng thp, dc ca knh tho tng i ln, mt ct knh c th nh v khi lng cng trnh c th gim. Cng do ct nc trn ngng trn nh m c th gim c cao ca p v gim c tn tht ngp thng lu.

Loi ng trn ngang c th dng cho tt c cc loi cng trnh ln, va v nh.

III. Cc b phn ca ng trn ngang ng trn ngang gm: ca vo, ngng trn, mng bn, knh tho (dc nc, bc

nc), thit b tiu nng, knh dn v sng c (hnh 7 - 30).

1. Ca vo:

Thng c cu to nh mi ca vo cc cng trnh tho nc khc nhng n rng m hn, chiu di ngn hn (do ngng trn men theo ng ng mc).

2. Ngng trn:

Ngng trn b tr men ng ng mc. Hnh thc hay dng l ngng thc dng v ngng nh rng, c hay khng c ca van. Trn mt bng tuyn ngng thng thng hoc cong hoc gy khc, i khi c phn u mng cng b tr nc trn vo (hnh 7 - 30). Vt liu lm ngng l gch xy, b tng (vi lu lng trn, ct nc trn nh); b tng ct thp (khi lu lng v ct nc trn ln).

Khi b tr ngng cn ch m bo n nh: t trn nn t nguyn th hoc nn , mi a hnh khng b trt st xung pha h.

3. Mng bn:

Mng bn c mt ct ngang hnh ch nht hoc hnh thang. y mng theo phng dng chy c th thng vi dc no hoc cong ph hp vi tnh ton thu lc. Mi ca mng bn thng chn t 1:1 n 1:2 tu theo iu kin a cht nhm m bo mi khng b trt, st. c bit mi pha i ni cn ch ti c mi o pha trn mng bn.

Mng c th c bo v bng gch xy, b tng, b tng ct thp. y mng thng lm tch ri vi hai thnh bn. Knh tho tip theo mng, bn thit b tiu nng cui knh tho, knh dn v sng c cng tng t nh ng trn dc.


304

Hnh 7 - 30. ng trn ngang.

1. Mng bn; 2. Knh tho; 3. Cng trnh ni tip;

4. Knh dn; 5. p; 6. Cu; 7. Ngng trn.

IV. Tnh ton thu lc Ni dung tnh ton thu lc ca vo, knh tho (dc nc, bc nc), tiu nng... nh

trnh by ng trn dc. Trong phn ny ch yu trnh by tnh ton thu lc ngng trn v mng bn.

1. Tnh ton thu lc cho ngng trn:

Ta c th dng loi p trn ngng rng hay thc dng. Qua so snh kinh t - k thut, chng ta c th xc nh c cao trnh ca ngng v ct nc H trn ngng. trnh chy ngp v lm cho kh nng tho khng b gim, su mc nc trong mng trn ngang khng nn vt qu 0,4H. Vi b , t yu cu kinh t cng nn bo m cho dng chy qua p trn khng ngp.

Chiu di ngng trn c th tnh theo biu thc nh p trn:

- Lc chy khng ngp:

L = 2/3

0 Hg2mQ

(7 - 74)

- Lc chy ngp:

L = gz2h

Qj

, (7 - 75)

trong : H - ct nc trn ngng trn (m).

j - h s lu tc. h - chiu su nc trn ngng (m).

z - chnh mc nc thng h lu (m).

5

3

II

6

I

7II 1

I

2

4

1

A

A1

7

I - I II - II


305

2. Tnh ton thu lc cho mng bn

Sau ngng trn l mng bn (mng bn dc theo ngng trn). Dng nc chy trong mng bn rt phc tp. Nc qua ngng trn s chy dc theo tuyn mng bn xung h lu (knh dn hoc dc nc). mt ct thit k ca mng bn tho mn c yu cu tho l, chng ta phi xc nh c ng mt nc trong mng bn, ri cn c vo xc nh cao trnh y ca mng bn.

Dng chy trong mng bn l dng bin lng ba hng. y l mt vn thu lc

phc tp, cho n nay vn cha gii quyt c trit . y ch c th gii thiu mt vi phng php tnh gn ng.

a) Phng php dng u: (coi dng chy trong mng bn l dng chy u - y l phng php n gin nht).

t ti din tch chy nh nht, phi cho lu tc dng chy trong mng bn t ti lu tc cho php. Do trc lc tnh phi xc nh lu tc ln nht trong mng (ly gn bng lu tc cho php, tu theo vt liu ca mng bn).

Trnh t tin hnh nh sau:

1. Chia mng bn ra tng on, vi khong cch ca cc mt ct k t u mng bn l: x1, x2, x3,... xn,...

2. Lu lng chy qua mt ct x c xc nh theo biu thc:

ixQ = m0xi

2/3Hg2 (m3/s), (7 - 76)

trong : m0 - h s lu lng ca ngng trn.

3. Xc nh chiu rng b ca y mng bn v dc mi ca mt ct mng bn (mt ct ngang ca mng bn). Ta bit

ixQ , v nn c th tnh mt cch d dng chiu su

trung bnh h ti mt ct xi.

n y s b ta tnh ra chiu rng ca y v chiu su cc mt ct ca mng bn. Nhng quan h gia cc mt ct th phi qua vic tnh ng mt nc mi xc nh c.

4. Tnh dc thu lc:

i = 3/4

22

2

2

Rnv

RCv

= , (7 - 77)

trong : R - bn knh thu lc.

n - H s nhm ca mng bn, thng ly theo cc bng.

(do dng chy mng bn l dng chy xit).

5. Bit dc thu lc ti tng mt ct, ta c th xc nh tr s trung bnh cho hai mt ct:

itb = )ii(21

1nn ++ (7 - 78)

6. Coi chnh mc nc gia hai mt ct bng tn tht ct nc gia hai mt ct:

hw = l. itb , (7 - 79)


306

trong : l - khong cch gia hai mt ct.

Nh vy ton b ng mt nc trong mng bn c xc nh. Ta bit su ti mi mt ct, do c th xc nh cao trnh y ca ton b mng bn (c th lp thnh biu tnh cho r rng). Do xut pht t gi thit dng chy trong mng bn l dng chy u, nn kt qu tnh ra y ch l s b v gn ng s b chn c dc ca y, sau phi kim tra li bng mt phng php tnh ton chnh xc hn.

b) Phng php dng bin lng:

Gi thit dng chy l n nh, chng ta c th dng biu thc sau y v ng mt nc trong mng bn:

0vdQgQ

)v(dxidZdpg2vd f

20 =

q-a+++

g+

a, (7 - 80)

trong :

v - lu tc trung bnh ca dng chy ti mt ct nh.

Q - lu lng ti mt ct.

Z v p - to v p lc thu ng ti mt im ca mt ct.

If - dc ma st (c th xc nh theo dng chy u).

q - hnh chiu ca lu tc ca dng nhp vo phng ca dng c bn trong mng bn.

a0 - h s lu tc. Biu thc (3 - 76) cng c th vit di dng:

.QdQg

v1

dxRC

Qg2vdZpd 2

0

22

220

w

q-a

+w

+

a=

+

g- (7 - 81)

Gi y l to ca mt nc ti mt ct x (so vi mt chun nm ngang):

y = Z + gp

(7 - 82)

Ly hai mt ct tng i gn nhau, cch nhau mt on Dx, vit phng trnh (7 - 81) di dng sai phn ta c:

y1 - y2 = ( )

2tb

21

22

tb0

tb2tb

2tb

2tb

21

220

g2QQ

V1x

R..CQ

g2vv

w-

q-a+D

w+

-a (7 - 83)

y ch s "tb" dng ch tr s trung bnh cho c on, tc l trung bnh cng ca hai tr s 2 mt ct.

Trong trng hp dng nhp vo vung gc vi trc ca dng c bn ca mng bn th q = 0, lc phng trnh (7 - 83) c dng:

y1 - y2 = ( )

2tb

21

22

0tb

2tb

2tb

2tb

21

220

g2QQx

RCQ

g2vv

w-

a+Dw

+-a

. (7 - 84)


307

Chng ta c th tnh ng mt nc mt cch d dng bng my tnh, hoc c th tnh tay bng cch lp bng theo phng php th dn.

Gi s h1 l chiu su ti mt ct th nht m ta bit, h2 l chiu su ti mt ct th hai m ta cn phi tnh ra. Trc ht ta gi thit h2 tnh th. Ta s tnh ton theo th t trong (bng 7 - 13).

Nu ng thc: y1 - y2 = Dh + i0Dx (7 - 85) c tho mn (theo yu cu chnh xc ca thit k) th vic gi thit h2 l ng. Nu ng thc (7- 81) cha tho mn th ta phi gi thit li h2 cho n lc ng thc (7 - 85) tho mn. Trong ng thc (7- 81) th:

y1- y2 l chnh mc nc 2 mt ct tnh ton gn nhau tnh theo biu thc (7 - 84):

Dh = h1 - h2 , (7-86) trong : i0 - dc y mng bn.

Dx - khong cch gia hai mt ct.

Bng 7-13. Tnh ton ng mt nc trong mng bn

theo phng php th dn.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

. Dx

v 22 -

v 12

2g

Q2 t

b

C2 t

bw2 t

bRtb

Q22

- Q

12

2gw

2 tb

S th

t

mt

ct

Kho

ng

cch

k

t

u x

(m)

Lu

ln

g t

i mt

ct

tnh

ton

Qx(

m3 /

s)

Chi

u s

u h

(m)

Chi

u r

ng

mn

g b

n b(

m)

Di

n tc

h m

t c

t w

(m2 )

Lu

tc

v(m/s

)

Chu

vi

t c

(m

)

Bn

kn

h th

ul

c R9m

)

H

s S

zi C

v2

Q2

a0

Q2 b

t

C2 t

b

w2 t

b

Rtb

Dx

a0 y 1

- y

2

y 1 -

y2 =

Dh +

i 0D

x

Cao

trn

h

y (m

)

Cao

trn

h m

t n

c

(m)

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Thng vic tnh th bt u t mt ct cui cng ca mng bn (mt ct ni tip gia mng bn v on knh ni tip). Chiu su mt ct cui ny l mt iu kin bin quan trng m ta cn xc nh trc. Chiu su ny tu thuc vo quan h gia dc ca y mng bn v knh dn tip mng bn. Trng hp ta thng gp nht l trng hp dc y mng bn nh hn dc phn gii v dc ca on knh ni tip ln hn dc phn gii. Trong trng hp , theo kt qu hng lot th nghim chuyn m chng ti tin hnh, ta c th ly chiu su ca mt ct cui ca mng bn bng chiu su phn gii ng vi lu lng tnh ton ti mt ct (hcui = hpg).

Nhng kt qu tnh ton ra theo phng php ny vn l tng i, v thc ra, dng chy trong mng bn l dng bin lng 3 hng rt phc tp. Mt nc ti mi mt ct


308

cng khng u, mc nc ti pha thnh i din vi ngng trn thng cao hn cc mc nc khc trn cng mt ct. C nhiu vn m ta c th tip tc nghin cu.

c) Cc phng php khc:

rt ra biu thc tnh ton ng mt nc trong mng bn, c th chia hai loi xut pht im:

- Xut pht t nguyn l ng lng.

- Xut pht t nguyn l ng nng.

1. T.R.Camp v W.E.Howlomd xut pht t nguyn l ng lng, cho b khng i, rt ra biu thc:

Dy = 2D tb

2tb

2

.

g2V2

g2V

+ , (7-87)

y cc k hiu cng nh trn. 2. I.M.Konovalov xut pht t nguyn l ng nng rt ra phng trnh c bn ca

dng bin lng sau y:

Dh = bxqgxq.2ig

2tb

23tb

tb2

tb3tb

-ww-w

Dx, (7-88)

hoc: Dy = D

D+

tb

2tb

2

xx2

y2V

g2V

, (7-89)

trong : Qx = q.x

Cc k hiu khc nh trn.

7.5 Ging tho l

I. iu kin s dng Ging tho l l mt loi cng trnh tho l b mt, thng c xy dng ngoi thn

p, l cng trnh c s dng lu di tho l trong h thng u mi thy li. Ngi ta thng s dng cc tuy nen, ng ng c dng dn dng thi cng trong thi gian xy dng kt hp vi ging ng tho l trong thi gian vn hnh. Ging tho l c s dng trong cc trng hp sau y:

1. B p l , c a hnh dc v hp khng thun li cho vic xy dng cc cng trnh tho l khc.

2. Bn thn ng hm hoc ng ng l cng trnh dn dng thi cng.

3. Ging tho l thng c xy dng trong cc u mi thy li ca h cha c p dng bng vt liu a phng, c trng hp p dng l p vm, p bn ta v cc loi p khc.

4. Lu lng tho qua ging ng c th n 5.000m3/s v c ct nc n 100m.


309

II. Cc b phn ca ging tho l 1. Ming vo:

Trn mt bng l ngng trn hnh trn ng tm vi ging ng, c trng hp do a hnh phi b tr lch tm. Ming trn c th l hnh elp, mt phn ca hnh trn. Trn nh trn c th khng c ca van hoc c ca van. Ca van thng dng l van phng, van cung, van tr; a s trng hp dng khng c ca van. Trc ca van c th c kt cu hng dng.

2. Ging:

L phn ni tip gia on ming vo vi ging. Ni chung ging c dng thng ng, tm ca ming vo trng vi tm ca ging. C trng hp do s ni tip gia ging vi ng hm dn nc c kh khn mi bt buc t ging hi nghing. T ming vo n ging bao gm cc on: on loe trn (ti ming vo), on thu hp ni tip, on vo vi ging, on ging ng ni tip vi ng hm (hnh 7-31).

Hnh 7-31. Cc b phn ca ging tho l

Hnh 7-32. Ging tho l trong cng trnh Melrin Iran

(kch thc trong hnh ghi theo m)

Ging tho l t ca vo n ng hm dn nc c th b tr ton b trong ni (hnh 7-32) l ging tho l Melrin Iran thuc loi trn.

Ging tho l c th b tr mt phn trong v mt phn on vo hnh thc thp bng b tng ct thp (hnh 7-33).

P

yy

H

h

Ro

Td

dn

y

n

do=dmin

max

xR

O Hp

226,6233,5

90

22,8

R=23

,090

8,0

4,0

189,6

8,0

23,5

192,8

270

b)

a)


310

Hnh 7-33. Ging tho l Pontezi ()

Ging tho l c th b phn ng ng dn nc b tr di p vt liu a phng, phn ging ng l thp bng b tng ct thp t trong h cha (hnh 7-34).

Hnh 7-34. Ging tho l c ct nc cao

a. ytem - Malaixia

b. Trn sng Relipka (Ch. Sc)

3. ng hm hoc ng ng:

L on nm ngang ni tip vi ging ng bng on cong c bn knh r>(2,5-4)d; (d - ng knh ca on ni tip). ng knh ca ng hm hoc ng ng c thit k theo yu cu dn dng thi cng v c dng lm cng trnh tho l vi kh nng tho theo yu cu. Cui ng hm, ng ng cn c kt cu tiu nng.

III. Kh nng tho nc Lu lng chy qua ging ng ph thuc vo ch chy vo ming trn ngp hay

khng ngp (cn gi l trng thi c p hay khng p).

1,45

1,25

2,25

1:21:287,3

1:3.2

27,5

24,0

2,

50

43,1

79,3

b)

1:1,251

:3

1:1,5

1: 1,2

1:2

1:2

1:2,5

1:2

234,7

167,6 a)

81,3

236,5

21,223,72

1,1

1:2

2770,0

0,35,6

20,0

1

802,0800,0


311

1. Trng hp chy khng ngp vi ngng vo l hnh trn:

23

0Hg2R2.mQ pe= (7-90)

trong : Q - lu lng

H0 - ct nc trn nh trn.

2pR - chu vi ngng trn (R - bn knh loa trn). m - h s lu lng.

e - h s co hp. 2. Trng hp chy ngp c xem l chy c p vo ging ng:

)ZH(g2Q h +mw= (7-91)

trong :

m - h s lu lng;

wh- din tch mt ct ngang ca ng hm. Z- khong cch t nh trn n mc nc h lu, khi ca ra b ngp, hoc tnh Z l

khong cch t nh trn n pha di trn cng ti ca ra mt on bng (0,15-0,2)d; (d - ng knh ca ng hm).

Hnh 7-35. S tnh ton thy lc ging tho l.

H s lu lng m c tnh:

dl1

1

l+x+

=m

(7-92)

trong :

x+1 - tng h s tn tht cc b. l - h s cn theo chiu di. d, l - ng knh v chiu di ng hm c p.

H Q

QmQn

Ho

d

z(0

,15

0,2

) a

Q=mw 2g(H+z)

DR

H

bnO

3/2Q=mL 2g HmC


312

H s khng cc b x gm h s khng ch ming vo (xv), h s khng ch ca ra (xr), h s khng ch un cong (xc) v h s khng do thay i tit din (xt), y ch nu bng tnh h s khng ca ming vo (xv), v l c th ca ging tho l (bng 7-14).

Bng 7-14. H s khng ming vo ging tho l.

H/R 0,1 0,2 0,3 0,4 0,5 0,6

xv 0,123 0,075 0,049 0,043 0,040 0,039

Nhn xt: Qua s tnh ton kh nng tho ca ging, thy rng ng quan h Q = f(H) gm 2 phn (hnh 7-35).

- Phn th nht: on ob tng ng vi ch dng chy khng ngp ca ming loa trn hoc ca ging tho l ni chung.

- Phn th hai: on bc tng ng vi ch chy ngp ca ming loa trn hoc ca ging tho l ni chung.

Trong phn th hai, dc ng cong kh ln, tc l ct nc tng nhanh khi lu lng tho tng. Trong trng hp ny kh nng d tr tho l ca cng trnh nh hn ca cc cng trnh trn h khng p. cng l nhc im chung ca cc cng trnh tho l c p.

V d: Vi lu lng tho Q = 200m3/s; ng trn c B = 22,8m; H0 = 3m; m = 0,38m; ng hm c p c w = 12,57 m2; Z = 27m; m = 0,69. Khi mc nc cao hn mc nc trn 1,5m th kh nng tho ca ng trn s l Q = 490 m3/s (do Z + 1,5m), tc l ch tng 7,5%.

V vy khi thit k ging tho l, thng theo ch chy khng ngp, nh vy p dng nc khng phi lm qu cao. Mun t c yu cu ny, c th s dng hai biu thc (7-82) v (7-83) xc nh cc tr s R, d, H hp l.

Cho nhng tr s R khc nhau v lp cc h ng cong Q = f(H, R) i vi ming loa trn theo ch chy khng ngp. Cho nhng tr s d ca hm ngang khc nhau, t tnh ra h s m v lp cc h ng cong Q = f(H + Z) i vi hm ngang v vi ging tho l ni chung theo ch chy ngp. Ta c th hnh 7-33, trong c hai h ng cong: ng vi chy khng ngp v ng vi chy ngp. Dng th ny c th vi tr s H v Q cho trc chn c R, d, hoc ngc li t R, d, chn c H trong phm vi chy khng ngp.

Kh nng tho nc trc ht ph thuc vo ch lm vic ca ngng trn, c th xc nh nh sau:

46,0RH

< - p trn chy khng ngp.

46,0RH

= 1,0 - p trn chy ngp.

0,1RH

= 1,6 - phu b ngp.


313

6,1RH

> - phu ngp lng.

Hnh 7-36. Biu tng hp tnh ton thy lc.

- Khi p trn chy khng ngp, kh nng tho c tnh theo cng thc (7-90) hoc

23

0Hg2)nR2(mQ d-pe= ,

trong : m - h s lu lng;

R - bn knh phu.

g2VHH

2

0a

+= , H - ct nc trn nh phu.

V0 - vn tc n gn nh phu.

n, d, e - s tr pin, chiu rng tr pin, h s co hp ly gn bng 0,90 (khi khng c tr pin e = 1).

H s lu lng tnh theo cng thc ca N.I.Romank (khi c kt cu hng dng v

RH

= 0,20 0,38, RP

= 0 1,0):

--

-=

32

21

RP103,0

RH068,0490,0m , (7-93)

trong : P - chiu cao ngng so vi y knh dn vo.

Khi khng c kt cu hng dng, h s lu lng vn c th tnh theo cng thc trn, nhng gim i 6%.

- Khi phu b ngp

> 6,1

RH

, kh nng tho nc c tnh theo cng thc (7-91).

z+H

xz

0,5Q 0

Q

Q


314

iv. chn khng v bin php chng chn khng Trong nhng iu kin thy lc khng ph hp, vi iu kin thit k, hoc do cha

xt ht cc nh hng, trong ging tho l s hnh thnh chn khng. Nhng vng d tn ti chn khng c nu hnh 7-37.

Hnh 7-37. Vng chn khng trong ging tho l ct nc cao.

Nhng vng chn khng pha di ngng trn l c li, tng c h s lu lng, cn nhng vng khc cn phi trnh.

Theo N.P.Rozanov, tr s chn khng cho php c tnh theo biu thc

( )g2

vPPh2

0hk

cpck d-g-

g= , (7-94)

trong : (hck)cp - chn khng trung bnh cho php (m);

gkP - p sut kh tri (m);

ghP - p sut hi nc (m);

v - vn tc nc trn vng chn khng(m/s);

d0 - h s, ly bng 0,1. C th tnh gn ng chn khng trung bnh cho php theo biu thc

( )g2

v1,075,9h2

cpck - (7-95)

Trong cc vng xy ra chn khng, phi bo m iu kin:

(hck)max< (hck)cp

Theo N.P.Rozanov, tr s chn khng trong cng trnh khng nn vt qu 7,5-8,5 (mt ct nc); vi tr s nh th, khng gy nn nguy him v kh thc.

Cc nghin cu v chn khng trong ging tho l cho bit:

- Chn khng trong phm vi ngng trn c tr s ln nht khi tho lu lng bng (0,75-0,85) lu lng tnh ton;

BP


315

- Trong phm vi ging ng v trong on cong ni tip gia ging ng v ng hm ngang thng tn ti chn khng.

Chn khng c tr s ln nht mp trn ca on un cong v thng gy nn xm thc btng. chng cc tc hi do chn khng vt qu tr s gii hn gy nn, ngi ta tm cch a khng kh vo cc ni ny.

A.N.Ahutin kin ngh tng din tch tit din cc ng a khng kh vo trong ging ng ct nc cao nn ly bng (10-15%) tit din ngang ca ging.

C th dng biu thc gn ng tnh tng din tch ng a khng kh.

( )

)m(h

g2

WF 2

k

cpckk g

m

= , (7 -96)

trong : W = k0Q - khi lng khng kh cn a vo;

k0 - h s hm kh;

Q - lu lng nc chy qua ging;

mk - h s lu lng ca h thng a khng kh vo ; (hck)cp - tr s chn khng trung bnh cho php tnh theo biu thc gn ng (7-95) ;

gk - dung trng khng kh di p lc khng kh bnh thng, ly bng 12,3 N/m3 ;

Tuy nhin vic dng nhiu ng a khng kh vo cng gy nhng kh khn v thi cng, qun l. Bin php tt nht l lm ging ng c tit din thu nh dn t trn xung theo quy lut ngc vi s tng ca lu tc.

V. Thit k cc b phn ca ging tho l. 1. Thit k ming loa trn.

Loa trn l mt loi p trn tuyn trn, h s lu lng ca n, nh trn ni, ph thuc vo loi ngng trn: kiu p trn thc dng, p nh rng hoc cc loi khc.

Vic thit k ming loa trn, khng nhng ch xc nh dng ngng m phi tnh c on loa ni tip t ngng n ging.

Ging tho l c 2 loi loa trn (hnh 7-38): c mt phng nghing v khng c mt phng nghing.

a) Loa trn c mt phng nghing (hnh 7-38a). Nu gi R l bn knh ngoi ca ngng trn, loi loa trn ny c dng khi R (6-8) H v khi c a hnh thun li, loa trn ny c on phng nghing v on loa ni tip.

on phng nghing c tc dng lm vic nh p trn nh rng, t nghing mt gc b = 6-80 a nc vo ming ging c tt. H s lu lng ging p trn nh rng m = 0,36.

Chiu di ca on phng nghing L = (3-4) H (0,4-0,5) R. Kh nng tho l tnh theo biu thc (7-90).

Nu c tr van th tnh theo biu thc (7-92).


316

Chiu su dng chy cui on phng nghing ly bng 0,65H0 v lu tc ti tnh theo biu thc:

00

0 H)65,0(r2Qv

p= , (7-97)

trong : r0 - bn knh ca loa:

b--= sin2hLRr 00

Hnh 7-38. Tnh ton loa trn ca ging tho l:

a) c mt phng nghing; b) khng c mt phng nghing.

ng vin ca loa a nc xung ging, phi tha mn yu cu khng c hin tng tch dng khi y. c nhiu th nghim chn ng vin ca loa trn dng elip, nhng dng thch hp nht l dng parabol theo phng trnh ng tia nc vi lu tc ban u l

0v v c xt ti hng ca vect lu tc (nghing mt gc b so vi mt phng ngang).

Trc ht tnh qu o ca tia dng nm gia lp nc trn c gc ta ti cui mt phng nghing v c lu tc 0v (h.7-38a). T c:

b= cosvv 0x v tcosvtvx 0x b== ;

b= sinvv 0y v 2

02

y gt21tsinvgt

21tvy +b=+= ;

Kh t trong cc phng trnh trn, s c phng trnh ca qu o tia dng gia lp nc trn nh sau:

b+b

= xtgcosv2

gxy 20

2

(7-98)

Khi x = r0 ta c y = ymax v do xc nh c im gp nhau ca qu o tia dng t hai bn, cng l ch kt thc khu vc ming loa trn. Lu tc ti mt ct ny c tnh theo biu thc:

0,325.H.sinb

Rb

L

H

R

y

x

r

x

y

y

vn

v

hn

n

n

h

y

H OO x

n nh

no

o xVo

n

a) b)


317

20maxmax0r vgy297,0vv +== (7-99)

Do ng knh cn thit cui loa tnh theo biu thc:

max

0 vQ4d

p= (7-100)

Khi bit phng trnh qu o tia gia lp nc, v nu bit chiu dy hn ca lp nc ti cc im, c th v c ng mt nc v y, tc l ng vin loa trn (h.7-38a) vi:

nn0

n v)xr(2Qh-p

= , (7-101)

trong : 20nn vgy297,0v += (7-102)

T qu o tia dng v ng vung gc ln trn mt on bng 0,5hn s c ng mt nc, v ly xung pha di 0,5hn s l im trn ng vin loa trn.

b) Loa trn khng c mt phng nghing (h.7-38b). Trng hp ny thng gp khi ngng trn cao, c dng p trn thc dng, dng trong trng hp R5H.

H s lu lng trong tnh ton s b ly bng m = 0,46. Chiu su trn ngng trn.

h0 = 0,75H (7-103)

Lu tc ti )H75,0(r2

Qv00

0 p= , (7-104)

trong : r0 = R v vect lu tc c hng ngang.

ng vin ca loa trn cng c tnh ton nh trng hp trn, nhng n gin hn (h.7-38b).

Trc ht tm phng trnh v qu o ca tia dng gia lp nc c gc ta ti nh ngng trn v lu tc ti bng 0v (b = 0).

Ta c: 0x vv = v tvtvx 0x ==

0vy = v 2gt

21y =

Kh t trong cc phng trnh trn, ta c phng trnh qu o nh sau:

20

2

v2gxy = (7-105)

Lu tc v su lp nc ti im bt k tnh theo cc biu thc sau y:

nn

20nn

v)xR(2Qh

vgy297,0v

-p=

+= (7-106)

Khi bit nv , hn s v c ng mt nc v ng vin loa trn.


318

Ging nh trn, loa trn kt thc ti v tr khi: y = ymax ng vi x = r0 = R So snh khi tr s lu ng tho bng nhau, loa trn khng c mt phng nghing c

ng knh ngng trn nh, do d b tr. Nhng khi ct nc ca hai loi ngng trn bng nhau, ng mt nc ca loi loa ny gp nhau di su, do c th c hin tng khng gp nhau trong phn ging ng. Do cn phi thit k mt cch hp l. Loa trn c mt phng nghing c th d b tr ca van.

Trong trng hp c ca van, ct nc trn ln nht t ti 4,5-5m; cn khi t trn ct nc trn thng ly bng 1,0-1,5m.

tng kh nng tho nc ca ging tho l, ngoi vic tm cch tng h s lu lng, mt phng hng quan trng l tng chiu di trn nc. Mt s tc gi a ra loi ming trn hnh hoa hng trn mt bng.

Cc ging tho l loi ny c xy dng ti h cha nc Sacno (Angiri) v trn sng Vorotok (Nga).

c im ca n l cc cnh hoa lm cho chiu di trn nc tng ln nhiu ln, do c th tho c lu lng ln vi ct nc kh nh, rt thch hp vi loi t trn. Ti h cha nc Sacno vi ct nc 1,2m tho c 360m3 /s.V.K.Xanoyan ngh phng php tnh dng ca hoa hng nh sau:

Ti mi im trn cnh hoa lu tc u phi vung gc vi ngng trn. Mun gii quyt vn ny, tc gi dng l thuyt hm s phc tnh c m hnh dng chy th, m ng vin ca cc cnh hoa, tc l mp ngng trn tng ng vi ng th (hnh 7-39).

Hnh 7-39. S tnh ming trn hnh hoa hng

Tc gi ngh phng trnh lin h gia cc ta ca cnh hoa nh sau:

n

n2

2C

cosa

-a=q , (7-107)

trong : ar

=a

r - khong cch t tm ging n cnh hoa; C - h s; n - s cnh hoa, Trnh t tnh ton nh sau:

- trc ht chn bn knh rmin v rmax ca ging;

r

r

qr mi

n

maxx

M(r,q)

y


319

- tnh C v a theo h thng phng trnh (qua din ton ca tc gi);

++=r

=a

-+=r

=a

;)1C1(a

;)1C1(a

n1max

max

n1min

min

(7-108)

- Thay cc tr s C, a vo phng trnh (7-107), s tnh v v c ta dng ca tng cnh hoa ca ging.

Sau khi nc trn qua cc cnh hoa, mi cnh lm vic nh mt mng trn nc chy t ba pha. Do theo chng ti, c th dng phng trnh tng dn lu lng ca phng php tnh mng trn ngang tnh chiu su cn thit ca mi cnh.

2. Kt cu hng dng ca loa trn.

Nh trn ni, do nh hng ca b nn nc chy vo loa trn khng vung gc vi ngng trn, m c th thnh dng chy xon lm cho lu tc v lu lng n v phn b khng u quanh ngng. iu khng nhng lm gim h s lu lng ca cng t

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