hot wire construction

8/12/2019 Hot Wire Construction

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App l. sci. Res. Vol. A2

O N T H E C O N S T R U C T I O N O F H O T - W I R EA N E M O M E T E R S F O R T H E I N V E S T I G A T I O N

O F T U R B U L E N C Eb y B . G . V A N D E R H E G G E Z I J N E N

Kollinklijke/Shell Labo ratoriu m Delftumma r y

A descr ipt ion is g iven of the construct ion of a hot-wire anemometer forthe inves t iga t ion o f tu rbu lence and of the meth od of spo t weld ing tungs tenwire to the supp or t s o f th i s anemom eter . The weld ing equ ipme nt hasbeen developed by the Royal Dutch /Shel l -Labora to ry a t Del f t .Th is descr ip t ion i s p receded b y a r ev iew of the r equ i rements which haveto be sa t is f ied ; the p r inc ipa l d imens ions o f the , ,Del f t anem om eter : w i relength, wire diame ter and the dimensions of the su ppor ts follow f romthese r equ i rements . I t seems tha t the wi re leng th shou ld fo r the p resen tpurpose no t exceed 0.5 m m and tha t the wi re d iame ter shou ld be 0 .005 m mor less. 1 Intro ductio n I n r e c e n t y e a r s e x t e n s i v e r e s e a r c h e s h a v e b e e n

i n p r o g r e s s , b o t h i n E u r o p e a n d i n t h e U . S . A . , i n t o t h e r a n d o m ,s e c o n d a r y m o t i o n a c c o m p a n y i n g t u r b u l e n t f l ow . T h e i r m a i n p u r -p o s e is t o p r o v i d e t h e b a s i c i n f o r m a t i o n a s t o , a n d n u m e r i c a l d a t a o f,t h e m a g n i t u d e o f t h e v a r i o u s v e l o c i t y c o m p o n e n t s , t h e i r ro o t m e a ns q u a r e v a l u e s ( i n t e n s i ty o f t u r b u l e n c e ) , t h e t u r b u l e n t s h e a r i n gs t r e s s e s, t h e d i m e n s i o n s o f t h e s m a l l e s t e d d i e s r e s p o n s i b l e f o rd i s s i p a ti o n , a n d t h e d i m e n s i o n s o f b o d i l y m o v i n g m a s s e s o f f lu i d( sc a le o f t u r b u l e n c e ) . I n v e s t i g a t i o n s o f c o r r e l a t io n s b e t w e e n f l uc -t u a t i o n s o f v e l o c i t y a n d t e m p e r a t u r e - - o r c o n c e n t r a t i o n o f m a t t e r- - a r e r e q u i r e d i n t h e c a s e o~ i n t e r c h a n g e i n fi e ld s o f f l o w w i t h h e -t e r o g e n e o u s t e m p e r a t u r e (o r c o n c e n t r a t i o n o f m a t t e r ) . T h e s e s ta t i s-t i c a l q u a n t i t i e s h a v e t o b e d e t e r m i n e d i n o r d e r t o a r r i v e a t a f u l lu n d e r s t a n d i n g o f t h e m e c h a n i s m o f d i ff u si o n b y t u r b u l e n ce .

T h e d e v i c e f o r m e a s u r i n g t h e l o c a l i n s t a n t a n e o u s v e l o c i t y in th et u r b u l e n t f lo w c e r t a i n l y i s n o t t h e l e a s t i m p o r t a n t p a r t o f t h e e x -

3 5 1


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35 B G VAN DER HEGGE ZIJNEN

perimental setup, because it must yield results that come as nearas possible to true point measurements. Moreover, it must be able toreproduce the velocity fluctuations accurately up to frequencies ofmany thousands per second and cause the smallest possible distor-tion of the field of flow.

The so-called hot-wire anemometer has proved to be an instru-ment that fulfills the requirements. Only a few dat a on the construc-tion of hot-wire anemometers for the investigation of turbulence Callbe traced in literature, and none on the way of attaching tungstenwire to the supports by welding, as is done at present in some la-boratories in the U.S.A. For this reason we have drawn up the fol-lowing description of the tungsten hot-wire anemometer and theequipment for spot welding fine tungsten wires, both developed inthe Royal Dutch/Shell Laborat ory at Delft.

The method for spot welding adopted has been Suggested by H.C. R i e s (Chem. Eng. Dept ., Universi ty of Delaware), who kind lyfurnished details of his setup.

2. Requ irem ents imp osed upo n a hot-wire anemometer /or theinvestigation o] turbulence. In designing an anemometer the questionarises: what scale of turbulence is to be expected ? The experimentsby D r y d e n and co-workers 1) on the scale of turbulence pro-duced by screens with a mesh of ~,~,11,, ,,,~qx,,and 5 (with corre-sponding wire diameter) show that the scale of turbulence at adistance x of 10 times mesh downstream of the screen is roughly

of the mesh and increases with increasing x (compare fig. 7 of5D r y d e n's report). This means for the finest screen investigatedby D r y d e n and co-workers a scale of turbulence of about 2 mmat a distance x ~ 100 mm downst ream of the screen.

On the other hand, experiments on the distribution of the time-mean velocity across turbulent jets suggests a mixing leng th , orconvection path, of the bodily moving fluid masses of about1 ~-0.017 x, when x denotes the distance from the cross-sectionconsidered to the hypothetical origin of the jet. Again, withx ~ 100 mm, I turns out to be roughly 1.7 mm. I t appears thereforethat the part of the anemometer intended for spot-measuringthe velocity must be smaller than 1.5 mm, preferably much smallerwhen details within the scale of turbulence are to be detected. Infact, the dimensions of the anemomete r ought to be reduced to the


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H O T W I R E A N E M O M E T E R S 353d i m e n s i o n s o f t h e m i c r o v o r t i c e s w h i c h a r e - - a c c o r d i n g t o T a y -1 o r ~ - - r e s p o n s i b l e f o r d i s s i p a t i o n . T h e s e m i c r o v o r t i c e s m a y h a v ea d i a m e t e r o f a b o u t 0 .5 m m o r m o r e f o r v e l o c i ti e s b e lo w 5 0 m / s e c .H e n c e , i n g e n e r a l , t h e d i m e n s i o n s o f t h e a n e m o m e t e r f o r t h e i n -v e s t i g a t i o n o f t u r b u l e n c e s h o u l d n o t e x c e e d a b o u t 0 .5 m m . S t il ls h o r t e r w i r e s c o m e i n t o c o n f l i c t w i t h o t h e r r e q u i r e m e n t s , s u c h a sr i g i d it y , b e c a u s e s h o r t w i r e s r e s u lt i n a c o r r e s p o n d i n g l y s m a l l w i red i a m e t e r a n d s u p p o r t s , a n d i n a d e l ic a t e i n s t r u m e n t .

C o r r e c t i o n s t o t h e r e s u l t s o b t a i n e d w i t h w i r e s l o n g e r t h a n t h es c a le o f t u r b u l e n c e a r e p o s s ib l e i n p r in c i p le . T h e s e c o r r e c t i o n s a r eg i v e n b y S k r a m s t a d i n t h e r e p o r t o f 1) i n t h e c h a p t e r o n t h em a t h e m a t i c a l t h e o r y p e r t a i n i n g t o th e c o r r e c t i o n o f t h e m e a s u r e -m e n t s , b o t h i n s c al e a n d i n t e n s i t y , f o r l a c k o f c o m p l e t e c o r r e l a t i o no f t h e f l u c t u a t i o n s o v e r t h e en t i r e l en g t h o f t h e w i re : A ls o F r e n -k i e 1 3) s t u d i e d t h es e co r r e c t i o n s .

B e s i de s , t h e t h e r m a l i n e r t i a o f t h e v / * ~ c a us e s a d i s t o r t e d r e s p o n s eo f b o t h t h e a m p l i t u d e a n d t h e p h a - ~ ._. s he v e l o c i t y f l u c t u a t i o n s ;t h e m o r e s o a s th e f r e q u e n c y is h i g h e r a n d t h e t h e r m a l i n e r t i a isg r e a t e r . T h e c o n d i t i o n o f s m a l l t h e r m a l i n e r t i a a ls o r e q u i r e s a v e r yf i n e w i r e ( a n d a l o w w i r e t e m p e r a t u r e t o o ) . T h e d i s t o r t i o n i n t h er e s p o n s e o f a v e r y f i ne w i r e t o t h e v e l o c i t y f l u c t u a t i o n s , h o w e v e r ,c a n b e c o m p e n s a t e d t o a l a r g e e x t e n t b y a s u i t a b l e e l e c t r i c a l c i r c u i ti n t h e e q u i p m e n t .

3 Th e W ire P r i o r t o th e m a n u f a c t u r e o f v e r y f i ne tu n g s t e nw i r e t h e s i l v e r - c o a t e d p l a t i n u m w i r e d r a w n a c c o r d i n g t o t h e W o l -l a s t o n p r o c e s s , w a s e x t e n s i v e l y us e d . A p l a t i n u m c o r e o f s u i t a b l el e n g t h i s p r e p a r e d b y d i s s o l v i n g t h e s i l v e r c o a t i n g i n a m i x t u r e o f2 5 cm 3 n i t r i c ac i d an d 5 0 cm 3 d i s t i l led wa t e r t o wh i c h 10 g ram o fa m m o n i u m n i t r a t e is a d d e d . S o m e e x p e r i m e n t e r s p r e f e r a w e a k e rs o l u t i o n : t h r e e p a r t s o f d i s ti l le d w a t e r t o o n e p a r t o f n i t r i c a c id ,b e c a u s e a r i c h s o l u t i o n g i v e s w i r e s w h i c h a r e u n d e r s t r e n g t h , p o s -s i b ly a s t h e r e s u l t o f e t c h i n g t h e p l a t i n u m c o r e. T h e a c i d s h o u l d b ef r e e f r o m c h l o r i d e , s o a v o i d i n g t h e p o s s i b i l i t y o f e t c h i n g t h e p l a t i -n u m i ts e l f o r o f f o r m i n g s i l ve r c h l o r id e . A n e l e c t r o l y s i n g p o t e n t i ao f 2 V ac ro ss t h e w i re an d t h e d i l u t e ac i d r em o v e s t h e s i l v e r co a t i n gi n o n e m i n u t e o r l e s s .

G o o d r e s u l t s a r e a l s o o b t a i n e d w h e n a t i n y j e t o f d i l u t e a c i d i sAppl . sci. Res. A 2 23


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35 4 B G VAN DER HEGGE ZIJ NEN

made to impinge on the wire, the removal of the coating again beingassisted by electrolysis.

The delicate platinum wires cannot be kept straight under ten-sion; deformation of the wires and consequent change of the cali-bration constants, or even breakage of the wires, results from col-lision with ti ny solid particles in the air stream. For these reasons itis not advisable to use platinum wires of 0.0025 mm for air speedsexceeding 12 m/sec; wires of 0.00625 mm can withs tand speeds of25 m/sec.

Platinum-iridium has far superior mechanical qualities. The ad-vantage of superior mechanical strength offsets the disadvantageof a lower temperature coefficient of tile electrical resistance. Thisalloy, however, cannot be drawn to a smaller diameter than 0.015m m

W e s k e ~ pointed out t ha t tungsten , because of its excellentmechanical strength, is very attractive for hot-wire anemometers.The temperature coefficient of the electrical resisfance is higher fortungsten than for platinum or platinum-iridiuin; this too is anadvantage. Tungsten wires down to 0.003 mm and less, are at pre-sent commercial ly available.

Tungsten wire can be kept st raight under tension; breakage rare-ly occurs because the wire is less sensitive to collision with solidparticles in tile airstream, and the anemometer is rugged enough topermit the removal of accumulated dust with a small brush, or bywashing, without changing the calibration constants. The durab ili tyof the calibration constants is promoted by the low operational tem-perature of the wire which, for practical reasons, is restricted. According to S c h u b a u e r 5 tungsten wire can be subjected

for an indefinite length of time to a temperature of about 300Cwithou t showing deterioration or weakening; above 350C, however,deter iorat ion sets in and rupture finally occurs. Of course the avera -ge operational temperature must be kept appreciably below 300C.

Because the sensitiv ity of a hot-wire anemomete r increases withtile average wire temperature, the sensitivi ty of a tungst en hot-wireanemomete r cannot be raised to t hat of a plati num hot-wire anemo-meter, which permits of a maximum operational temperature ofabout 500C. This is a drawback, but not a very serious one, sincethe loss of sensitivity can be made good by increasing the amplifi-cation of the voltage fluctuations across the wire. It tells, however,


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HOT WIRE ANEMOMETERS 35 5

when the hot-wire anemometer is applied to the measurements offluctua tions as well as of the time-mean values of the velocity in thefield of turbulent flow.

4 The wire supports The fine and short wires of anemometersintended for measuremen ts of turbulence require the use of delicatesupports. The flexural rigidity of the supports sets a lower limit tothe support diameter. A lower limit is also set by the condition tha tthe temperature of the support be equal, or nearly equal, to that ofthe ambient air. This is the case when the ratio of cross-sectionalarea of support and wire exceeds a certain value. This value dependson the kind of material used and on the absolute dimensions of wireand support. For a wire diameter of 0.005 mm this value amou nts toroughly 100. This means a suppor t diameter of 0.05 ram. Suchsupports are too fine for practical use.

An upper limit to the support diamete r is dictate d by the require-ment that the field of flow suffer the least possible distortion.

The general practice in the U.S.A. and in Europe is to use stra ightsupports: sewing needles, phosphor-bronze wire or other suitablemetal with a uniform diamete r of 0.2 to 0.3 mm or over. When tung-sten wire is used, the supports must be flexible enough to keep thewire straigh t. When Wollaston wire is used, t he silver-coated plati-num often serves as support diameter of coated wire 0.15 to0.25 ram).

Whereas the ratios of wire length to wire diameter employed byvarious experimenters appear to be high enough to ensure favour-able operational conditions, the ratio of wire length to support dia-mete r is sometimes astonishingly low compare the table with da taon hot-wire anemometers). A value of 2 is found for the anemomete remployed by S c h u h and W i n t e r 6) Royal Aircraft Establish-ment, Farnborough, England) ; the higher ratios of the a nemometersof Sc hu ba ue rS ) and Co rr si nT ) are due to a wire lengththat is presumably too great for accurate measurements. A ratio ofwire length to support diameter of about 10 should be aimed at.Indeed, it will hardly be possible, to achieve higher ratios with wireswith a length of 0.5 mm.

5 Att ach ing the wire Wires of platinum, or the silver-coatedWollaston wire, are soldered onto the supports with ordinary lead-


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3 5 6 13 G V A N D E R H E G G E Z I J N E N

t i n so lde r. Tungs ten wi re, howe ver , ca nno t be so ldered wi th o rd ina rylead-t in so lder, a t leas t not in a d i rec t way. I t seem s tha t We skef i r s t u sed tungs t en wi re wh ich was coa ted ove r it s en t ire l eng th w i tha th in laye r of a me ta l th a t could be so ldered wi th lead-t in . Ele c t ro-p la t ing , howev er , increases the therm al iner t ia of the wire .

In order to av oid the inconve nience of increasing the w ire d ia-m et er b y plat ing, S c h u b a u e r 5) de ve lop ed an ingeniou s tec h-n ique fo r p l a t ing the tung s t en wi re on ly where con tac t i s t o be ma dewi th the suppo r t s , l eav ing the pa r t o f t he wi re be tw een the sup-por t s ba re . I t m ay be in fe r red tha t t h i s me th od o f p l a t ing requ i resconsiderable sk i l l

Ano the r w ay o f a t t ach ing the tungs t en wi re to the suppor t s i sby weld ing . This process of a t taching f ine wires i s a t present carr iedou t i n a few l abora to r i e s i n the U .S .A . and a lso by the Ro ya l Du tch /She l l Labora to ry a t De l f t acco rd ing to R ies ' me thod . A desc r ip t ionof th i s p rocess and the equ ipmen t deve loped fo r spo t we ld ing f inetungs t en wi res i s g iven in the l a s t pa rag raph .

6. Pre tension o/t he wire and/ unda me ntal/r equ enc y of the support and the wire. Opin ions d ive rge on the ques t ion whe the r a ho twi re should , o r shou ld no t , b e kep t s t ra igh t under t ens ion . In thecase of p la t inum wire th is quest ion does not a r i se because f ine p la-t i num wi res s imply canno t be kep t under p re t ens ion ; i t does a r i se ,however , w i th tungs t en wi re .

In measu r ing low l eve l s o f t u rbu lence the d i f f i cu l ty of t he v i -b ra t io n o f t he ho t -wi re ane mo mete r i s encoun te red . In gene ra l t hev ib ra t ion s a r i se e i the r f rom the moun t ing (wind- tunne l wa l l, t hefoundat ion of t i le se tup , e tc . ) , the wire supports , or t i le wire i t se l f .T h e i m p o r t a n c e o f v i b r a t io n s c a n b e d e mo n s t r a t e d b y a n e x a mp l e :a ssum e a wire v ib ra t ing w i th an am pl i tude o f 0 .0025 mm a t afre qu racy of 1000 pe r second; le t the ve lo ci ty of the f low be 30 m/sec .-Th~s v :bra t ion produces a spurious effec t equivMent to an in tensi tyo f t u rbu lence o f 0 .04% ,whereas the in t ens i ty o f t u rbu lence in a w ind-tunne l m ay be a s l ow as 0 .1% . I t fo llows tha t t he fou nda t ion , o rmo ,an ting, o f t he ho t -wi re an em om ete r mu s t b e a s f ree f rom v ib ra -t io-~ a~ possible; this is an external problem.

In o rde r to avo id re sonance , t he funda me n ta l f requenc y of t hesuppor t s shou ld no t co inc ide wi th the f requency o f t he v ib ra t ionso~ the mou n t lng o r o f t he vo r t i ces shed b y the suppor t s.


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H O T W I R E A N E M O M E T ER S 3 5 7

T h e f r e q u e n c y f o r t h e f u n d a m e n t a l m o d e o f v i b r a t i o n o f a b a rw i t h u n i f o r m c r o s s s e c t i o n , w i t h o n e e n d b u i l t i n a n d t h e o t h e re n d f r e e , n e g l e c t i n g d a m p i n g , i s

~ / f ig (1)/1 ~ 0.56 X~d2~L

a n d f o r t h e s e c o n d m o d e o f v i b r a t i o n :/ EIg/2 = 3 48 ~d2~ L4 2)

w h e n E1 = f i exu ra l r ig id i ty ,g = acce le r a t ion o f g r a v i ty ,y = w e i g h t o f t h e m a t e r i a l p e r u n i t o f v o l u m e ,d = d ia m e te r o f ba r ,L ---- le ng th of ba r.

(com pare S. T i m o s h e n k o : Vi bra t ion prob lem s in engin eer -ing, 2nd ed. , New York 1937, p. 344) .

Ins er t ing : 7 = 8 10-3 kg /c m 3 (chrome1), d = 0.02 cm, L = 1cm, the formulae (1) and (2) y ie ld

A = 1400/sec,/2 = 8700 /sec.I f t h e v a l u e o f / i f o u n d i n t h i s w a y c o m e s d a n g e r o u s l y n e a r to t h e

f r e q u e n c y o f e x t e r n a l v i b r a t i o n s , i t a p p e a r s t o b e w is e e i t h e r t o d e -c reas e the s uppor t d i ame te r (o r inc reas e i t s l eng th ) , o r to inc reas e d ,a t t h e s a m e t i m e r e d u c i n g L , in o r d e r to i n c r e a s e / i b e y o n d t h e h i g h -e s t f r e q u e n c y to b e e x p e c t e d .

S c h u b a u e r S ) u se d a n a n em o m e te r w i th d = 0 . 3 2 m m ,= 14 m m (phos phor -b ro nze w i r e ), y i e d ing fo r /1 abo u t t l 40 / sec .

T h i s f r e q u e n c y m i g h t c o i n ci d e w i t h t h e f r e q u e n c y o f t h e v i b r a t i o n sg e n e r a t e d b y a w i n d - t u n n e l m o t o r o r f a n .

S c h u h a n d W i n t e r 6 ) , o n t h e o t h er h a n d , u se s u p po r tsw i t h a n e f f e c t i v e l e n g t h o f 2 .5 m m a n d a d i a m e t e r ( a t t h e t o p o f t h es u p p o r t ) o f 0 . 2 5 m m . T h i s y i e l d s f o r / 1 a b o u t 28000/sec, wel l abovet h e f r e q u e n c y le v e l e n c o u n t e r e d i n p r a c t ic e . Y e t t h i s a n e m o m e t e ra p p e a r s t o b e so b u l k y , t h a t i t i n e v i t a b l y c a u s e s a co n s i d e r a b le d i s-to r t ion o f the f i e ld o f f low. Here f l exu ra l r ig id i ty and the l eas tpos s ib le d i s to r t ion o f the f i e ld o f f low a re s ha rp ly con f l i c t ing r e -q u i r e m e n t s


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358 B.G. VAN DER HEGGE ZIJNEN

As for the f und ame nta l freq uenc y o f the wire , a ssume i t to be as tr i n g wi th b o th en d s b u i l t i n . T h e f req u en cy fo r th e fu n d a m en ta lmode of v ibration of a string is]11 2~ /ga/? 3 )where : g = acce lerat ion o f grav i ty ,

a = ten sion,7 = specif ic we ight ,l = ien gth of s tring.

No w, assume the wire to hav e a d iamete r o f 5 10 -4 cm anda lengt h o f 0 .05 cm. A t ens io n in the wire a o f 1 kg /cm 2 resul ts ina wire load of abo ut 0 .2 10 -a gram . W it h y = 19 10 -3 k g/c m 3(tungsten) formula (3 ) y ie lds

/1 : a b o u t 2 3 0 0 / s e c .T h e a s s u m ed v a l u e o f a m a y b e i n crea s ed m a n y t i m es b e fo re ru p tu reo ccu rs . I t m a y s a fe l y b e a s s u m ed th a t i n a t ta ch i n g th e w i re to th esupports a suf f ic ient wire load i s generated; so no spec ia l care ap-pears to be needed in regard to pre- tens ion .

7 . D a ta on ho t w i r e anemome te r s . Only a few part icu lars haveb een co m m u n i ca ted i n th e l i t era tu re a b o u t th e a n em o m eters u s ed b yv a r i o u s ex p er i m en ters for th e i n v es t i g a t i o n o f tu rb u l en ce .T h e fo l lo w-i n g ta b l e co n ta i n s th e co n s tru c t i o n a l d a ta th a t co u l d b e t ra ced ;th e d a ta o f th e De l f t tu n g s ten -wi re a n em o m eter h a v e b een a d d ed(co m p a re n ex t p a ra gra p h ) .

TABLE IDimensions in mm

WireLaboratory

Aeron. Res C.G.A.L.C.I.T.N.A C.A.Roy Aircr. Est.Nat. Luchtv. LabTechn Un. DelftKon./SheI1 Lab.

ExperimenterHallCorrsinSchubauerSchuh andWinterBenthemWellingv. d. Hegge

Zijnen

Mat.PtPtWoPtPtPtPt IrWo

Supp.Le/ gth Diam.d diam D

1 2521.760.51.02-420.6

0.0025 0.2?0.0063 0.3 ?0.00751 0.320.00251 0.250.005 ] 0.250.009 I 0.3?0.015 ?0.005 0.05X

0.13 *)

li lI Ref.

500 16.251 832 17 / 7/ 5 5 / 5200 12 1 6200 4 / 6

200 400 7 13 9133 j ?120 10 - -

* at top of taper ed sup.port.


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H O T - W I R E A N E M O M E T E R S 3 5 9

8 . T h e D e l / t a n e m o m e t e r . In compliance with the require-ments as to the dimensions of the hot-wire anemometer, the lengthselected for the wire was l = 0.6r amand for the diameter:

d = 0.005 ram,yielding a length/ diameter ratio of about 120, in accordance withthe requirements of good operational conditions. Because of itsgreat tensile strength tungsten wire has been employed.

The wire supports are made from chromel wire with an initialdiameter of 0.3 mm; they taper from about 0.3 0.2 mm 2 at thebase to about 0.12 0.05 mm 2 at the top, where the tun gst en wireis attached. The length of the support is about 10 ram. In order toreduce as far as possible the distortion of the field of flow at thepoint where turbulence is to be measured, the tops of the supportshave been ben t in t he form of an arch (compare fig. 1 and 2). Be-cause the end of the support is near ly para .. .. ~ the wire, this formfacilitates the att ach men t of the wire.

T y~ '/ ./~A; J , /:~TO lY~ 8R * ' D G

F i g . 1. S k e t c h o f D e l f t h o t - w i r e a n e m o m e t e r .Chromel is an excellent metal for supports: it is resilient, good to

work on and resistant to corrosive effects. It cannot, however, besoft soldered; hence the built-in end of the support must be copper-plated before soldering it to the holder.

The copperplating is performed as follows: the support is soakedin a hot concentra ted K OH solution, which will remove grease, andthen dipped in a mixture of concentrated H2S04, HNOa and He1,in the proportions 2 : 1 : 0.015. The acid mixture, if free of water,will leave the support bright and not pitted. Then the end of thesupport is dipped in an acid sulphate plating bath consisting of 10gram CuSO 4. 5aq and 10 cm 3 H;SO 4 added to 50 cm 3 dist illed water.According to C a r t e r 11) this solution yields strong, dense depo-sits when it is operated at room tem pera ture and a current densi ty of0.05 A/cm 2.


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360 B G V A N D E R H E G G E Z I J N E NThe wire holder compare figl 1), to which the supports are at-

tached, is made from strips of hard brass. The upper and thelower half of this holder are electrically insulated from each o therby a strip of paper. The left end in fig. 1 has a slight ly tapered rect-angular cross section ; it serves as a plug for connecting the hot-wireanemometer to the Wheatstone-bridge arrangement in which it isinserted. This construct ion proved to be convenient because if thewire breaks, it is a simple m at te r to remove and replace the holderor substitute a new one.

9 The equi pme nl/o r spot welding the tungsten wire to the supportsThe author has chosen the Ries method of electrically spot welding

0

C

F i g 4 R e e l f o r p r e p a r i n g a p i e ce o f t u n g s t e n w i r e

the tungsten wire to the supports. Because of the good results ob-tained with it, a description of this method and of the equipmentdeveloped by the author may follow here.

The tungsten wire, with a diamater of 0.005 mm, as supplied bythe Sigmund Cohn Corp. of New York, N.Y., is wound on a bo bb in .


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Fig. 2. Delft hot-w ire anemometer, nea rly full size

Fig. 3. We ldin g equipment


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Fig. 7 . Tun gsten wire of 0 .005 mm diameter spot welded tosupports 15 times enlarged)


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HOT WIRE ANEMOMETERS 36

Befo re weld ing , a p iece of t h i s w i re mu s t be s t re t che d ove r t he topof the su ppor ts . T his is a ra th er de l ica te , bu t not a d i ff icu l t opera-t ion.The tungs t en -w i re bobb in A (see the pho to o f t he equ ipm en t ,f ig . 3, and the d i ag r amm at i c ske t ch showing the p repa r a t ion o f ap iece of tung sten wire , f ig . 4 ) i s m ou nte d on a s tan d B; th is s tan dalso carr ies a smal l ree l C wi th s ix horizon ta l sp okes . N ow the end ofthe wi re i s a t t ached wi th wax o r o the r su i t ab le ma te r i a l t o one o fthe sp okes of the ree l . By turn in g the ree l th ro ugh 1 /6 th of i t s ci r -cumference the wi re i s b rough t i n to con tac t w i th the nex t spoke ;he re aga in the wi re is a t t ached . The p i ece o f t ungs t en wi re be tw eenthe tw o spokes , fo r i n s t ance 10 mm long , is a t t ache d wi th w axnear the tops of the prongs D carr ied by the s tan d E ( in the r ight ofthe photo f ig . 3 , compare f ig . 4 and 5) . By cut t ing the wire be tweenthe pron gs a nd the spokes of the reel , a short p iece of s t ra ig httungs t en wi re remains be tween the p rongs .

+ M

~WPPOI T

Fig. 5. Posit ion of wire dur ing welding.

Now the s t an d E i s m ove d to the le f t of t he s t an d F and in to apos i t ion such tha t t he tun gs t en wi re is j u s t ove r t he tops o f t he wi resuppor t s . The ad ju s tm en t o f t he wi re w i th re spec t t o t he top o f t hesupport i s ass i s ted by a screw sp indle H in the s tand F for horizon-ta l d isp lacem ent of the h ot -wire hold er I (f ig . 3 ), and by the screwspindle K in the s tan d E for ver t ica l d is p lace me nt of the p iece oftungs t e n wi re. W hen the wi re i s exac t ly pa ra l l e l t o, and ju s t ove r t hetops o f t he supp or t s , i t is l owered by tu rn ing the kno b L o f t hescrew sp indle K in the s tand E in order to br ing the wire in to con-t ac t w i th the suppor t s . When the l i gh t i s good , t h i s p rocess o f ad -jus t ing i s no t d i f f icu l t .


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362 B G VAN DER HEGGE ZIJNENThe we ld ing o f t he tun gs t en wi re to t he sup por t s i s done by m eans

o f an e l ec t rode M (a p i ece o f red copper w i re) a t t ach ed to the s t andN (see pho to f ig . 3 and the sk etch f ig. 5 ). By turn in g the knob 0 inthe s t an d N the e l ec t rode is b roug h t i n to con tac t w i th the tungs t enwi re and a t t he same t ime the l a t t e r i s p ressed aga ins t t he top o fthe suppor t . Con tac t i s de t ec t ed b 'y an e l ec tr i c pocke t t o rch P seenat the left of fig . 3 .

W hen a su f f ic i en t ly s t rong e l ec t ri c cu r ren t is passed th roug h thee l ec t rode M, the wi re ough t t o be we lded f i rmly to the top o f t hesuppor t .

E lec t r i c cu r ren t o f su f f ic i en t s t reng th i s gene ra t ed in the spo t -weld ing uni t m ark ed Q in the pho to f ig . 3 , a c ircu i t d iagram of whichis g iven in f ig . 6. To the le f t is the conn ect ion to th e 220 V m ains(50 c .p .s . ; conne ct ion not v is ib le in the pho to f ig . 3) and the ma inswi t ch R . The de t a il s o f t h i s un i t have been spec if i ed in the cap t ion

io K t o wa t t sa V W

. ;~ 6o. ~2

~xso .= I= * ~ to o ~ =

R = , . ~ . 4 / ~ ~wt r cU a C A P A C / T Y - $ E L f C T / H 6 S W l T ~ 2, = t S O ~ F . - ~ t ~ . Z O O / O - FW = p M ~ H - ~ G T T O H W E L ~ I H 5 ~ W I r C H~ = P O T E H T / O / ~ E T E R / : O R V A R y / t / d : T N ~ W E l_ D l f f G YO L T A G E 2 R G AD ABL E O f f / ~J r~ WT R TV = A D d U S T F I E I Y 7 0 ~ S N O R T C I R C U / 7 W E L D I I ' ~ O C M R R E H T7 = mE Tt /~R

Fig 6 Circuit diagr am of spot welding unit

t o f ig . 6. Su ff i ce i t t o say th a t t he we ld ing vo l t age i s ad jus t ed by thepo ten t iom ete r S , wh ich vo l t age i s read o f f on the me te r T ; t he ca -pac i ty in t he c i rcu i t i s ad jus t ed by means o f t he se l ec t ing swi t ch U .T h e c a p a c i t y r a n ge s - - b y s te p s - - f r o m 50 t o 70 0 F . T h e re s is t a nc eV se rves fo r ad jus t ing the sho r t c i rcu i t cu r ren t wh ich remains a f t e rd i scha rge o f t he capac i ty un i t .

D i scha rge ~ th i s i s t he ac tua l we ld ing p rocess - - occu rs when thebu t to n W i s pushed . Wh en the e l ec t rode M has been co r rec t ly ad -ju s t ed , no spa rks shou ld occu r a t t he we ld ing spo t .


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H O T - W I R E A N E M O M E T E R S 6

Good result s have been obta ined with a welding vo l tage o about30 to 40 V and a capacity of 500/ ,F. The photo f ig . 7 shows the re-sul t s obta ined in this wa y with a tungsten wire o f 0 .005 mm dia-meter, welded to chromel supports. The wire is reproduced 15 t imesenlarged.R e c e i v e d 3 0 t h A u g u s t , 1 9 50 .

R E F E R E N C E S

1) D r y d e n , H . L . , G . B . S c h u b a u e r , W . C . M o c k a n d H . K . S k r a m -s t a d , M e a s u r e m e n t s o f i n t e n s i t y a n d s c al e o f w i n d - t u n n e l t u r b u l e n c e a n d t h e i rr e l a t i o n t o t h e c r i ti c a l R e y n o l d s n u m b e r o f s p h e r e s , N a t . A d v . C o m m . f . A e r o n .T e c h n . R e p . 5 8 1 , 1 9 37 .

2 ) T a y 1 o r , G . I . , P r o c . L o n d o n m a t h e m . S o c . ~ 0 ( 1 9 2 2 ) 1 9 6.3 ) F r e n k i e 1, F . N . , E t u d e s t a t i s t i q u e d e l a t u r b u l e n c e .

I . M e s u r e d e l a t u r b u l e n c e a v e e u n f il c h a u d n o n c o m p e n s 6 .I I . I n f l u el l c e d e l a l o n g u e n r d u n I l l c h a u d c o m p e n s 6 s u r l a m e s u r e d e l a t u r b u l e n c e ,O f f . N a t . E t u d e s e t R e c h . A 6 r o n . R e p . T e e h n . 3 7 , 1 9 48 .

4 ) W e s k e , J . R . , M e t h o d s o f m e a s u r e m e n t o f h i g h a i r v e l o c it i e s b y t h e h o t - w i r em e t h o d , N a t . A d v . C o m m . f . A e r o n . , T e c h n . N o t e 8 8 0 , 1 9 4 3.

5 ) S c h u b a u e r , G . B . a n d P . S . K 1 e b a n o f f , T h e o r y a n d a p p l i c a t i o n o f h o t -w i re i n s tr u m e n t s i n t h e i u v e s t i g a t io n o f t u r b u l e n t b o u n d a r y l a y e rs , N a t . A d v . C o m m .f . A e r o n . , W a r t i m e R e p . W . 8 6 ( A C R 5 K 2 7) 1 9 4 6 .

6} S e h u h , H . a n d K . G . W i n t e r , R . A . E . 4 I t 3 I t e x p e r i m e n t a l l o w t u r b u -l e n ce w i n d t u n n e l , R o y . A i r c r . E s t a b l . R e p . A e r o 2 2 8 5 , 1 9 4 8.

7 ) C o r r s i n , S . , I n v e s t i g a t i o n o f f l o w i n a n a x i a l l y s y m m e t r i c a l h e a t e d j e t o f a i r .N a t . A d v . C o m m . f. A e r o n . , W a r t i m e R e p . A C R W . 9 4 , 19 4 3.

8 ) H a 1 1, A . A . , M e a s u r e m e n t s o f t h e i n t e n s i t y a n d s c a l e o f t u r b u l e n c e , A e r o n . R e s .C o u n e . R e p . a n d M e m . 1 8 4 2, 19 3 8 .

9 ) B e n t h e m , J . P . , O n d e r z o e k n a a r d e i n v l o e d v a n g a z e u o p e e n g e l i j k m a t i g e e ne e n n i e t g e l i j k m a t i g e s t r o m i n g , N a t . L u c h t v a a r t L a b . , A m s t e r d a m , R a p . A 1 1 68 ,1949 .

1 0 ) W e l l i n g , W . A . , I n g e n i e u r B l ( 1 94 9 ) C h . 3 3.1 ) C a r t e r , H . J . , R e v . s c i. I n s t r . 1 ,9 ( 1 9 48 ) 9 1 7 .

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