川のながれ river flows mechanism of spiral flow in a curved channel a b a 水圧 遠心 水圧...
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川のながれRiver Flows
1. Classification of river flows (free surface flows in open channels) Sub-critical vs Super-critical, Stable or Unstable流れの分類(常流・射流, 安定・不安定)
2. Enjoy watching vortices !! Kelvin Helmholtz type instability渦を楽しむ(ケルビン・ヘルムホルツの不安定)
3. Spiral motion induced stream line curvature 湾曲の2次流
Outline
River Eng. By Hosoda, T.
年に数回しか流れない川もある(Wadi Channel in Oman)Water flows a few times a year in arid areas, bur sever floods occur in the downstream area.
ミシシッピー川の閘門Lock & Dam in the Upper Mississippi
(輸送の道としての川)
川の流れは分類できるー速い流れ(射流)と普通の流れ(常流)ー
Super-critical or Sub-critical
普通の流れ
速い流れ
普通の流れ
跳水
斜め衝撃波
流れの遷移
Sub-critical flow
Flow transition
Super-critical flow
Hydraulic jump
Oblique shock waveSub-critical flow
速い流れから普通の流れへの遷移
常流:下流の影響を受ける.
射流:上流の影響のみ受ける.
跳水
定義速い流れ:フルード数が1を超える流れ普通の流れ:1以下の流れ
Transition from super-critical flow to sub-critical flow
Sub-critical flowSuper-critical flow
Hydraulic jump
波状跳水 Undular jump
動く跳水→段波 たとえばポロロッカ(アマゾン大逆流)
A hydraulic jump moving downstream or upstream is called bore.A bore moving upstream is generated near the river mouth of the Amazon due to the large tidal oscillation and the peculiar shape of the river mouth.
速い流れの特徴 Basic features of super-critical flows
流れは上流から下流に向かって順番に決まっていく.(下流の影響を受けない)Hydraulic variables such as depth and velocity can be calculated from the upstream region to the downstream region.
速い流れと普通の流れの説明Q: The flow shown in the lower photo is super-critical or sub-critical
when you observe the flow standing on a moving boat ?
時間をあけて水面を叩いてみたときの波紋の様子Two patterns can be observed when you beat the water surface standing on a moving boat.
速い流れ Super-critical flow 普通の流れ Sub-critical flow
波紋(water ring)の拡がる速さ(propagation speed) は√gh (津波の伝波速度)流れの速さV
速い流れV/ √gh >1普通の流れV/ √gh <1
V
速い流れの特徴 Typical features of super-critical flows
速い流れ Super-critical底が上に凸ならは水面も上に凸
普通の流れ Sub-critical
もっと速い流れの特徴もっと速い流れは不安定になる - 転波列(てんぱれつ)
Roll waves are generated when Froude number exceeds the critical value (1.5-2.0).
渦を楽しむEnjoy watching eddies !!
ケルビン・ヘルムホルツの不安定 Kelvin-Helmholtz type instability
by the courtesy of MLIT
The Super-Presentation(TED) by NHKCloudy with a chance of joy(曇時々 喜び)Mr. Gavin Pretor-Pinney, the Cloud Appreciation Society
The Kelvin-Helmholtz cloud.
This looks like a series of breaking waves, and it’s caused by shearing winds.
The wind above the cloud layer and below the cloud layer differ significantly, and in the middle, in between, you get this undulating of the air,
and if the difference in those speeds is just right, the tops of the undulations curl over in these beautiful breaking wave-like vortices.
by Lauraine Ayers-Brie, Maui, Hawaii, US
by Jason Cleverly, Bryher, Scilly Isles, UK
鳴門の渦潮も同じ現象Naruto Eddies are generated due to K-H instability
Simulation
橋脚周辺の3次元的流れ3-D flow structures around a bridge pier with vortex shedding
川の蛇行とらせん流Spiral flow(secondary currents) caused by stream line curvature
Stream-wise velocities
Transverse velocities
Plan view of depth averaged stream-wise velocities
湾曲流路の中の螺旋流発生のメカニズム(実用的に重要)Generation mechanism of spiral flow in a curved channel
A
BA
水圧 水圧遠心力
主流の流速分布(遠心力の元)
小 大
小 大
水圧と遠心力の差
せん断応力
横断方向の流速分布
Longitudinal velocity distribution
Transverse velocity distribution
Hydro-staticpressure
Centrifugal force
Imbalance between pressure and centrifugal force
Shear stresses
B
湾曲流路の外岸側で局所洗掘が発生The local scouring is caused due to secondary currents along the outer bank of a curved channel, resulting in the collapse of the river structure.
湾曲流路の内岸側で土砂堆積が発生Sediments deposit along the inner bank of a curved channel.
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