River Eng. By Hosoda, T. 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
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 Sub-critical flow 流れの遷移 Flow transition 跳水 Hydraulic jump 速い流れ Super-critical flow 斜め衝撃波 Oblique shock wave 普通の流れ Sub-critical flow
速い流れから普通の流れへの遷移 Transition from super-critical flow to sub-critical flow 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 跳水 常流 : 下流の影響を受ける. 射流 : 上流の影響のみ受ける. Hydraulic jump Sub-critical flow Super-critical flow
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. 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 速い流れ Super-critical flow 普通の流れ Sub-critical flow 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 NHK Cloudy with a chance of joy( 曇時々喜び ) Mr. Gavin Pretor-Pinney, the Cloud Appreciation Society by Jason Cleverly, Bryher, Scilly Isles, UK 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, by Lauraine Ayers-Brie, Maui, Hawaii, US and if the difference in those speeds is just right, the tops of the undulations curl over in these beautiful breaking wave-like vortices.
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
Plan view of depth averaged streamwise velocities 川の蛇行とらせん流 Spiral flow(secondary currents) caused by stream line curvature Stream-wise velocities Transverse 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 B A B Longitudinal velocity distribution 主流の流速分布 ( 遠心力の元 ) 水圧 小 A 遠心力 大 Hydro-static pressure 水圧 Transverse velocity distribution 横断方向の流速分布 せん断応力 Shear stresses Imbalance between pressure and centrifugal force 水圧と遠心力の差 小 大 Centrifugal force
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. 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.