Turbulent flow and drag over fixed two‐ and three‐dimensional dunes
Turbulent flow and drag over fixed two‐ and three‐dimensional dunes
复制标题
DOI：
10.1029/2006jf000650
复制
发表时间：
2007-12
影响因子：
--
通讯作者：
J. Venditti
中科院分区：
文献类型：
--
作者：
J. Venditti
[1]在固定的平床、二维沙丘和四种三维沙丘形态(1)全宽鞍、(2)全宽叶、（3)弯曲峰和(4)不规则峰）上获得了湍流的详细测量结果。固定平床上的时间平均湍流结构在动力上与泥沙运动的平床上的流动相似。二维沙丘上的流场与以往对流动和固定河床形式的流场的观测结果一致。三维床型显著改变了二维沙丘上的流场。与在二维沙丘上观察到的相比，叶状的沙丘峰顶线，地形高度向下游弯曲的弯曲峰顶，增强了湍流水平，产生了更好的尾流结构和更强烈的分离单元混合。马鞍状的沙丘峰顶线，地形高度向上游弯曲的弯曲峰顶，减少了湍流的水平，抑制了一个明确的尾迹结构和流动分离单元中的混合。带有叶状和鞍状的弯曲峰状河床的流动与全宽河床的流动在动力上相似。沿下游方向的成对鞍状体和叶状体的发育受近层速度梯度控制，鞍状体上的速度梯度较大，叶状体上的速度梯度较小。这些梯度控制着局部边界剪切应力模式，这些模式应该促进冲刷和鞍上的高输运率以及叶上的沉积。相同高度和长度的二维和三维床型的流动阻力不同。流动观测支持这样的观点，即相对于二维沙丘，鞍状沙丘减少流动阻力，叶状沙丘增加流动阻力。由不规则间距的叶状和鞍状组成的床状峰顶抑制了湍流结构的发展，并使二维或弯曲峰顶沙丘的阻力降低了20%。结果表明，在流动阻力计算中需要考虑床型波峰形状，而在床型可以经历二维、三维和不规则三维床型状态的情况下，单一阻力系数是不够的。
[1] Detailed measurements of turbulent flow were obtained over a fixed flat bed, two-dimensional (2-D) dunes and four types of three-dimensional (3-D) dune morphologies including (1) full width saddles, (2) full width lobes, (3) sinuous crests, and (4) irregular shaped crests. The time-averaged turbulence structure over the fixed flat bed was dynamically similar to flow over a flat bed with active sediment transport. The flow field over 2-D dunes conforms with previous observations of flow over mobile and fixed bed forms. Bed form three dimensionality significantly altered the flow field observed over the 2-D dunes. Lobe-shaped dune crest lines, curved crests with topographic highs that bowed downstream, enhanced the level of turbulence producing a better defined wake structure and more vigorous mixing in the separation cell than observed over 2-D dunes. Saddle-shaped dune crest lines, curved crests with topographic highs that bowed upstream, diminished the level of turbulence suppressing a well-defined wake structure and mixing in the flow separation cell. Flow over sinuous-crested bed forms with lobes and saddles was dynamically similar to flow over full width bed forms. Development of in-line paired saddles and lobes in the downstream direction appears to be controlled by near-bed velocity gradients, which were larger over saddles and smaller over lobes. These gradients control local boundary shear stress patterns that should promote scour and high transport rates over saddles and deposition on lobes. Two-dimensional and 3-D bed forms with the same height and length offered different levels of flow resistance. The flow observations support the idea that saddles decrease flow resistance and lobes increase flow resistance relative to a 2-D dune. Bed form crests composed of irregularly spaced lobes and saddles suppressed turbulent flow structure development and reduced drag by 20% below levels for 2-D or sinuous crested dunes. The results suggest that bed form crest shape needs to be accounted for in flow resistance calculations and that a single drag coefficient is inadequate where the bed can evolve through 2-D, 3-D, and irregular 3-D bed form states.