紊流中的底形.第1部分：地形上的湍流
Bedforms in a turbulent stream.Part 1: Turbulent flow over topography
摘要：
在水下波纹和沙丘形成的背景下，我们在这里提出了一个雷诺平均计算的湍流的地形。我们执行一个弱非线性膨胀的速度场，足够准确地恢复分离的流线和形成的再循环气泡以上的一些纵横比。基底应力进行了详细的研究，特别是，我们表明，相对于地形的剪切应力的相移，负责形成的底形，出现在一个内部的边界层的剪切应力和压力梯度的平衡。我们研究了计算的灵敏度与（i）湍流闭合的选择，（ii）底部的运动（生长或传播），（iii）在表层中起作用的物理学，负责底部的流体动力学粗糙度，（iv）底形的纵横比和（v）自由表面的影响，其可以根据地形激发的驻波重力波来解释。最重要的影响是（iii）至（v）点的影响。我们表明，动力学机制控制的水动力学粗糙度（混合由于粗糙度元素，粘度，泥沙输运等）有影响的基础剪切应力时，表层的厚度是可比的内层。我们的证据表明，非线性效应往往反对线性的，是相同的顺序为1/10的顺序的床型长宽比。我们表明，自由表面上的基底剪切应力的影响是占主导地位的波长是大的相比，流的深度，使内层延伸到整个流动和共振条件下，当下游材料的速度平衡上游波的传播。
英文摘要：
In the context of subaqueous ripple and dune formation, we present here a Reynolds averaged calculation of the turbulent flow over a topography. We perform a weakly non-linear expansion of the velocity field, sufficiently accurate to recover the separation of streamlines and the formation of a recirculation bubble above some aspect ratio. The basal stresses are investigated in details; in particular, we show that the phase shift of the shear stress with respect to the topography, responsible for the formation of bedforms, appears in an inner boundary layer where shear stress and pressure gradients balance. We study the sensitivity of the calculation with respect to (i) the choice of the turbulence closure, (ii) the motion of the bottom (growth or propagation), (iii) the physics at work in the surface layer, responsible for the hydrodynamic roughness of the bottom, (iv) the aspect ratio of the bedform and (v) the effect of the free surface, which can be interpreted in terms of standing gravity waves excited by topography. The most important effects are those of points (iii) to (v). We show that the dynamical mechanisms controlling the hydrodynamical roughness (mixing due to roughness elements, viscosity, sediment transport, etc) have an influence on the basal shear stress when the thickness of the surface layer is comparable to that of the inner layer. We evidence that non-linear effects tend to oppose linear ones and are of the same order for bedform aspect ratios of the order of 1/10. We show that the influence of the free surface on the basal shear stress is dominant when the wavelength is large compared to the flow depth, so that the inner layer extends throughout the flow and in the resonant conditions, and when the downstream material velocity balances the upstream wave propagation.
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