Vortex ripples induced by oscillatory flow display straight crests at equilibrium, but they develop transient three‐dimensional (3‐D) bedform geometries when transitioning from one equilibrium state to another due to changes in the oscillatory flow. Large‐eddy simulations were performed of oscillatory flow over 3‐D bedform defects associated with an increase of the orbital motion amplitude, similar to what might happen during the waxing phase of a storm. The objective is to determine the intercorrelation between flow processes and evolution of 3‐D bedform defects toward an equilibrium two‐dimensional (2‐D) state of vortex ripples. Results are presented for oscillatory flows over fixed ripples with sinusoidal crests in the spanwise direction, which introduce slope asymmetries on opposite sides of the crests, during different transient states. It was found that steep‐sloped regions of the bedforms are associated with thicker recirculation areas, whereas in more gently sloped regions thinner recirculation vortices exist. Furthermore, streamwise vortical structures and significant secondary flow develop in the vicinity of sinusoidal crests, relative to straight crests. These flow structures might be responsible for the gradual reformation of the rippled bed, as they induce higher bed shear stresses, that is, increasing bed sediment transport, and they diffuse turbulent kinetic energy to higher levels above crests, that is, increasing suspended sediment transport, relative to equilibrium 2‐D ripples. Finally, the period‐averaged currents around the sinusoidal crests exhibit higher velocities on the steep‐sloped side than on the mild‐sloped one, also likely contributing to sediment transport and bed reformation toward a 2‐D configuration.

中文翻译：

---

准平衡中三维瞬态涡旋波纹几何上湍流振荡流动的大涡模拟

由振荡流引起的涡旋波纹在平衡时显示出笔直的波峰，但由于振荡流的变化而从一种平衡状态过渡到另一种平衡状态时，它们会形成瞬态三维（3D）床形几何形状。大涡模拟是在3D床形缺陷上进行振荡流动的，这与轨道运动幅度的增加有关，类似于风暴加蜡阶段可能发生的情况。目的是确定流动过程与3D床形缺陷向平衡的二维（2D）涡旋波纹状态演变之间的相互关系。给出了在跨展度方向上具有正弦波峰的固定脉动上的振荡流的结果，这在不同的瞬态状态下在波峰的相对两侧引入了斜率不对称性。研究发现，床形的陡峭倾斜区域与较厚的再循环区域相关，而在较平缓的倾斜区域中，存在较薄的再循环涡流。此外，相对于笔直的波峰，在正弦波波峰附近形成了水流涡旋结构和明显的二次流。这些流动结构可能会引起波纹床的逐渐重整，因为它们会引起更高的床层剪切应力，即增加床层沉积物的输送，并将湍流动能扩散到波峰上方的更高水平，即增加悬浮物的输送相对于平衡2D波纹。最后，正弦波峰周围的周期平均电流在陡坡一侧比在缓坡一侧显示出更高的速度，