The dynamical effects of roughness geometry on the response of a half-height turbulent channel flow to an impulse acceleration are investigated using direct numerical simulations. Two rough surfaces different in the surface height spectrum are compared between themselves and with a smooth-wall baseline case. Both rough cases develop from a transitionally rough state to a fully rough one. Results show that on rough walls the thickness of the roughness sublayer (RSL), defined as the layer with significant form-induced stresses, stays almost constant. The ensemble-average flows inside the RSL stays close to equilibrium throughout the transient. This is shown by the form-induced perturbations largely scaling with the mean velocity at the edge of the RSL. Inside the RSL, turbulence develops rapidly to the new steady state, accompanied by substantial changes in the Reynolds stress balance. In contrast, the flow above the RSL recovers long after the sublayer is fully developed, without a significant change in Reynolds stress balance. The geometry of the roughness plays an important role in determining the rate of response of turbulence throughout the boundary layer. This work provides detailed explanation of the suppression of reverse transition by surface roughness in response to a mean flow acceleration.

使用直接数值模拟研究粗糙度几何对半高湍流通道流动对脉冲加速度的响应的动力学影响。将表面高度谱不同的两个粗糙表面与光滑壁基线情况进行比较。两种粗糙情况都从过渡粗糙状态发展到完全粗糙状态。结果表明，在粗糙壁上，粗糙亚层 (RSL) 的厚度（定义为具有显着形状诱导应力的层）几乎保持不变。RSL 内的整体平均流量在整个瞬态过程中保持接近平衡。这通过形状引起的扰动在很大程度上与 RSL 边缘的平均速度成比例显示。在 RSL 内部，湍流迅速发展到新的稳态，伴随着雷诺应力平衡的实质性变化。相比之下，RSL 上方的流动在子层完全开发后很长时间内恢复，雷诺应力平衡没有显着变化。粗糙度的几何形状在确定整个边界层的湍流响应率方面起着重要作用。这项工作详细解释了响应平均流动加速度的表面粗糙度对反向过渡的抑制。