Roughness characterization is important for modeling its effects in simulations and engineering tools. A common approximation is that the physical roughness height is equivalent to the sand grain height. However, this can lead to under- or over-prediction of flow quantities such as the wall shear stress and heat flux. In this work two rough surfaces were evaluated for flow modifications in a fully-developed turbulent channel flow. The first was a hexagonal, dimpled geometry meant to represent an ablated thermal protection system surface. The second surface was an unstructured granular surface with nominally the same roughness height as the dimpled surface. Differences from classical sand grain behavior were observed for both surfaces, with overall higher surface shear stress and larger shifts in the log-law when compared to smooth-wall cases. Conformance to Townsend’s hypothesis for the mean flow and velocity variance was only observed between the two rough surfaces in the outer layer for similar Reynolds numbers. An investigation into outer-layer flow structures using spectral maps indicated a significant decrease in larger-wavelength energy far from the wall for both rough surfaces. This departure from pure Townsend-like behavior in the outer-flow suggests that other roughness length scales are important for both surface geometries when modeling the transport and exchange of mass and energy between the surface and free-stream.

粗糙度表征对于在仿真和工程工具中对其影响进行建模非常重要。常见的近似是物理粗糙度高度等于砂粒高度。然而，这可能会导致流量（例如壁剪切应力和热通量）的预测不足或过高。在这项工作中，评估了两个粗糙表面在完全发展的湍流通道流中的流动修改。第一个是六边形、凹坑几何形状，代表烧蚀的热保护系统表面。第二表面是非结构化颗粒表面，其名义上具有与凹坑表面相同的粗糙度高度。两个表面都观察到与经典砂粒行为的差异，与光滑壁情况相比，整体表面剪切应力更高，对数定律的变化更大。仅在外层的两个粗糙表面之间观察到相似的雷诺数，才符合汤森关于平均流量和速度方差的假设。使用光谱图对外层流动结构的研究表明，两个粗糙表面远离壁的较大波长能量显着下降。这种与外流中纯粹的汤森德式行为的背离表明，在模拟表面和自由流之间的质量和能量的传输和交换时，其他粗糙度长度尺度对于表面几何形状都很重要。