Turbulent flow over a surface with streamwise-elongated rough and smooth stripes is studied by means of direct numerical simulation (DNS) in a periodic plane channel with fully resolved roughness. The goal is to understand how the mean height of roughness affects the characteristics of the secondary flow formed above a spanwise-heterogeneous rough surface. To this end, while the roughness texture as well as the width and spacing of the rough stripes are kept constant, the elevation of the smooth stripes is systematically varied in different simulation cases. Utilizing this variation three configurations representing protruding, recessed and an intermediate type of roughness are analysed. In all cases secondary flows are present and the skin friction coefficients calculated for all the heterogeneous rough surfaces are meaningfully larger than what would result from the area-weighted average of those of homogeneous smooth and rough surfaces. This drag increase appears to be linked to the strength of the secondary flow. The rotational direction of the secondary motion is shown to depend on the relative surface elevation. The present results suggest that this rearrangement of the secondary flow is linked to the spatial distribution of the spanwise-wall-normal Reynolds stress component which carries opposing signs for protruding and recessed roughness.

中文翻译：

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展向非均质粗糙度二次流的重排

通过直接数值模拟 (DNS) 在具有完全解析粗糙度的周期性平面通道中研究了具有流向拉长的粗糙和光滑条纹的表面上的湍流。目标是了解粗糙度的平均高度如何影响在展向非均质粗糙表面上方形成的二次流的特性。为此，在粗糙度纹理以及粗糙条纹的宽度和间距保持不变的情况下，在不同的模拟情况下，平滑条纹的高度是有系统的变化。利用这种变化，分析了代表突出、凹陷和中间类型粗糙度的三种配置。在所有情况下都存在二次流，并且为所有异质粗糙表面计算的皮肤摩擦系数明显大于均匀光滑和粗糙表面的面积加权平均值。这种阻力增加似乎与二次流的强度有关。次级运动的旋转方向显示为取决于相对表面高程。目前的结果表明，二次流的这种重新排列与展向壁法向雷诺应力分量的空间分布有关，该分量带有突出和凹陷粗糙度的相反符号。次级运动的旋转方向显示为取决于相对表面高程。目前的结果表明，二次流的这种重新排列与展向壁法向雷诺应力分量的空间分布有关，该分量带有突出和凹陷粗糙度的相反符号。次级运动的旋转方向显示为取决于相对表面高程。目前的结果表明，二次流的这种重新排列与展向壁法向雷诺应力分量的空间分布有关，该分量带有突出和凹陷粗糙度的相反符号。