Reynolds-number dependence of the near-wall flow over irregular rough surfaces
Reynolds-number dependence of the near-wall flow over irregular rough surfaces
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DOI：
10.1017/jfm.2016.680
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发表时间：
2016-11
影响因子：
3.7
通讯作者：
A. Busse;M. Thakkar;N. Sandham
中科院分区：
文献类型：
--
作者：
A. Busse;M. Thakkar;N. Sandham
采用直接数值模拟的方法，研究了两个不规则粗糙表面上的湍流通道流动与雷诺数的关系。其目的是描述粗糙表面附近和内部的流动变化，这是一个很难获得实验测量的流动区域。两种表面的平均粗糙度高度和粗糙度特征的空间相关性相似，但两种表面的高度分布的偏斜度有显著差异，石墨样品为正偏(峰为主)，而喷砂表面为负偏(谷为主)。对于这两种情况，在7个不同的雷诺数下进行了数值模拟，范围从$Re_{\Unicode[STIX]{x1D70F}}=90$到$Re_{\Unicode[STIX]{x1D70F}}=720$。在所有情况下，正倾斜表面都比负倾斜表面产生更高的摩擦因数。对于最高的雷诺数，流动的粗糙度函数$\UNICODE[STIX]{x0394}U^+}$在两个表面上都远远超过$7$，除了粗糙度的直接附近，主体流剖面在整个通道高度上都达到了一个恒定的形状，这将表明完全粗糙的流动。然而，粗糙表面内部和正上方的平均流动轮廓仍然表现出相当大的雷诺数依赖性，形式与粘性阻力之比继续增加，这表明至少对于某些类型的粗糙表面，流动保留了过渡粗糙区域的某些方面，其值远高于传统上假设的过渡到完全粗糙的阈值。这也反映在近壁流动随着雷诺数的增加而发生的变化：表面粗糙度最初埋在其中的粘性亚层破裂，逆流区加剧。在最高雷诺数时，观察到一层近壁流动沿着局部表面的轮廓线。这种覆盖层的厚度分布具有混合标度，表明粘性效应在近壁流动中仍然很明显。
The Reynolds-number dependence of turbulent channel flow over two irregular rough surfaces, based on scans of a graphite and a grit-blasted surface, is studied by direct numerical simulation. The aim is to characterise the changes in the flow in the immediate vicinity of and within the rough surfaces, an area of the flow where it is difficult to obtain experimental measurements. The average roughness heights and spatial correlation of the roughness features of the two surfaces are similar, but the two surfaces have a significant difference in the skewness of their height distributions, with the graphite sample being positively skewed (peak-dominated) and the grit-blasted surface being negatively skewed (valley-dominated). For both cases, numerical simulations were conducted at seven different Reynolds numbers, ranging from $Re_{\unicode[STIX]{x1D70F}}=90$ to $Re_{\unicode[STIX]{x1D70F}}=720$ . The positively skewed surface gives rise to higher friction factors than the negatively skewed surface in all cases. For the highest Reynolds numbers, the flow has values of the roughness function $\unicode[STIX]{x0394}U^{+}$ well in excess of $7$ for both surfaces and the bulk flow profile has attained a constant shape across the full height of the channel except for the immediate vicinity of the roughness, which would indicate fully rough flow. However, the mean flow profile within and directly above the rough surface still shows considerable Reynolds-number dependence and the ratio of form to viscous drag continues to increase, which indicates that at least for some types of rough surfaces the flow retains aspects of the transitionally rough regime to values of $\unicode[STIX]{x0394}U^{+}$ or $k^{+}$ well in excess of the values conventionally assumed for the transitionally to fully rough threshold. This is also reflected in the changes that the near-wall flow undergoes as the Reynolds number increases: the viscous sublayer, within which the surface roughness is initially buried, breaks down and regions of reverse flow intensify. At the highest Reynolds numbers, a layer of near-wall flow is observed to follow the contours of the local surface. The distribution of thickness of this ‘blanketing’ layer has a mixed scaling, showing that viscous effects are still significant in the near-wall flow.