离散粗糙元对壁湍流影响的实验研究

Rough-wall turbulence exists widely in nature and engineering. In general, the small-scale continuous roughness on the wall will lower the whole mean velocity profile by a constant value, i.e. the roughness function, based on which, the classical theory uses wall similarity law to correlate the roughness function to the characteristic roughness length-scale. Wall similarity law regards that the influence of the roughness cannot extend into the outer layer, but only confines in the near-wall region. However, the recently experiment and DNS challenge this idea severely. Moreover, wall similarity law fails to explain the enhancement or inhibit effect of wall roughness to the near-wall turbulence self-sustaining circle with different roughness length-scale. In this background, the present project is aimed at the following two scientific questions: how the roughness modify the near-wall turbulence self-sustaining circle, and what mechanisms leads to the failure of wall similarity law. Here, we focus onto discrete independent roughness to avoid the complicated roughness interaction scenario. Experimental PIV measurement in water tunnel will be conducted, in combination with two-point conditional correlation, morphological analysis, low-dimensional mode analysis and so on. The primary task is to study how the single roughness element alters the turbulence statistics and structural characteristics in both inner layer and outer layer. By comparing the different influence of transitional roughness and full roughness, we hope to cast light onto both the mechanisms of the roughness-based drag-reduction control method and the reason that leads to the failure of wall similarity law.

粗糙壁湍流广泛存在于自然界和工程实际中。壁面上存在的小尺度连续粗糙将整体拉低壁湍流的平均流向速度。基于此发展出了壁面相似率：粗糙函数与粗糙特征尺度存在确定的函数关系，粗糙的影响仅限于近壁流区，而不影响到外区湍流的统计特性和流动结构。但最新的研究成果对壁面相似率提出了挑战。此外，壁面相似率也无法解释壁面粗糙对近壁湍动能自维持循环的增强或抑制作用。基于上述背景，本课题拟围绕“壁面粗糙如何改变近壁湍动能自维持循环”、“导致壁面相似率失效的机理”这两个科学问题，聚焦于单一粗糙元，避开粗糙间相互干扰的复杂情况。以实验研究为手段，通过PIV流场测量，结合两点条件相关、形态学辨识、特征模态分析等，研究单一粗糙元引起的湍流流动特性与拟序结构的改变，辨析过渡粗糙和完全粗糙对近壁/外区湍流的不同影响规律，以期认识离散粗糙控制在不同特征参数下有效/失效的流动机理，发现导致壁面相似率失效的根本原因。

本课题围绕“壁面粗糙如何改变近壁湍动能自维持循环”、“导致壁面相似率失效的机理”这两个科学问题，依据原计划开展研究，完成了所有的预定研究内容。发展了抑制壁面反光的背景去除算法和单像素系综相关和单行互相关算法，实现了对近壁流动及瞬时壁面摩擦速度的高精度测量。发展了准二维变分模态分解（QB-VMD）和多分量变分模态分解（MCQB-VMD）等新方法，为研究尺度间相互作用提供了更加合适的尺度分离工具。在北航低速水槽、大尺度水洞和低速风洞中分别对光滑壁湍流和圆柱状离散粗糙元影响下的壁湍流进行了多相机大视野范围二维PIV测量，建立了雷诺数范围Reτ=400~10000的高质量速度场数据库。.对于光滑壁面壁湍流，发现在300<Reτ<4000范围内，壁湍流壁面摩擦速度的脉动强度遵循lnReτ的标度规律，其偏度近似为1，其对流速度在内尺度下约为11个壁面速度单位；进一步给出了涡包结构存在的统计证据，发现了涡结构空间分布的标度规律。对于单排离散粗糙元而言，通过两点条件相关、形态学辨识、特征模态分析等研究，发现当边界层厚度与粗糙元高度的比值δ/k=10~30时，粗糙尾迹对速度场和流动结构的影响随流向距离的增大逐渐扩展至更高流层，如：在145倍粗糙元直径的下游，粗糙的影响可达6倍粗糙元高度的法向位置。因此，连续粗糙壁中壁面相似率的概念难以在此适用。进一步得到了粗糙元对壁湍流影响存在分区行为的机理：在近场以切割高低速条带、规则脱落发卡涡的形式调制平均速度场；在远场以抑制原有的小尺度旋涡的方式减少近壁区的雷诺切应力。.相关研究工作在Journal of Fluid Mechanics、Physics of Fluids、AIAA Journal等国内外相关权威学术刊物上发表SCI论文12篇，在航空学报、空气动力学报等国内期刊上发表论文5篇，取得发明专利2项。研究成果为理解壁湍流不同流层、不同尺度的湍流结构的相互作用提供了新的视角，为发展调制壁湍流中典型含能结构的新策略提供了依据。