Study of Wall Turbulence Response to Large-scale Homogeneous and Heterogeneous Surfaces


August 2023


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Fully-rough wall-sheared turbulence consists of an inner and outer layer, each with its own distinct characteristics. The inner layer is composed of sinuous structures sustained by an autonomous cycle, while the outer layer boasts inclined parcels of relative momentum deficit and excess. The stair-case pattern of successive uniform momentum zones (UMZs) necessitates the existence of an interfacial shear layer of abrupt velocity change on the wall- normal direction. This phenomenon has been established in prior research and highlights the importance of understanding the structure and behavior of fully-rough wall-sheared turbulence. A conditional sampling procedure has been leveraged in the LES statistics of fully rough channel flow to generalize the positions of UMZs, which prevail where sublayer interactions create large-scale and arbitrary momentum excesses or deficits. By observing the distribution of the interfacial shear layer during different conditions when fast or slow parcels of fluid pass the sampling location, gained are the insight into the flow structures and their interactions within the roughness sublayer. These observations are consistent with previous studies and provide a new perspective on the structure of fully-rough wall-sheared turbulence. The relationship between wall roughness obliquity and the flow regime of the fully-rough wall-sheared turbulence has been investigated through parametric assessments. This research estimates between the flow pattern of the internal boundary layer (IBL) when there is large- scale orthogonal roughness heterogeneity, and the secondary flow of the second kind, i.e., the counter-rotating secondary cells, under the circumstance of the heterogeneity parallel to the streamwise direction. Of particular interest is the transition at the obliquity angle, 22π/56, where a critical obliquity is observed and the sheltering area abruptly changes. These findings provide valuable insights into the behavior of fully-rough wall-sheared turbulence and can be used to improve future turbulence models. Additional research has found that the radial spacing between roughness elements in fully- rough wall-sheared turbulence is a crucial factor in determining the critical obliquity of the wall roughness heterogeneity. By adjusting the radial spacing between adjacent roughness blocks in a row, it is possible to shift the critical obliquity in a predictable way. This theory was discovered by studying turbulent channel flow cases of rows of roughness blocks with different oblique angles and radial spacing. The predictable influence of radial spacing on critical obliquity further highlights the interplay between successive element sheltering, the flow patterns of IBL, and secondary cells, which are all key factors that determine the structure and behavior of turbulence in fully-rough wall-sheared flows, and further to contribute to refining turbulence models and improving the understanding of fully-rough wall-sheared turbulence.



Engineering, Mechanical