Roughness Effects in Non-Equilibrium Turbulent Flows

Abstract

Shark dermal denticles have inspired the development of drag-reduction surface microstructures (i.e., roughness) such as riblets and new fabrication techniques. Recent literature reports that the elastic anchoring of denticles enables them to bristle when subjected to the reversing flow that occurs during flow separation. This motion leads to a flow-dependent anisotropic separation control function of the denticles that remains unclear. Multiple types of deviations from the canonical TBL happen in this process, including separation, the wavelike rhythm of the stream, and the dynamic response of the movable roughness to local turbulence. In this talk, findings from several investigations of roughness effects in each of these non-equilibrium situations are discussed. DNS and wall-resolved LES are employed to capture the physics within the roughness layer. A) Despite often considered a mixing enhancer, roughness is found to promote the separation induced by an adverse pressure gradient. The cause is identified as the momentum deficit due to the roughness which cannot be compensated by the enhanced mixing. Separation criteria can be less appropriate due to the decoupling of the flow within the roughness canopy and the one detaching over it. B) DNS of a pulsatile channel flow over roughness further highlights the critical role of the local flow around roughness elements in modifying the turbulence. The pulsating wake significantly increases the ability of the roughness to convert mean kinetic energy to TKE. C) Channel flows over stationary denticles and the ones bristling in an assigned pattern are simulated by DNS. The bristling further increases the drag by 67%. Reynolds stresses and turbulent structures in the TBL are altered by the streamwise vortices generated at the grooves over the denticle crown and shed off during the bristling. Rich dynamics are observed under the crown of the denticle. The new understanding and tools developed from these investigations will be utilized to integrate bristling denticles with unsteady flow separation and reveal their separation control capability.

Bio

Dr. Wen Wu is an Assistant Professor at the University of Mississippi (UM) since 2020. He is visiting Prof. Moin’s group to work on bio-inspired surface roughness for turbulent flow modulation. Before joining UM, he obtained his Ph.D. degree from Queen’s University, Canada, and worked as a postdoctoral fellow at Johns Hopkins University. Dr. Wu combines high-fidelity simulations and theoretical analysis for his research, with an emphasis on the multiscale and non-equilibrium aspects of turbulence. His recent research has focused on vortex-dominated flows, flow separation, and turbulence-surface structure interactions.