Effects of roughness on turbulent boundary layers under non-zero pressure gradient conditions
United States Naval Academy
Abstract
Surface roughness has a significant effect on boundary layer behavior in many flows of practical interest, including naturally occurring flows such as those in the atmospheric boundary layer and flows of engineering interest such as those over naval vessels and aircraft or within turbomachinery. Roughness typically increases drag, can induce transition from laminar to turbulent flow, and in adverse pressure gradients (APG) increases the likelihood of boundary layer separation. Prediction of flows over rough surfaces is, therefore, of critical importance. The most studied and well understood boundary layer, in contrast, is the canonical, zero pressure gradient (ZPG) boundary layer on a smooth flat plate. Most turbulence models are based on the canonical case, with the assumption that roughness does not change the turbulence structure in the outer part of the boundary layer, and roughness effects can be captured through the boundary condition at the wall. For the ZPG case, there is considerable evidence that supports the assumption of outer layer similarity between rough- and smooth-wall cases. For non-ZPG flows, the validity of the assumption and the conditions needed for similarity are less clear. This seminar will present current experimental work that documents the flow over smooth and rough surfaces for a range of favorable (FPG) and adverse pressure gradient conditions. The results indicate that similarity largely holds in FPG cases unless the FPG is strong enough to induce relaminarization, but the APG case is considerably more complicated.
About the speaker
Dr. Ralph Volino is a Professor in the Department of Mechanical and Nuclear Engineering at the United States Naval Academy. He received his B.S. in Mechanical Engineering from Michigan State University, and his M.S. and Ph.D. in Mechanical Engineering from the University of Minnesota. He has held visiting summer positions at the NASA Glenn Research Center, Naval Research Laboratory, and the Air Force Research Laboratory. He studies the fluid mechanics and heat transfer in turbulent and transitional boundary layers. Current efforts are directed at the combined effects of roughness and pressure gradients, with a particular focus on boundary layers approaching separation, and is funded by the Office of Naval Research. His prior work focused mainly on turbomachinery applications, including boundary layer separation and transition on low pressure turbine airfoils, tip leakage flows, and film cooling under unsteady conditions. He is a Fellow of the ASME.
