Byron Short Seminar Series

Abstract

The presentation has two parts, both focusing on experimental characterization of boundary layers over complex surfaces. First, dual-view tomographic holography is used to measure the 3D mean flow, Reynolds stresses, and hydrodynamic forces on cylindrical roughness elements (k⁺=45.9, d/k=46.2) embedded in the inner part of a rough wall turbulent boundary layer. The mean pressure distribution is calculated by integrating the Reynolds Equations, and the wall shear stresses from velocity gradients in the viscous sublayers. Dominant flow features include a horseshoe vortex, 3D open separation in the near wake, and a vortical canopy engulfing most of the roughness and its wake. The latter consists of vertical vorticity on the sides and spanwise vorticity just above the cylinder and in the shear layer. The turbulence peaks in a shear layer extending behind the cylinder’s top.  The form drag contributes 69.9% of the total drag, and viscous drag on the bed, top and side wall of the cylinder contribute 10.6%, 14%, and 5.5%, respectively. Combined, they agree with the wall stress estimated from a log fit to the boundary layer profile. In the second part, the interactions between compliant surfaces and boundary layers are investigated by simultaneously measuring the time-resolved, 3D flow field and the 2D surface deformation at Re_t varying between 2,300 to 9,000. The optical setup integrates high speed tomographic PTV for measuring the flow with Mach-Zehnder interferometry for mapping the deformation. The time-resolved 3D pressure field is calculated by spatially integrating the material acceleration. For cases with two-way flow-deformation coupling, the interactions cause momentum deficits in the inner part of the boundary layer and increase the near-wall turbulence. The deformation amplitudes scaled by the compliant wall thickness collapse and have a linear relationship with the pressure fluctuations rms scaled by the compliant material shear modulus. The preferred wavelength of wall deformation is about three times the wall thickness, consistent with theoretical predictions. The flow-deformation correlations peak when the deformation wavelength matches that of the energy containing eddies in the boundary layer. The highest correlations occur at the ‘critical layer’, where the mean flow speed is equal to the surface wave speed. It is located within the log layer, increasing in elevation with the Reynolds number. For the entire region below the critical layer, the turbulence is phase locked and travel with the deformation, even for deformation amplitudes much smaller than a wall unit. In contrast, above the critical layer, the turbulence is advected at the local mean streamwise velocity, and its coherence with the deformation decays rapidly.

About the Speaker

Joseph Katz received his B.S. degree from Tel Aviv University, and his M.S. and Ph.D. from California Institute of Technology, all in mechanical engineering. He is the William F. Ward Sr. Distinguished Professor of Engineering, and the director and co-founder of the Center for Environmental and Applied Fluid Mechanics at Johns Hopkins University. He is a Member of the National Academy of Engineering, and a Fellow of the American Society of Mechanical Engineers (ASME), the American Physical Society, and American society of Thermal and Fluids Engineering. He has served as the Editor of the Journal of Fluids Engineering, and as the Chair of the board of journal Editors of ASME. He has co-authored more than 460 journal and conference papers. Dr. Katz research extends over several fields, with a common theme involving experimental fluid mechanics, and development of optical and ultrasonic diagnostics techniques for laboratory and field applications. His group has studied laboratory and oceanic boundary layers, turbomachinery flows, flow-structure interactions, cerebral and cardiac vascular flows, plankton swimming, as well as cavitation, bubble, and droplet dynamics.