Weekly Seminar: Fall 2012
Speaker: Dr. Xiaofeng Liu (MechE | Johns Hopkins University)
Title: "Vortex-Corner Interactions in a Cavity Shear Layer Elucidated by Time Resolved Measurements of the Pressure Field"
Date: Friday, December 7, 2012
Time: 11:00 a.m.
Location: Gilman Hall 50 (Marjorie M. Fisher Hall)
The flow structure and turbulence in an open cavity shear layer at a Reynolds number of 4.0 x 104 is experimentally investigated using time resolved PIV, with an emphasis on interactions of the unsteady pressure field with the cavity corners. The pressure is obtained by spatially integrating the in-plane components of the measured material acceleration. (Liu and Katz, 2006).
Conditional sampling, low-pass filtering and time correlations among variables enable us to elucidate two processes with distinctly different frequency ranges, which dominate the shear layer interactions with the corners. The first process, with Strouhal number range of 0.5-3.2, involves the traveling Kelvin-Helmholtz shear layer eddies. Their interactions with the trailing corner introduce two sources of vorticity fluctuations above the corner, i.e., the advected shear layer vorticity and the locally generated vorticity, with the latter one intrinsically associated with the local streamwise pressure gradients. The local vorticity production is strongly affected by the streamwise location of the large scale shear layer vortices, periodically creating a lingering region with peak vorticity and pressure minima just above the trailing corner, making the place there most prone to cavitation inception.
The second distinct unsteady flow process, with characteristic Strouhal numbers of ~0.05, is characterized by the low frequency flapping of the shear layer and the boundary layer upstream of the leading corner, periodically strengthening or weakening all of the flow and turbulence quantities around both the leading and the trailing corners. Time dependent correlations of the shear layer elevation show that the flapping starts from the boundary layer upstream of the leading corner and propagates downstream at the freestream speed. The high negative correlations of shear/boundary layer elevation with the streamwise pressure gradient above the leading corner introduce a plausible mechanism that sustains the flapping: When the shear layer is low, the boundary layer is subjected to high streamwise adverse pressure gradients that force it to widen, and when the shear layer is high, the boundary layer is subjected to favorable pressure gradients, causing it thin down. Flow mechanisms that would cause these pressure changes and their relation to the flow within the cavity will be discussed.
*This project was sponsored by ONR and NSF.
Dr. Xiaofeng Liu is currently an assistant research scientist at the Department of Mechanical Engineering at Johns Hopkins University. He was a postdoctoral fellow at the same department, with Prof. Joseph Katz as his advisor. He received his Bachelor and Master degrees in aerodynamics from Beijing University of Aeronautics and Astronautics, and a Ph.D. degree in aerospace engineering from the University of Notre Dame. Prior to coming to the U.S. for his Ph.D. studies, he was a lecturer at the Department of Engineering Mechanics at Tsinghua University. His research interests include turbulent boundary layer and shear layer flows, vortex dynamics, high-lift aerodynamics, cavitation, fluid-structure interactions, acoustics, turbulence modeling, biofluid mechanics, image processing, and development of optics-based flow field measurement techniques.