Thursday, September 25 at 3:00pm - 4:00pm
Phillips Hall 736
Karen A. Flack, Ph.D.
Professor & Chair
Mechanical Engineering Department
United States Naval Academy
Annapolis, Maryland
Hosted by: Professor Murray Snyder
Abstract: The importance of surface roughness is well known for wall-bounded flows. Roughness typically increases drag in turbulent boundary layers due to pressure forces on the roughness elements. While rough-wall flows are ubiquitous in engineering practice, the issues of modeling the roughness in computations and accurately predicting the increase in frictional drag remain elusive goals. In this talk, the effect of roughness on the mean flow, turbulence statistics, and turbulence structure will be discussed. In particular, rough-wall flows will be examined in light of Townsend’s Reynolds number similarity hypothesis, which states that the turbulent motions in the outer layer are independent of surface roughness when the Reynolds number is sufficiently high. Many results over a range of roughness types lend support to Townsend’s hypothesis. However, it has been shown that two-dimensional roughness can have a significant impact on the outer layer turbulence, even when the roughness height is very small compared to the boundary layer thickness. Additionally, the presentation will include recent work on the estimation of frictional drag due to surface roughness. Detailed experiments have been performed in the transitionally rough and fully rough regimes. This research is part of an effort to determine the relevant predictive scales based solely on the roughness topography.
Biographical Sketch: Karen A. Flack is a Professor and Chair of the Mechanical Engineering Department at the United States Naval Academy in Annapolis, Maryland. She received a bachelor’s degree from Rice University, a master’s degree from the University of California, Berkeley and a Ph.D. from Stanford University, all in Mechanical Engineering. Professor Flack teaches courses in thermodynamics, fluid mechanics, heat transfer, as well as wind and tidal power. Her research focuses on turbulent boundary layer physics with a concentration on rough wall boundary layers and frictional drag prediction. Recent work also includes performance characteristics of tidal turbines in unsteady flow conditions. She is secretary/treasurer of the American Physics Society, Division of Fluid Dynamics and on the editorial boards of the International Journal of Heat and Fluid Flow and the Journal of Turbulence. She is the recipient of the ASME award for best paper in the Journal of Fluids Engineering, a Pi Tau Sigma teaching award, the Naval Academy Research award and a United States government meritorious service medal.