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Fluid Seminar: On the Wall Pressure Signature of Vortical Structures in Unsteady Separated Flows

Event Type
Mechanical Science and Engineeering
2005 Mechanical Engineering Bldg. (Deere Pavilion)
Nov 1, 2013   12:00 - 1:00 pm  
Prof. Ahmed M. Naguib, Michigan State University
Debbie Richardson
Originating Calendar
MechSE Seminars


Knowledge of the unsteady wall‐pressure (p'w) field is significant for the prediction and control of flow‐induced noise and vibration as well as for the estimation of the flow state from surface measurements for feedback flow control. In this work, the characteristics of the unsteady surface pressure and the mechanisms leading to its generation are investigated in two different flow fields that are dominated by coherent vortical structures: an axisymmetric backward facing step (BFS) and an axisymmetric impinging jet. Common to both of these flows is the presence of vortical structures that convect past a solid wall. However, a fundamental difference between the two flows is that in the impinging jet case, the vortical structures interact with each other and the wall causing boundary layer separation and the formation of secondary vortices having opposite‐sign vorticity. To study the BFS flow, measurements are conducted using a surface‐pressure sensor array simultaneously with Particle Image Velocimetry (PIV) downstream of the step at a Reynolds number of approximately 8000, based on step height. To facilitate examination of the spatiotemporal characteristics of the surface‐pressure generating structures, multi‐point, linear, stochastic estimation (mLSE) is used to estimate the velocity field from the time‐resolved wall-pressure signature. On the other hand, simultaneous time resolved flow visualization and wall-pressure sensor array measurements are employed to conduct similar examination in the impinging flow at a jet Reynolds number of nearly 7000 based on the jet diameter. Results show that in the BFS flow, the wall‐pressure fluctuations are associated with a vertical flow structure similar to that found in the wake of bluff bodies. The p'w footprint of this structure could be explained in straightforward manner through examination of the “source terms” in Poisson’s equation for pressure. In contrast, a similar straightforward relationship between the vortical structures and p'w signature does not exist in the impinging‐jet case where the vortex‐vortex and vortex‐wall interactions create additional pressure‐generating sources, which result in a more dynamically evolving and complex pressure signature and make the inference of the flow features from the surface pressure unfeasible. The nature of these additional sources is clarified and the physics underlying the observed behavior of p'w signature is explained through a computational study of a model problem involving an axisymmetric vortex ring impinging on a wall.

About the Speaker

Ahmed Naguib is currently Professor and Associate Chair for the Graduate Program in Mechanical Engineering at Michigan State University (MSU). His BS degree is from Ain Shams University, Cairo, Egypt, and Master and Ph.D. degrees are from Illinois Institute of Technology, Chicago, USA. His research interests are in experimental fluid dynamics and associated applications, particularly in the field of turbulence and instability, physics and control, unsteady aerodynamics as well as development of advanced measurement techniques. Prof. Naguib’s work has resulted in more than seventy conference and journal publications, two book chapters, and three patents. He has also been an Associate Editor of the American Institute of Aeronautics and Astronautics (AIAA) Journal since January 2009 and he was awarded the AIAA Associate Fellow grade in 2012.

Host: Professor Ken Christensen

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