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11th Annual T. Francis Ogilvie Young Investigator Lecture Prof. Pavlos P. Vlachos, Mechanical Engineering Department, School of Biomedical Engineering and Sciences, Virginia Tech PDF Flyer
ABSTRACT: Recent numerical results of flow control for turbulent boundary layers provide evidence that small, spanwise wall motions can modify the character of the near wall turbulence and even eliminate the near-wall streaks responsible for the turbulent skin friction resulting in significant viscous drag reduction1. Experimental implementation and validation of such flow control methods is not a trivial exercise. Near wall measurements are notoriously challenging and complicated. Our understanding of the physical mechanisms associated with wall turbulence is hampered by the lack of accurate non-invasive measurement techniques with high spatiotemporal resolution. In this talk, advancements on the accuracy of resolving near wall turbulence non-invasively using an in-house developed Time-Resolved Digital Particle Image Velocimetry (TRDPIV) system with kHz sampling rates and performing shear stress measurements will be demonstrated. Subsequently, we will present the development of a new class of mini/micro sensors for direct measurements of fluctuating wall shear stresses. These sensors provide a unique combination of features. They can operate in air, water or multi-phase flow environments, they are minimally invasive, flush mountable, immune to vibration and pressure effects and more importantly deliver high sensitivity, linearity and high frequency response with accuracy better than 4%. The above presented tools are employed to validate the performance of a novel approach for controlling the near wall turbulence and reducing viscous drag. The method is based on the hypothesis that a disturbance introduced in the flow via the motion of an active compliant surface and in the form of a traveling wave can interrupt the turbulence production cycle and result in significant reduction of turbulent skin friction. The mechanism of this flow control approach -referred to as “traveling-wave skin”- is based on a force generated from the combination of viscous effects via the oscillating wall motion and body/pressure forces resulting from the pressure gradients introduced by the curvature of the traveling wave. The character of the flow is analyzed and significant drag reduction in excess of 40% is estimated. Pavlos P. Vlachos graduated from Virginia Polytechnic Institute and State University (Virginia Tech) with a M.S. and Ph.D. in Engineering Mechanics in 1998 and 2000, respectively. He had received his B.S. in Mechanical Engineering from the National Technical University of Athens in 1995. He was first appointed as research assistant professor in 2000 and then (2003) he assumed his current position as assistant professor in the Department of Mechanical Engineering at Virginia Tech. In spring of 2005 he received the Deans Award for Outstanding Assistant Professor. His research focuses on experimental fluid mechanics addressing a variety of flows, primarily wall bounded flows, vortex dynamics, bio-fluid mechanics and multi-phase flows. He has attracted significant government and industry funding. Prof. Vlachos has authored over 50 technical papers that appeared in archival journals and conference proceedings. He also holds two patents. 1Karniadakis and Choi (2003): Mechanisms of transverse motion in turbulent boundary layers, Ann. Rev of Fluid Mechanics Vol. 35: 45-62 |
