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11.3.15 Iyer

67th NIA CFD Seminar:
HIGH-SPEED BOUNDARY-LAYER TRANSITION INDUCED BY A DISCRETE ROUGHNESS ELEMENT

Prahladh S. Iyer, Research Scholar, NIA
November 3, 2015, 11:00 am, NIA, Rm 137

Abstract:
Surface protuberances can cause laminar to turbulent transition in high-speed vehicles leading to a higher heating load and skin-friction drag. Hence, understanding the mechanism of transition is important for the design of such vehicles. Direct numerical simulation (DNS) is used to study laminar to turbulent transition induced by a discrete hemispherical roughness element in a high-speed laminar boundary layer using an unstructured finite volume methodology. The simulations are performed under conditions matching the experiments of Danehy et al. (AIAA Paper 2009-394, 2009) performed at the NASA Langley Mach 10 Air Tunnel, for free-stream Mach numbers of 3.37, 5.26 and 8.23. Qualitative comparison to experiment shows good agreement. It is observed that the Mach 8.23 flow remains laminar downstream of the roughness, while the lower Mach number flows undergo transition. A phenomenological mechanism is proposed for the observed behavior. For Mach 3.37 and 5.26, mean statistics downstream of the roughness is compared with available turbulent boundary layer data and show good agreement. The effect of boundary layer thickness on Mach 3.37 flow past a hemispherical bump is also studied keeping all other parameters constant. While the essential mechanism of transition is similar for the conditions studied, differences are observed in the number of trains of hairpin vortices downstream of the roughness element. A Reynolds number based on the skin friction velocity and wall properties is seen to correlate with the onset of transition for the cases considered.

Speaker Bio:
Dr. Prahladh S. Iyer is currently working at NIA as a Research Scholar. He obtained his PhD from the Dept. of Aerospace Engg. & Mechanics, University of Minnesota in February 2015 and B.Tech in Chemical Engineering from the National Institute of Technology, Surathkal, India in 2008. His research interests include DNS/LES of complex flows, transition to turbulence and turbulence modeling.

Additional information, including the webcast link, can be found at the NIA CFD Seminar website, which is temporarily located at http://www.hiroakinishikawa.com/niacfds/index.html

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