Direct numerical simulations of spatially developing turbulent boundary layers over riblets are conducted to examine the effects of riblets on skin friction and heat transfer at high speeds. Zero-pressure gradient boundary layers under two flow conditions (Mach 2.5 with Tw/Tr = 1 and Mach 7.2 with Tw/Tr = 0.5) are considered. Simulations are conducted for boundary-layer flows over a clean surface and symmetric V-groove riblets. The DNS results at Mach 2.5 confirm the few existing experimental observations and show that a drag reduction of approximately 7% can be achieved for riblets with proper spacing. The comparisons in turbulence statistics and flow visualizations between a drag-reducing configuration (s + ≈ 20) and a drag-increasing configuration (s+ ≈ 40) demonstrate that drag reduction mechanisms proposed for incompressible flows can still apply for highspeed boundary layers. DNS studies at Mach 7.2 further show that riblets remain effective in reducing drag in the hypersonic regime. The Reynolds analogy holds with 2Cf/Ch approximately equal to that of flat plates for the drag-reducing configuration. © 2012 by the American Institute of Aeronautics and Astronautics, Inc.


Mechanical and Aerospace Engineering

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01 Jan 2012