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Accession Number ADA601132
Title Wall-based Actuation for Transition Delay and Drag Reduction.
Publication Date Feb 2014
Media Count 21p
Personal Author T. Zaki
Abstract Subjecting a boundary layer to sudden or changing strain can have a profound effect on the flow. In turbulent boundary layers, oscillating the wall has been shown in previous studies as a viable mechanism to reduce drag. At lower Reynolds numbers, and in particular in the transition regime, the flow response is complex. On the one hand, the three dimensionality of the base state can lead to new instabilities. On the other hand, the streaks which are often observed in bypass transition can be weakened, akin to observations in fully-turbulent shear flows. This work investigates the influence of spanwise wall oscillation on bypass transition in zero-pressure-gradient boundary layers. Direct numerical simulations are performed in order to examine the impact of the wall forcing on the non-linear transition process. The simulations demonstrate that appropriate choice of the oscillation amplitude and frequency can delay transition. The non-linear computations are complemented by linear analysis of a simple model that explains the influence of the unsteady shear on the penetration of free-stream vortical disturbances into the boundary layer. This effect, and the weaker streaks in the pre- transitional flow, ultimately lead to a delay in the secondary instability of the streaky base flow and a downstream shift in transition onset.
Keywords Boundary layer
Boundary layer transition
Drag reduction
Mathematical models
Transition delay
Turbulence
Wall-based actuation


 
Source Agency Non Paid ADAS
NTIS Subject Category 46B - Fluid Mechanics
51A - Aerodynamics
51C - Aircraft
Corporate Author European Office of Aerospace Research and Development, FPO New York 09510.
Document Type Technical report
Title Note Final rept. 1 Sep 2011-31 Aug 2013.
NTIS Issue Number 1421
Contract Number FA8655-11-M-4002

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