## OPTIMAL FORCING FREQUENCY FOR ENHANCEMENT OF NEAR-WALL VORTICAL STRUCTURE IN A TURBULENT BOUNDARY LAYER
## AbstraktEffects of localized periodic blowing on a turbulent boundary layer are investigated using direct numerical simulation. The time-periodic blowing through a spanwise slot is given by varying the wall-normal velocity in a cyclic manner from 0 to 2A ^{+}=0.5. Simulations of spatially-evolving turbulent boundary layer are carried out at the Reynolds numbers of 300 and 670 based on the momentum thickness and free-stream velocity. The effects of the blowing frequency are scrutinized by examining the phase- or time-averaged turbulent statistics. A most effective blowing frequency is obtained at f=0.035 for both Reynolds numbers, where the maximum increases of Reynolds shear stress, streamwise vorticity fluctuations and energy redistribution are made. Reynolds stress budget shows that the effective blowing frequency most enhances the pressure-strain term, which is closely related with the energy redistribution. The analysis of the phase-averaged stretching and tilting terms reveals that the stretching term is significantly enhanced in the 'downward' motion which is induced by the spanwise vortical motion.^{+} |

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