Steady and time-periodic blowing/suction perturbations in a turbulent channel flow

Direct Numerical Simulations (DNS) of an incompressible turbulent channel flow with given local perturbations at the walls are performed. Steady and time-periodic blowing/suction are applied by means of narrow spanwise slots located at the lower and upper walls. Our previous investigations on time-periodic blowing/suction (Araya et al. (2008) [13,14] and Araya (2008) [15]) have revealed the existence of a characteristic forcing frequency of f ¯ = 0.64 or f + = 0.044 . At this frequency, the largest local increases of the skin friction in the region downstream from the slots, followed by the highest augmentation of the Stanton number, were achieved. In this study, we seek to compare the different effects of constant blowing/suction with time-periodic blowing/suction on the flow and thermal fields. For the cyclical perturbation, the normal disturbing velocity is varied sinusoidally in time at the characteristic frequency of f ¯ = 0.64 . In both cases, the forcing amplitude, A o , is fixed to 0.025. The Reynolds number of the unperturbed case is Re τ = 394 and the molecular Prandtl number is Pr = 0.71 . Isothermal conditions are assumed for the lower wall (hot) and the upper wall (cold), respectively. For the forcing amplitude considered, steady disturbances induce higher modifications on the mean time quantities than time-periodic blowing/suction does. In addition, the effects of steady perturbations were observed to last longer downstream. However, time-periodic blowing/suction disturbances at the characteristic forcing frequency of f + = 0.044 have been demonstrated to be more effective in promoting local pressure fluctuations despite its short-distance influence. Furthermore, the energy spectra analysis of fluctuations indicates that time-periodic perturbations at this forcing frequency would be more efficient to excite the very small scales in a turbulent channel flow than by steady disturbances at the same forcing amplitude.