Passive scalar statistics in a turbulent channel with local time-periodic blowing/suction at walls

Direct Numerical Simulations (DNS) of an incompressible turbulent channel flow with local forcing at the walls are performed. Time-periodic blowing/suction is applied by means of narrow spanwise slots located at the lower and upper walls in x / L x = 0 (where L x is the channel length). The normal perturbing velocity is varied sinusoidailly in time at several perturbing frequencies between 0.16 < f ¯ < 1.6 and at a fixed amplitude of A o = 0.2 . The temperature field is also computed and assumed to be a passive scalar. The Reynolds number of the unperturbed case is R e τ = 394 and the Prandtl number is P r = 0.71 . It is concluded that the forcing frequency of f ¯ = 0.64 or f + = 0.044 produces the largest local increase of the skin friction in the region 0.1 < x / L x < 0.3 , followed by the highest augmentation of the Stanton number. Furthermore, budgets of the passive-scalar variance and wall-normal turbulent heat fluxes at this frequency demonstrate a significant enhancement of the molecular diffusion at the wall and pressure-related terms, respectively. The latter confirms the importance of pressure fluctuations on the transport of passive scalars and redistribution of energy.