Active control of impinging jet for modification of mixing

The objective of the present study is to modify mixing and heat transfer in impinging jets using a single-frequency excitation imposed at the jet exit. The excitation frequency is selected to be St_θ = fθ/U_J,max = 0.017 where θ is the jet-exit momentum thickness and U_J,max is the jet-exit maximum velocity. In free jets, this excitation results in turbulence suppression in a downstream location. On the other hand, in impinging jets, the effect of excitation significantly depends on the distance (H) between the jet exit and the impinging wall. For large H (e.g. H / D = 10, D is the jet exit diameter), the Nusselt number near the stagnation point (Nu_stag) decreases due to turbulence suppression by the excitation. For small H (e.g. H / D = 2), Nu_stag is almost unchanged but the secondary peak much suppressed. On the other hand, Nu_stag increases for H / D = 6 due to turbulence enhancement by the excitation. The different behaviors of Nusselt number with respect to H / D are closely related to the changes in vortical structures by excitation.