Combined effects of groups of vortices generated by a pulsating turbulent plane jet impinging on a semi-cylinder: Effects of the forcing frequency

A submerged pulsating plane turbulent jet was simulated using Large Eddy Simulation at different forcing frequencies. Based on a previous work, interesting phenomena, related to the interaction of coherent primary vortices with the dynamical and thermal fields along the target curved wall, were elucidated for a forcing frequency equal to 600 Hz. As a systematic continuation, the present work extends the study to the effects of more forcing frequencies. The main objective remains the same which is investigating the time-dependent responses of Nusselt number, friction coefficient and pressure profiles to the passage of the coherent structures along the curved impingement wall. The distance s, along the target wall, is measured from the impact point and normalized by the jet exit width W. It was found that for a forcing frequency equal to 200 Hz, coherent forced primary vortices induced by the pulsations are separated by less organized vortices naturally induced as in the unforced jet. The results showed clearly that the natural vortices have moderate effects on the boundary layer development on the impingement surface starting at relatively short distances from the stagnation point, s/W < 3. On the other hand, the forced vortices seem to have a more influential role on the wall jet behavior but further downstream. Increasing the forcing frequency to 400 Hz reduces the distance separating successive forced vortices causing the pairing phenomenon to occur at s/W = 6 along the target wall. Increasing the forcing frequency further makes the pairing phenomenon followed by vortex breakdown to occur at shorter distances along the target wall. The smaller forcing frequencies such as 400 Hz generate large and strong distant vortices that affect the dynamic field noticeably with an important deterioration of heat transfer due to their strong mixing effect and entrainment of cold air from the surroundings towards the target wall. On the other hand, high frequencies (1500 Hz and 2200 Hz) generate smaller closely successive vortices, with a weaker effect, that do not disturb the boundary layer on the target wall considerably up to a distance of s/W = 4 where the lowest heat transfer is observed. In fact, the cold air from the surroundings is not entrained and does not approach the target wall over a long distance which generates the highest heat transfer away from the impingement region.