Response of the sphere wake to freestream fluctuations

Direct numerical simulations have been used to investigate the response of the wake of a sphere to freestream fluctuations. This study has been motiva ted by the need to understand particle-induced turbulence enhancement in pa rticulate flows. A sequence of simulations of flow past a sphere have been carried out where the frequency and amplitude of the freestream fluctuation s and the flow Reynolds number has been varied systematically. It has been suggested that turbulence enhancement is primarily caused by vortex sheddin g from particles (Gore and Crowe, 1989; Hetsroni, 1989). Our simulations of the forced wake indicate that turbulence enhancement may be attributed to natural vortex shedding only when the freestream fluctuation level is low a nd the Reynolds number is greater than about 300. In addition to natural vo rtex shedding, the current simulations also suggest another mechanism for t urbulence enhancement. It is found that in the presence of freestream fluct uations, the wake behaves like an oscillator and returns large amounts of k inetic energy to the surrounding fluid at resonance. This mechanism is not associated with natural vortex shedding and is therefore capable of enhanci ng freestream turbulence even at Reynolds numbers less than 300. Simulation s also indicate that when the turbulence intensity of the carrier fluid is high, this resonance mechanism might be solely responsible for turbulence e nhancement. Finally, our simulations also suggest a possible explanation fo r the correlation between turbulence enhancement and the ratio of the parti cle size to the size of energy containing eddies of turbulence found by Gor e and Crowe (1989).