(298e) Counter-Rotating Vortex Shedding Generated By Acoustic Excitations in Confined Mixing Layers

Novel ultrafast mixing enhancement has been realized in confined mixing layers at an optimal forcing frequency, under which the flow exhibits strongest velocity fluctuations (Wang GR. Chem Eng Sci. 2003, 58:4953-63; Wang, GR, AIChE J, 2006, 52:111-24). Previous investigations showed that, the optimal forcing frequency is attributed to an acoustic resonance which is dominated by the overall geometric scale of water tunnel (Zhao and Wang, Chemical Engineering and Processing: Process Intensification, 111 (2017) 67–78). We also found, the fast mixing was always accompanied with the so-called counter-rotating vortices (CRVs). In this investigation, with particle imaging velocimetry (PIV), we quantitatively study the velocity fluctuations in the nozzle section. It is found, under the optimal frequency, the flow exhibits highest velocity fluctuations due to acoustic resonance. We also find, CRVs can be generated are while two conditions are satisfied simultaneously, which are: (1) The acoustic particle displacement must be larger than the summation of the curvature radius of the trailing edge and the thickness of Stokes boundary layer, and (2) the convective transport of mean vorticity of unforced mixing layer should be conquered by the variation of shedding vorticity. These conditions are experimentally validated under different forcing frequencies, Reynolds number of basic flow and velocity ratios etc. Our investigations could be very useful on understanding forced vortex shedding mechanism. The results are also inspiring on designing novel and fast mixer, heat exchanger and sound attenuator etc.