Experimental Investigation On The Large Scale Coherent Motions In A Backward Facing Step Flow

Backward facing step(BFS)flow has received great attentions due to the complex dynamics and simple geometric.Several unsteady modes have been identified in the BFS flow including the shear-layer mode,step mode,shedding mode and flapping mode.However,there are still open problems remaining unsolved,like the cause of shear layer flapping motion and the transition process from step mode to shedding mode.An experimental study of BFS flow was performed in both low-speed circulating water tunnel and open-circuit wind tunnel with Reynolds numbers form 992 to 21688.Methods include muti-exposure smoke-wire flow visualization,planar and stereo Particle Image Velocimetry and hotwire array were used.Data was analysed by statistical tools such as correlation,spectra and Proper Orthogonal Decomposition.Small amplitude periodic perturbations were also used to study the large scale coherent motions.The results show that the spanwise vortices emerges in the shear layer due to KelvinHelmholtz instability were quickly engulfed by step mode structures,resulting in a characteristic frequency St?0.2 near the reattachment location.The step mode appears to be the 'preferred'mode of the BFS flow,with is widely seen in literature and its scaled with step height,H.Downstream of the reattachment,step mode structure become fully three-dimensional and appear in ?-shape.The order of ?-shape structures tend to be staggered after reattachment due to the influence from the previous structure.This mechanism lead to"X" shape in the distribution of the correlation coefficients,making characteristic frequency reduced by half to St ?0.1.It is the core contribution from this thesis by proposing a staggered arrangement of A-shape structures.Long streamwise vortices were also found in the BFS flow when Re? 2946.Counterrotating adjacent streamwise vortices with a spanwise scale of H lead to a spanwise wavelength?z? 2H.Characteristic frequency St<0.05 for the spanwise wave-like movement of the shear layer causes the low frequency flapping motion.Neither Reynolds number nor aspect ratio influence the dimensionless spanwise wavenumber and frequency spectra when 3232?Re?21688 and 12?w/H ?20.