| A set of controlled numerical experiments was performed to verify the role of (mean) secondary vortices (SV) in the separation dynamics of three-dimensional asymmetric diffusers. Large Eddy Simulations with instantaneous turbulent inflow data were performed. At the inflow, the mean SV were manipulated, e.g. their sense of rotation was switched or their strength was enhanced or attenuated in a steady or unsteady fashion. The results show a strong sensitivity of the flow to SV and that the performance and geometric sensitivity of the diffusers can be controlled effectively by these flow structures with little energy input (of the order of one percent of the mean flow energy). The results also explain why eddy-viscosity models, which cannot account for mean SV, fail to predict the separated flow in such diffusers. |
