Fractal/multiscale trippings for passive control of turbulent flow separation.*

An experimental investigation was performed on the control of turbulent flow separation by fractal/multiscale tripping at the LMFL boundary layer (BL) wind-tunnel. The passive devices have a height equal to 0.125 δ, where δ is the BL thickness. They are located at approximately x = -3.85h upstream of a rounded backward-facing step where h is the step height. The Reynolds number based on h is Reh= 100,000 and the expansion ratio is 1.15, based on the step height h. The recirculation region is studied by 2D2C particle image velocimetry (PIV) in the streamwise-wall-normal (x–y) plane. Time-resolved velocity fields are acquired at 192 Hz allowing the characterization of the unsteady nature of separated flows at high Reynolds number. The baseline configuration (without devices) is compared to three fractal/multiscale trippings, all having the same properties except for the number of fractal/multiscale iteration. All obstacles have the same frontal area. The recirculation region is smaller in size for the fractal/multiscale trippings than for the straight baseline configuration, and the size of the recirculation region decreases as the fractal iteration number increases.