Detailed Characterization of Turbulent Separated Flow Dynamics and Boundary Layer Evolution Over a Speed-Bump Geometry

Abstract

The separated flow over a speed-bump was studied using planar particle image velocimetry(PIV) and high frequency surface pressures to investigate the separation and reattachment dynamics. The source of low frequency unsteadiness has yet to be determined and is of great interest for aerospace structures. The upstream boundary layer dynamics were dominated by the pressure gradient when in the presence of changing surface curvature. Conditional averaging of PIV fields based on reverse flow fraction yielded the mean separation and reattachment locations. For ReL = 2.56∗106, the mean separation point x/L = 0.105 varied between 0.089 < x/L < 0.120 (±2σ) and the mean reattachment point x/L = 0.352 varied between 0.300 < x/L < 0.403 (±2σ), yielding a mean separation bubble length Lsep = 0.226m. Velocity perturbations in the upstream boundary layer were correlated to reverse flow fractions, indicating a relationship with the separation location. Proper orthogonal decomposition of the velocity fluctuations indicated the largest contribution to total energy and turbulent shear stresses came from the low frequency motion of the separation bubble, while other modes showed convecting structures and the shear layer. Surface pressures revealed a spectral peak downstream of the apex at St = f ∗ Lsep/U∞ ≈ 0.05 and peaks downstream of separation at St ≈ 0.62, consistent with the presence of low-frequency bubble breathing and shear layer vortex shedding, respectively.