An Experimental Investigation of Smooth-Body Flow Separation over a Tapered Gaussian Bump

The results of a three year experimental campaign aimed at comprehensively documenting the separated flow over a three-dimensional bump are presented with the purpose of generating a benchmark experimental database useful in validating computational fluid dynamics (CFD) flow simulations. The bump model geometry was designed to provide well-defined and repeatable smooth-body flow separation conditions that were suitable for both experiments and simulations. The bump had a Gaussian streamwise profile with a constant height equal to 8.5% of its width over the central 60% of the test section width. The remaining 40% were outboard spanwise portions that gradually taper to zero using an error function profile to minimize side-wall boundary layer effects. The model was immersed in a turbulent boundary layer that was developed on a suspended flat plate in the Notre Dame Mach 0.6 Wind Tunnel. In order to document the effect of the incoming boundary layer thickness on the flow separation, the bump model could be located at two streamwise positions. The mean velocity and turbulence intensity of the wind tunnel freestream flow field and approaching turbulent boundary layer were fully documented. The measurements of the flow separation region included surface visualization, wall shear stress using oil-film interferometry, mean and dynamic surface pressure, and planar and stereoscopic particle image velocimetry. The experiments were conducted over a range of Mach numbers from 0.05 to 0.2 corresponding to a range of Reynolds numbers based on the test section spanwise dimension (0.914m) of 1.0 × 10^6 ≤ Re_L = U_∞L/ν ≤ 4.0 × 10^6. The bulk of the results are presented for the higher Mach number conditions of 0.1 and 0.2 with Re_L = 2.0 × 10^6 and 4.0 × 10^6, respectively. Extensive uncertainty analysis of the data was performed. The data are archived in the NASA Langley Turbulence Modeling Resource website at https://turbmodels.larc.nasa.gov/Other exp Data/speedbump sep exp.html. In addition to the experiments, a computational effort was made in parallel by the CFD group at Boeing Research & Technology highlighting the usefulness of the data set, and is outlined in an accompanying CFD report. Mean surface streamlines were highlighted to show the footprint of two symmetric counter rotating vortices that lift away from the surface and propagate downstream. Cross-plane measurements using stereoscopic particle image velocimetry were taken to track the development of the vortices and identify off-surface topological patterns in the separated shear layer region. A combination of time-averaged measurements reveal the presence of a lifting arch vortex. The ends of the arch vortex, originating at the symmetric spiral node footprints on the surface, lift away from the wall and join together with spanwise oriented clockwise vorticity. Reynolds number sensitivity to the separated flow is discussed, with sufficient tunnel speed resolution to identify how the flow transitions from an attached case to a fully separated shear layer by increasing the freestream velocity. Evidence of relaminarization in the upstream turbulent boundary layer was found to be a critical factor in the lack of separation in the attached flow case. Similarity of the mean streamwise velocity profiles using embedded shear layer scaling (Schatzman, D. M., and Thomas, F. O., J. Fluid Mech 2017, Vol. 815, pp. 592–642) supports the claim that an instability in the outer inflection point of the mean velocity profile is present in the flow. Progress towards the scaling of turbulence intensity profiles within a turbulent shear layer with surface curvature was made. The dynamic measurements of the surface pressure field using Kulite sensors were analysed using Fourier transforms to isolate shared frequency modes between streamwise sensor pairs. Turbulent detachment states along the centerline were computed and the onset of separation was found to be in spatial alignment with the location of the peak adverse pressure gradient. An SPIV measurement using an acquisition frequency of 1.5 kHz was used to investigate the temporal evolution of the decomposed Reynolds stresses as well as the center of the separation bubble within the backflow region.