Roughness-Induced Transition in High Speed Flows

Abstract

Roughness elements in a laminar boundary layer can cause the flow to transition. The effects of discrete and distributed roughness are explored using direct numerical simulation on unstructured grids. Velocity profiles for Mach 8.12 flow past a cylindrical roughness element are compared to experiment. 1 Flow features produced by an isolated hemispherical bump are studied for three Mach numbers [3.37, 5.26, 8.23] whose simulation parameters are chosen to match the experiments performed by Danehyet al. 2 at NASA Langley. The vortices formed upstream of the roughness due to boundary layer separation wrap around the hemisphere generating coherent streamwise vortices. A significant increase in wall skin friction coefficient along with the presence of unsteady structures far downstream of the bump for the lower Mach number cases indicated the transitional/turbulent nature of the flow while M∞=8.23 remained laminar. These observations are consistent with those in experiment. The effect of a sinusoidally varying distributed roughness was also investigated at M∞=2.9. The flow transitioned to a fully developed turbulent boundary layer that shows good quantitative agreement with experimental data. The cumulative effect of multiple roughness elements is to decelerate the near wall fluid, and setup inflexional velocity profiles. For both types of roughness it was seen that: (1) coherent streamwise vortices were produced which broke down far downstream, (2) prominent hairpin shaped structures were observed as the flow transitioned and (3) wall normal and spanwise inflexion points in streamwise velocity are observed.

Hypersonic Boundary-Layer Instabilities due to Near-Critical Roughness

“…Thanks in large part to advancements in computational methods, researchers are recently beginning to characterize the mechanisms responsible for transition due to roughness at hypersonic speeds [11][12][13][14][15][16][17][18][19][20][21][22][23][24]. However, even direct numerical simulation must construct an appropriate grid and make assumptions regarding the input disturbance field when simulating these flows.…”

Direct numerical simulation of hypersonic transition induced by an isolated cylindrical roughness element

“…The grid density of the Baseline grid is comparable with the typical mesh employed in the similar DNS study on the RIT (for instance, Iyer et al [16,17], Bernardini et al [18] and Wheaton et al [20]). Figure 4 presents the horseshoe vortices and the shear layer after the roughness element using Q criterion.…”

“…Iyer et al [16,17] numerically studied the transition triggered by a hemispherical bump on a flat plate using DNS method based on unstructured girds, with about 40 million grid cells. The Mach number effects (Ma=3.37, 5.26, and 8.23) on the transition location were investigated.…”

Review of Fluorescence-Based Velocimetry Techniques to Study High-Speed Compressible Flows

“…A subsequent comparison between this data set and computations 12 showed relatively good agreement between the measured and predicted streamwise velocity for most of the profiles. However, unpublished comparisons between computations and measurements of the velocity immediately upstream of the tripping element showed discrepancies, with the measured velocities consistently lower than those predicted.…”