Prahladh S. Iyer

Transition of hypersonic flow past flat plate with roughness elements

Roughness elements in high speed flows can cause laminar-turbulent transition leading to higher heating rates and drag. Transition of flow past a hemispherical bump placed on a flat plate is explored in this paper for three Mach numbers [3.37, 5.26, 8.23] using direct numerical simulation on unstructured grids. The simulation parameters are chosen to match the experiments carried out by Danehy et al. The wall is at a constant temperature of 300K. For flow conditions corresponding to Ma=3.37 and 5.26, unsteady flow structures were observed while for Ma=8.23, the flow remained laminar downstream of the trip. The location of transition was closer to the trip for the lowest Mach number. Qualitative comparison between the computation and experiment show good agreement. Based on the computed skin friction coefficient values, Ma=3.37 appeared to become turbulent in nature, Ma=5.26 was transitional and Ma=8.23 was laminar. Quantitative comparisons of flow profiles downstream of the trip were made with theoretical turbulent profiles to confirm transition to turbulence. The effect of distributed roughness on transition was also studied at Ma=2.9. A laminar boundary layer at Ma=2.9 was observed to transition to a turbulent boundary layer that shows good quantitative agreement with experimental data. The freestream Mach number and roughness amplitude were seen to strongly influence whether or not the flow transitions. A local Reynolds number based on bump/roughness amplitude is seen to correlate the tendency to transition for both single bump and distributed roughness cases.

Roughness-Induced Transition in High Speed Flows

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. 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. 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.

Boundary layer transition in high-speed flows due to roughness

Direct numerical simulation (DNS) is used to study the effect of individual (hemispherical) and distributed roughness on supersonic flat plate boundary layers. In both cases, roughness generates a shear layer and counter–rotating pairs of unsteady streamwise vortices. The vortices perturb the shear layer, resulting in trains of hairpin vortices and a highly unsteady flow. Mach 3.37 flow past a hemispherical bump is studied by varying the boundary layer thickness (k/δ = 2.54, 1.0, 0.25 & 0.125). Transition occurs in all cases, and the essential mechanism of transition appears to be similar. At smaller boundary layer thickness, multiple trains of hairpin vortices are observed immediately downstream of the roughness, while a single train of hairpin vortices is observed at larger δ. This behavior is explained by the influence of the boundary layer thickness on the separation vortices upstream of the roughness element. Mach 2.9 flow past distributed roughness results in a fully turbulent flow. Mean velocity profiles show spanwise inhomogeneity in the transitional region, with the flow becoming more homogenous downstream. Spanwise spectra initially exhibit only the wavelength of the roughness surface. Then, the energy at smaller wavelengths increases resulting in a broadband spectra downstream. Temporal spectra in the transitional region are characterized by the frequency of the unsteady vortices and a higher frequency corresponding to the shear layer breakdown. The magnitude of wall–pressure fluctuations is observed to be greater in the transitional region than in the turbulent region, where a good agreement with recent experiments is obtained.

Comparing Experiment and Computation of Hypersonic Laminar Boundary Layers with Isolated Roughness

‡‡ Streamwise velocity profile behavior in a hypersonic laminar boundary layer in the presence of an isolated roughness element is presented for an edge Mach number of 8.2. Two different roughness element types are considered: a 2-mm tall, 4-mm diameter cylinder, and a 2-mm radius hemisphere. Measurements of the streamwise velocity behavior using nitric oxide (NO) planar laser-induced fluorescence (PLIF) molecular tagging velocimetry (MTV) have been performed on a 20-degree wedge model. The top surface of this model acts as a flat-plate and is oriented at 5 degrees with respect to the freestream flow. Computations using direct numerical simulation (DNS) of these flows have been performed and are compared to the measured velocity profiles. Particular attention is given to the characteristics of velocity profiles immediately upstream and downstream of the roughness elements. In these regions, the streamwise flow can experience strong deceleration or acceleration. An analysis in which experimentally measured MTV profile displacements are compared with DNS particle displacements is performed to determine if the assumption of constant velocity over the duration of the MTV measurement is valid. This assumption is typically made when reporting MTV-measured velocity profiles, and may result in significant errors when comparing MTV measurements to computations in regions with strong deceleration or acceleration. The DNS computations with the cylindrical roughness element presented in this paper were performed with and without air injection from a rectangular slot upstream of the cylinder. This was done to determine the extent to which gas seeding in the MTV measurements perturbs the boundary layer flowfield.