Computations are performed to examine the effects of a spanwise periodic array of trip elements on a high-speed laminar boundary layer, so as to identify the potential physical mechanisms underlying an earlier transition to turbulence as a result of the trip(s). In the context of a 0.333 scale model of the Hyper-X forebody configuration, the time accurate solution for an array of ramp shaped trips asymptotes to a steady flow at large times, indicating the likely absence of a strong absolute instability in the mildly separated flow due to the trips. A prominent feature of the wake flow behind the trip array corresponds to streamwise streaks that are further amplified in passing through the compression corner located downstream of the trips. Stability analysis of the streaks using a spatial, 2D eigenvalue approach reveals the potential for a strong convective instability that might explain the earlier onset of turbulence within the array wake. For the ramp trips considered in this paper, the dominant modes of streak instability are primarily sustained by the spanwise gradients associated with the streaks and lead to integrated logarithmic amplification factors (N factors) approaching 7 over the first ramp of the scaled Hyper-X forebody, and substantially higher over the second ramp. Additional computations are presented to shed further light on the effects of both trip geometry and the presence of a compression corner on the evolution of the streaks. I.

The flow topologies of gas and liquid injection into hypersonic crossflow are compared with the flow topology around 3D rigid obstacles in hypersonic flow. Experimental techniques such as oil flow visualization, infrared thermography, sublimation technique and high speed Schlieren photography are applied for a Mach 6 crossing flow on a flat plate. The shock system and the vortex system are visualized and discussed for all cases. Pressure ratio for gas injection, momentum flux ratio for liquid injection and Reynolds number for 3D obstacles are taken into consideration. The effects of non-dimensional numbers on the flow topologies are analyzed. Similarities and differences among different topologies are explained by studying the vortical aerodynamics, making use of CFD simulations..omenclature a = diameter or width of 3D roughness element, mm AR = aspect ratio Cf = non-dimensional skin friction coefficient d, dj = injector diameter, mm fps = frame per second, s-1 k = height of 3D roughness element, mm K = ratio of experimental heat flux to theoretical turbulent heat flux, % P0 = total pressure, bar PR = ratio of total injection gas pressure to freestream static presssure Re = Reynolds number T0 = total temperature, K TPS = thermal protection system q = momentum flux (dynamic pressure) ratio of injected liquid to freestream V ∞ = Freestream velocity, m/s Vj = Injected jet velocity, m/s ρ ∞ = Freestream density, kg/m

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

NO PLIF study of hypersonic transition over a discrete hemispherical roughness element

The importance of discrete roughness and the correlations developed to predict the onset of boundary layer transition on hypersonic flight vehicles are discussed. The paper is organized by hypersonic vehicle applications characterized in a general sense by the boundary layer: slender with hypersonic conditions at the edge of the boundary layer, moderately blunt with supersonic, and blunt with subsonic. This paper is intended to be a review of recent discrete roughness transition work completed at NASA Langley Research Center in support of agency flight test programs. First, a review is provided of discrete roughness wind tunnel data and the resulting correlations that were developed. Then, results obtained from flight vehicles, in particular the recently flown Hyper-X and Shuttle missions, are discussed and compared to the ground-based correlations.

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