Freestream

About: Freestream is a research topic. Over the lifetime, 3428 publications have been published within this topic receiving 56147 citations.

An Investigation of Contoured Wall Injectors for Hypervelocity Mixing Augmentation

Abstract: A parametric study of a class of contoured wall fuel injectors is presented. The injectors were aimed at enabling shock-enhanced mixing for the supersonic combustion ramjet engines currently envisioned for applications on hypersonic vehicles. Short combustor residence time, a requirement for fuel injection parallel to the freestream, and strong sensitivity of overall vehicle performance to combustion efficiency motivated the investigation. Several salient parametric dependencies were investigated. Injector performance was evaluated in terms of mixing, losses, jet penetration and heating considerations. A large portion of the research involved a series of tests conducted at the NASA Langley High - Reynolds Number Mach 6 Wind-Tunnel. Helium was used as an injectant gas to simulate hydrogen fuel. The parameters investigated include injector spacing, boundary layer height, and injectant to freestream pressure and velocity ratios. Conclusions concerning injector performance and parameter dependencies are supported by extensive three-dimensional flow field surveys as well as data from a variety of flow visualization techniques including Rayleigh scattering, Schlieren, spark-shadowgraph, and surface oil flow. As an adjunct to these experiments, a three-dimensional Navier-Stokes solver was used to conduct a parametric study which closely tracked the experimental effort. The results of these investigations strongly complemented the experimental work. Use of the code also allowed research beyond the fairly rigid bounds of the experimental test matrix. These studies included both basic investigations of shock-enhanced mixing on generic injectors, and applied efforts such as combining film-cooling with the contoured wall injectors. Location of an oblique shock at the base of the injection plane was found to be a loss-effective method for enhancing hypervelocity mixing through baroclinic generation of vorticity and subsequent convection and diffusion. Injector performance was strongly dependent on the displacement effect of the hypersonic boundary layer which acted to modify the effective wall geometry. Strong dependence on injectant to freestream pressure ratio was also displayed. Mixing enhancement related to interaction of the unsteady component of the boundary layer with both steady and unsteady components of the flow field was found to be secondary, as were effects due to variation in mean shear between the injectant and the freestream in the exit plane.

Aerodynamic properties of some simple bodies in the hypersonic transition regime.

Abstract: A Monte Carlo simulation technique has been used to conduct numerical experiments with a model gas on a digital computer. Drag, heat transfer, and general flowfield information was obtained for cylinders, spheres, and zero incidence flat plates. Speed ratios of 5 and 10 were treated, while the Knudsen number ranged from 0.1 to 30. It was found that drag coefficient results give a poor indication of the onset of collisional effects, since the fall in the streamwise momentum of the molecules striking the body is initially compensated by an increase in the flux of these molecules. For cylinders and spheres with cold surfaces, the drop in momentum may be overcompensated? and the drag coefficient may rise above the free molecule value before falling, with Knudsen number, towards the continuum value. The shear stress on a flat plate was found to rise above the free molecule value for both hot and cold surfaces. Some results were obtained for the pressure distribution on finite length flat plates, and these indicate that, contrary to a common assumption, there is no free molecule region at the leading edge of such plates. Nomenclature d = parameter related to the mean free path e = probability of event I = separation distance n = number flux p = pressure r — random number s = molecular speed ratio t = time tm = time interval u = streamwise velocity component v = molecular speed vr = relative velocity x = probabilistic variable C = counter CD = drag coefficient Kn = Knudsen number N = number density No = total number of molecules Q = heat transfer per unit area per unit time R = gas constant St = modified Stantori number T = temperature Tr = free molecule recovery temperature X = mean free path p = density a- = molecular radius r = shear stress Subscripts oo = freestream value continuum stagnation value surface value leading-edge value free molecule value o = w = le fm =

Calculation of Thermal Response of Ablator Under Arcjet Flow Condition

Abstract: An integrated computational method is developed to calculate thermal response of ablator under an arcjet flow condition. In the method, the arcjet freestream condition in the test section is evaluated by calculating the flows in the arcjet wind tunnel fully theoretically. The thermal response of the ablator is calculated by loosely coupling the shock layer computational fluid dynamics code and the 2-D version of ablation code using the arcjet freestream condition so evaluated. The method is applied to the heating tests conducted in the 1 MW arcjet wind tunnel for one operating condition. The influence of catalytic conditions of ablating surface and the effect of nitridation reaction and surface roughness on the thermal response of the ablator are investigated. Comparison of the temperature profile at the ablating surface between calculation and measurement suggests that the measured temperature profile can be reproduced with a low catalytic efficiency of the surface. It is found that the nitridation reaction increase the surface temperature moderately, and that the effect of the roughness on the surface were small for the present operating condition.

Effects of upstream coherent structures on low-frequency motion of shock-induced turbulent separation

Abstract: x Streamwise direction z Spanwise direction y Wall-normal direction U Streamwise velocity (m/s) U∞ Streamwise freestream velocity (m/s) uτ Skin friction velocity M Mach number δ Boundary layer thickness θ Momentum thickness δ∗ Displacement thickness Um Mean streamwise velocity at any given location in the boundary layer σu R.M.S streamwise velocity at any given location in the boundary layer xsep Separation location (located using a threshold technique) Ul Mean streamwise velocity along a streamwise line upstream of the separation location T Static temperature Cp Specific heat at constant pressure