Freestream

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

Receptivity and linear stability of stetson's mach 8 blunt cone stability experiments

Abstract: Currently, the mechanisms leading to hypersonic boundary layer transition are still poorly understood. The transition in the boundary layer depends on the receptivity process, which is the process of environmental disturbances initially entering the boundary layers and generating disturbance waves. The receptivity of hypersonic boundary layers to free stream disturbances is altered considerably by the presence of bow shocks in hypersonic flow fields and by the entropy layers created by the blunt nose. This paper conducts a numerical simulation study of the receptivity to freestream acoustic disturban& waves for Mach 7.99 axisymmetric flow over a io half-angle blunt cone, and compares the numerical results with experimental results by Stetson et al. (1984) and with those obtained from linear stability. Both steady and unsteady flow solutions of the receptivity problem are obtained by computing the full Navier-Stokes equations using a high-order accurate shock-fitting finite difference scheme, which can accurately account for the effects of bow-sho&/free-streamsound interactions on the receptivity process. In addition, a normal-mode linear stability analysis is also used to study the stability and receptivity properties of the boundary layer affected by the entropy layer. The main focus of this study is on the excitation of the second mode waves in the receptivity process in the presense of freestream acoustic waves. One of the major findings of this study is that, for the case of receptivity to fast freestream acoustic waves with a blunt nose, the second mode waves are not excited in the early region dong the cone surface where the second modes are pre dicted to be unstable by the linear stability analysis. It is shown that the delay of the second mode excitation is due to the unique stability characteristics of the current flow which are affected by the entropy layer produced by the nose bluntness. The understanding of such receptivity processes may lead to a better understanding of nose bluntness effects on hypersonic boundary layer transition and the accuracy of the LST analysis.

High-enthalpy, hypersonic compression corner flow

Abstract: The results of an experimental investigation of high-enthalpy, hypersonic flow over sharp leading-edge compression corners are presented and discussed. In particular, the possible effects of real gas behavior are examined. Measurements have been made of the heat transfer and pressure distributions for flat plate and compression corner flow. Some flow visualization data have also been obtained. Test flows were generated using a free-piston shock tunnel operating in the reflected mode. The reservoir enthalpy ranged from 3 to 19 MJ kg -1 , giving freestream speeds of 2.3-5.5 km s -1 . For these conditions, the flow remains laminar throughout. The flat plate data for both high- and low-enthalpy flows are in agreement with the reference enthalpy method for heat transfer and the weak interaction theory for pressure. Also, the measured flat plate boundary-layer thickness compares well with an expression strictly valid for perfect gas flows only. The high- and low-enthalpy compression corner flows have upstream influence and plateau pressure behavior similar to perfect gas flow. That is, real gas effects for the present flows appear to be negligible. This is consistent with the essentially chemically frozen viscous and inviscid flow upstream of the interaction.

Application of FLEET Velocimetry in the NASA Langley 0.3-Meter Transonic Cryogenic Tunnel

Abstract: Femtosecond laser electronic excitation and tagging (FLEET) velocimetry is demonstrated in a large-scale transonic cryogenic wind tunnel. Test conditions include total pressures, total temperatures, and Mach numbers ranging from 15 to 58 psia, 200 to 295 K, and 0.2 to 0.75, respectively. Freestream velocity measurements exhibit accuracies within 1 percent and precisions better than 1 m/s. The measured velocities adhere closely to isentropic flow theory over the domain of temperatures and pressures that were tested. Additional velocity measurements are made within the tunnel boundary layer; virtual trajectories traced out by the FLEET signal are indicative of the characteristic turbulent behavior in this region of the flow, where the unsteadiness increases demonstrably as the wall is approached. Mean velocities taken within the boundary layer are in agreement with theoretical velocity profiles, though the fluctuating velocities exhibit a greater deviation from theoretical predictions.

The Influence of Freestream Turbulence Freestream Nuclei Populations and a Drag-Reducing Polymer on Cavitation Inception on Two Axisymmetric Bodies.

Abstract: The influence upon the basic viscous flow about two axisymmetric bodies of (i) freestream turbulence level and (ii) the injection of small amounts of a drag-reducing polymer (Polyox WSR 301) into the test model boundary layer was investigated by the schlieren flow visualization technique. The changes in the type and occurrence of cavitation inception caused by the subsequent modifications in the viscous flow were studied. A nuclei counter using the holographic technique was built to monitor freestream nuclei populations and a few preliminary tests investigating the consequences of different populations on cavitation inception were carried out. Both test models were observed to have a laminar separation over their respective test Reynolds number ranges. The separation on one test model was found to be insensitive to freestream turbulence levels of up to 3.75 percent. The second model was found to be very susceptible having its critical velocity reduced from 30 feet per second at a 0.04 percent turbulence level to 10 feet per second at a 3.75 percent turbulence level. Cavitation tests on both models at the lowest turbulence level showed the value of the incipient cavitation number and the type of cavitation were controlled by the presence of the laminar separation. Cavitation tests on the second model at 0.65 percent turbulence level showed no change in the inception index, but the appearance of the developed cavitation was altered. The presence of Polyox in the boundary layer resulted in a cavitation suppression comparable to that found by other investigators. The elimination of the normally occurring laminar separation on these bodies by a polymer-induced instability in the laminar boundary layer was found to be responsible for the suppression of inception. Freestream nuclei populations at test conditions were measured and it was found that if there were many freestream gas bubbles the normally present laminar separation was elminated and travelling bubble type cavitation occurred - the value of the inception index then depended upon the nuclei population. In cases where the laminar separation was present it was found that the value of the inception index was insensitive to the free stream nuclei populations.

Linear stability of the compressible reacting mixing layer

Abstract: We investigate the linear stability of mixing layers with special emphasis on the effects of heat release and compressibility. Multiple supersonic modes exist for both nonreacting and reacting flows when the disturbance phase velocity is supersonic relative to the freestream. These supersonic modes become less unstable with increasing Mach number but more unstable with increasing heat release. The most unstable supersonic modes are three-dimensional (oblique) for nonreacting flows but two-dimensional for reacting flows.