Freestream

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

The effect of contaminant source momentum on a worker's breathing zone concentration in a uniform freestream.

Abstract: Several factors affecting breathing zone concentration were examined in a paint spray booth by using a tracer gas method. The variables in the study include contaminant momentum, the presence of a flat plate downstream of the worker, the distance between the contaminant source and the body, and the worker's motion. A dramatic reduction in breathing zone concentration was observed when the spray gun emitted contaminants with high momentum. Reductions of 30-50% were observed because of the other variables. The source momentum effect was studied, subsequently, in a wind tunnel by measuring the breathing zone concentration of a mannequin with various flows through jets of different diameter, at varying freestream velocities. A functional relationship was determined between nondimensional breathing zone concentration and contaminant source momentum. This relationship is supported by numerical simulations. The effect of contaminant momentum on the near-wake flow field is discussed in conjunction with results fr...

Boundary layer turbulence and freestream turbulence interface, turbulent spot and freestream turbulence interface, laminar boundary layer and freestream turbulence interface

Abstract: We study the boundary-layer turbulence and freestream turbulence interface (BTFTI), the turbulent spot and freestream turbulence interface (TSFTI), and the laminar boundary-layer and freestream turbulence interface (LBFTI) using direct simulation. Grid spacings in the freestream are less than 1 Kolmogorov length scale during transition. Probability density functions of temperature and its derivatives are used to select the interface identification threshold, corroborated by a vorticity-based method. The interfaces so detected are confirmed to be physical a posteriori by the distinctive quasi-step-jump behavior in the swirling strength and temperature statistics along traverses normal to the BTFTI and TSFTI. No interface-normal inflection is detected across the LBFTI for either swirling strength, temperature, vorticity magnitude, Reynolds shear stress, streamwise velocity, normal velocity, or turbulence kinetic energy. The present direct numerical simulation data thus cast serious doubts on the shear-sheltering hypothesis/theory, which asserts that a subset of freestream fluctuations is blocked by the LBFTI. In the early stage of transition, quasi-spanwise structures exist on the LBFTI. The TSFTI shape is dominated by head prints of concentrated hairpin vortices. Further downstream, the BTFTI geometry is strongly modulated by groves of hairpin vortices (the boundary layer large-scale motions) with a distinct streamwise preferential orientation. Streamwise velocity and turbulence kinetic energy only exhibit minor plateaus (rather than quasi-step-jump) across the BTFTI and the TSFTI. We emphasize that it is more meaningful and important to acquire reproducible and reliable interface-normal statistics prior to considering any plausible substructures and elusive transient dynamics of the BTFTI, TSFTI, and LBFTI.

A Turbulent Boundary Layer with Mass Addition, Combustion, and Pressure Gradients

Abstract: : A subsonic turbulent boundary layer with mass addition and combustion is studied to investigate the effects of combustion on the velocity field in constant pressure and accelerating flows. Particular attention is given to determining (1) the extent to which combustion alters the flow and (2) the mechanism whereby combustion interacts with the flow field. The experimental results demonstrate that combustion alters the velocity profiles in both constant pressure and accelerating flows. The velocity gradients at the surface in combusting flows differ markedly from those of corresponding isothermal flows and the velocity in the flame regions of accelerating flows actually exceed the freestream value. The results of analysis indicate that in a subsonic turbulent boundary layer with combustion the Reynolds stress is essentially kinematic and does not explicitly involve density fluctuations. This in turn indicates that the experimentally observed changes in the velocity profiles are attributable to the temperature dependence of the local mean density and molecular viscosity. Analytical results also indicate that the combustion-induced changes in velocity profile are strongly dependent on the axial pressure gradient. The consequence of combustion with regard to skin friction is also examined and it is found that effects on the skin friction coefficient are similar to those on the wall velocity gradient. A method of determining the velocity profiles in a combusting turbulent boundary layer is also presented. (Author)

Study of the Afterbody Radiation during Mars Entry in an Expansion Tube

Abstract: Recent work has shown that a significant contributor to the afterbody aeroheating during Mars entry is radiation1-3. However, relevant ground test data is not available to help assess the uncertainty associated with prediction of the radiation when designing the thermal protection system for the aeroshell afterbody. The present work is aimed at studying the afterbody radiation experienced during Mars entry through experiments. The X2 expansion tube at the University of Queensland is used to generate the relevant experimental freestream flow conditions. Analysis is carried out to characterize the generated experimental freestream conditions. A two dimensional wedge model is used to produce the expanding flow which simulates aspects of the afterbody flow around Mars entry vehicles. This generated expanding flow has a test time of about 50-110 μs, thus, allowing flow measurements to be conducted. Horizontal emission spectroscopy measurements at 3.25 mm above the test model was taken for the carbon dioxide 4.3 μm band at freestream velocities of 2.8, 3.4, and 4.0 km/s. The measurements were compared to numerical calculations of the radiation emission generated from three-dimensional flowfield calculations. The comparison showed that the numerical result was significantly overpredicted for the 2.8 km/s condition and underpredicted for the 4.0 km/s condition.

A new consistent spatial differencing scheme for the transonic full-potential equation

Abstract: A new spatial differencing scheme for the transonic full-potential equation in conservative form has been developed. Three consistency conditions for the full-potential equations are derived and are satisifed by the new scheme. This scheme guarantees zero truncation error on any curvilinear mesh for freestream flows in either two- or three-space dimensions. Solutions obtained with this new differencing scheme, away from freestream regions, exhibit greatly improved accuracy, especially for nonsmooth or singular meshes. The computing times associated with the new scheme are approximately the same as the less accurate old scheme when computations are performed on the same mesh.