Freestream

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

Boundary-layer profile measurements in a combustion driven MHD generator

Abstract: The velocity, temperature, and electron number density profiles were measured in the electrode wall boundary layer of a combustion driven MHD generator. Both subsonic and supersonic conditions were run. The experimental results are compared with predictions of a two-dimensional turbulent boundary-layer computation. For the subsonic condition, high levels of freestream turbulence were measured, about 10-12 percent. The measured velocity profile was fatter than that predicted, although the temperature and electron number density profiles were in agreement. This difference is tentatively ascribed to the high freestream turbulence levels. There was no measurable MHD effect for the subsonic case. For the supersonic condition, the measured velocity, temperature, and electron number density profiles fell under the predicted profiles. The discrepancy may be due to three-dimensional recirculation effects. There was a small amount of MHD interaction, the degree of which was in agreement with predictions. Electron number density nonequilibrium was not identified, but the degree of nonequilibrium predicted was small. Under the appropriate supersonic conditions, primarily at freestream temperatures below 2400K, ionization nonequilibrium is predicted to occur.

Mixing layer produced by a screen and its dependence on initial conditions

Abstract: This paper aims to elucidate the structure of the turbulent mixing layers, especially, its dependence on initial disturbances. The mixing layers are produced by setting a woven-wire screen perpendicular to the freestream in the test section of a wind tunnel to obstruct part of the flow. Three kinds of model geometry are treated; these model screens produced mixing layers which may be regarded as the equivalents of the plane mixing layer and of two-dimensional and axisymmetric wakes issuing into ambient streams of higher velocity. The initial disturbances are imposed by installing thin rods of various sizes along the edge of the screen or at the origin of the mixing layer. Flow features are visualized by the smoke-wire method. Statistical quantities are measured by a laser-Doppler velocimeter. In all cases large-scale transverse vortices seem to persist, although comparatively small-scale vortices are superimposed on the flow field in the mixing layer. The mixing layers are in self-preserving state at least up to third-order moments, but the self-preserving state is different in each case. The growth rates of the mixing layer are shown to depend strongly on the initial disturbance imposed.

Particle turbulence interaction

Abstract: The interaction between the carrier and the dispersed phases is bi-directional: the carrier-phase turbulence influences the dispersion and preferential accumulation of particles and bubbles, and particles and bubbles in turn modulate the fluid turbulence. At the level of a single particle, the effect of freestream turbulence is to modify the drag force compared to that in a steady uniform flow. On the other hand, the particle can modify freestream turbulence by the formation of a wake, periodic shedding of vortices, and wake turbulence. The collective effect of a distribution of particles can further modify the effective drag force on a particle due to the screening effect and thereby influence the mean settling and dispersion characteristics. Similarly, the collective effect of the dispersion of particles will determine the attenuation or augmentation of the turbulence intensity. Thus, many of the mechanisms of multiphase flow turbulence are quite distinct from those that have been well established in single-phase turbulence and therefore deserve detailed investigation. Here we present results from fully-resolved direct numerical simulations (DNS) of turbulent multiphase flow. In addition to resolving the wide range of length and time scales associated with turbulence, which would exist even in the absence of the particles, we now need to resolve all the length scales associated with the particles and the small-scale flow features generated by them. Thus, direct numerical simulations of multiphase flow turbulence pose greater challenge than the DNS of corresponding single-phase turbulence. Here we plan to present results for the interaction of a single spherical with ambient turbulence. We will consider the cases of both isotropic freestream turbulence and wall-bounded channel flow turbulence. For the case of isotropic turbulence the particle Reynolds number is varied from about 50 to 600, and the particle diameter is about 1.5 to 10 times the Kolmogorov scale of the undisturbed turbulent flow. The Taylor microscale of the freestream turbulent flow is 164 and the relative intensity of freestream turbulence to mean crossflow is varies from 10% to 25%. The DNS technique employed here resolves both the smallest scales in freestream turbulence and the thin shear layers and complex vortical structures associated the particle wake. The present DNS study is similar to the experimental study by Wu & Faeth (1994a, 1994b), and agreement between the DNS results and the experimental measurement are presented. Figure 1a summarizes the drag coefficient for a sphere in the presence of freestream isotropic turbulence compiled from many different experimental measurements. Also plotted for reference as the solid line is the standard Schiller-Neumann drag law corresponding to a turbulence-free uniform flow. The scatter in the experimental data clearly illustrates the large discrepancy between the different results. Also plotted in the figure are the DNS results which show that freestream isotropic turbulence does not have a substantial and systematic effect on the time-averaged mean drag on the particle. Standard drag correlation based on instantaneous relative velocity between the particle and the undisturbed fluid velocity at the center of the particle results in a reasonable prediction of the mean drag. However, the accuracy of prediction of the instantaneous drag decreases with increasing particle size. For the smaller particles, the low frequency oscillations in the DNS drag are well captured by the standard drag, but for the larger particles significant differences exist even for the low frequency components. Inclusion of the added-mass and history terms, computed based on the undisturbed ambient flow at the center of the particle, does not improve the prediction of instantaneous forces. Fluctuations in the drag and lift forces are shown to scale with the mean drag as well as the freestream turbulence intensity (see figure 1b). The effect of freestream turbulence on the mean and instantaneous wake structure is studied. The mean wake in a turbulent flow shows reduced velocity deficit and a flatter profile. However, the mean wake in a turbulent flow behaves like a self‐ preserving laminar wake. At low Reynolds numbers the wake in a turbulent flow oscillates strongly without any vortex shedding, but at higher Reynolds numbers vortex shedding starts. The nature of the vortices are very different from that in a uniform flow. Increasing the freestream turbulence intensity suppresses the process of vortex shedding, and only marginally increases the wake oscillation. The modulation of freestream turbulence in the wake is studied in terms of the distribution of the distribution of the kinetic energy and RMS of velocity fluctuation. The freestream energy lost in the wake is recovered faster in a turbulent flow than in a uniform flow. The energy of the velocity fluctuation is enhanced in the wake at low freestream intensities, and is damped or marginally increased at higher intensities. The fluctuation energy is not equi-partitioned among the streamwise and cross-stream components. The RMS of the streamwise fluctuation is always enhanced, whereas the RMS of the cross-stream fluctuation is enhanced only at low freestream intensities, and damped at higher intensities.

Determination of atmospheric properties for STS-1 aerothermodynamic investigations

Abstract: A procedure for determining an approximation to the freestream atmospheric properties along the Shuttle entry trajectory is presented. Meteorological data as input is obtained by rawinsondes from surface to 70 km, and meteorological spheres from 60-90 km, launched from Hawaii and California. The Langley Atmospheric Information Retrieval System (LAIRS) developed to approximate the atmospheric freestream properties along the flight path, is outlined, noting temperature and wind data are interpolated in altitude, while gradients and diurnal and semidiurnal coefficients are taken from the COSPAR reference atmosphere. The data are input to a model to project temperature profiles for the Shuttle descent, and the input atmospheric parameters are listed. Efforts are continuing in order to correct discrepancies in the generated profiles for regions below 3 km.

Experimental Investigation of Unsteady Transition Processes on High-Lift T106A Turbine Blades

Abstract: Periodic wake-boundary layer interactions on the T106A high-lift low-pressure turbine blade cascade at enginerepresentative flow conditions are described. Through a comparison with previously published moderate Reynolds number/low freestream turbulence data, the influence of elevated freestream turbulence intensity and Reynolds number is assessed and the following conclusions are drawn. At elevated freestream turbulence, the mechanism of turbulence production outside of the boundary layer did not change. Although, an enhanced diffusion of turbulence in the wake was responsible for small decreases in the maximum value of the turbulent kinetic energy in the freestream. For both freestream turbulence cases, the wake from upstream interacted with the inflexional or separated shear layer and forced an inviscid Kelvin-Helmholtz type of breakdown, resulting in the formation of roll-up vortices. At corresponding Reynolds numbers, the turbulence in the wake induced a bypass transition, at a similar streamwise location and phase of the unsteady wake interaction cycle. Furthermore, the higher freestream turbulence delayed the appearance of the inflexional profiles in space, and the transition onset occurred farther upstream. The latter is related with the effect of changes in the Reynolds number. Reduced Reynolds numbers extended the length of the inflexional shear layer, resulting in the formation of a greater number of vortices. Furthermore, the delayed transition allowed these vortices to penetrate farther downstream. Increasing the Reynolds number, at the higher freestream turbulence level, led to the limiting case, where roll-up vortices were not seen to be formed.