Freestream

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

Effects of freestream disturbances on boundary-layer transition.

Abstract: T effects of freestream disturbances on boundary-layer transition in flows with zero pressure gradient have been studied by several investigators. Some of the notable results have been published by Dryden, Hall and Hislop, and Schubauer and Skramstad. A study similar to that of Schubauer and Skramstad was performed and reported by Wells. In the two latter studies the transition Reynolds number was found to increase systematically with decreasing freestream disturbance intensity until a maximum was reached which was not affected by further reductions in disturbance intensity. The maximum transition Reynolds number found by Wells was 80% greater than that reported by Schubauer and Skramstad, being 5.0 X 10 and 2.8 X 10, respectively, for disturbance intensities less than 0.1% of the freestream velocity Examination of the energy spectra of the freestream disturbances indicated that most of the energy in Wells' experiments occurred at frequencies below 150 cps with acoustic content less than 10% of the total energy. The tests of Schubauer and Skramstad involved significant energy levels out to 400 cps, and in addition the spectrum exhibited large acoustic energy peaks at 60 and 95 cps which accounted for approximately 90% of the total disturbance energy that was measured for intensities less than about 0.05%. These results suggest an influence of both the spectral energy distribution and the nature of the disturbances in determining the transition Reynolds number. The effects of acoustical noise are particularly interesting in the light of recent studies by Pfenninger, Bacon, and Carlson" and Jackson and Heckl concerning the stability of laminar boundary layers in the presence of intense acoustical fields. This report describes a recent series of experiments designed to extend the study reported in Ref. 4. In particular, the effects of acoustic noise fields of discrete frequencies and the broad-band turbulence created by coarse-mesh grids placed in the freestream have been investigated. These experiments are meant to be only illustrative of the problem and do not attempt to explain completely the relation between transition and free-stream disturbance energy. The results do emphasize the importance of the frequency spectra and nature of the disturbance, as well as the rms intensity of disturbance energy in predicting transition.

Large structure convection velocity measurements in compressible transverse injection flowfields

Abstract: Temporally correlated Rayleigh/Mie scattering images were used to examine the convection characteristics of the large-scale vortex structures developing in flowfields created by sonic transverse injection through circular and elliptical nozzles into a Mach 1.98 crossflow. Both compressibility and injector geometry were found to have significant influences on the convection characteristics of the large eddies. High compressibility injection cases produce eddies having dramatically larger near-field convection velocities than those in low compressibility cases. Farther downstream, the large-scale vortices tend to travel at velocities nearer the freestream veocity. Injector geometry primarily affects the near-field behavior.

Transition on swept leading edges at Mach 3.5

Abstract: Data and correlations for transition from laminar to turbulent flow on 45and 60-deg swept cylinders are presented. The data were obtained at Mach 3.5 in the Pilot Low-Disturbance Wind Tunnel at NASA Langley. Freestream noise levels were varied during the test program from extremely low values that were essentially in the instrument noise range to much higher values approaching those in conventional wind tunnels. The results show that end plates or large trips near the upstream end of the cylinders cause turbulent flow along the entire attachment line of the models over the freestream test Reynolds number range (based on cylinder diameter) of approximately 1.0 X10

Freestream capturing for moving coordinates in three dimensions

Abstract: A detailed formula is developed that can be used in both finite-volume (FV) and finite-difference (FD) methods for constructing freestream capturing metrics in space and time. It is shown that, considering an FV cell on the FD grid, the freestream capturing metrics in space and time can be constructed from the FD formulation. The approach is costly but guarantees the global conservation for an arbitrary motion of the grid.