Thomas J. Beutner

Control of Edney IV Interaction by Pulsed Laser Energy Deposition

An experimental investigation was conducted to examine the effect of a pulsed Nd:YAG laser energy addition on the shock structures and surface pressure in a Mach 3.45 flow past a sphere. Two configurations were considered: 1) a sphere in a uniform freestream and 2) an Edney IV interaction generated by impingement of an oblique shock on the bow shock of the sphere

Response of supersonic turbulent boundary layers to local and global mechanical distortions

A series of experiments were conducted to investigate the response of a Mach 5 turbulent boundary layer (Reθ ≈ 38,000) to periodic diamond roughness elements in combination with a series of curvature-driven favorable pressure gradients. Flow visualizations were obtained using schlieren photography and quantitative data were obtained using planar particle image velocimetry. It is found that streamwise variations in the Reynolds-averaged normal and shear stresses occur due to the shock waves and expansions fans generated by the roughness elements. The Reynolds-averaged shear stress in particular, was found to vary by up to 35% in the streamwise direction, in comparison to the equivalent zero-pressuregradient case. Overall, results indicate that the favorable pressure gradients considered have a dominant stabilizing effect on the flow, reducing Reynolds-averaged normal and shear stresses by up to 50%. Profiles of the shear stress indicate appreciable structural changes within the boundary layer, with the peak shear stress moving closer to the wall with increasing favorable pressure gradient strength.

Supersonic Boundary Layers with Periodic Surface Roughness

In the present study, the effects of large-scale periodic surface roughness on a high-speed (M = 2.86), high Reynolds number (Re θ ≈ 60,000), supersonic turbulent boundary layer was examined. Two roughness topologies (square and diamond) were compared with an aerodynamically smooth wall. The measurements included planar contours of the mean and fluctuating velocity, pitot pressure profiles, pressure-sensitive paint, and schlieren photography. The local strain-rate distortion parameters for the square roughness pattern were small (∼-0.01), and the mean and turbulent flow properties followed the canonical rough-wall boundary-layer trends. The diamond-shaped roughness topology produced a pattern of attached oblique shocks and expansion waves that led to strong distortion parameters. The distortions varied from -0.3 to 0.4 across the roughness elements, which resulted in localized extra turbulence production that generated large periodic variations in the turbulence levels across individual roughness elements that spanned the boundary-layer thickness; for example, the Reynolds shear stress varied by ∼100%. This result demonstrated a mechanism for altering the turbulence in supersonic boundary layers.

Characterization of Schlieren Light Source Using Laser -Induced Optical Breakdown in Argon

Introduction S CHLIEREN imaging systems are widely used for both qualitative and quantitative flow visualization in compressible flows and active index of refraction flowfields such as two-index mixing problems or combustion flowfields. A common implementation of schlieren imaging uses a pulsed light source to provide instantaneous measurements in unsteady flowfields. A variety of pulsed light sources has been used in the past with schlieren imaging systems, including arc lamps, incandescent bulbs, flash tubes, spark gaps, and light-emitting diodes.1 Lasers have been used to provide a narrow linewidth illumination source, which is useful for filtering broad spectral emissions from plasmas or flames, but at the cost of image degradation due to laser speckle. A recent technical note described the use of a laser-induced spark as a point source, which could be inserted in the flowfield avoiding the need to integrate through the density fluctuations associated with the boundary layers on the wind-tunnel walls.2 Recent papers have also described the use of a laser-induced spark as a light source for schlieren imaging in a plasma flow3 and an exploding wire bridge,4 both applications that benefit from a very high-intensity schlieren light source. Our objective in this Technical Note is to characterize a laserinduced spark schlieren imaging technique that provides a very highintensity light source, with short time duration and with repeatable temporal and spatial characteristics. Spatial and temporal variations in intensity are reported for this light source, as well as a comparable light source using a laser discharge in air. This light source has

Evaluation and optimization of a multi-component planar doppler velocimetry system

A Planar Doppler Velocimetry system has been developed for multi-component velocity measurements in large scale subsonic wind tunnel facilities. System components, methodologies and improvements are evaluated and discussed. Data is presented on two component measurements conducted in the flow field above a 70 degree delta wing at an angle of attack of 23 degrees in a Mach 0.2 (69 m/s) free stream. Although only two components of velocity could be resolved, axial and spanwise measurements were made characterizing the velocity field associated with the vortex cores as they developed downstream. Detector placement and system performance was improved in a second test to measure the three dimensional velocity field above a Boeing UCAV model again at 20 degrees angle of attack operated in a Mach 0.2 free stream. The evolution of the three dimensional velocity field created by vortices from the sharp leading edge of the body and the outboard vortex above the wing was characterized. Further optimization of the current system indicated that current levels of uncertainty can be improved by proper camera placement to reduce the sensitivity to laser frequency fluctuations and by improving the linear independence of the measured velocity components as characterized by the condition number of the coefficient matrix. Other considerations of camera placement and considerations to make measurements are also discussed.

Determination of solid/porous wall boundary conditions from wind tunnel data for computational fluid dynamics codes

A computational and experimental study has been undertaken to investigate methods of modelling solid and porous wall boundary conditions in computational fluid dynamics (CFD) codes. The procedure utilizes experimental measurements at the walls to develop a flow field solution based on the method of singularities. This flow field solution is then imposed as a pressure boundary condition in a CFD simulation of the internal flow field. The effectiveness of this method in describing the boundary conditions at the wind tunnel walls using only sparse experimental measurements has been investigated. Verification of the approach using computational studies has been carried out using an incompressible flow solver. The current work demonstrates this technique for low speed flows and compares the result with experimental data obtained from a heavily instrumented variable porosity test section. Position and refinement of experimental measurements required to describe porous wall boundary conditions have also been considered for application to other porous wall wind tunnels. The approach developed is simple, computationally inexpensive, and does not require extensive or intrusive measurements. It may be applied to both solid and porous wall wind tunnel tests. Some consideration is given to the extension of this method to three dimensions.

Computational Method for Describing Porous Wall Boundary Conditions Based on Experimental Data

A computational and experimental study has been undertaken to investigate methods of modeling solid and porous wall boundary conditions in computational fluid dynamics (CFD) codes. The procedure utilizes experimental pressure measurements at the walls to develop a flow-fleld solution based on the method of singularities. This solution is then imposed as a pressure boundary condition in a CFD simulation of the internal flowfield. The effectiveness of this method in describing the boundary conditions at the wind-tunnel walls using only sparse experimental measurements has been investigated. Verification of the approach using computational studies has been carried out using an incompressible flow solver. The current work demonstrates this technique for low-speed flows and compares the result with experimental data obtained from a heavily instrumented variable porosity test section. Position and refinement of experimental measurements required to describe porous wall boundary conditions has also been considered for application to other porous wall wind tunnels. The approach deveioped is simple, is computationally inexpensive, and does not require extensive or intrusive measurements. It may be applied to both solid and porous wall wind-tunnel tests.

Mechanically Distorted Supersonic Boundary Layers

θ ) supersonic turbulent boundary layers were examined. Large localized distortions ( d = -0.5 to 0.4) were generated from the shock and expansion structure produced by diamond roughness elements. Weak ( dmax = 0.05) and strong global favorable pressure gradients ( dmax = 0.25) were studied. The results were compared to similar flows with canonical surface patterns (smooth and square roughness). The measurements included planar contours of the mean and fluctuating velocity, Pitot pressure profiles, pressure sensitive paint and schlieren photography. The canonical flows followed established trends. However, their inclusion provides (1) a basis for comparison for the non- canonical flows and (2) new high-speed experimental data with turbulence. The diamond roughness element produced substantially different flows that were characterized by strong local distortions ( d = -0.5 to 1.8 across the element) and highly varying turbulence properties, where the shear stress levels varied by ~100%. The present data showed that combining the global pressure gradient to the local gradients associated with the diamond roughness element produced regions of flow over a rough surface with turbulence levels reduced to 70% of the undisturbed zero pressure boundary layer. These data and trends have important implications in controlling the turbulence in high-speed boundary layers.

Modeling of solid/porous wall boundary conditions for the validation of computational fluid dynamics codes

A computational study has been undertaken to investigate method of modeling solid and porous wall boundary conditions in computational fluid dynamics (CFD) codes. The procedure utilizes experimental measurements at the walls to develop a flow field solution based on the method of singularities. This flow field solution is then imposed as a boundary condition in a CFD simulation of the internal flow field. The effectiveness of this method in describing the boundary conditions at the wind tunnel walls using only sparse experimental measurements has been investigated. Position and refinement of experimental measurement locations required to describe porous wall boundary conditions has also been considered.