Scott J. Peltier

Velocimetry Measurements of a Scramjet Cavity Flameholder with Inlet Distortion

Particle image velocimetry measurements were made along the center plane of a scramjet cavity flameholder to analyze simulated inlet flow distortion in the direct-connect test environment. Mach 3 nonreacting tests examined an oblique shock impinging upon locations in- and upstream of the cavity, including cases with wall-normal air injection upstream of the cavity to simulate fuel injection. Addition of flow distortion altered the size and shape of the primary recirculation region within the cavity by deflecting the bounding shear layer: the recirculation region was compressed by shock impingement upstream of the cavity, and shock impingement on the cavity itself expanded it. Air injection upstream of the cavity thickened the shear layer and produced a stronger effect on velocity direction than magnitude, preventing the formation of a large-scale recirculation region in two of the three shock locations studied. Flow distortion and upstream air injection both increased flow unsteadiness, with the greatest ...

Visualization of Hypersonic Turbulent Boundary Layers Negotiating Convex Curvature

The effects of convex curvature on the outer structure of a high-Reynolds number Mach 4.9 turbulent boundary layer are visualized using condensate scattering and analyzed using twopoint spatial correlations, intermittency, and fractal theory. Consistent with other studies, the large-scale flow structures appear to survive the initial expansion. They appear to increase in size, consistent with the effects of bulk dilatation, and undergo a reorientation, leaning farther away from the wall. The intermittency, however, does not change significantly. There is a decrease in the fractal dimension, indicating that the boundary layer interface becomes more regular. Because fractal scale similarity does not encompass the largest scales, this suggests that the change in fractal dimension is due to the action of the smaller-scales, thereby supporting the notion that the small-scale structures are quenched during the expansion. Thus, the effects of convex curvature appear to create larger-scale structures, which lean farther away from the wall, and which exhibit a more regular interface. Time-resolved visualizations using a 500 kHz laser system enable the volumetric reconstruction of the boundary layer’s instantaneous structure.

PIV of a Mach 5 Turbulent Boundary Layer over Diamond Roughness Elements

The effects of a periodic diamond roughness (k/ = 0.07) pattern on a Mach 4.9 turbulent boundary layer were examined experimentally. Particle image velocimetry and schlieren photography were used to quantify the flow structure over the roughness elements. On average, the roughness elements resulted in increased turbulent fluctuations in the lower half of the boundary layer (compared to a smooth baseline model), and decreased values above y/δ ~ 0.5. The turbulent structures within the rough-wall flowfield were stretched along the major axis, while maintaining a nearly constant minor axis (when normalized by δ). The structure angles for the rough-wall were increased for y/δ 0.3. It was observed that the rough and smooth Reynolds transverse and shear stresses nearly overlapped, when rotated to their respective local structure angle. This finding suggests that Reynolds stress redistribution is a predominant mechanism that should be included when modeling this class of flow.

Multi-angular Flame Measurements and Analysis in a Supersonic Wind Tunnel Using Fiber-Based Endoscopes

This paper reports new measurements and analysis made in the Research Cell 19 supersonic wind-tunnel facility housed at the Air Force Research Laboratory. The measurements include planar chemiluminescence from multiple angular positions obtained using fiber-based endoscopes (FBEs) and the accompanying velocity fields obtained using particle image velocimetry (PIV). The measurements capture the flame dynamics from different angles (e.g., the top and both sides) simultaneously. The analysis of such data by proper orthogonal decomposition (POD) will also be reported. Nonintrusive and full-field imaging measurements provide a wealth of information for model validation and design optimization of propulsion systems. However, it is challenging to obtain such measurements due to various implementation difficulties such as optical access, thermal management, and equipment cost. This work therefore explores the application of the FBEs for nonintrusive imaging measurements in the supersonic propulsion systems. The FBEs used in this work are demonstrated to overcome many of the practical difficulties and significantly facilitate the measurements. The FBEs are bendable and have relatively small footprints (compared to high-speed cameras), which facilitates line-of-sight optical access. Also, the FBEs can tolerate higher temperatures than high-speed cameras, ameliorating the thermal management issues. Finally, the FBEs, after customization, can enable the capture of multiple images (e.g., images of the flow fields at multi-angles) onto the same camera chip, greatly reducing the equipment cost of the measurements. The multi-angle data sets, enabled by the FBEs as discussed above, were analyzed by POD to extract the dominating flame modes when examined from various angular positions. Similar analysis was performed on the accompanying PIV data to examine the corresponding modes of the flow fields. The POD analysis provides a quantitative measure of the dominating spatial modes of the flame and flow structures, and is an effective mathematical tool to extract key physics from large data sets as the high-speed measurements collected in this study. However, the past POD analysis has been limited to data obtained from one orientation only. The availability of data at multiple angles in this study is expected to provide further insights into the flame and flow structures in high-speed propulsion systems.

The Influence of Favorable Pressure Gradients upon the Coherent Motions in a Mach 5 Turbulent Boundary Layer

The effects of streamline curvature-induced favorable pressure gradients on a Mach 4.9 turbulent boundary layer were investigated. The goal was to provide a physical explanation for the response of the Reynolds stresses, by examining the distortion, re-orientation, and attenuation of the turbulent structures. Pre-multiplied power spectra of the streamwise component displayed a peak at Λx + = 200 – 300 for all cases below y/δ = 0.1. The strong pressure gradient showed a greater relative amount of energy concentrated at that wavelength, suggesting that this length scale plays a more significant role in the SPG case. The swirling strength criteria, weighted by the sign of the vorticity to differentiate vortex orientations, was applied to each vector field. An initial inspection of the instantaneous distribution of turbulent structures shows strong similarities with incompressible flow, suggesting that the same hairpin packet model is applicable to the flowfields in question. Conditional averaging of the velocity fields was performed using prograde and retrograde vortices as the conditional event. The strong pressure gradient re-oriented the interface region to 20°, reduced the intensity of the induced low-speed fluid, and produced a more isotropic velocity distribution around the vortex. These observations suggest that the pressure gradient is attenuating the fluctuating velocity associated with the turbulent structures, thereby limiting their contribution to the Reynolds stress.

Multi-Angular Flame Measurements and Analysis in a Supersonic Wind Tunnel Using Fiber Based Endoscopes

This paper reports new measurements and analysis made in the Research Cell 19 supersonic wind-tunnel facility housed at the Air Force Research Laboratory. The measurements include planar chemiluminescence from multiple angular positions obtained using fiber based endoscopes (FBEs) and the accompanying velocity fields obtained using particle image velocimetry (PIV). The measurements capture the flame dynamics from different angles (e.g., the top and both sides) simultaneously. The analysis of such data by proper orthogonal decomposition (POD) will also be reported.Non-intrusive and full-field imaging measurements provide a wealth of information for model validation and design optimization of propulsion systems. However, it is challenging to obtain such measurements due to various implementation difficulties such as optical access, thermal management, and equipment cost. This work therefore explores the application of FBEs for non-intrusive imaging measurements in supersonic propulsion systems. The FBEs used in this work are demonstrated to overcome many of the practical difficulties and significantly facilitate the measurements. The FBEs are bendable and have relatively small footprints (compared to high-speed cameras), which facilitates line-of-sight optical access. Also, the FBEs can tolerate higher temperatures than high-speed cameras, ameliorating the thermal management issues. Lastly, the FBEs, after customization, can enable the capture of multiple images (e.g., images of the flowfields at multi-angles) onto the same camera chip, greatly reducing the equipment cost of the measurements.The multi-angle data sets, enabled by the FBEs as discussed above, were analyzed by POD to extract the dominating flame modes when examined from various angular positions. Similar analysis was performed on the accompanying PIV data to examine the corresponding modes of the flowfields. The POD analysis provides a quantitative measure of the dominating spatial modes of the flame and flow structures and is an effective mathematical tool to extract key physics from large data sets such as the high-speed measurements collected in this study. However, past POD analysis has been limited to data obtained from one orientation only. The availability of data at multiple angles in this study is expected to provide further insights into the flame and flow structures in high-speed propulsion systems.Copyright

Periodic Surface Roughness in High-Speed Turbulent Flow

Experimental data were collected for a periodic roughness pattern, consisting of an array of distributed diamond elements. The experiments were performed in a high-Reynolds number (Re/m ≈ 52 x 10 6 ) Mach 5 tunnel. Data were collected using schlieren photography and PIV. Preliminary results indicate that the diamond elements produce a strong response within the boundary layer. The diamond roughness produces a cyclical pattern of attached oblique shocks and expansion fans, which penetrate into the outer region of the boundary layer. Difficulties in seeding a high-Mach number rough-wall flow are discussed.