Katya M. Casper

Instability and transition measurements in the Mach-6 quiet tunnel

The Boeing/AFOSR Mach-6 Quiet Tunnel achieved quiet flow to a stagnation pressure of 163 psia in Dec. 2008, the highest value observed so far. It remains quiet at pressures above 160 psia. Under noisy conditions, nozzle-wall boundary-layer separation and the associated tunnel shutdown appear to propagate slowly upstream, whereas under quiet conditions, the propagation is very rapid. A new diffuser insert has been designed, fabricated, and installed in the tunnel in order to start larger blunt models and increase run time. A flared cone with a circular-arc geometry was designed to generate large second-mode N factors under quiet flow conditions. When the computed N factor was 13, large instability waves were measured under quiet flow conditions using fast pressure sensors, but the flow remained laminar. Transition was observed only under noisy conditions. A laminar instability was detected in the wake of an isolated roughness element in the boundary layer on the nozzle wall; this appears to be the first such measurement at hypersonic speeds.

Effect of Freestream Noise on Roughness-Induced Transition at Mach 6

The effects of freestream noise on roughness-induced transition were measured for the HIFiRE forecone model. Temperature-sensitive paint measurements were used to visualize the wake of the roughness elements under quiet and noisy flow. The transition Reynolds number increased under quiet flow, for a less-than-effective trip, by a factor of up to 6.4 when referenced to the trip location. The difference in transition location between quiet and noisy flow conditions is significantly reduced when effective trips are used, but quiet flow still delays the transition location for an effective trip. STABL calculations of the flow over the model at 0 and 6 angle of attack were used to quantify the results. The effect of an isolated roughness on the nozzle wall of the BAM6QT is also being studied. The nozzle wall features a thick laminar boundary layer, allowing measurements of the growth of instabilities. Temperature-sensitive paint measurements of the wake behind an isolated roughness element were obtained. In quiet flow, several hot streaks are visible.

Comparison of Experimentally Measured and Computed Second-Mode Disturbances in Hypersonic Boundary-Layers

Abstract : Experiments were carried out in the Boeing/AFOSR Mach 6 Quiet Tunnel at Purdue University and the Sandia National Laboratories Hypersonic Wind Tunnel at Mach 5 and 8. The purpose was to measure second-mode boundary-layer instabilities on a 7 deg half-angle cone at zero angle of attack using surface pressure sensors. Second-mode waves were successfully measured at all three Mach numbers, including under both noisy and quiet conditions. The most amplified second-mode disturbance frequencies compared well to the linear Parabolized Stability Equations computed by the STABL software suite. The eigenfunctions are also reported, to aid in the development of new instrumentation methods. The e(expn N) method is used in an attempt to determine the N factor for transition onset prediction in both tunnels.

Effect of freestream noise on roughness-induced transition for a slender cone

The effect of freestream noise on roughness-induced transition was studied on a blunt 7 deg half-angle cone in the Boeing and U.S. Air Force Office of Scientific Research Mach-6 Quiet Tunnel. Temperature-sensitive paints were used to visualize the wake of an isolated roughness element at 0 and 6 deg angles of attack. Transition onset was determined from the rise in centerline temperature downstream of the roughness. Transition was always delayed under quiet flow compared with noisy flow. For example, at 0 deg angle of attack, transition was up to 6.3 times further downstream from the trip location. The difference in transition location between quiet and noisy flow conditions was significantly reduced when an effective trip height was reached. However, quiet flow still delayed transition by a factor of 2.4 in those cases. Because quietflowdelays transition behind a roughness element, the height of trips sized in a conventional noisy tunnel should be increased for quiet flightlike conditions. On the other hand, any naturally occurring roughness that causes transition under noisy flow might not cause transition in a quiet environment.

Starting Issues and Forward-Facing Cavity Resonance in a Hypersonic Quiet Tunnel

Blunt 70° sphere-cones with a 2.5-in. diameter were successfully started in the Boeing/AFOSR Mach-6 Quiet Tunnel when the flow was quiet and the nozzle-wall boundary layer was laminar. However, under noisy flow with thicker turbulent boundary layers, similar models with a 2.0-in. diameter barely started. A porous Apollo capsule with a 2.0-in. diameter at 24° angle of attack failed to start. In an effort to start larger models with stronger bow shocks, new sting-support and diffuser sections were installed. Downstream of the nozzle exit, the diameter was increased from 9.5 to 14.1 in. A series of inserts are being installed behind the backward-facing step in attempts to control the shock/boundarylayer interactions induced by models. These efforts have not yet been successful. During this time, the maximum stagnation pressure for quiet flow has typically ranged from 120 to 145 psia. Pressure measurements near the exit of the Mach-6 diffuser indicated that quiet flow may extend far downstream under some circumstances. A laser differential interferometer was rebuilt for the Mach-6 tunnel with an improved signal-to-noise ratio. Quiet-flow experiments were also carried out at Mach 4 with a pressure transducer at the base of a forward-facing cavity. Computational predictions of self-resonance in deep cavities were confirmed experimentally. For moderately deep cavities, small freestream disturbances were amplified even under quiet flow.

Joint Experimental/Computational Investigation Into the Effects of Finite Width on Transonic Cavity Flow.

Recently acquired experimental data on pressure fluctuations in cavities of equal length (L) to depth (D) ratio but varying length to width (L/W) ratio have shown substantial variations in the dominant modes in the cavity. These observations have been carried out at subsonic and transonic Mach numbers at cavity L/D=5, which puts the cavity flow in the “open” category. This paper presents results from a joint computational and experimental investigation undertaken at Sandia to explain these observations. To this end, simulations of L/D=5.0 cavity at L/W=1.0,1.67 and 5.0 have been carried out and analyzed. The results show strong differences in the mean flow structure between the three widths. The widest cavity shows significantly higher turbulence intensities across the cavity. The unsteady wall pressures reveal that in this case, significant tunnel wall interactions are present, intensifying the pressure fluctuations and the shear layer oscillations. The differences in the wall pressures and turbulent flow field are smaller for the L/W=1.0 and 1.67 cavities. The L/W=1.67 cavity is strongly influenced by the streamwise vortices at the spanwise edges of the cavity, resulting in strong three dimensional variations in mean flow across the width of the cavity. In the case of the narrowest cavity, this effect is minimal, with the resultant flow field showing predominantly two-dimensional character.

Complex Geometry Effects on Cavity Resonance

The flow over an aircraft bay is often represented using a rectangular cavity; however, this simplification neglects many features of actual flight geometry that could affect the unsteady pressure field and resulting loading in the bay. To address this shortcoming, a complex cavity geometry was developed to incorporate more realistic aircraft-bay features including shaped inlets, internal cavity structure, and doors. A parametric study of these features was conducted based on fluctuating pressure measurements at subsonic and supersonic Mach numbers. Resonance frequencies and amplitudes increased in the complex geometry compared to a simple rectangular cavity that could produce severe loading conditions for store carriage. High-frequency content and dominant frequencies were generated by features that constricted the flow such as leading-edge overhangs, internal cavity variations, and the presence of closed doors. Broadband frequency components measured at the aft wall of the complex cavities were also sig...

Spanwise Growth of the Turbulent Spot Pressure-Fluctuation Field in a Hypersonic Boundary Layer.

The pressure-fluctuation field beneath wave packets and turbulent spots in a hypersonic boundary layer was studied on the nozzle wall of the Boeing/AFOSR Mach-6 Quiet Tunnel. For a controlled study, the breakdown of disturbances created by spark perturbations was measured at various freestream conditions. A disturbance first grows into a linear instability wave packet and then quickly becomes nonlinear. At this point, the wave packet is still concentrated near the disturbance centerline, but weaker disturbances are seen spreading from the center. Breakdown to turbulence begins in the core of the wave packets where the wave amplitudes are largest. Second-mode waves are still evident in front of and behind the breakdown point and can be seen propagating in the spanwise direction at a spreading angle. The turbulent core grows downstream resulting in a turbulent spot with a typical arrowhead shape. However, the spot is not merely a localized patch of turbulence; instability waves are still an integral part of the disturbance.

Pulse-Burst PIV Measurements of Transient Phenomena in a Shock Tube

Time-resolved particle image velocimetry (TR-PIV) measurements were made in a shock tube using a pulse-burst laser. Two transient flowfields were investigated including the baseline flow in the empty shock tube and the wake growth downstream of a cylinder spanning the width of the test section. Boundary layer growth was observed following the passage of the incident shock in the baseline flow, while the core flow velocity increased with time. The measured core flow acceleration was compared to that predicted using a classical unsteady boundary layer growth model. The model typically provided good estimates of core flow acceleration at early times, but then typically underestimated the acceleration. As a result of wall boundary layers, a significant amount of spatial non-uniformity remained in the flow following the passage of the end-wall reflected shock, which could be an important factor in combustion chemistry experiments. In the transient wake growth measurements, the wake downstream of the cylinder was symmetric immediately following the passage of the incident shock. At later times (≈ 0.5 ms), the wake transitioned to a von Karman vortex street. The TR-PIV data were bandpass filtered about the vortex shedding frequency to reveal additional details on the transient wake growth.

Relationship between Acoustic Tones and Flow Structure in Transonic Rectangular Cavity Flow

Particle image velocimetry (PIV) measurements quantified the coherent structure of acoustic tones in a Mach 0.94 cavity flow. Stereoscopic PIV measurements were performed at 10-Hz and two-component, time-resolved data were obtained using a pulse-burst laser. The cavity had a square planform, a length-to-depth ratio of five, and an incoming turbulent boundary layer. Simultaneous fast-response pressure signals were bandpass filtered about each cavity tone frequency. The 10-Hz PIV data were then phase-averaged according to the bandpassed pressures to reveal the flow structure associated with the resonant tones. The first Rossiter mode was associated with large scale oscillations in the shear layer, while the second and third modes contained organized structures consistent with convecting vortical disturbances. The spacing between coherent structures suggested the convective velocity of downstream-propagating disturbances was less than that typically used in the Rossiter relation. The time-resolved PIV data were bandpass filtered about the cavity tone frequencies to reveal flow structure associated with specific resonant modes. Similar to the phase-averaged results, two-point correlations of vertical velocity gave convective velocities lower than those typically used in the Rossiter relation, whose magnitudes appear to be dependent on cavity mode number.