Brandon C. Chynoweth

Instability Measurements in the Boeing/AFOSR Mach-6 Quiet Tunnel

A new 7 half-angle cone at 6 angle of attack was used to investigate the growth of secondary instabilities on hypersonic stationary crossflow waves. The new cone allows rotation of the sensor array before each run, independently of the rest of the cone. Good repeatability of the cone flow with respect to rotation was established. Measurements of secondary instabilities show the existence of at least two modes, each maximum in different areas of the stationary vortex. Experiments were also performed on a 30 half-angle cone probe equipped with two different pressure transducers to measure the disturbance level of a Mach 6 Ludweig Tube and compare the disturbance levels measured to another facility. It was found that the low frequency disturbance levels differed greatly between the two facilities. Third, RIM insert with 45 roughness elements was tested on a flared cone geometry. Peak second-mode pressure fluctuations of nearly 35% were measured, similar to previous results with a smooth wall. Temperature sensitive paint measurements showed a modified hot-cold-hot heating pattern. Fourth, preliminary measurements on a 7 half angle cone with a slice and flap were made to study free-shear layer transition in a low noise environment. Boundary-layer transition within the free-shear layer has been observed and is altered by the freestream noise levels.

Boundary-Layer Transition Measurements in the Boeing/AFOSR Mach-6 Quiet Tunnel

Recent results from several projects in the BAM6QT are presented. An infrared camera system was used to image a circular cone at an angle of attack, and the results are compared to previous TSP measurements. The IR images show clear streaks and demonstrate repeatability and low noise levels compared to TSP. Oil flow and surface pressure sensor measurements are presented for a cone with a slice and ramp. Separation and reattachment are discussed, along with the amplification and dampening of instabilities at various locations on the model. The temperature distribution along the BAM6QT nozzle wall was varied to study the relationship between heating and the percentage of a run which was quiet. No apparent correlation was observed. Pitot-probe measurements were taken at various locations on the nozzle centerline to investigate an increase in noise levels that occurs roughly two seconds into runs. The magnitude of the increase and the time at which it started depended on the Reynolds number. Development of higher-Reynolds number hypersonic quiet tunnel facilities may require the use of suction on the nozzle wall. Initial computations are presented for the design of a flared inlet centerbody that can be tested in the Boeing AFOSR/Mach-6 Quiet tunnel to determine the feasibility of creating sufficiently uniform suction. A stability analysis is performed to determine the most unstable second-mode frequencies and to compute the Görtler numbers on the flared aft-body portion. Finally, the 3 inch shock tube used for PCB calibration has been upgraded with high accuracy sensors and an automated pressure control system.

Mach 6 Quiet Tunnel Laminar to Turbulent Investigation of a Generic Hypersonic Forebody

This paper presents an experimental boundary layer transition investigation of the windward side of a generic hypersonic forebody performed in April 2015 in the Boeing/AFOSR Mach 6 quiet tunnel facility at Purdue university (BAM6QT). At 0 and 4 degrees of angle of attack, Reu=11x10 6 /m, flow was fully laminar in quiet conditions. Under noisy conditions, an early transition front (Reθ~200) was observed, even when dividing the unit freestream Reynolds number by 6. In quiet conditions, several diamond-shaped roughness trips were found to efficiently trip the laminar boundary layer when Rekk > 1000. Temperature-Sensitive Paint (TSP) enabled a global measurement of the heat flux distribution and detection of the transition front. PCB sensors confirmed the state of the boundary layer : laminar, turbulent or transitional. Transition results with a single continuously blowing sonic air jet was also collected at various pressure ratios, giving the laminar to turbulent threshold.

Instability and Transition Experiments in the Boeing/AFOSR Mach 6 Quiet Tunnel

This paper presents results for three projects in the Boeing/AFOSR Mach 6 Quiet Tunnel (BAM6QT) at Purdue University. The second-mode instability was measured on a straight 3 half-angle cone under quiet conditions using pressure sensors and temperature sensitive paint. Natural transition was observed near the aft end of the model at a unit Reynolds of 12.1×10/m. Maximum pressure fluctuation magnitudes prior to the onset of transition were measured to be 27% of the mean surface pressure. Secondly, experiments on a 7 half-angle cone at 6 angle of attack were performed to verify the presence of a secondary instability of a stationary crossflow wave. Using TSP imaging techniques and PCB pressure transducers, a possible secondary instability with a peak frequency near 300 kHz was measured at azimuthal angles between 125 and 140 at Reynolds numbers near 10.7×10/m. Lastly, Roughness elements and an air jet were tested as boundary layer trips on a generic hypersonic forebody.

Measurements in the Boeing/AFOSR Mach-6 Quiet Tunnel on Hypersonic Boundary-Layer Transition

This paper presents results from four different research projects currently ongoing at Purdue University. (1) Different criteria for detecting the edge of the boundary layer were investigated on the flared cone geometry. It was determined that a method based on the total enthalpy profile would be used for future edge-detection computations on the flared cone geometry. A Rod Insertion Method (RIM) roughness insert was measured using a Zygo ZeGage optical profiler. Experimental results with a single RIM insert are presented. Maximum second-mode magnitudes of nearly 27% were measured 2.5 cm upstream from where spectral filling and intermittency algorithms compute that transition has begun. (2) Preliminary data from a new model shows that high-frequency secondary instabilities of stationary crossflow waves are localized under the troughs of the stationary vortices. Measurements of the growth of the secondary instabilities are reported for two different vortices. (3) In order to better understand the effects that probe geometry has on measured pressure fluctuations, pitot measurements were taken using various sleeves which alter the forward-facing diameter of the probe. The results indicate a clear effect of probe size on the measured power spectral densities. Furthermore, it was found that the geometry effects are Reynolds number dependent. (4) Experiments on a cone with a slice and ramp were completed to determine if transitional shock wave-boundary layers interactions can be measured within the Boeing/AFOSR Mach-6 Quiet Tunnel. Initial experiments showed that with a newly designed model it is possible to measure transitional interactions.

Compilation and Analysis of Second Mode Amplitudes on Sharp Cones in Hypersonic Wind Tunnels

This research effort coordinated by the NATO AVT-240 specialists’ group compiles and analyzes second-mode amplitudes on sharp slender cones at 0 degrees angle of attack. The analysis focuses on pressure fluctuations measured with piezoelectric sensors in 11 hypersonic wind tunnels operated by 9 organizations located in 3 NATO countries (Belgium, Germany, and USA) and Japan. The measurements are at freestream Mach numbers between 5 and 14, unit Reynolds numbers Re/m between 1.5 and 16 million per meter, and wall-to total temperature ratios between 0.1 and 0.8. The study shows that second-mode growth rates can be predicted with Parabolized Stability Equations (PSE) over the wide range of conditions. The maximum second-mode amplitudes vary weakly at edge Mach number Me greater than ~5.8, but significantly decrease at lower Me. The maximum N factor envelope from PSE and the measured amplitudes are used to estimate the initial amplitudes A0. At each Mach number, A0 varies approximately as Re/m^(-1). This leads to transition N factors that increase with Re/m. This behavior is consistent with the results from Marineau (AIAA Journal, Vol. 55, No. 2, 2017).