DEVELOPMENT OF A MICRO-PITOT TRAVERSE SYSTEM FOR PRESSURE MEASUREMENTS IN THE BOEING/AFOSR MACH 6 QUIET TUNNEL
Creators: name: Overpeck, Samuel J
Publisher: name: Purdue University Graduate School
Year: 2022
Subjects: Aerodynamics (excl. hypersonic aerodynamics)

description: Hypersonic boundary-layer transition greatly affects aerodynamic heating, skin friction, aircraft stability and other characteristics on flight vehicles. Understanding the factors leading to laminar-turbulent transition is pivotal in hypersonic aircraft design. Various instabilities and modes may facilitate transition at hypersonic speeds including first and second-mode waves, Görtler vortices, and cross-flow which may be stationary or traveling. The research presented here will focus on investigating traveling cross-flow instabilities on a 7° half-angle cone at 6° angle of attack. The experiments were conducted in the Boeing/AFOSR Mach-6 Quiet Tunnel (BAM6QT) at Purdue University. The low freestream noise of the quiet tunnel facility made it ideal for studying boundary layer transition due to its more, ”flight like” environment when compared to traditional tunnel environments. Previous experiments by Ryan Henderson, Chris Ward, and Joshua Edelman focused on studying the cross-flow instability on right circular cones at angle of attack (AoA) in the BAM6QT. From these experiments it was decided that a means for taking off-surface pressure measurements on a cone was needed. This work sets out to create a micro-pitot traverse system capable of doing such. The system is able to measure pressure fluctuations within the boundary layer of cone models at precise axial, azimuthal and wall-normal locations. The design for the traverse was based off a traverse used at Notre Dame which was designed by David Cavalieri in his PhD dissertation for Illinois Institute of Technology. Micro-pitot probes created using hypodermic tubing and Kulite sensors were created to attach to the end of the traverse and take pressure measurements. The micro-pitot probes were placed such that they formed two distinct spatial pairs capable of measuring both the phase speed and propagation angle of traveling cross-flow instabilities using the difference in time of arrival of the traveling instability between the sensor pairs. The micro-pitot probes developed were made from telescoped hypodermic tubes housing Kulite XCE-061-15A sensors. The telescoped tubing assembly caused attenuation at higher frequencies affecting the micro-pitot probes ability to measure pressure fluctuations at higher frequencies. It was necessary to increase the dynamic performance of the micro-pitot probes in order to capture the cross-flow instability. To accomplish this a custom built frequency 17 compensator was designed to correct for this attenuation. The process for designing the compensator utilized a Mach 4 supersonic jet system (SSJ) to estimate a transfer function model for the tubing assembly. This was done by comparing the spectral content of an untubed Kulite sensor and a micro-pitot sensor in the SSJ. The transfer function model was then used to develop the compensator improving measurements made with the micro-pitot up to 50 kHz. The micro-pitot traverse system was then used in a series of tests in the BAM6QT to validate its ability to function as designed. The traverse needed to provide a rigid platform for the micro-pitot probes during tunnel operation. The deflection of the pitot head was recorded using a shadowgraph system. This allowed real time measurements for the deflection of the pitot head during tunnel operation to be taken. These measurements were compared to theoretical calculations to ensure deflections were within acceptable limits. Also, of key importance was the survivability of the traverse system after repeated runs in the BAM6QT. This focused on the ability of the traverse to continue providing movement in all three-directions and its ability to resist wear in the tunnel environment. The only cause for concern noted over the course of three tunnel entries centered around the motor used for wall-normal movement. This motor suffered repeated damage impairing the traverses ability to function as intended. Observations regarding this issue and solutions implemented to mitigate the impact of this damage are discussed. Finally, the micro-pitot was combined with the traverse system and used in conjunction with surface mounted sensors on a axisymmetric cone to measure traveling cross-flow instabilities. Damage to Kulites needed for the micro-pitot prohibited three sensors from being used in the tunnel. For this reason only propagation angles and phase speed calculations for traveling cross-flow waves were calculated using the surface mounted sensors. However, one micro-pitot sensor was used to measure spectral content near the surface mounted sensors. The spectral content of the micro-pitot was compared to the surface mounted sensors in order to validate that the micro-pitot could measure the desired instability once more are acquired