Characterization of the Quiet Flow Freestream and a Flat Plate Model in the Boeing/AFOSR Mach 6 Quiet Tunnel

The ambient pressure fluctuations within a wind tunnel test environment can severely affect the boundary layer transition observed on test articles. The Boeing/AFOSR Mach 6 Quiet Tunnel was designed to minimize these fluctuations, also referred to as noise, and is the world's premier facility for studying hypersonic boundary layer transition in a quiet flow environment. All experiments performed for this work were conducted at this facility.

The freestream flow field of this tunnel has been characterized multiple times since its creation, but an extensive three-dimensional spatial sweep has never been conducted. A pitot rake model was designed and manufactured in order to perform an extensive spatial survey of tunnel noise. This model created measurement capabilities that were previously unknown to the BAM6QT facility, including the ability to take multiple concurrent freestream pitot probe measurements. The performance of this new measurement instrument was evaluated and suggestions for future verification tests are made.

This new measurement apparatus was used to investigate the effect that cavities in the tunnel wall had on the freestream noise level. These cavities are created by the installation of optical access windows in the tunnel nozzle wall. The experiments conducted provide evidence that the tunnel wall cavities do increase the noise downstream of their location by approximately 100%, however a control dataset is needed to verify this finding.

In addition to tunnel characterization, a novel flat plate model was evaluated for use in the BAM6QT. This model was intended for use as a platform for observing second-mode instability growth. These experiments show that the initial flat plate geometry used was incompatible with the BAM6QT, as the tunnel could not achieve nominal flow conditions with the model installed. The flat plate model was streamlined to rectify the startup issues experienced, but no evidence of the second-mode instability was found. A 3° half-angle cone is being designed to replace the flat plate model as a platform for the continuation of this project.