<div>The quiet-flow capabilities of the Boeing/AFOSR Mach-6 Quiet Tunnel have been well established in the last decade, but a full characterization of the nozzle flow is an ongoing project. Pitot probes outtted with Kulite pressure transducers were used to further the investigation of the tunnel's flowfield. Noise levels were calculated by integrating the power spectral densities of the measured pitot pressure fluctuations, and experiments were performed to investigate several aspects of the flow.</div><div><br></div><div><div>First, the temperature distribution along the nozzle was varied to determine if heating had an effect on the stability of the laminar nozzle-wall boundary layer. Runs made with initial stagnation pressures slightly above the maximum quiet pressure determined that additional nozzle-wall heating did not have an effect on the amount of runtime which experienced quiet flow. In addition, pitot-probe measurements were taken at various locations to better determine the axial dependence of the noise levels. Experiments were also performed using pitot probes of varying forward-facing diameters to determine the effects of probe geometry on the measured fluctuations. The results were found to differ signicantly from simulations and from a previous set of experimental data, but a likely cause of the discrepancies was not found. A pitot probe mounted on the base of a cone confirmed that the aft end of a model does experience quiet flow. Finally, characterization of the flowfield was attempted when the tunnel is run using helium. The measured pressures for these experiments have a signicant level of uncertainty because the sensor calibration changes as helium diffuses across the diaphragm. Nevertheless, the measurements suggest that there may be periods of uniform flow, although these periods remain unstable and unsteady.</div></div><div><br></div><div><br></div>

<div>The ambient pressure fluctuations within a wind tunnel test environment can severely affect the boundary layer transition witnessed 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.</div><div><br></div><div> </div><div> The freestream flow field of this tunnel has been characterized multiple times since its creation, however an extensive three-dimensional spatial sweep has never been conducted. A pitot rake model was designed to allow for 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 concurrent freestream pitot probe measurements. The performance of this new measurement method was evaluated, and suggestions for future verification tests are made. The pitot rake appears to suffer from probe-probe interactions in certain configurations, and has demonstrated variation in measurements that depends on the individual sensor used.</div><div><br></div><div> </div><div> This new measurement apparatus was used to investigate the effect that cavities in the tunnel wall created by the installation of new optical windows had on the freestream noise level. A control dataset corresponding to a perfectly conformal tunnel wall was not collected during this work. 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.</div><div><br></div><div> </div><div> 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 proved 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 issue, but no evidence of the second mode instability was found. A 2.5° half angle cone is being designed to replace the flat plate model as a platform for the continuation of this project.</div>