Boundary layer instabilities in high and low enthalpy hypersonic facilities using high speed schlieren
Keywords: hypersonic boundary layer
hypersonics
enthalpy
instabilities
schlieren
hypersonic facilities
T6 Stalker Tunnel
reflected shock tunnel

This thesis presents results from the first experiments conducted in the Oxford T6 Stalker Tunnel in Reflected Shock Tunnel mode and observations on the boundary layer instabilities established over a cone model at Mach 7 and Mach 8 conditions characterised using high speed schlieren image processing techniques.
The Oxford T6 Stalker Tunnel was developed and commissioned in a new mode of operation, Reflected Shock Tunnel mode which was crucial to the establishment of boundary layer instabilities over a model. The process by which this was done is outlined, from an initial analytical model to a quasi one-dimensional facility simulation. Piston dynamics were modelled and characterised by blank-off experiments, followed by validation of the diaphragm properties with tests done in shock tube mode. Finally, reflected shock tunnel experiments at half and then full pressure were conducted. This was used iteratively to improve the test conditions, bringing them to within 1% of the required nozzle supply enthalpy and 2.6% of the required nozzle supply pressure. A nitrogen driver condition was developed to improve testing time and these conditions were used with both a Mach 7 and Mach 8 nozzle.
Traditional sensors were not reliable at the high frequencies of instabilities expected (∼1 MHz) and would be damaged in the harsh flow environment. Thus, optical diagnostic techniques were explored as a means of extracting both quantitative and qualitative information from the flow. These techniques were developed based on high speed schlieren imaging and first applied to the Oxford High Density Tunnel, which is a cold flow facility for which pressure transducers could be used as a means of validating the developed techniques. Development of schlieren image processing techniques both for frequency and wavepacket convection speed detection as well as feature detection was undertaken. These methods were applied to experiments on a 7deg half-angle cone with a nose tip radius of 1.25 mm at Mach 7. Second mode instabilities were detected at a frequencies of 208-216 kHz with pressure transducers which allowed for validation of the image processing techniques.
Nonlinear interactions were explored with the use of selected image processing techniques showing significant second harmonics in the flow. An investigation was conducted using three different high speed cameras and the effect of varying camera parameters on the quality of the data extracted. Methods of extracting the frequencies of second mode instabilities using camera frame rates at much lower and much higher sampling rates than the expected frequency of the instabilities were developed and applied with varying levels of success.
Useful results were gained from these experiments, both as a review of the application of current schlieren processing methodologies, novel methodologies, and a comparison of different camera parameters which would inform future testing. Finally, both the commissioning work on the facility and the developed image processing techniques culminated in establishing and observing excellent first images of boundary layer instabilities in T6, namely high frequency second mode instabilities and non-modal instabilities at Mach 7 and Mach 8. Frame rates of 1-2 million frames per second allowed for capture of these instabilities. The extracted frequencies of these instabilities ranged from 0.8-1.4 MHz, at significantly higher frequencies than those observed in HDT. The methodologies allowed for the extraction of frequencies within 1-7% from the theoretical prediction. A blunt nose tip of 3.4 mm was used, allowing for the establishment of non-modal instabilities which could be visualised.
These were fully characterised in terms of their convection speed and structure angle. These instabilities were swept along at just under the edge velocity (0.93-0.94Ue). Interesting results were obtained by tracking and identifying the structure angle variation in time, which clearly showed the instability collapsing from a maximum angle of 30deg to a more linear feature at approximately 8deg to the surface.
The results show significant progress in the development and study of hypersonic boundary layers in the T6 high enthalpy facility and allows for the extraction of accurate boundary layer information in harsh flow conditions without the use of traditional pressure sensors. This enables the UK to have the capability of high enthalpy testing for the study of hypersonic vehicle models and future boundary layer instability research at flight representative conditions.