In-model free-flight instrumentation for hypersonic shock tunnel testing
Last modified: July 16, 2011
The paper presents preliminary testing of a microcontroller-based data-acquisition system for heat flux measurements in shock tunnels. A measurement system has been developed involving an ARM-based microcontroller, the ADuC7020, programmed as a data-acquisition device used with a signal processing circuit, custom-made to amplify the heat flux signal sensed by a platinum thin-film gauge. The gauge was mounted near the shoulder of a hemisphere cylinder model designed to be dropped into the hypersonic flow, simulating unsuspended hypersonic flight.
In standard aerodynamic ground testing, models are mounted in a wind or shock tunnel near the nozzle axis using support structures such as a sting or wire. These support structures also act as conduits for transferring data from the data-acquisition system which is generally situated outside the tunnel. While this technique is routinely used to acquire forebody flow data, the presence of a sting or wire disturbs the low-density afterbody flow. A support mechanism in the form of a sting could completely alter the velocity distribution in the wake by introducing a zero-velocity boundary condition at the sting surface that would not exist in flight. Wire mounts may form localised regions of stagnant flow, producing a different thermal flow field to the one nominally under investigation. These problems are overcome by drop-testing an unsuspended model, but, if wake flow properties such as base pressure or heat flux must be measured, the problem of acquiring and storing the data from the sensors on the model becomes important.
To eliminate the problems associated with these mounting methods, a free-flight model deployment system has been developed at the UNSW@ADFA, which allows the model to drop into the centre of the nozzle during the tunnel's steady flow time. To overcome the problem of getting data from the sensor to the data-acquisition system, an onboard microcontroller-based data-acquisition system has been developed and embedded within the model. The data-acquisition system is triggered by a MEMS-based accelerometer and is capable of taking 8 ms worth of data from one platinum thin-film gauge, with a sampling rate in excess of 100, 000 samples per second. Data is retrieved from the microcontroller after the test. The presentation will discuss the design and preliminary bench testing of the data-acquisition system. High-speed schlieren visualisations showing the repeatability of the drop testing procedure will also be presented.