Laser-Induced Diaphragm Rupture for Improved Sequencing and Repeatability in a Hypersonic Facility

Laser-Induced Diaphragm Rupture for Improved Sequencing
and Repeatability in a Hypersonic Facility

For hypersonic facilities where the flow conditions are established through the rupture of a diaphragm, such as in the University of Southern Queensland’s hypersonic wind-tunnel facility, the variability in the flow conditions is related to the uncertainty of the pressure at which the diaphragm ruptures. Variability in the diaphragm rupture pressure also results in uncertainty of the time at which the diaphragm will rupture. For experiments that require knowledge of when the test flow will be initiated, the sequencing of events relative to the flow onset is difficult when the flow is initiated using the natural rupture of a diaphragm. The challenge of experiment sequencing that arises due to rupture pressure variability is addressed by introducing a laser for rapid thermal weakening of the diaphragm. Event sequencing challenges are discussed in the context of free-flight testing, including model release strategies for such testing. The work proceeds through a review of Ludwieg tube flow initiation strategies and a discussion of the present context, which requires a reliable method for sequencing the retraction of the free-flight model holder. The natural variability of strength of the Mylar diaphragms in the present work is found to result in around ±6% uncertainty in rupture pressure. This rupture pressure variability is demonstrated to have a significant temperature dependence through empirical results and engineering models. Implementation of the laser-induced diaphragm rupture method is demonstrated to enhance repeatability in generating the flow conditions; the variability in rupture pressure was reduced to ±2% when the laser method was used. Based on the remaining sequencing uncertainties with the laser-induced rupture method and practical speeds for model platform retraction, uncertainty in the positioning of the free-flight models at the time of flow onset is shown to be ±2  mm.

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