In the design of control system of a wind tunnel, it is important to understand the importance of operations of mechanical systems in order to ensure that any emergencies that may arise during the running of the wind tunnel are properly handled by the system. In the NAL 0.6m blowdown wind tunnel, it is proposed to incorporate a Variable Mach number Flexible Nozzle (VMFN), using which the tunnel13; can be started at a low Mach number and the required test Mach number (up to 4) can be reached by continuously (on-line) changing the nozzle contour using an Electroservohydraulic drive. Before stopping the tunnel, the VMFN would be reverted to the initial low Mach number condition. During the entire operation of the tunnel in such a scenario, it is essential that the Pressure Regulating Valve (PRV) maintains the specified or minimum stagnation pressure in the test section by following a safe trajectory. In order to determine a safe trajectory, a basic understanding of the variation of stagnation pressure in the settling chamber is called for, before the PRV control system is designed. In this paper, the problem is13; formulated based on quasi-steady isentropic equations and programmed in C language to predict the timehistories13; of settling chamber pressure and storage tank pressure for a given trajectory of the opening of PRV, as the VMFN throat is changed from Mach 1 to Mach 4 condition and vice versa. In order to validate the present methodology, the PRV trajectory measured during an experiment in the NAL 0.6m13; wind tunnel was used as input to the program and the measured time-histories of settling chamber pressure and storage tank pressure are compared with the predicted values. Predictions of settling chamber pressure and the storage tank pressure relevant to VMFN operation indicate that settling chamber pressure rapidly builds up towards the storage tank pressure, apparently due to the constriction offered at the VMFN throat as VMFN throat is changed from M=1 contour to M=4 contour during the13; start of the run. Minimum PRV opening area has been predicted to maintain the settling chamber pressure13; higher than the estimated minimum value at high Mach numbers. For smaller openings of the PRV (0.065m2), the predicted pressure falls below the minimum and is hence unsafe. From the predicted time history of free-stream dynamic pressure (q), it is noted that alarming increase in q can occur during starting and stopping transients, which can damage a test model in the tunnel, balance and such other components. Therefore, the design of test object and support system need to be adequately designed.
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