The peak overpressure field resulting from the discharge of an open-ended shock tube
Newman, A. Jensen
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A low cost shock tube was constructed and its effectiveness as a blast wave generator was evaluated. This was done to provide Neuroscientists at the Center for Hearing and Deafness at The University at Buffalo with an economical method to conduct consistent blast exposure experiments. Effective blast wave simulation was defined as the capability to consistently produce blast waves that were qualitatively and quantitatively similar to those produced from muzzle blast. Blast wave consistency was determined by first taking repeated overpressure measurements at a range of shock tube driver pressures (21.4 to 74.6 PSIA). The range of variation in produced overpressures was determined to be 13% of mean on average for all driver pressures. Approximate isobar plots were created from collected data to compare with those produced in the firing of artillery. Qualitative and quantitative agreement was good. Based on these observations, the low cost shock tube was found to be an effective blast wave generator. In addition, having a cost of about 100 times less than that of other shock tube facilities, the one developed in this research removes the financial requirement previously associated with shock tube research. The overpressure field resulting from the discharge of this shock tube was also examined. Initially theoretical predictions were made using blast scaling laws and the assumption of isentropic expansion. Agreement between experimental data and theoretical predictions was not found to be in good agreement. Therefore, dimensional analysis was used to suggest parameters on which peak overpressure was dependent. Further experimental data was collected for the range of shock tube driver pressures specified above along the 0, 45 and 90 degree radials to provide the form of the equation which would describe the pressure field. A general equation based on curve fitting experimental data was determined. The problem was then re-examined from a physical point of view. Theory from Whitham and experimental results from Skews were used to determine the initial shape of the emerging shock wave. This shape was seen be very similar to a cardoid and it was assumed that this shape remains constant as the wave propagates. Physical considerations were used to provide the details of the cardoid's variation with distance from the shock tube exit. Using experimental data from only the 0 and 90 degree radials, isobar plots were generated and found to provide a reasonable approximation for the pressure field based on measurements along the 45 degree radials. This approximation allows the entire pressure field resulting from a shock emerging from an open-ended tube to be estimated based only on measurements at two polar angles.