Shock wave interactions with porous plates

The idea of creating a material or barrier that attenuates shock and blast waves has long been investigated. Considerable work has been performed on the interactions of shock and blast waves with various materials. The application of permeable solids, porous materials and textiles for the attenuation and reflection of destructive shock waves have been studied extensively. The studies presented herein examine the interaction of shock waves and porous plates in order to ameliorate the hazardous effects of these waves particularly in ducts or channels leading to protected areas or objects. A number of tests were performed in an automated shock tube to determine the effects that a series of directional porous plates had on the initial peak pressure and impulse amelioration experienced by the end wall. Mild steel test specimens, ranging in porosity values from 6.6 % to 41.1 %, were mounted two at a time in the test section of the shock tube. Each plate had directional properties and since four plates were used in the study, a total of forty eight plate configurations were tested. Six pressure transducers were located along the side of the test section and two pressure transducers were located in the end wall of the shock tube in order to measure initial peak pressure and impulse amelioration values experienced by the end wall and to identify the wave interactions involved in the amelioration process. Schlieren photographs were also taken in order to investigate these wave interactions. Tests were run at three different Mach numbers viz. 1.23, 1.35 and 1.42. The separation distances between the plate specimens were varied between 30 mm and 60 mm; however the distance between the downstream plate and the end wall was kept constant at 140mm for all tests. It was found that significant initial peak pressure and impulse amelioration was achievable. The Back & Back plate arrangement produced the greatest initial peak pressure and impulse amelioration with averages values of 73.7 % and 20.45 % respectively. Both the initial peak pressure and impulse amelioration values were found to be dependant on the plate combination porosity. As the porosity of the combination increased, the amelioration values decreased. Complementary plate combinations produced differing results as different wave interactions occur when plate positions were interchanged. The porosity of the combined plates were found to have an overriding influence on the end wall initial peak pressure and impulse amelioration values when compared to the effect that plate arrangement (i.e. geometrical influences) had. For all tests performed in this study, the time period used for the integration of the end wall pressure traces was 9 250 μs. As an acceptable closing time for a blast valve in a shelter’s ventilation system is approximately 4 000 μs, the impulse amelioration values for certain plate combinations were recalculated. It was found that using this time period greater impulse amelioration values were produced as the rate of pressure rise, dp/dt, was initially lower at the beginning of the end wall pressure trace. Therefore, the lowest impulse amelioration value (7.9 %) achieved in this study, would produce significant impulse amelioration (20.3 %) if it were to be used in a shelter’s ventilation system. Impulse amelioration values were found to increase as the separation distance between v plates were increased. The average impulse amelioration value was found to increase approximately 1 % for a 15mm increase in the distance between plates. The amplitude of the entire end wall pressure trace was found to increase as the incident Mach number was increased. This resulted in greater initial peak pressure and impulse experienced by the end wall. The significant attenuation of the incident shock wave obtained during this study is attributed to the system of multiple reflected and transmitted waves that are produced by the presence of the plate specimens in series. This increases the frequency of shock wave and barrier interactions, when compared to just using a single barrier, creating regions of highly unsteady flow, especially in the air space between the plate specimens. Furthermore, the presence of the series of plates also allows for wave resonance to occur which may further attenuate the strength of the incident shock wave. It is suggested that future studies include numerical techniques in order to further investigate the complex wave processes that occur upon interaction with the plate specimens and confirm the major loss mechanisms of the system