Tailoring the Blast Exposure Conditions in the Shock Tube for Generating Pure, Primary Shock Waves: The End Plate Facilitates Elimination of Secondary Loading of the Specimen

Abstract

The end plate mounted at the mouth of the shock tube is a versatile and effective implement to control and mitigate the end effects. We have performed a series of measurements of incident shock wave velocities and overpressures followed by quantification of impulse values (integral of pressure in time domain) for four different end plate configurations (0.625, 2, 4 inches, and an open end). Shock wave characteristics were monitored by high response rate pressure sensors allocated in six positions along the length of 6 meters long 229 mm square cross section shock tube. Tests were performed at three shock wave intensities, which was controlled by varying the Mylar membrane thickness (0.02, 0.04 and 0.06 inch). The end reflector plate installed at the exit of the shock tube allows precise control over the intensity of reflected waves penetrating into the shock tube. At the optimized distance of the tube to end plate gap the secondary waves were entirely eliminated from the test section, which was confirmed by pressure sensor at T4 location. This is pronounced finding for implementation of pure primary blast wave animal model. These data also suggest only deep in the shock tube experimental conditions allow exposure to a single shock wave free of artifacts. Our results provide detailed insight into spatiotemporal dynamics of shock waves with Friedlander waveform generated using helium as a driver gas and propagating in the air inside medium sized tube. Diffusion of driver gas (helium) inside the shock tube was responsible for velocity increase of reflected shock waves. Numerical simulations combined with experimental data suggest the shock wave attenuation mechanism is simply the expansion of the internal pressure. In the absence of any other postulated shock wave decay mechanisms, which were not implemented in the model the agreement between theory and experimental data is excellent.

Using Gas-Driven Shock Tubes to Produce Blast Wave Signatures

“…Chandra et al studied the shock evolution within and outside shock tubes, concluding that the placement of a test article outside of the open end of a shock tube exposed the test article to complex flow phenomena that were not representative of blast waves in the field (3). Kuriakose et al, Yu et al, and Needham et al, also studied the effects of the test article's placement and whether end-jet testing provided representative results (8)(9)(10). Kuriakose et al (8) and Needham et al (10) agreed with Chandra's conclusions, while Yu et al (9) concluded that end-jet testing was acceptable under specific constraints.…”

“…Kuriakose et al (8) and Needham et al (10) agreed with Chandra's conclusions, while Yu et al (9) concluded that end-jet testing was acceptable under specific constraints. Specifically, Yu et al concluded that testing inside is acceptable as long as the test article is placed at least 8-10 shock tube diameters down the driven section, measured from the diaphragm, and testing outside is acceptable as long as the test article is placed within ½ a tube diameter from the exit of the shock tube (3,8,9). When testing inside, another important consideration is the blockage, "the ratio of the total 'presented area' of the obstruction relative to the crosssection of the tube, " and Needham et al found that the blockage should not exceed 10-25% depending on the exact nature of the experiment (10).…”

The evolution of secondary flow phenomena and their effect on primary shock conditions in shock tubes: Experimentation and numerical model

“…The rapid expansion of the shock front at the shock tube exit causes a density gradient to form, which initiates the rarefaction wave. Previously, we have shown that a reflection generated from a reflector plate at an appropriate offset can largely nullify the impact of the rarefaction wave on the incident waveform within the shock tube [29]. A normal reflection of the shock on a perpendicular endplate causes a reflected compressive wave that greatly reduces the effect of the tensile rarefaction wave.…”

“…The shock tube used in this work has been validated to reproduce free-field explosions within the test section accurately at sensor location I3 in Fig 2 (230 x 230 mm 2 square cross-section, 6 m in length) [29,30]. Briefly, compressed helium within a 55 cm length, 10 cm diameter chamber is separated from atmospheric-pressure air by Mylar membranes.…”

“…The rate of decay within the shock tube and the following expansion was found to depend on the shock strength. that the peak pressure decays as the shock wave propagates down the shock tube [29,32]. Energy loss occurs due to the expansion of the high-pressure shock front as it interacts with low-pressure upstream air, increasing the shock duration while maintaining a comparable impulse [29].…”