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

Compressed gas-driven shock tubes are widely used for laboratory simulation of primary blasts by accurately replicating pressure profiles measured in live-fire explosions. These investigations require sound characterization of the primary blast wave, including the temporal and spatial evolution of the static and dynamic components of the blast wave. The goal of this work is to characterize the propagation of shock waves in and around the exit of a shock tube via analysis of the primary shock flow, including shock wave propagation and decay of the shock front, and secondary flow phenomena. To this end, a nine-inch shock tube and a cylindrical sensing apparatus were used to determine incident and total pressures outside of the shock tube, highlighting the presence of additional flow phenomena. Blast overpressure, impulse, shock wave arrival times, positive phase duration, and shock wave planarity were examined using a finite element model of the system. The shock wave remained planar inside of the shock tube and lost its planarity upon exiting. The peak overpressure and pressure impulse decayed rapidly upon exit from the shock tube, reducing by 92-95%. The primary flow phenomenon, or the planar shock front, is observed within the shock tube, while two distinct flow phenomena are a result of the shock wave exiting the confines of the shock tube. A vortex ring is formed as the shock wave exited the shock tube into the still, ambient air, which induces a large increase in the total pressure impulse. Additionally, a rarefaction wave was formed following shock front expansion, which traveled upstream into the shock tube, reducing the total and incident pressure impulses for approximately half of the simulated region.