Shock wave interactions with porous beds
Doyle, Graham Kevin Einar
A study was performed on the movement of particles in a porous bed which had been impacted by a shock wave travelling parallel to the surface. The effect of the shock wave on the bed is greatest inside the bed. This study concentrates on the mechanisms occurring inside the bed which lead to the unique particle motion. Initial tests used a seven metre long shock tube with a steady flow testing time of approximately ten milliseconds (ten millisecond shock tube). Results with this tube showed that particle motion only began up to 50 ms after the passage of the shock wave. A new 51 m shock tube with a steady flow testing time of 100 ms was then designed and built. The shock tube has a larger cross-sectional area for longer visualisation of the particle movement. The test section has also been increased in size over that of the ten millisecond shock tube to allow for greater versatility. This new one hundred millisecond shock tube has been calibrated for Mach 1.2 - 1.4. Calibration tests have shown that the shock tube develops a thicker boundary layer than expected due to surface roughness and the presence of an area change at the diaphragm station. This boundary layer restricts the flow, but as the Mach number is reduced, this attenuation also reduces. Through careful diaphragm and Mach number selection this attenuation can be minimised. Tests on both shock tubes showed a high bed permeability. The pressure rise in the bed was found to move from a shock wave to a steep pressure wave as the bed depth was increased. The shock takes longer to move to the floor of the bed as the bed depth is increased. The shock in the bed stabilises quickly and then maintains a constant speed along the floor of the bed. The flow in the bed is highly complex due to the close proximity of particles to one another. The pressuie in the bed was measured with pressure transducers located in the floor.The local packing arrangement of particles around these transducers affects the flow over the ansducers.This ultimately determines the pressure which is measured. A distinct surface wave like phenomena was seen to form with the sm aller test specimens. This wave moves down the test section A model describing the physical processes which cause this wave has been proposed. The movement of the shock into the bed causes a vortex to be shed at the leading edge of the bed. This causes the particles to swirl inside the bed. The presence of the leading wall of the test section forces the particles out of the bed and into the airstream behind the shock. The airstream causes the particles to adopt this wave like structure. The effect o f the vortex is localised and only affects the particles towards the front of the bed. An increase in Mach number leads to greater particle movement. Increasing particle density steepens the wave profile and decreases the overall particle movement and response. The Magnus effect was found to have no influence on the gross particle motion. The time for particles to begin lifting off from the bed was found to be independent of the bed depth. Particle lift-off depends on the properties o f the particles tested and the manner in which they were placed in the bed. Particles were tracked at velocities in excess o f 20 ms- 1 . It is believed particles can achieve velocities up to 90% of gas velocity.