Examination of the forces controlling dust dispersion by shock waves

The interaction between a shock wave and a thin layer of inert dust is studied by solving unsteady, multidimensional Navier-Stokes equations representing the interactions between a compressible gas and incompressible particles. The system studied consists of a layer of densely packed limestone dust containing particles of uniform diameter (40 mu m) that interact with a shock of strength M-s = 1.4. Particle dispersion is investigated by comparing vertical particle accelerations due to Archimedes, gravitational, intergranular, and aerodynamic drag and lift forces. The simulations show that the shock produces two dust regions: a compacted layer and a dispersed region. The layer compaction, which increases the intergranular particle stress, is produced by drag and Archimedes forces. The dispersed dust is produced by forces that change in time as the shock passes. Initially, the dispersion is caused by intergranular forces. Later it is driven by a tradeoff between lift and drag forces. Eventually, drag forces dominate. Comparisons of the computations to experimental shock-tube data reproduced the observed initial growth of the dispersed dust and later leveled off. Particle agglomeration in the experiments made it difficult to determine a true particle size experimentally, although the computations for 40-mu m particles explain the experimental data.