FLOW VISUALIZATION OF SMOKE SCREEN FORMATION FOR A RECTANGULAR VEHICLE IN DYNAMIC MOTION

摘要

This study uses computational fluid dynamics (CFD) and the dynamics mesh technique to simulate and visualize the unsteady vortex shedding of a smoke screen produced by a smoke-generating vehicle in dynamic motion. The effects of atmospheric wind speed and the speed of the smoke-generating vehicle on the smoke screen transport phenomena are examined. In this study, the smoke screen is released from the base of the vehicle as two parallel jets with a ratio of centerline spacing to nozzle diameter equal to 52. Once the smoke concentrations are obtained from the numerical results, the Bouger-Lambert law is applied to calculate the percentage of visibility degradation at particular axial positions. Two-dimensional flow variables are solved using the unsteady Reynolds-Averaged Navier-Stokes equation (RANS) with the control volume method. The computational domain is constructed with the conformal hybrid mesh system combined of rectangular grids around the vehicle and triangular grids in the remaining open domain. The buoyancy effect of air and smoke are approximated using the Boussinesq assumption, and the Reynolds stress terms are treated with the RNG κ-ε turbulence model with standard wall functions. The smoke-generating vehicle was simplified as a rectangular cylinder body. The tested atmospheric wind velocity ranged from 1 ms~(-1) to 9 ms~(-1) and the speed of the smoke-generating vehicle relative to the leeward wind varied from 3 ms~(-1) to 9 ms~(-1) . The velocity ratio (IR) of released smoke to the wind speed ranged within 3.3-30. Simulation results reveal that the wake flow pattern behind the smoke-generating vehicle is similar to the von Karman street vortex. The amplitude of vortex shedding decreases and gradually changes into a single jet as the speed of the smoke-generating vehicle increases. The percentage of visibility degradation decreases with an increase of the oncoming atmospheric wind speed, regardless of whether the smoke-generating vehicle is in stationary parking or forward motion mode. When the oncoming atmospheric wind speed is below 1 ms~(-1) , driving the vehicle forward at a speed below 7 ms~(-1) can effectively enhance the performance of smoke screen production within a certain time period. However, there are limits on the speed of the smoke-generating vehicle with respect to the atmospheric wind flow in establishing a smoke screen over a large area.