An experimental study of the flow emerging from porous walls

The fluid mechanics of air flow exiting from porous materials is investigated. Porous walls with filter ratings differing by about three orders of magnitude are studied. The flow behavior is investigated for its spatial and temporal stability. The measurements show that coalescence effects cause jet development which gives rise to a stable, spatial velocity nonuniformity termed pseudo-turbulence. Beyond a critical mean velocity, the pseudo-turbulence shows a leveling trend and the flow exhibits jets whose dimensions are independent of wall porosity and provide a unique length scale for this type of flow field (1 mm). A correlation is developed to predict the onset of fully developed jets. The impact of using porous wall materials in cold flow solid rocket motor testing is addressed.This study was supported by the Experimentation Branch, Aerophysics Division, NASA Marshall Space Flight Center. The useful discussions with Mr. Werner Dahm are deeply appreciated. The authors thank the useful comments of one of the reviewers and acknowledge the help of the associated technicians and NASA/ED-31 personnel during the conduct of this experiment.