A Second Look at Geometric Porosity as Revealed by Computational Fluid Dynamics (CFD)

We report on the results of a series of CFD simulations aimed at calculating a parachute’s geometric porosity. This is achieved with a definition of the coefficient of discharge adapted for use with CFD-generated flow speed and pressure data inside and outside a parachute canopy. The approach is illustrated with the calculations of the porosity and discharge coefficients of seven parachute-like solid shells based on hemispheres and cylinders (pillboxes), all outfitted with apex vents, and many with an additional concentric annular gap. The apex vent openings have radii equal to 5% of the projected diameter and the annular gaps have widths equal to 5% of the same diameter. The overall shell/canopy size and freestream speeds are typical of those encountered in personnel parachute applications. The discharge coefficients are found to be in the range 95 97% for apex vents and 92-94% for annular gaps. It is found that the fluid viscosity effects that reduce the mass flow rate near the edges of vents and gaps are most important over distances that are about 10% of the vent and gap span.