Geometric Porosity as a Dynamically-Driven Process: Inflation versus Steady Descent

The geometric porosity of a parachute is an all-important design characteristic that determines fall rates and stability during steady descent. Herein the concept is further explored in the context of inflation, where the complexity of the flows within the expanding canopy is likely to change the discharge of the air through a parachute’s vent and gaps. Here a simple example of an apex-vented, flat-circular parachute built out of non-porous fabric and inflating quasi-steadily is used to show how geometric porosity could indeed vary. This is numerically demonstrated from the analysis of CFD-generated flows through the vent. The numerical data suggest the discharge coefficient of the entire canopy (i.e., of the whole parachute seen as a nozzle) to be in the 0.93 – 0.95 range early during the inflation process when the canopy contains a relatively small about of air. This coefficient is seen to decrease to about 0.88 – 0.89 when it is nearly inflated and containing a lot more air. In other words, one has a scenario where the geometric porosity decreases over time, as the result of increasing viscous friction within an ever-increasing air mass. The CFD data was also used to study vent discharge, i.e., with the vent itself seen as a nozzle. In this case the discharge coefficient appears to remain constant at 0.95 – 0.96 throughout the inflation process.