Fluid-structure interaction simulations of the inflated shape of ram-air parachutes

Abstract

Historically, the development of ram-air parachutes, or parafoils, has been mostly empirical. The use of computational methods has the potential to provide a greater level of insight into the underpinning mechanisms of parachute flight than that offered by experiments alone. To this end, a loosely coupled fluid-structure interaction approach has been developed, and is utilised to predict the equilibrium shape of a parafoil during steady glide, thus permitting an analysis of the associated flow-field. In order to reduce computational cost, a novel method for modelling a single cell of a ram-air canopy is derived, which utilises periodic boundary conditions to produce a response representative of a full canopy. The fabric membrane of the structure is modelled using the non-linear Finite Element Method, and the fluid dynamics are characterized by a finite-volume discretization of the steady Reynolds-Averaged Navier-Stokes equations with a k-epsilon turbulence model.

Two-dimensional flow simulations reveal a strong influence of the ram-air inlet on the lift and drag performance, greater than that caused by the blunt shape of the leading edge cut alone. The results of the three-dimensional FSI analysis show key differences between the inflated parachute structure and the original cut pattern design. These include rounding of the trailing edge and an increase in the parafoil cross-sectional thickness and the size of the inlet. The results highlight a number of instances of interaction between fluid and structure, such as high structural stresses near the inlet, flow separation from the lower leading edge, and a large degree of separation towards the rear of the upper surface. The importance of spanwise forces on the shape of the canopy has been identified, in addition to the significance of internal pressure forces on lift and drag. Finally, the separated nature of the flow means that flow-field results show dependence on the chosen turbulence model, outlining the requirement for a comprehensive experimental dataset to enable the necessary validation of the fluid dynamics simulation approach.