Prediction of terminal descent characteristics of parachute clusters using CFD

Abstract

Introduction A computational study has been performed to determine the aerodynamics of a cluster of round parachutes using computational fluid dynamics (CFD). The results given here are the first predictions of descent characteristics for a cluster of three half-scale C-9 parachutes. In particular, the results include the aerodynamic flow field and geometry of the parachute cluster with assumed shapes for the individual canopies. Computed results have been obtained using both structured and unstructured numerical techniques. The computed pressure over the inner and outer surfaces of a single canopy in the cluster is used to calculate the net forces and moments acting on the canopy. A manual iterative process is used to determine the expected stable configuration for the cluster geometry by determining the condition at which the net moment about the payload (origin) is zero. The corresponding angular location of the canopies predicted by the CFD computations is compared with the available experimental results and is found to be in good agreement with the data. It is shown that significant progress has been made in determining the terminal descent flow field characteristics of a particular parachute cluster configuration. These computational solutions provide, for the first time, an understanding of the flow field in and around parachute clusters. * Aerospace Engineer, Associate Fellow AIAA ** Aerospace Engineer, Member AIAA This paper is declared a work of the U.S. Government and is' not subject to copyright protection in the United States. Parachute clusters have been utilized in a wide range of applications throughout their history, and the process utilized for their development and design has significantly improved over the years. Parachute cluster systems were developed to avoid the use of excessively large single canopies. A practical limit for the utility of single-round canopies is approached, as the users' desire for recovering larger payload weights increases. In many applications, a cluster of parachutes has a number of benefits over a large single canopy. These include the ability to rig and manufacture the system. Clusters also have the advantage of backup protection. This is evident when a single cluster chute failure is considered. The system under a smaller subset of the canopies still has a lower impact velocity than a free fall. Clustered parachute systems have excellent stability characteristics compared to most single-canopy systems. This is due in part to the terminal descent shape of multiple-bluff bodies versus a single-bluff body. Of course there are some undesirable features of parachute clusters which include the difficulty in obtaining a time-sequenced opening of all canopies together. This phenomena is known as lead-lag, in which one canopy opens quicker than the next canopy. This generally forces the individual parachutes in the cluster to be stronger than