The flow past a parachute with and without a vent hole at the top is studied both experimentally and numerically. The effects of Reynolds number and vent ratio on the flow behaviour as well as on the drag coefficient are examined. The experiments were carried out under free-flow conditions. In the numerical simulations, the flow was considered as unsteady and turbulent and was modelled using the standard - turbulence model. The experimental results reveal good agreement with the numerical ones. In both the experiments and numerical simulations, the Reynolds number was varied from 85539 to 357250 and the vent ratio was increased from zero to 20%. The results show that the drag coefficient decreases by increasing the Reynolds number for all the cases tested. In addition, it was found that at low and high Reynolds numbers, the parachutes, respectively, with 4% vent ratio and without vent are deemed more efficient. One important result of the present work is related to the effect of vent ratio on the stability of the parachute. 1. Introduction The study of bluff bodies involves the consideration of complex aerodynamic phenomena such as semiwake and vortex shedding. The analysis of parachute dynamics is one of the most interesting problems, which would lie within this context. One important feature that may distinguish the computational fluid dynamics (CFD) modelling from the experimental analysis of the parachute behaviour is the geometry flexibility in the latter allowing large variations in the experiments [1]. A parachute is generally equipped with a venthole at the apex to allow some air to flow through the open canopy. The canopy can be represented by a hemispherical shell, which has been shown schematically in Figure 1. In the numerical simulations carried out in the present work, the canopy is considered solid and its direction of motion is from left to right (i.e., the flow direction is from right to left in the positive direction). The flow structure near the wake of canopy is responsible for the aerodynamic forces and moments it experiences. The relationship between the Reynolds number and flow field structure around spherical bodies in incompressible flow regime has been studied extensively. The numerical study carried out by Natarajan and Acrivos [2] indicated that the wake of a sphere became unstable at a Reynolds number of 105. The experimental results of Sakamoto and Haniu [3] showed that the hairpin-shaped vortices begin to be periodically shading when the Reynolds number reaches about 350. Their research indicated that when the Reynolds
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