J. Fluid Mech. (2005), vol. 536, pp. 185–207. c  2005 Cambridge University Press doi:10.1017/S0022112005004490 Printed in the United Kingdom 185 Vortex shedding in the near wake of a parachute canopy By HAMID JOHARI AND KENNETH J. DESABRAIS Mechanical Engineering Department, Worcester Polytechnic Institute, Worcester, MA 01609, USA (Received 14 January 2004 and in revised form 16 February 2005) The dynamics of flexible parachute canopies and vortex shedding in their near wake are studied experimentally in a water tunnel. The velocity field was measured by particle image velocimetry for two different canopy diameters. The periodic oscillation of the canopy diameter about a mean value which is referred to as ‘breathing’ has a non-dimensional frequency, based on the free-stream velocity and the mean canopy projected diameter, of approximately 0.55 for the range of Reynolds numbers examined. The dimensionless breathing frequency observed in the experiments is consistent with the values for larger canopies. The shear layer emanating from the canopy rolls up and sheds symmetric vortex rings. The frequency of vortex shedding was measured to be the same as the canopy breathing frequency. This Strouhal number is unique in the sense that it is much higher than those associated with rigid axisymmetric bluff bodies such as disks and spheres. The canopy breathing is shown to stem from the cyclical variation of suction pressure, resulting from the passage of vortex rings, on the exterior surface of the canopy. The added mass associated with the breathing of the canopy is found to be accountable for up to 40% of the canopy drag fluctuations in the range of parameters investigated. 1. Introduction Round parachute canopies pose challenging problems for the understanding of axisymmetric bluff-body wakes. Two features specific to fabric canopies differentiate them from rigid bluff bodies such as cups, disks or spheres. First, and most importantly, fabric flexibility allows large variations in the canopy geometry not only in the inflation phase, but also during terminal descent. Secondly, there is a small mean flow through the canopy surface owing to the fabric permeability, amounting to a few per cent of the free-stream velocity. Given that bluff-body wakes are unsteady, coupling of the time-dependent vortical flow in the near wake with the flexible fabric geometry results in strong fluid–structure interaction. Moreover, full-scale canopies operate typically at Reynolds numbers exceeding several millions. The complexity of these issues has so far precluded a thorough examination of the flow field in the near wake of parachute canopies. The flow structure in the near wake is also responsible for the aerodynamic forces and moments experienced by the canopy. We are interested in examining the dynamics of flexible canopies and the relationship between the near-wake flow field and the canopy motions. Since there is a general lack of data in the literature on the flow about flexible bluff bodies and rigid hemispherical shells, the key flow features in the wake of disk and sphere are presented below to provide a basis for comparison with the results of our study.