Vortex shedding in the near wake of a parachute canopy

Abstract

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.