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.

Experimental study of current forces and deformations on a half ellipsoidal closed flexible fish cage

“…This behaviour leads to a fluctuation in the drag force, and is associated with a clear vortex shedding frequency in the response. The fluctuations in the drag have been reported to be up to 27 % of the mean, see Johari and Desabrais (2005). The standard deviations compared to the mean drag force for the presented results were 10-16% above the mean.…”

“…The shape of parachutes are known to oscillate under some unfavourable conditions (Hoerner, 1958). The oscillation of the shape/diameter is called "breathing", and is caused by the interaction between the near-wake fluid forces and the fabric (Johari and Desabrais, 2005). This behaviour leads to a fluctuation in the drag force, and is associated with a clear vortex shedding frequency in the response.…”

“…For parachutes, drag have been shown to be dependent on the permeability of the fabric (Hoerner, 1958). The permeability of the nylon used in the experiments were not measured, but since parachute nylon were used it will be assumed that the permeability is not far from 1-2% (Johari and Desabrais, 2005). Permeability at this level can lead to a small reduction in the drag compared to non permeable structures.…”

Verification and Validation of the Spring Model Parachute Air Delivery System in Subsonic Flow

“…If we use a parachute with larger size or lower Reynolds number, the breathing period will increase. For example, the average period of breathing for the 10.7 m T-10 parachute is 2.3 s in experiment [6], while it is about 2 s in our simulation. Figure 13: Simulation of cross parachute unfolding and inflation.…”

“…However, the main difference between fabric canopies and rigid bluff bodies are fabric flexibility and permeability [5]. Thus the coupling of the vertical flow with the permeable and flexible fabric may have significant difference with rigid bluff bodies.…”