Flow Field Characteristics around Bluff Parachute Canopies

The objectives of the present investigation are to determine the nature of the flow field around bluff parachute canopies, considering the effects of canopy shape parameters on this flow field and hence on the resulting aerodynamic forces and moments which are developed on the canopy surface. In order to relate the flow field developed around bluff parachute canopies to their aerodynamic characteristics, a series of experiments in the Leicester University wind tunnel has been conducted on a family of particularly significant canopy shapes. These cross-shaped canopies have excellent drag and stability characteristics if arm ratios of about 4:1 are selected. Flow visualisation, using both helium bubbles and wool tufts, was used to determine the flow field around the canopy. The most probable description of the wake flow is chains of irregularly-shaped vortex loops which move at about 0.7 times the undisturbed free stream velocity. Aerodynamic forces and moments measured on the various canopies correspond with the observed flow characteristics. Statistical correlation analyses made with hot wire anemometers in their wake indicate the periodic structure of the wakes formed behind these bluff bodies and reveal their basic similarities. Strouhal numbers of about 0.15 were obtained in the wake formed behind an imporous rigid hemispherical canopy. These are increased as canopy porosity is made larger. A useful comparison between a semi-theoretical blockage correction applied to aerodynamic forces and test results was obtained from an ancillary test programme conducted under water in a large cross-sectional area ship tank.