Kenneth J. Desabrais

Vortex shedding in the near wake of a parachute canopy

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.

Measurement of the Geometry of a Parachute Canopy using Image Correlation Photogrammetry

The systematic application of a stereo-image based correlation photogrammetry technique was used to measure the geometry of a fully-inflated model parachute canopy. The laboratory scale experiments were conducted in a low-speed water tunnel at a freestream velocity of 0.2 m/s. The model canopy had a constructed diameter of 0.3 m, and was made from a single piece of fabric. A stochastic pattern of small markers was applied to the canopy surface to permit the use of a correlation-based processing scheme. Measurements of the flexible model canopy appeared to present reasonable values for the imaged portion of the canopy. The experiments have demonstrated that the correlation-based technique has the potential of providing quantitative data on the canopy surface geometry with acceptable uncertainties.

Vortex Shedding in the Near Wake of Rigid and Flexible Bluff Bodies

The velocity profiles and shedding frequencies of three axisymmetric bluff bodies consisting of a disk, a cup, and a rigid canopy model were examined in the near wake, 0.25 ≤ z/D ≤ 9.0. The measurements were conducted at a Reynolds number of 1.93 × 10 5 using a single element hot-film anemometer. The data revealed that the mean velocity deficit profiles became self-similar beyond z/D ≥ 3.0, and are identical in self-similar coordinates. However, the cup and rigid canopy models recover more quickly than the disk. At least two shedding frequencies were observed in the near wake of these axisymmetric models; one corresponding to the dominant downstream mode at a Strouhal number ≈ 0.15 and the other at low Strouhal numbers which disappears after z/D ≥ 3.0. The dominant mode appears at z/D ≥ 1.6 and persists at all locations further downstream. This mode has typically been associated with a helical mode present for axisymmetric bluff bodies with a fixed separation point. A high Strouhal number mode (≈ 0.55) seen in flexible parachute canopies does not appear to be present in the near wake of these rigid models including the rigid canopy.

Aerodynamic Investigations of a Ram-Air Parachute Canopy and an Airdrop System

Wind tunnel and airdrop tests were conducted to characterize the aerodynamic forces and flow fields of an MC-4/5 parachute. The wind tunnel tests were completed at the MIT Wright Brothers Wind Tunnel using a semi-rigid model consisting of fabric upper and lower surfaces attached to aluminum airfoil sections. The ram-air model had a 2-meter span, and an angle of attack sweep was completed at Re = 8.2 × 10 5 . Surface flow features were visualized using string tufts. Tufts were also used on the full-scale airdropped MC-4/5 to compare with wind tunnel results. Both sets of tests documented the lower surface flow separation predicted in numerical simulations, and the agreement in relative sizes of the recirculation region served to validate the wind tunnel model. The string tufts were also used to record slow flight and dynamic stall characteristics for the airdrop tests. Additional wind tunnel testing to analyze the observed decrease in time of L/D for models made of F111 fabric also establish a dependency between the canopy’s aerodynamic performance and fabric permeability properties.

Low Cost HALO Cargo Airdrop Systems

A low cost high-altitude and low-opening (HALO) cargo airdrop system was developed to airdrop 2,500 - 10,000 lb payloads with an objective of providing precision airdrop capabilities. The system consists of the Army standard Low Cost Low Velocity parachutes and Low Cost Container that have been used in humanitarian and other Army airdrop applications. The system was successfully designed, developed and full-scale tested. Performance tests were first conducted to generate Calculated Aircraft Release Point (CARP) data to examine the basic system performance characteristics. Operation Utility Evaluation (OUE) tests were then performed using the CARP data to determine system airdrop accuracy. The mean value of 32 OUE tests was found to be 662 ft (202 m) with an uncertainty of 51 ft (16 m).

Effects of Inflation Dynamics on the Drag of Round Parachute Canopies

This paper addresses the issue of the source of peak opening force, and whether it can be determined solely by the instantaneous canopy geometry. A set of rigid canopy models were fabricated which had geometries resembling those of a flexible, round canopy model undergoing an infinite mass inflation. The drag coefficient of rigid models were measured in a wind tunnel and compared with that of the fabric canopy. Data indicated that the two drag coefficient trends were quite different. It was concluded that the canopy geometry alone cannot be responsible for the peak opening forces during inflation, even when the apparent mass is accounted for. The time history of the viscous flow field about the canopy determines the peak opening force for the most part.

Added Mass of a Model Round Parachute Canopy during Inflation

The added mass of a model round parachute canopy during various stages of inflation was computed using a commercial finite element-based solver. Images from an experiment in which a small scale round canopy model was inflated under infinite mass conditions were employed to create a set of solid models. Eight instants during inflation were selected and the potential flowfield about the models at these instants was computed assuming a uniform freestream. Subsequently, the added mass component associated with translation along the symmetry axis was calculated. The computed added mass for the eight cases was greater than the enclosed fluid mass and could not be correlated with the latter. The ratio of added mass to the enclosed mass varied from 1.2 at the ‘sock’ stage to 4.1 at the ‘over-inflated’ stage. A correlation was developed to estimate the added mass as a linear combination of the enclosed fluid mass and the added mass of a disk having the same area as the canopy mouth.

A Novel Parachute Canopy Geometry for Airdrop Applications

Based upon past work on bluff bodies, it is hypothesized that parachute canopies with rectangular parallelepiped constructed geometries may have significantly greater drag coefficients than current circular designs. A series of wind tunnel tests were conducted in which model parachute canopies with rectangular parallelepiped geometries, i.e. a cross parachute with the sides attached together, were examined. All models had a lateral dimension of 0.2 m, and aspect ratios ranged from 0.2 to 1.2. The Reynolds number, based on the model dimension and the freestream velocity, spanned the range of 20 to 190 thousands. The models did not have a central vent, or any other geometric porosity. A line passing through the center of models prevented any off-axis motions. The data indicate that the drag coefficient has a maximum value of approximately one for the constructed aspect ratios of 0.4 and 0.6. The drag coefficient is less for smaller and larger aspect ratio models. The inflated aspect ratio for the two models with the largest drag coefficient is in the range of 0.55 – 0.66. The maximum drag coefficient is 45% larger than a similarly sized flat, circular canopy. These findings are consistent with the data on rigid bluff bodies.