Parks College Parachute Research Group

Modeling the Interaction of Sub-scale Parachutes With Atmospheric Turbulent Eddies

Jean Potvin
Saint Louis University
St. Louis, MO 63103, USA

Robert Wright
Planning Systems Inc. Reston, VA 20191, USA

Presented at the 18th AIAA Aerodynamic Decelerator Systems Conference and Seminar, Munich, Germany, May 23-26, 2005, Paper AIAA 2005-1675




Abstract

By virtue of being light and featuring small canopy diameters (i.e. < 10 lbs & < 3ft), sub-scale parachute systems such as hand-thrown wind-sensing sondes are most susceptible to the turbulent "gusts" that populate the atmosphere, even when the latter is characterized by low or moderate turbulence levels. Depending on parachute and payload design, atmospheric turbulence will generate descending flight trajectories that include small and large random transverse (i.e. tilting) motions of the parachute and/or of the payload. Gusts may even trigger undesirable flight modes such as parachute coning. As a result, these unsteady flight characteristics may reduce stability, which for small atmospheric-sampling probes may translate into data-collection performance degradation. This paper discusses a simple calculation of the time scales associated with the sometimes random, sometimes cyclical transverse displacements that are caused by atmospheric turbulence. The calculation will be based a simple picture of turbulence, namely as islands of turbulent eddies, gusts or vortices distributed randomly in a vast expense of air moving in a "laminar" fashion. The interaction between the system and atmospheric turbulence will be assumed to arise whenever the parachute-payload is entering an atmospheric eddy and is exchanging energy with portions of the eddy. By using Kolmogorov's turbulent energy density equation, this theoretical framework will be used to assess to atmospheric-gustiness sensitivity of several parachute systems, including a sub-scale system and a full-scale, military cargo airdrop system.



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