WPI Professor Builds a Better Parachute

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WORCESTER, Mass. -- Andre-Jacques Garnerin was a fearless Frenchman who catapulted out of a hot air balloon and into the history books in 1797 after he bumped and swayed his way to the ground supported by a homemade parachute. Since Garnerin took his leap into the record books 201 years ago, parachutes have been used extensively to drop people and cargo from aircraft for sport and to save lives.

Hamid Johari of Worcester, Mass., an associate professor of mechanical engineering at WPI, recently began looking at parachutes in a new way to explore the aerodynamics of canopy inflation. Under a three-year, $186,093 grant from the Army Research Office (ARO), Johari and mechanical engineering doctoral candidate Kenneth J. Desabrais of Monson, Mass., are testing their own homemade parachutes in a water tunnel.

Using the same material the Army uses, the researchers are building parachutes that are reduced in scale by a factor of 30 to 60 times. "Nothing is stitched. Heating and melting methods are used to attach fine nylon strings to the canopy," says Johari. Each model will be evaluated in an experimental rig in the water tunnel in Higgins Laboratories, where the flow field can be measured. The flow field is the 'map' of the fluid velocities in the vicinity of each parachute. "The incoming flow inflates a parachute when it is first deployed," Johari explains. "They exert the forces that slow jumpers down. Doing our experiments in the water tunnel, with velocities slower than in the atmosphere where parachutes open in real-world situations, gives us more time to acquire data."

That data will provide a better understanding of the flow field-and that, in turn, will lead to a more systematic approach to designing parachutes, specifically as it relates to their opening time and opening shock. Johari and Desabrais will also explore any differences that may exist in models tested in the water tunnel and those tested in wind tunnels-until now the traditional way of looking at parachutes.

"We are the first to attempt to measure the flow field around an inflating parachute canopy in a water tunnel setting," notes Johari. The Army currently has computer models that predict the performance of their parachutes; the WPI study can be used to verify the accuracy of those models. "The measurements will also allow for the creation of a physics-based model for the parachute opening," he explains. "Because the opening process is poorly understood at this time, designers have to rely on 'cut-and-try' methods to create parachutes with specific characteristics. A simple, physics-based model (one of the anticipated products of our research) can provide accurate information to designers before any fabric is cut."

Johari earned his B.S. at the California Institute of Technology and his M.S. and Ph.D. at the University of Washington. He has been a member of the WPI faculty since 1989. "Our research on parachutes will help with modeling the opening times and shock loads that jumpers or cargo experience during inflation. Having reliable models will significantly reduce development time and costs. As computer simulations become more sophisticated and robust, the design process will also benefit from such simulations."

An independent technological university founded in 1865, WPI is renowned for its project-based educational program.