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Pressure Sensitive Paint Testing and Tennis Ball Research

by Rabi Mehta

March 24, 1998

We just completed a wind tunnel test on a 40% commercial van model in the NASA Ames 7 X 10 foot low-speed wind tunnel. The main goal of the test was to identify noise sources --- a lot of the noise heard on the inside of a vehicle is aerodynamic noise generated by wind flow patterns around the vehicle. On this particular test, an array of microphones was installed on the test section side wall to measure the noise levels. This part of the test was run by another research group. The main task of our group was to measure the surface pressure distribution on certain parts of the van. The pressures on the surface are determined by the flow patterns and they give a good idea of the locations and strengths of the noise sources.

The surface pressures on the van model were measured using a relatively new measurement technique which involves the use of a special "pressure sensitive" paint. The paint was excited by a special light source and by measuring the intensity of the emitted light, we determined the pressures on the model. Two high grade scientific CCD cameras were used to image the surface and the data were then processed on work stations. We obtained data over a speed range of 100 to 150 mph --- the relatively high speeds are needed to compensate for the fact that the model is scaled down. We investigated the effects of side mirrors and rain gutters on the pressure distributions. The preliminary results look very promising and we are particularly excited because this is the first time that this measurement technique has been used at such low speeds --- the technique typically works better at higher speeds (transonic to supersonic). The data from this test will hopefully help in designing quieter vehicles in the future.

For more details of this new measurement technique and some sample data, check out the following website:


As part of my continued interest and research in sportsball aerodynamics, I am currently participating in a project on tennis balls. This project is mainly involved with the aerodynamics and dynamics of the tennis ball, although some studies also include the bio-mechanics of players in action. High-speed videos have been shot at professional tournaments generating data on the speed and spin rates of the ball at various stages of its trajectory. The bounce properties of the various balls used in Grand Slam events on the different surfaces (grass, clay, hardcourt) are also being investigated. A computational program is underway which we hope will eventually be able to predict the flight of a ball, given the initial conditions. A wind tunnel test program is also planned to study the flow features over a tennis ball and to measure the aerodynamic forces acting on the ball. Check out the following website for complete and regularly updated details of this research program: http://wings.ucdavis.edu/Tennis

For the forthcoming web chat scheduled for April 2, 1998 I will be happy to answer questions on both topics (pressure sensitive paint and tennis ball aerodynamics). I have also studied the aerodynamics of other sportsballs (e.g. cricket, baseball and golf) and questions on these topics are also welcome.


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