The effect of spin on the flow behind a sphere.
In the following photographs, you can "see" the flow behind a non-spinning and spinning golf ball. The experiments were performed in the small water tunnel at syracuse University, with a mean flow velocity of 3.8 centimeters (1.5 inches) per second, corresponding to a Reynolds number of 1.6*10^3. In the first two pictures, the golfball isn't spinning. In the third, it's spinning with 7 revolutions per minute (rpm), and in the fourth with 12 rpm. These spin rate correspond to the surface speed, 41% and 71% of the flow speed, respectively.
In photo 1, small particles are suspended in the water and the streaks are made visible by a laser light sheet. The exposure time was 1/8th of a second, so that the paths the particles follow during that time become visible. You can clearly see a region behind the golf ball called the wake where the flow is very different from the flow around it. Here, the flow right behind the ball moves back toward the ball surface. This causes much larger drag compared to the flow past more streamlined geometries.
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When the dye is introduced, keeping the conditions equal to those in the first photo, more details become visible. In the second photo, you can see that vortices are being shed from the ball.
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In photographs 3 and 4, you can clearly see the effect of spin. Now, the golfball is spinning in a clockwise direction, and the wake is directed downwards. In Photo 4 the ball is spinning faster, and the angle of the wake deflection is steeper, indicating a larger lift.
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Pictures taken by Michel van Rooij,
under supervision of dr. H. Higuchi,
Dpt. of Mechanical, Aerospace and Manufacturing Engineering, Syracuse University.
Copyright reserved.
