~Abstract~

The intent of this experiment is to observe the correspondence between a sphere's drag characteristic and the properties of its wake region. These properties were measured for both a smooth sphere and a golf ball in order to observe the change in drag force caused by the dimples in the golf ball.

The wake region is a low pressure area directly behind an object moving through a fluid. Using a Pitot probe, the wake region was surveyed in order to obtain a position versus pressure coefficient curve from which the size of the this region was determined. Comparing the size of each sphere's wake region should indicate which sphere experiences a greater drag force.

Drag measurement results indicated that the smooth ball experienced less drag than the golf ball, and pressure measurements indicated that the wake region behind the golf ball was smaller than that of the smooth sphere, meaning a lower drag on the golf ball. Thus, the results were in agreement, and it can be concluded that there is a correlation between drag coefficient and wake region size. Agreement between the smooth sphere's drag characteristic and the smooth sphere reference data suggests these results are reasonable.

[I. Introduction]
[II. Equipment]
~III. Procedures~

DAT DRAG
Drag characteristics for the sphere were obtained by putting dem ballz on a beam in the wind tunnel. Dat beam got a pressure transducer connected to an instrumentation system connected to an ELVIS unit interfacing with LabView on the computer so it can feel the drag force pushing on dem ballz. Drag force was measured at between fan speeds of 25 Hz and 55 Hz, with 2.5 Hz increments. To ensure the i.id requirement was met as the speed setting was changed, We carefully put numbers on washers and threw them in a bucket. Then we shuffled the bucket and picked out a number to use for the fan speed setting. Before turning on the fan, we made sure the instrumentation system and the LabView interface were calibrated at zero measurement. The fan was turned on and the fan was allowed to reach its specified speed setting. Once the specified speed was reached, the system was allowed to settle for 60 seconds before taking the measurement. The LabView interface output the average of 10 data points. Once the data point was saved on a spread sheet, the fan was turned off, and the system was recalibrated at zero. Then a new number was selected from the bucket, and the measurement procedure was repeated for the chosen speed setting. This procedure was repeated for the golf ball.

DAT WAKE
To survey the wake region, a Pitot probe connected to the instrumentation system was placed 0.68 inches (1/2 of the sphere diameter) behind the centerline of the sphere. The fan was turned on at a speed of 30 Hz, and the probe was used to measure the pressure beginning at the center of the sphere and moving outward in 0.1 inch increments. This procedure was repeated for the golf ball.

[IV. Analysis]

DRAG
Raw:: drag force; fan setting
Refined:: drag coefficient; Reynolds number
CD(Fd) = Fdrag / 0.5 density vinf^2 A
Re(f) = fit polynomial curve from lab "Wind Tunnel 1"

WAKE (PRESSURE)
Raw:: Local pressure; y position
Refined:: Pressure coefficient; Vertical position
Cp(P) = P / 0.5 density vinf^2

[V. Results]
[VI. Discussion]
[VII. Conclusions & Recommendations]