Channels, Fall 2020

Channels • 2 020 • Volume 5 • Number 1 Page 3 Fig. 1 Cross section of circular (a) Model A and (b) Model B for = 90° Physical Experimentation A. Experimental Procedure and Approach The experimental analysis consisted of a calibration assessment of the equipment, fabricating physical models, and testing the manufactured circular and elliptical cylinders. The force measurement device was designed by M. Bird [6] for reading small forces in a wind tunnel. The measurement apparatus, referred to as the low drag cart, (Fig. 3) used a pulley system to transfer a drag force to a weight resting on a scale outside of the wind tunnel. Suspended by a steel fork, the test specimen was held in front of the testing apparatus to receive uniform flow. The post holding the fork was cased in a symmetric air foil to reduce the drag influence from the mounting elements. The weight on the scale was recorded before the test began and while the wind tunnel ran at the desired speed. The difference of the readings on the scale was the drag force on the mounting elements and the test specimen combined. Then the drag without the specimen was measured and subtracted from the previous value, eliminating the bias error from the vertical post’s drag. To verify that the low drag cart accurately measured the drag force of the mounted specimen, two types of objects were tested. The first was a round, thin disk with a face perpendicular to the flow. Because thin disks are well studied shapes in fluid mechanics [7], this was the primary means of assessing the testing procedure. The first verification test yielded results within 10% of the published values, indicating that the low drag cart was nominally accurate. The second verification procedure included 150 mm long cylinders with outer profiles correlating with the smooth model, Model A, and Model B. The physical models had the same cross section as those studied in [1], but the diameter was scaled up by 1.5 (from 25.4 mm in [1] to 38.1 mm) in order to achieve the same Reynolds number used in [1] while keeping the required wind speed within a range acceptable for the wind tunnel in use (23.3 m/s instead of 35 m/s in [1]). After verifying through dimensional analysis [7], the groove parameters were scaled proportionally to the cylinder diameter.