1. The models were tested in a low speed, low turbulence, return type wind tunnel. where the air speed can be varied from 4-150 ftlsec. with a turbulence level less than 0.1%. The pressure differential across the contraction section of 7:l ratio can be measured on a Betz micromanometer with an accuracy of 0.2 m. of water. The rectangular cross-section, 36 in. x 27 in., is provided with 45' corner fillets which vary from 6 in. x 6 in. t o 4.75 in. Y 4.75 in. t o partly compensate for the boundaq layer growth. The spatial variation of mean velocity in the test section was found t o be less than 0.25%. The tunnel is powered by a 15 horsepower direct current motor driving a commercial axiflow fan with a Ward-Leonard system of speed control. Fioure 1. Constructional details of models:
2. Test Procedures
3. The Strouhal number based on projected height (h) showed comparatively less dependence on the angle of attack (a) in the Reynolds number range investigated. An observation concerning the Stmuha 1 number variation at low angles of attack is pertinent here. For the thicker ellipse a slow rise in the projected height with a small drop in shedding frequency leads to an almost uniform Strouhal number in this range. On the other hand, for the thinner ellipse the very sharr, drop in shedding frequency off-sets the relatively greater rise in projected height thus leading to a decrease in Strouhal number. The similar tendency was also observed by Schramm 15. In all cases except one Schramm's measurements were limited t o 0-45" range due to, as reported by him, "lack of stable vortex street". This is in contrast t o the strong and w ell defined fluctuating signals observed in the test v results presented here.
4. For both ellipses, the effect of varying the Reynolds number in the range 3 x lo 4 - 105 was investigated for 01 = 0, 30', 60', 90". The fluctuating pressure measurements were taken at four taps, two in the laminar and other two i n the separated flow regions. The results are platted in Figure 10. These curves represent the percentage variation about the mean of the'pressure coefficient at each of the ports i n question.