Fluctuating Lift Coefficient for a Circular Cylinder in Cross Flows

Abstract

Fluctuating lift forces experienced by cylinders of diameters 1, 1.125, 2, 3 and 4 in, and length-to-diameter ratios ( l/d) of 0.25 to 9 have been measured for Reynolds numbers in the range Re = 104to 2.5 times 105. Fluctuating lift and drag coefficients for the limiting case of l/d ŕ 0 were obtained by integrating measured values of the fluctuating surface pressure at points distributed around a horizontal section of the tube. At lower Reynolds numbers, lift coefficients for cylinders having l/d < 1 were found to agree within the limits of experimental accuracy with the values obtained by surface pressure measurements. However, at the Reynolds number of 105, the lift coefficient for cylinders of l/d = 0 were much higher than the lift coefficients for cylinders of l/d = 1; this clearly demonstrates the incoherence of the flow at Re = 105. A measure of the extent to which vortex shedding is in phase along the length of a tube is obtained as a length scale for the coherence of vortex shedding. This was inferred from the variation of the measured value of the lift coefficient as a function of l/d. Within this length scale there is a high degree of correlation between signals from two points. Direct measurement of the length scale obtained by measuring u' correlations gave a value of the same order of magnitude but some 50 per cent lower than the length scale obtained from the measurements of the lift coefficient.