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.

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