Force on a circular crylinder in an elliptical orbital flow at low Keulegan-Carpenter numbers

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Force on a circular crylinder in an elliptical orbital flow at low Keulegan-Carpenter numbers

Chen, M.M. / Dalton, C. / Zhuang, L.X.

The orbital flow of a fluid past a circular cylinder is just one example of the many different flow situations that occur when the structural members of an offshhore platform are subjected to wave action. The most common fluid loading problem occurs when a vertical member is subjected to wave motion which is typically represented as an oscillating planar flow. Orbital flow occurs when a horizontal member is exposed to wave action which provides both horizontal and vertical components of velocity and acceleration. The flow structure and force acting on a horizontal cylinder exposed to uniform orbital flows od varying ellipticity have been determined by a finite-difference solution of the 2-D problem using vorticity/stream-function variables. The Keulegan-Carpenter number (KC) for the analysis is two or less with frequency parameter (beta) values of 483 and 2300. For a uniform circular orbital flow, the calculated results are compared to previous analytical and computational results with excellent agreement in most respects. For an uniform elliptical orbital flow, a series of calculations was done for three sets of parameter values, letting only the orbital ellipticity vary from circular to rectilinear. No change in the maximum total force occurred at KC = o.1 for increasing values of ellipticity, but, for KC of 1 and 1.02, the maximum total force decreased with increasing values of ellipticity (E) at a given value of beta and E has a strong influence on the rate of decrease. The maximum total force also decreased at a given value of E as beta increased for constant KC; this decrease in force may be explained by the difference in flow patterns between the two values of beta. The formation of a small separation and reattachment region is observed for E >= 0.8 when KC = 1.02 and beta = 2300; this flow feature is evident from the spikes present in the force curve and from the surface vorticity distribution.