Forces on cylinders in relative fluid motion

Ashley, Richard Martin (1980) Forces on cylinders in relative fluid motion. MPhil thesis, Thames Polytechnic.

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Abstract

This thesis reviews current knowledge concerning the forces between circular cylinders and relatively flowing fluids. The kinematic conditions considered are these of steady, unidirectional unsteady, oscillatory, and that of monochromatic surf act- water waves incident upon a vertical surface piercing cylinder. The complicating effects of cylinder surface roughness and freestream turbulence are also considered.

Detailed investigation of an oscillating cylinder experiment carried out by the author, at high Stokes number (Reynolds divided by Keulegan Carpenter number), but covering a low range of Keulegan Carpenter number is presented. Following this, a discussion of wave tank and generator design, culminates in the design of such a facility for the testing of cylinders in progressive waves. These experimental examples are used in conjunction with model theory to explain the deficiencies and similarities between the various kinematic conditions above. The experimental utilization of simpler fluid kinematic conditions to model those that are more complex is shown to be possible within certain limitations. Guidance is given to facilitate selection of the most suitable experimental technique for the investigation of specific fluid-cylinder dynamic problems.

An original picture of vortex behaviour throughout a cycle of relative planar oscillatory motion is postulated based upon recorded circumferential pressure distribution history, and resultant in-line and lift forces.

The Morison equation, considered as a mathematical model which describes the cylinder-fluid dynamics, is shown to be reasonable for Keulegan Carpenter numbers less than 5 (in the inertia dominant regime), or greater than 25 (in the drag dominant regime). The equation is misleading in the intermediate region (5 to 25) where the drag and inertia force components are each of comparable importance. The neglect of the important transverse (with respect to relative flow direction) force component and the effects of flow history, contained in residual vorticity, are also shown to be important deficiencies.

The implications of this work for the understanding of the fluid mechanics of vertical circular cylinders in the sea are also considered.

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