The complex vibrations and implied drag of a long oceanographic wire in cross-flow |
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| Authors: | C M Alexander |
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| Institution: | University of California, San Diego, Marine Physical Laboratory of the Scripps Institution of Oceanography, La Jolla, CA 92093, U.S.A. |
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| Abstract: | A cylinder in cross-flow experiences periodic forcing related to the shedding of vortices in its wake. Long wires in the ocean vibrate in response to this forcing, but their length and damping is such that boundary conditions do not generally apply to solutions of the equations of motion and their vibratory behavior at one point is of little consequence at remote points. The bulk of previous studies of vibrating wire characteristics have utilized only short wires or cylinders in the laboratory, so have not considered or even permitted this phenomenon.During a typical deployment of the Scripps Deep-Tow survey system to 2800 m a small 2-axis accelerometer package was attached to the tow wire at a depth of 30 m and its output recorded in a diver-operated vehicle about 1 m downstream. Analysis of this data produced sharply peaked spectra with the frequencies of vibration in the direction of flow twice those across the flow. Good correlations were found between mean amplitudes across the flow and the corresponding peak frequencies, and between mean amplitudes across and aligned with the flow. Preference for a specific phase relation between motions in the two planes suggests that vortex shedding occurs progressively and forcing has the form of a complex wave travelling down the wire.To investigate the implications of the preceding, a tow-tank fixture was built which permitted reproduction of amplitude, frequency and phase relations in a cylinder representing a point on the wire. A series of runs were then made to measure drag force directly and obtain an empirical drag law. A constant drag coefficient of 1.8 was found in the Reynolds number range 7000–12000, contrary to other investigations that have ignored vibrations in the flow direction. |
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