Hydrodynamic force coefficients for rectangular cylinders in waves and currents

Venugopal, Vengatesan (2002) Hydrodynamic force coefficients for rectangular cylinders in waves and currents. PhD thesis, University of Glasgow.

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The research into hydrodynamic loading on ocean structures is concentrated mostly on circular cross section members and relatively limited work has been carried out on wave loading on rectangular sections, particularly in waves and currents. This research work is therefore carried out focussing on the evaluation of hydrodynamic force coefficients for sharp edged rectangular cylinders of various cross-sections (aspect ratios), subjected to waves and currents. Three cylinders with three different cross-sections are constructed and tested vertically, as surface piercing and horizontally, as fully submerged with the cylinder axis parallel to the wave crests. The aspect ratios considered for this investigation are 1.0, 112, 2/1, 3/4 and 4/3. The length of each cylinder is 2000mm. The sectional loadings are measured on a 100mm section, which is located at the mid-length of the cylinder. The forces are measured using a force measuring system, which consists of load cells, capable of measuring wave and current forces. The in-line & transverse forces (for vertical cylinders) and horizontal & vertical forces (for horizontal cylinders) have been measured. For horizontal cylinder, to study the effect of depth of variation on submergence of the cylinder, the tests are carried out for two depths of submergence. The experiments are carried out at the Hydrodynamic Laboratory, Department of Naval Architecture and Ocean Engineering, University of Glasgow. The tests are carried out in a water depth of 2.2m with regular and random waves for low Keulegan-Carpenter (KC) number up to 4.5 and the Reynolds number varied from 6.397xl03 to 1.18xl05 • The combined wave and current effect has been produced by towing the cylinders in regular waves, along and opposite to the wave direction at speeds of ± 0.1 mis, ± 0.2 mls and ± 0.3 mls. Based on Morison's equation, the relationship between inertia and drag coefficients are evaluated and are presented as a function of KC number for various values of frequency parameter, {3. For the vertical cylinders, the drag coefficients decrease and inertia coefficients increase with increase in KC number up to the range of KC tested for all the cylinders. For the horizontally submerged cylinders, the drag coefficients showed a similar trend to vertical cylinders, whereas the inertia coefficients decrease with increase in KC number for all the cylinders. This reduction in inertia force is attributed to the presence of a circulating flow [Chaplin (1984)] around the cylinders. The random wave results are consistent with regular wave results and the measured and computed force spectrum compares quite well. While computing the force coefficients in the case of combined waves and currents, only the wave particle velocity is used, as the inclusion of current velocity tends to produce unreliable drag force coefficients. For vertical cylinders, the drag and the inertia coefficients in combined waves and currents are lower than the drag and the inertia coefficients obtained in waves alone. For horizontal cylinders the drag coefficients are larger than those obtained for waves alone and the inertia coefficients are smaller than those measured in waves alone. The Morison's equation with computed drag and inertia coefficients has been found to predict the measured forces well for smaller KC numbers. However, the comparison between measured and computed positive peak forces indicate that the computed forces are underestimated. It is suggested that if the wave particle kinematics are directly measured, this discrepancy between measured and computed forces might well be reduced. Wave excitation forces are also reported in non-dimensional forms in the diffraction regime, using 3D-Green function method. Wave induced pressure distribution around the cylinder in regular waves have been measured and are reported as normalised pressures. Wave run-up on the cylinder surfaces has been measured and simple empirical formulae are presented for run-up calculations on the cylinder surfaces. The results of this investigation show that the cylinder aspect ratio plays major role on hydrodynamic force coefficients, dynamic pressure distribution and on wave run-up on cylinder surfaces.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Subjects: T Technology > TC Hydraulic engineering. Ocean engineering
Colleges/Schools: College of Science and Engineering > School of Engineering > Naval Architecture and Marine Engineering
Funder's Name: UNSPECIFIED
Supervisor's Name: Varyani, Dr. K.S.
Date of Award: 2002
Depositing User: Ms Anikó Szilágyi
Unique ID: glathesis:2002-6351
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 13 May 2015 08:45
Last Modified: 13 May 2015 08:52
URI: http://theses.gla.ac.uk/id/eprint/6351

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