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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Abstract

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
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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