ICCAM2009

14
Streamwise oscillations of a cylinder beneath a free surface:
Part 2. Free surface effects on fluid forces
Canan Bozkaya
email: canan@mun.ca
Department of Mathematics and Statistics
Memorial University of Newfoundland
A1C 5S7, St. John’ s
Canada
(Joint work with: Serpil Kocabiyik)
Abstract: This study presents the results of a two-dimensional computational study of
the free surface flow past a circular cylinder forced to perform streamwise oscillations in
the presence of an oncoming uniform flow. In Part 1, we have examined the effects of the
inclusion of the free surface on the vortex-shedding modes in the near wake region for
the same problem at the Reynolds number of R = 200 for fixed displacement amplitude
A = 0.13 when the forcing frequency-to-natural shedding frequency ratio, f/f0, ranges
1.5-3.5. The numerical simulations are carried out using basically the same two-phase
flow model and finite volume code as that used in Part 1 for numerical simulation of the
unsteady Navier-Stokes equations in their primitive variable formulation. The objective
of this study is to examine the effect of the cylinder submergence depths, h = 0.25, 0.5,
0.75 and the frequency ratios, f/f0 =1.5, 2.5, 3.5 on fluid forces as well as the total
mechanical energy transfer at two values of the Froude numbers F r = 0.2, 0.4. The time
histories of the in-line (drag) and transverse (lift) force coefficients are plotted as well as
their power spectrum densities and Lissajous trajectories. The mean and the root-meansquare
lift and drag force coefficients, are also predicted to determine the free surface
effects on the fluid forces. It is interesting to note that irrespective of the values of h
and f/f0, the total mechanical energy transfer is negative, indicating the energy transfer
from the cylinder to the fluid unlike transverse oscillation case. However, the changes
in the absolute values of the energy transfer is observed depending on the values of h
and f/f0, resulting in variations in the amount of the mechanical energy transfer from
cylinder to fluid at each F r.