Twenty-First Symposium on Naval Hydrodynamics (1997) / Chapter Skim
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Bluff Body Hydrodynamics
Bluff Body Hydrodynamics
Pages 561-579
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From page 561... ...
Vortex shedding patterns from a circular cylinder in oscillatory flow are reviewed. The potential for applying CFD in bluff body hydrodynamics is discussed and results are presented for the forces on a fixed cylinder and for the response of a flexible cylinder.
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From page 562... ...
, . , ~ ~ 1.0 2.0 S.0 4.0 logic Re 6.0 7.0 Figure 1 Base Pressure Coefficient versus Reynolds Number for a Circular Cylinder (Roshko (11)
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From page 563... ...
For example, Szepessy (5) has investigated the correlation of vortex shedding from a circular cylinder at a high subcritical Reynolds number.
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From page 564... ...
on a circular cylinder at Reynolds numbers up to about 150, have all shown that two-dimensional vortex shedding may only be obtained if the cylinder end conditions are very carefully controlled. At higher Reynolds numbers it is still possible to influence the flow by adjusting the end conditions but it is no longer possible to preserve the instantaneous flow in a two dimensional state.
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From page 565... ...
Flow visualisation was performed at a Reynolds number of 2,500 in a water flume. As an initial comparison between the various models, the spanwise distribution of mean base pressure was measured.
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From page 566... ...
The existence of a dual frequency characteristic has been demonstrated by measuring power spectra of fluctuating velocity but what does this means from a physical point of view? We cannot be sure that both of the main peaks in a power spectrum of velocity necessarily reflect the frequency of vortex shedding (at some time instants, at least)
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From page 567... ...
One shedding cycle later, at (b) , Me cell at the peak has delayed its vortex shedding by a small amount.
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From page 568... ...
to (h) antisymmetric vortex shedding mode In the antisymmetric mode, shown in figure 8 (e)
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From page 569... ...
For example, a wave field may be idealised as a regular train of unidirectional waves or, carrying the idealization a stage further, the ambient flow may be replaced by a planar, harmonic, oscillatory flow. The latter simplification is the equivalent of using a strip theory approach to predict the loading on a vertical, surface-piercing, circular cylinder in waves.
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From page 570... ...
(22) , have studied vortex shedding patterns developed by a circular cylinder in oscillatory flow.
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From page 571... ...
How best to predict the correlation length remains an open question. NUMERICAL SIMULATION OF OSCILLATORY FLOW PAST A CIRCULAR CYLINDER Several numerical solutions of the unsteady, two dimensional Navier Stokes equations for flow around a circular cylinder at low KC and relatively low ,8 parameter have been reported in the literature.
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From page 572... ...
Vortex shedding patterns at low values of ~ have been observed by Tatsuno and Beannan (38) using flow visualization and these are also available for co mpan son with numerical simulations.
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From page 573... ...
However, it is interesting to note from figure 12, where measured and computed root mean square inline force coefficients are plotted, that the predicted r.m.s force coefficients agree well with the experimental results throughout the KC range examined. At larger KC (16 - 30)
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From page 574... ...
These can then be compared with actual flow visualizations performed in water, such as those by Tatsuno and Bearman (38~. With the discrete vortex method, recording the positions of the point vortices provides a relatively simple method of visualising the flow.
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From page 575... ...
.. ~ .7 Figure 14 Numerical flow visualization for KC - 10 and ,3 = 70 THE RESPONSE OF FLEXIBLE CYLINDERS IN OSCILLATORY FLOW A flexible circular cylinder mounted normal to a steady current flow is susceptible to flow induced vibration, depending on the level of damping and the non-dimensional mass ratio M/p D2, where M is the mass per unit length of the cylinder and p is the density of the fluid.
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From page 576... ...
In the transverse direction the cylinder is forced to oscillate at an integer multiple of the flow frequency since the vortex shedding force is also at a multiple of the flow frequency.
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From page 577... ...
Vortex shedding is directly responsible for vibrations in the transverse direction and as the cylinder frequency ratio increases so the Keulegan Carpenter number for maximum response increases. CFD is being increasingly used to study circular cylinders in oscillatory flow and in order to validate these codes more experimental data is required, particularly at low values of p.
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From page 578... ...
& Bearman P.W., "Aspect Ratio and End Plate Effects on Vortex Shedding from a Circular Cylinder, J.Fluid Mech., Vol.
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From page 579... ...
and Mackwood P.R., "Measurement and Prediction of Response of Circular Cylinders in Oscillating Flow, Proc.
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