Twenty-First Symposium on Naval Hydrodynamics (1997) / Chapter Skim
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The Wake of a Bluff Body Moving Through Waves
The Wake of a Bluff Body Moving Through Waves
Pages 592-604
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From page 592... ...
gation into the effect of free surface waves on In the first part of the study, the flow the vortex shedding and subsequent developfield is simulated at low Reynolds numbers ment of the wake of a vertical cylinder transand assuming locally two dimensional flow. rating steadily under conditions such that the The rlumerical simulation is based on a vor- forward speed exceeds that of the particle ortex (particle)
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From page 593... ...
, and moreover there is only a single configuration of horizontal to vertical vortex spacing for which this neutrally stable configuration holds; all other spacing ratios are unstable. For an assumed two-dimensional flow and an infinite periodic point vortex model, the imposition of an additional tw~dimensional approximation to simulate the effect of the wave motion on the point vortices as described by mechanisms (a)
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From page 594... ...
The solution adopted ~ to split the tempm rat and spatial components of the phase of the wave so that the potential velocity field am plied at the centroids of the computational elements consists of a steacly free stream component plus a planar oscillatory component, whilst the velocity applied to the point vortices also includes the spatial component. The potential velocity field around a cylinder of diameter D = 2 is therefore given by: A — ~ — A — Y — up(x, t)
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From page 595... ...
Figures 3 to 10 are plots of point vortices for all weak wave cases presented here, and show the vortex structures of the nearwake. The wakes are shown to a downstream distance of about 30 diameters.
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From page 596... ...
Often, the vortex pair results from the interaction of three closely spaced vortices; such a vortex triplet may be seen at z/D ~ 24 in figure 8. In weak wave cases the vortex pairs are not able to travel too far in the traverse sense clue to the small differences in vortex strengths and inter-vortex spacings.
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From page 597... ...
The howisfromiefttoright,and the of the transverse velocity in the near wake in- waves are propagating from left to right. dicate that for cases 3,5 and 7, the vortex shedding is locked on to twice the wave frequency.
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From page 598... ...
the wave frequency is exactly twice the natural shedding frequency. The wake structure consists of alternately shed vortices of opposite sign; however, the transverse inter-vortex spacing is large and has a value of 2.75D in the near wake.
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From page 599... ...
This flow was iniding frequency the wake is seen to consist of tially studded experimentally by Caster (1969) , a regular line of closely formed vortex pairs, and has been shown to demonstrate threw which move in a transverse flow sense un- dimensional Y-type structures in the wake, der their mutually induced velocity field (fig- which arise through cellular shedding vortex ure 21~.
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From page 600... ...
Velocity vectors at various spanwise locations Figure 28 An experimental visualization of this Bow is performed for an identical tapered cylinder geon~etry. The method used is an electrolytic precipitation technique, as detailed in Honji, Taneda and Tatsuno (1980~.
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From page 601... ...
Precipitate in the wake of a tapered cylinder Bi-cellular shedding Figure 30 A three-dimensional numerical simulation of the towed cylinder in waves has also been carried out. The free surface is approximated by the usual linearised wave boundary condition.
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From page 602... ...
More insight may be gained by form ing iso-vorticity surfaces from the data, and examining the way in which these 'structures' behave in the wake. A series of flow visualisation experiments are carried out in the 20m wave flume in the Department of Aeronautics, Imperial College to examine the free surface vortices of the wake of a vertical cylinder translating steadily into waves.
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From page 603... ...
The formation of the first vortex pair in the numerical data occurs further from the cylinder than in the experimental data; indeed, the entire wake pattern is more stretched in the longitudinal sense, and the vortex pairs themselves travel a smaller distance in the transverse sense. This may be a result of the difference in Reynolds number between the two sets of data.
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From page 604... ...
These data are in good qualitative agreement with both data in the literature on vertical cylinders in steady flow undergoing in-line oscillations, and surface flow visualization data conducted in the Department of Aeronautics wave flume. A 3-D computation proved computationally expensive, but demonstrated reasonable agreement with the 2-D computations, and provides useful experience for future research in this direction.
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