Twenty-Fourth Symposium on Naval Hydrodynamics (2003) / Chapter Skim
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An Experimental and Computational Study of Three-Dimensional Unsteady Flow Features Found Behind a Truncated Cylinder
An Experimental and Computational Study of Three-Dimensional Unsteady Flow Features Found Behind a Truncated Cylinder
Pages 305-321
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From page 305... ...
Abstract The numerical prediction of three-dimensional turbulent separation regions and the resulting unsteady vertical flow patterns within these regions is still poor. This paper presents detailed experimental data for steady onset flow around a truncated cylinder of height/diameter ratio of 1.0, mounted on a ground plane.
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From page 306... ...
The mean flow around a truncated cylinder of aspect ratio 1 can be seen to be composed of three distinct flow features; that is the flow over the free end, the arch vortex and the horseshoe vortex, which interact strongly with each other, generating a fully three-dimensional flow. At a high Reynolds number, when the flow is fully turbulent there will be considerable unsteadiness in the wake region.
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From page 307... ...
Indeed restricting the domain reduces the uncertainty in the computations. Various measurements were made of the flow, including surface pressures, using pressure transducers, total forces using a strain-gauge dynamometer, surface flow visualisation, hot-wire anemometry in the wake, and particle image velocimetry to obtain details of the velocity
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From page 308... ...
3.2 Ground plate and upstream flow characteristics One of the problems of mounting a model directly on the floor of the wind tunnel is that of the boundary layer thickness having developed along the length of the contraction. This has been measured at 25mm or h/6 at the speed of these tests.
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From page 309... ...
The image size in the longitudinal plane was 292 x 233mm and in the transverse direction 225 x 180mm. In the transverse plane this was sufficient to cover the entire wake region near the model.
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From page 310... ...
Figure 5: Distribution of pressure coefficient around cylinder, UOO = 20m/s 4.3 Dynamic force measurements The mean drag coefficients were found at a range of speeds and are plotted in Figure 7. Here the drag coefficient uses the standard definition using the freestream velocity, UOO and the frontal area of the cylinder.
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Figure 4: Surface flow visualization pictures
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(f) Lift, UOO = 20m/s Figure 8: Power spectral density function of force signals 80 1 00 8c 100 80 1 00
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From page 313... ...
Figure 7: Mean drag coefficients 20 25 Figure 9: U velocity contours y/d = 0, measured by PIV ameters downstream they are quite large and in contact with the ground. In the instantaneous images the flow picture is much more chaotic as the counter-rotating vortices move around and other vortices appear in the wake region.
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From page 314... ...
;.,,, _ _._ -0.6 -0.4 -0.2 0 0.2 0.4 0.6 y/d (b) Velocity vectors x/d = 0.17 0.8 ~0.6 0.4 F 0.2 0.8 ~0.6 na 0.2 o Ficure 10: PIV measurements of vorticity and velocity v ...........
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From page 315... ...
(b) Instantaneous Figure 1 1: PIV measurements of velocity at x/d = 0.5 Figure 12: Numerical solution from CFX4.3 showing surface pressure contours and vector field behind cylinder played.
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From page 316... ...
The predicted local drag coefficients are widely scattered. The curves in Figure 15, which show the pressure distribution at z = 0.5 for the different cases, indicate that the pressure on the forward part of the cylinder is well predicted by all, but the base pressure at the rear is not.
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From page 317... ...
The flow separates from the cylinder at 70 degrees as can be seen from both the flow visualization and the pressure measurements. In the wake region the trailing vortices from the tip merge with the horseshoe vortex at the base, eliminating any von Karman vortex shedding.
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From page 318... ...
Luo, S., Gan, T., and Chew, Y., "Uniform flow past one (or two in tandem) finite length circular cylinders," Sumer, B., Christiansen, N., and Fredsce, J., "The Journal of Wind Engineering and Industrial Aerodynamics, horseshoe vortex and vortex shedding around a vol.
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From page 319... ...
4.0kamoto, S and Sunabashiri, Y., "Vortex shedding from a circular cylinder of finite length placed on a ground plane", Transactions of ASME~ Journal of Fluids Engineering, Vol.
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From page 320... ...
In the case of the low aspect ratio cylinder considered in this paper, there is no dominant periodic shedding pattern but the wake does contain large scale turbulent structures with apparently random behaviour. The advantage of LES over RANS methods is that it is able to simulate this unsteady behaviour, providing valuable information on the unsteady nature of the wake, which is entirely absent even in an unsteady RANS model.
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From page 321... ...
is being used to characterize the flow. Initial results suggest that the mean flow predicted by the URANS is never instantaneously realised perhaps emphasising again the necessity for carrying out LES calculations.
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