Article

Free end effects on the near wake flow structure behind a finite circular cylinder

Journal of Wind Engineering and Industrial Aerodynamics (Impact Factor: 1.7). 12/2000; 88(2-3):231-246. DOI: 10.1016/S0167-6105(00)00051-9

ABSTRACT The free end effect on the near wake of a finite circular cylinder in a cross flow has been investigated experimentally. Three finite cylinders with aspect ratios (L=D) of 6, 10 and 13 were tested in a subsonic wind tunnel at a Reynolds number of 20 000. A hot-wire anemometer was employed to measure the wake velocity. Mean pressure distributions on the cylinder surface were also measured. The flow near the free end was visualized to observe the flow structure qualitatively in a circulating water channel. The experimental results from these finite cylinder (FC) models were compared with those of a two-dimensional circular cylinder. The flow past the FC free end shows a complicated three-dimensional wake structure. As the FC aspect ratio decreases, the vortex shedding frequency is decreased and the vortex formation region is elongated. The free end effect becomes dominant close to the FC free end. The three-dimensionality of the FC wake may be attributed mainly to the strong entrainment of irrotational fluids, caused by the downwash of counter-rotating vortices separated from the FC free end. The downwash flow is concentrated in the central region of the wake. A peculiar flow structure having a 24 Hz frequency component was observed near the free end using spectral analysis and cross-correlation of the velocity signals. This 24 Hz frequency component is closely related to the counter-rotating twin vortices formed near the FC free end.

1 Follower
 · 
382 Views
  • Source
    Conference Paper: Heine_et_al_PIV11
    [Show abstract] [Hide abstract]
    ABSTRACT: The wakes of wall mounted circular cylinders of very low aspect ratio have been investigated in a wind tunnel. Planer PIV and high resolution PIV were used to capture the flow phenomena downstream of the cylinder. An extensive database of different flow conditions was obtained by changing the velocity U¥ and the cylinder height h at a constant aspect ratio L = h=D. The resulting relative height of the boundary layer thickness d was 0:6 � h=d � 2:5, while the Reynolds number based on the length l between leading edge and cylinder centre Rel was 57;000 � Rel � 188; 000. A pair of counter-rotating vortices was found in the wake of the cylinder. The rotation sense is such, that in the center of the wake a positive vertical velocity component results. Horseshoe vortices were found next to these trailing vortices. The relative height of the incoming boundary layer has a large impact on the development, strength and dissipation of the vortex system.
    9TH INTERNATIONAL SYMPOSIUM ON PARTICLE IMAGE VELOCIMETRY - PIV11; 07/2011
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The wakes of wall mounted circular cylinders of very low aspect ratio have been investigated in a wind tunnel. Besides planar PIV, also high resolution tomographic PIV was used to capture the flow phenomena downstream of the cylinder. By changing the velocity U_inf and the cylinder height h at a constant aspect ratio h/D, a wide database of different flow conditions was obtained. The resulting relative height of the boundary layer thickness was between h/delta=0.6 and h/delta=2.5, while the Reynolds number based on the length l between leading edge and cylinder centre Re_l was between Re_l=57,000 and Re_l=188,000. A pair of counter-rotating vortices was found in the wake of the cylinder. The rotation sense is such, that in the center of the wake a positive vertical velocity component results. Also horseshoe vortices were found next to these trailing vortices. The relative height of the incoming boundary layer has a large impact on the development, strength and dissipation of the vortex system.
    9th International Symposium on Particle Image Velocimetry - PIV'11; 07/2011
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Numerous factors contribute to the three-dimensional behavior of the wake behind a finite cylinder with relatively large aspect ratio in a cross flow; among the more important ones are the trailing vortex shed from the free end and the necklace vortex formed at the cylinder base. The trailing vortex interacts strongly with the flow around the cylinder; thus giving rise to oblique vortex shedding and changes the flow structures, such as the vortex pattern, and the pressure distribution around the cylinder. Formation of a necklace vortex near the base of the cylinder could give rise to interaction between this vortex and the shed vortices along the cylinder span. Besides, the necklace vortex could even interact with the trailing vortex, depending on the cylinder aspect ratio. For aspect ratios greater than a critical value a suppressed two-dimensional region exists with vortex shedding resembling the Karman vortex street. In this paper, a lattice Boltzmann method is used to numerically simulate the flow past a finite cylinder with an aspect ratio of 10 at Reynolds numbers (Re) varying from 100 to 200. A major finding is that, even in this limited Re range, the wake behavior and flow-induced forces are greatly affected by Re. A suppressed two-dimensional region exists but its size decreases as Re increases. In addition, the effects of the trailing and necklace vortices on the variation of the Strouhal number, and the mean and root mean square drag and lift coefficients along the span are found to be affected by Re.
    Journal of Fluids and Structures 05/2005; 20(4):589-609. DOI:10.1016/j.jfluidstructs.2005.02.009 · 2.23 Impact Factor

Preview

Download
8 Downloads
Available from