Vortex Shedding
Flow around rectangular cylinders

PhD student: Ahmad Sohankar
lada@chalmers.se
Supervisor: Lars Davidson
lada@chalmers.se
Cooperation: Dr. Christoffer Norberg
Sponsors: The Ministry of Culture and Higher Education of Iran
Publications: [1-8], see references below
Start of project: spring 1994
End of project: December 1998


THE PROJECT
The subject of flow past slender bluff bodies is of relevance to technical problems associated with energy conservation, structural design and acoustic emissions. In this work, calculations of unsteady 2D- and 3D-flow around rectangular cylinders, which are a type of slender bluff bodies, are carried out. Some useful physical quantities such as the dominating wake frequency (Strouhal number), mean and RMS values of lift, drag and moment, various surface pressure were calculated for different Reynolds number.
 
In 2D-calculations, flow around rectangular cylinders were performed. An incompressible SIMPLEC code is used employing non-staggered grid arrangement. The QUICK and Van Leer schemes are used for the convective terms. The time discretization is implicit and a second-order Crank-Nicolson scheme is employed. The influence of Reynolds number(Re=45-500), body side ratio (B/A=1-4) and angle of incidence (alpha=0-90) is investigated. Effect of the various numerical parameters such as time step, domain size, blockage, grid distribution and spatial resolution in both far- and near-body regions is investigated. At outlet of the computational domain, a convective Sommerfeld boundary condition is compared with a traditional Neumann condition. The onset of vortex shedding is investigated by using the Stuart-Landau equation, at various angles of incidence for a square cylinder.
 
In 3D-calculations, direct numerical simulation (DNS) of unsteady flow around a square cylinder at zero incidence for moderate-Reynolds numbers (Re=150 - 500) and large eddy simulation (LES) at Re=22000 are performed. A non-staggered grid arrangement, incompressible, finite-volume code is used employing an implicit fractional step method with a multi-grid pressure Poisson solver. A second-order central scheme is used for the convective and diffusion terms. The influence of the aspect ratio, finer grid and time step on the results are investigated for DNS. By DNS simulations, the study of transition from 2D- to 3D-flow, the wake structure, A- and B-mode of secondary vortices and a comparison of 2D and 3D results with experimental results are carried out. Also some dissimilarities with the flow around a circular cylinder are investigated.
 
In LES simulations, three different Subgrid-scale models, the Smagorinsky, dynamic and dynamic one-equation models for Re=22000, are applied and their results are compared with experimental results.
 
 

 
REFERENCES
 
- click to view postscript file
 
  1. A. Sohankar, L. Davidson, C. Norberg, Vortex Shedding: Numerical Simulation of Unsteady Flow Around a Square Two-Dimensional Cylinder, " 12th Australian Fluid Mechanics Conference", pp. 517-520 Sydney, Australia, Dec. 1995.
    View PDf file
     
  2. Sohankar, A., Norberg, C., Davidson, L.,
    "A Numerical Study of Unsteady Two-Dimensional Flow Around Rectangular Cylinders at Incidence", Rept. 96/25, Thermo and Fluid Dynamics, Chalmers University of Technology, Göteborg, 1996.
    View PDF file
     
  3. A. Sohankar, C. Norberg, L. Davidson, "Numerical Simulation of Unsteady Low-Reynolds Number Flow Around Rectangular Cylinders at Incidence", J. Wind Engng. and Ind. Aerodyn. Vol. 69-71, pp. 189-201, 1997.
     
  4. A. Sohankar, C. Norberg, L. Davidson, "Low-Reynolds Flow around a Square Cylinder at Incidence: Study of Blockage, Onset of Vortex Shedding and Open Boundary Condition, "Int. J. of Numerical Methods in Fluids", Vol. 26, pp. 39-56(1998).
     
  5. A. Sohankar, C. Norberg, L. Davidson, "Simulation of Unsteady Three-Dimensional Flow around a Square Cylinder at Moderate Reynolds Numbers", Phys. of Fluids, Vol. 11, No. 2, pp. 288-306, Feb. 1999
     
  6. Sohankar, A., Davidson, L. and Norberg, C.
    "A Dynamic One-Equation Model for Simulation of Flow Around a Square Cylinder", Engineering Turbulence Modelling and Experiments -- 4, pp. 227--236} W. Rodi and D. Laurence (Editors), 1999.
    View PDF file
     
  7. Sohankar A., Norberg, C. and Davidson, L.,
    "Numerical Simulation of Flow past a Square Cylinder", ASME-JSME Fluid Engineering Division Summer Meeting , San Francisco, July, 1999.
    View PDF file
     
  8. A. Sohankar, L. Davidson, C. Norberg,
    "Large Eddy Simulation of Flow Past a Square Cylinder: Comparison of Different Subgrid Scale Models", ASME J. Fluids Engng., Vol. 122, No. 1, pp. 39-47, 2000.
    (see also Erratum, ASME J. Fluids Engng., Vol. 122, No. 3, p. 643, 2000)
     

 


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