Turbulent flow over three square cylinders: Comparison between LES - - PowerPoint PPT Presentation

Turbulent flow over three square cylinders: Comparison between LES and DES

Naseem Uddin, Mahrukh Zaidi

Computational Thermo-fluid Laboratory Mechanical Engineering Department NED University of Engineering & Technology

• Introduction
• Some Applications
• Problem definition
• Computational Setup
• Results
• Conclusion
• N. Uddin, M. Zaidi

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• Flow separates
• Vortices formed
• Lift (L) and Drag (D) produces
• Vibration induces

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Streamline plot from LDV Time and spanwise average streamlines from LES

• Experimental
• Lyn et al. (1995)
• Numerical
• Sohankar (2006)

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Instantaneous vorticity contours for the tandem square cylinders

• Numerical
• Etminan et al. (2011)
• Numerical
• Nazari et al. (2009)

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• Stacks
• Tall buildings

(skyscrapers) in a city

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• Pipelines
• Electrical transmission

lines

• Bundles of tube in heat

exchanger

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• Computational Domain
• Domain Size
• 40w × 16w × 4w
• Computational Grid
• Hexahedral grids
• LES: 6.3 million
• DES: 3.2 million

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• INLET: Uniform Velocity inlet
• WALLS: No-slip at cylinder walls and top and bottom

walls of domain

• WALLS: Periodic boundary conditions in spanwise

direction

• For LES (Large Eddy Simulation)
• OUTLET: Convective outlet boundary condition
• For DES (Detached Eddy Simulation)
• OUTLET: Pressure outlet boundary condition

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• Dynamic Smagorinsky model is used. (Germano)
• FASTEST (Flow Analysis Solving Transport Equations

Simulating Turbulence)

• Space Discretisation: 2nd Order (Central Difference)
• Time Discretisation: 2nd Order Implicit (Crank-Nicolson

Method)

• Coupling of pressure & velocity fields: SIMPLE Algorithm
• Computations on HP Pro-Liant cluster in Thermo-fluid

laboratory of Mechanical Engineering department of NED University of Engineering & Technology

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• Spalart-Allmaras model is used
• Fluent ANSYS Inc. (version 12)
• Space Discretisation: 2nd Order (Central Difference)
• Time Discretisation: 2nd Order Implicit (Crank-Nicolson

Method)

• Coupling of pressure & velocity fields: SIMPLE Algorithm
• Computations on HP Pro-Liant cluster in Thermo-fluid

laboratory of Mechanical Engineering department of NED University of Engineering & Technology

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• The flow field over square cylinders is very complex due to

inherent flow unsteadiness and anisotropy.

• There are distinct coherent structures present in the separated flow

regions which are undulating in time and space.

• DES is capable of capturing 3D complex flow generated in the

wake of bluff bodies while consuming less time and computing resources as compared to LES.

• For the wall-to-wall spacing of 6w between the cylinders, the

drag coefficient and Strouhal number on the first square cylinder are similar to that of the experimental findings of Lyn et al. (1995) for the case of flow over single square cylinder.

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• D. A. Lyn, S. Einav, W. Rodi and J. H. Park. A laser-Doppler

velocimetry study of ensemble averaged characteristics of the turbulent near wake of a square cylinder. Journal of Fluid Mechanics 304:285-319, 1995.

• A. Sohankar. Flow over a bluff body from moderate to high

Reynolds numbers using large eddy simulation. Computers & Fluids 2006; 35:1154-1168.

• S. Stefan and T. Frank. Comparison of numerical methods applied

to the flow over wall mounted cubes. International Journal of Heat and Fluid Flow 23:330-339, 2002.

• M. R. Nazari, A. Sohankar, S. Malekzadeh, A. Alemrajabi.

Reynolds-averaged Navier-Stokes simulations of unsteady separated flow using the k-ɷ-v²-f model. Journal of Turbulence 34, 1-13, 2009.

• M. Germano, U. Piomelli, P. Moin, W. H. Cabot. A dynamic

subgrid-scale eddy viscosity model. Physics of Fluids A 3 (7):1760 - 1765, 1991.

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