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The prediction of vortex shedding past bluff and smooth bodies continues to receive much attention due to the frequent occurence of such flows in the field of structural, mechanical and offshore engineering. A major challenge for Computational Fluid Dynamics (CFD) is the reliable prediction of the fluctuating surface pressures and the magnitudes and frequency of the drag and lift forces associated with the shedding of vortices from such bodies.

PHOENICS V3.6.1 is used to make Large Eddy Simulations (LES) of the unsteady flow around a square cylinder at a Reynolds number of 22,000, as studied experimentally by Lyn [1992]. The Smagorinsky Sub-Grid-Scale (SGS) model, with an empirical coefficient of 0.1 and no wall damping, is used to model the unresolved part of the turbulent motion. For computational economy, two-dimensional rather than fully three-dimensional LES computations are carried out so as to cover roughly 14 shedding cycles. The cylinder dimension is 0.4m, and the free-stream water velocity is 0.535m/s.

The computations are initiated from solution fields produced by a 2000-sweep, unconverged steady computation. The linear upwind scheme is used for the momentum equations, and the near-wall region is bridged by standard wall functions. The simulation is advanced in time for 7 seconds using a uniform time step of 1ms. The INFORM facility is used to reduce gradually with time the number of sweeps per time step. The run takes 2.25 hours to complete the 7000 time steps on a 3GHZ PC with 2GB RAM.

The main results of the LES are compared with the measurements below:
LES      Data
 Strouhal number  0.15      0.13  
 Mean Drag Coefficient   2.17      2.10  
 RMS Drag Coefficient   0.14      0.12  
 RMS Lift Coefficient    1.17      1.20  

The fluctuating lift coefficient produced by the LES is displayed below:

For further information, please contact:

Mike Malin
Technical Support Manager
40 High Street,
London, SW19 5AU

Tel:   +44 (0)20 8947 7651
Email: support@cham.co.uk

December 2005: PHOENICS V3.6.1

The steady precursor Q1 file can be downloaded by clicking here.

The unsteady LES Q1 file can be downloaded by clicking here.