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Old   August 18, 2010, 09:17
Default Time Step in LES
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Saeed Salehi
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Hi every one. i am modeling a 3D airfoil at high Reynolds and high angle of attack in FLUENT.
I'm using LES for modeling turbulence. As you know LES is a transient model. My problem is that i don't know how my time step should be.
Is there any body help me?
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Old   October 21, 2010, 11:41
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I basically know nothing about LES but the CFX's Solver Modeling Guide states the following

First order fully implicit methods in time are usually too diffusive, and the turbulence is damped out. For highly unstable problems, such as cyclones, lower order methods may work, but the results will be very damped, unless very small timesteps are used.
For accuracy, the average Courant (or CFL) number should be in the range of 0.5-1. Larger values can give stable results, but the turbulence may be damped. For compressible flows where the acoustic behavior is being modeled (eg, for noise calculations), this conclusion still holds, but for the CFL number based on the acoustic velocity as well as the convective velocity.
1,000 - 10,000 timesteps are typically required for getting converged statistics. More steps are required for second order quantities (for example, variances) than for means. Check the convergence of the statistics. For a vortex-shedding problem, several cycles of the vortex shedding are required.
The implicit coupled solver used in CFX requires the equations to be converged within each timestep to guarantee conservation. The number of coefficient loops required to achieve this is a function of the timestep size. With CFL numbers of order 0.5-1, convergence within each timestep should be achieved quickly. It is advisable to test the sensitivity of the solution to the number of coefficient loops, to avoid using more coefficient loops (and hence longer run times) than necessary. LES tests involving incompressible flow past circular cylinders indicates that one coefficient loop per timestep is sufficient if the average CFL number is about 0.75. If the physics or geometry is more complicated, additional coefficient loops (3-5) may be required.

Good luck!
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Old   October 21, 2010, 17:32
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Michael P. Owen
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If you are using LES for a high Reynolds number model, you are doing it wrong.
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