I want to use LES to simulate the flow past a sphere at high Re by non-iterative PISO method, which is much faster than iterative method. However, the result of non-iterative PISO is far from the result in literatures, especially in Strouhal number. Can this method stable and accurate enough to apply to LES in turbulent flow? How can I improve the accuracy?
Try the NITA based on fractional time step and compare to see if you have better solutions.
The turbulence phenomenon is basically and inherently time-dependent. Indeed, an iterative calculation would obviously give acceptable results in terms of the real physics of the problem. However, a non-iterative method may be able to capture the results; but, to compare your simulation, both studies should be performed using the non-iterative scheme.
In my opinion, performing the FFT (to compute the Strouhal number) is more reasonable when the solution is converged with a rational number of iterations.
The selection of the pressure coupling scheme (i.e. PISO) may also affect the results in the case of non-iterative.
Besides, try setting acceptable convergence criteria as well as rechecking your computational grid.
I agree with the Ehsan Bakhtiari that the turbulence behavior is sensitive to the time as an independent variable. I would recommend using non iterative method and assigning an acceptable convergence criteria in solution controls.
I don't know about your geometry but if you have a complex geometry, you can try hybrid meshing (Hexahedral/Sweep meshing to uniform parts) along with convergence criteria.
Just to add more details:
https://www.afs.enea.it/project/neptunius/docs/fluent/html/th/node375.htm
The NITA version based on the fractional time-step is largely used to study incopressible turbulent flows. A lot of literature cab be found.
Filippo Maria Denaro, Ehsan Bakhtiari, Faheem Ejaz Thanks for your suggestions. I am running the LES on the case of grid number more than 5 million. Because of the limitation of computational resource, I can't try too many combinations in terms of schems, mesh, timestep size etc. on my cases to find out the best solution. Instead, in order to find the best solution of NITA, I apply the same case setting to much simpler cases (i.e. grid number much less) to fullfill the tests. Hopefully I can get the applicable solution asap.
Yes, you can use non-iterative methods for turbulent flows. As Filippo Maria Denaro has mentioned, you can go with NITA based on the fractional time-step. Earlier, I compared the mean as well as rmse velocity values, followed by the Strouhal number using both the iterative method (using SIMPLEC) as well as the non-iterative method (using NITA), and got identical results. I am sharing a few of my papers where I have used this approach (though you could find a lot of papers on non-iterative methods).
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