Hello All!
I am doing LES for flow around the 2d circular cylinder in fluent. I want to verify the Navier-stokes from my CFD results. Can somebody guide me on how I can do that? Thanks!
Do you want to check if your flow field satisfies NS equations?
You have your fields like u(x,y,z,t), p(x,y,z,t), rho(x,y,z,t), viscosity, etc. Plug them into the NS equations and see whether they are satisfied or how far from zero is the residual, that would give you the measure of how far off you are with your flow field from NS equations. Of course you will need a way to compute differential operators like grad, div. This can be done numerically (for example, using finite differences) or in some visualization software packages like ParaView or if you import your flow field into OpenFOAM you can use the available differential operators.
Anton Ishmurzin Thanks! I also want to ask that, do I need to unfilter the velocities because LES provides filtered velocities or viscosities?
There is no meaning in inserting the LES solution into NS equations...They are not satisfied by a theoretical point of view, until the filter width tends to zero. Furthermore, the numerical discretization of the NS time and space operators should be exactly the same of the LES solution.
Filippo Maria Denaro And how we can confirm that the filter width tends to zero? And secondly I want to confirm that is it good enough to perform 2D LES for 2D case or should I perform 3D LES and convert the results in 2D case. Since my work is mostly focus on 2D case, but LES perform spatial filters on 3D, so its mean 2D LES has no sense to be used. Thanks for your precious time!
The filter width depends on the formulation: implicit or explicit filtering. In a software like Fluent the approach to the filtering is implicit and the filter width tends to zero according to the grid size tending to zero. This way the LES solution tends to be a solution of the unfiltered NS equations.
The only congruent way to work is performing the 3D LES and then averaging in time the results to get a statistically steady solution according to the 2D RANS.
Filippo Maria Denaro Can you please explain little bit that how I can get 2D RANS solution from 3D LES? Thanks!
The 3D flow around a cylinder having an infinite extension in spanwise direction is 2D in statistical sense, that is the fluctuations in the third direction have zero statistical components. Therefore, the 3D solution (could be DNS/LES/URANS) when averaged in time over a long period will drive to 2D statistical fields
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