# LES channel flow

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 February 5, 2010, 11:52 LES channel flow #1 New Member   Ilia Popov Join Date: Feb 2010 Posts: 2 Rep Power: 0 Helllo all! I am trying to simulate channel turbulent flow with LES. I am using a box of size 6x2x4 ( x is flowwise, z is spanwise, y is normal to walls). Boundary conditions are cyclic in x and z directions. Noslip conditions at walls. I set mu = 1/180, pressure gradient gradP = 1, and delta t = 1e-3. (This gives Re_tau = 180). The mesh size is 32*32*32 I am using channelFoam solver modified to keep gradP constant. As initial conditions I use boxTurb utility. LES model i use is Smagorinsky with vanDriest damping. The question is: What discretisation schemes shall I use? I tried to use MUSCL scheme and CrankNicholson time scheme but there was some standing waves in flowwise with wavelength of 4 cells. Then I tried to use some simplier schemes, like Euler and linear, the solution becomes laminar. Could you suggest what schemes shall i use to both avoid standing waves and resolve turbulence?

 February 9, 2010, 05:10 #2 Senior Member   Eugene de Villiers Join Date: Mar 2009 Posts: 725 Rep Power: 14 For a channel flow, I suggest backward for time and linear for U.

 February 9, 2010, 06:00 #3 New Member   Ilia Popov Join Date: Feb 2010 Posts: 2 Rep Power: 0 Thank you! So, you recommend using the same schemes as in the channelFoam tutorial. But why MUSCL gives not good results in this case and you do not recommend using it?

 February 9, 2010, 12:42 #4 Senior Member   Eugene de Villiers Join Date: Mar 2009 Posts: 725 Rep Power: 14 I have never tried MUSCL for LES, but it includes upwind components, so it will dissipate a lot of energy from the system which makes it unsuitable for conventional LES.

 June 16, 2010, 16:46 #5 New Member   Matt James Join Date: Jun 2010 Location: Marinette,WI, USA Posts: 25 Rep Power: 9 Hi Ilia, How did you go about modifying channelFoam with regards to maintaining a constant pressure gradient?

 June 17, 2010, 06:12 #6 Senior Member   Eugene de Villiers Join Date: Mar 2009 Posts: 725 Rep Power: 14 The current channelFoam modifies the imposed gradP in a goal-seek fashion to try and maintain a fixed flow rate. To apply a fixed gradP, you just have to change the code to use a constant gradP. Very simple.

 April 8, 2011, 16:39 #7 New Member   David Thériault Join Date: Nov 2010 Posts: 8 Rep Power: 8 Hello everyone I think I'm at the right place. I try to maintain a constant pressure gradient in a solver derived from channelFoam. I encounter some difficulties and I want to know if it comes from my modification, maybe someone could help me. The pertinent part of this code is: while (runTime.run()) { #include "readTimeControls.H" #include "readPISOControls.H" #include "CourantNo.H" #include "setDeltaT.H" runTime++; Info<< "Time = " << runTime.timeName() << nl << endl; Info<< "Coriolis parameter = " << fc.value() << nl << endl; Info<< "pressure gradient = " << gradP.value() << nl << endl; sgsModel->correct(); fvVectorMatrix UEqn ( fvm::ddt(U) + fvm::div(phi, U) + sgsModel->divDevBeff(U) + (fc^U) == -gradP ); if (momentumPredictor) { solve(UEqn == -fvc::grad(p)); } ................... Where graP is declared as a constant vector. When I use this solver, several timeStep after reaching a turbulent flow, the third component of fvc::grad(p) accelerates as a formula 1 even if I had declared a zero in the third component of my constant gradP. My question is: knowing that I'm pretty sure that problem doesn't come from the other source term (fc^U), my modification appears to be legit or not? A hint could be very useful. Thanks

 April 18, 2011, 16:58 #8 New Member   David Thériault Join Date: Nov 2010 Posts: 8 Rep Power: 8 I found it, beginner's problem!

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