[gtg627e]

Dear Forum,

just as an observation that I forgot to attach to the previous message, Uz velocity is indeed symmetric about the x-y plane. This gives me confidence that cyclic boundary conditions are being enforced correctly.


Thank you again,

Alessandro

[gtg627e]

Sorry, I meant to say:

"Uz velocity is indeed cyclic about the x-y plane"

Alessandro

[ehsan]

Hello

Could I kindly ask you whether applying "LES Model: laminar" in Les Property folder will create implicit LES in OPENFOAM? If no, how we should apply implicit LES in OPENFOAM?

Thanks in advance

[Andy_bm]

Originally Posted by eugene

2. Initial conditions: k should have a small positive value (1e-10)

Can you explain why initial conditions for k should have small value? And how to calculate initial conditions for k?

[msuaeronautics]

Originally Posted by Andy_bm

Can you explain why initial conditions for k should have small value? And how to calculate initial conditions for k?

K should always tend to zero at the wall, as it represents your modeled portion of turbulence kinetic energy. In the case of LES, K is the (modeled) turbulence energy of the subgrid scales. Since velocity -> 0 as we get down to the wall, so should the energy.

[Andy_bm]

Thanks,

What about nusgs near wall? Maybe there is a ratio between nusgs and nu? In oneeqeddy nusgs = c*delta*sqrt(k). If k -> 0, delta -> 0 as y+ ~ 1 => nusgs -> 0.But nusgs may varried depending on k and y+.And I don't understand what value of nusgs should be near wall.

[msuaeronautics]

Originally Posted by Andy_bm

Thanks,

What about nusgs near wall? Maybe there is a ratio between nusgs and nu? In oneeqeddy nusgs = c*delta*sqrt(k). If k -> 0, delta -> 0 as y+ ~ 1 => nusgs -> 0.But nusgs may varried depending on k and y+.And I don't understand what value of nusgs should be near wall.

nuSgs -> 0

is the correct BC for the wall (if, for nothing else, the equation you wrote there) and really is the only reasonable one.

How nuSgs varies as we start to move away from the wall (as y+ increases) depends on your method. You could either be using a wall function for nuSgs or you could have a sufficiently fine grid (suspect this would be the case for most people using LES) to properly represent it.

Please let me know if I'm not being clear.

[Andy_bm]

Now I have BC on wall for nusgs - zeroGradient, so I must set fixedValue = 0 if y+ ~ 1?
Sorry,I don't understand initial condition for k on WALL must be near 0 or initial condition on INLET must ~ 0?

[msuaeronautics]

Originally Posted by Andy_bm

Now I have BC on wall for nusgs - zeroGradient, so I must set fixedValue = 0 if y+ ~ 1?
Sorry,I don't understand initial condition for k on WALL must be near 0 or initial condition on INLET must ~ 0?

No need to apologize.
Yes, for nuSgs and y+ ~1, you may use nuSgs_WALL ~ 0.
Your condition for K_WALL should also be near 0.

I am assuming you are using 1-equation eddy. The condition for inlet can be K_INLET ~ 1e-5 or 1e-6. In other words, some small value but not zero. It really depends on your case. The only time I've used 1-equation eddy I used that condition and got good results.

[Andy_bm]

Thanks for your answers!

My case is flow past cylinder.I have mesh with y+ 0.1-1. And I have following problem if I use k_inlet~0.96 (1.5(UI)^2) -> Cd ~0.8. if k_inlet~0.01-0.0001 -> Cd~0.3 but in experiment Cd~0.55+-.

[msuaeronautics]

Originally Posted by Andy_bm

Thanks for your answers!

My case is flow past cylinder.I have mesh with y+ 0.1-1. And I have following problem if I use k_inlet~0.96 (1.5(UI)^2) -> Cd ~0.8. if k_inlet~0.01-0.0001 -> Cd~0.3 but in experiment Cd~0.55+-.

How are you calculating Cd? Are you using an ensemble average of a list of outputs?

[Andy_bm]

I use libforces.lib and then plot result in labplot.For k~0.96 pulsation of coeffs occur near 0.8, for small k near 0.3.End time is sufficiently large.

[msuaeronautics]

Originally Posted by Andy_bm

I use libforces.lib and then plot result in labplot.For k~0.96 pulsation of coeffs occur near 0.8, for small k near 0.3.End time is sufficiently large.

That makes sense.

I have done study on sphere using DDES. I take the time average of the flow and compute the drag from the sum of pressure and viscous forces acting on the body from the mean solution. Maybe you could try that also for comparison. Drag gets a little tricky when using libforces, especially for an unsteady simulation.

[Andy_bm]

And what scheme for grad and div(phi,U) did you use?

[msuaeronautics]

Originally Posted by Andy_bm

And what scheme for grad and div(phi,U) did you use?

Based on my studies, limitedLinear works pretty well for DES. Avoid the upwind/2nd order upwind schemes like the plague. I use a k value of 0.1-0.3 based on trial and error using a simple case; i.e.

Gauss limitedLinear 0.3;

[Andy_bm]

What about internalField for k? Did you set this = k_inlet value or 0?

[msuaeronautics]

Originally Posted by Andy_bm

What about internalField for k? Did you set this = k_inlet value or 0?

I usually set this equal to k_inlet.

[openfoammaofnepo]

Dear Eugene,

Thank you for this suggestions. Do you know any papers or threads in which the LES results from linear and this filteredLinear2V for convection scheme are compared or validated? In the latter, we have two parameters that can be adjusted: "filteredLinear2V 0.2 0". If the first one (0.2 here) is small, this scheme seems equilevant to the linear. The comments from the source file are:

   // Scaling corefficient for the gradient ratio,
        // 0 = linear
        // 1 = fully limited

For the second parameter, in the source files, we have the following comments:

   // Maximum allowed overshoot/undershoot relative to the difference
        // across the face.
        // On input:
        //     0 = no overshoot/undershoot
        //     1 = overshoot/undershoot equal to the difference across the face
        // Note: After input 1 is added to l_

I am not sure I completely understand this usage of the second parameter. Could you please give an example? Thanks.

The link for the source file is here:

https://github.com/OpenFOAM/OpenFOAM-2.1.x/blob/master/src/finiteVolume/interpolation/surfaceInterpolation/limitedSchemes/filteredLinear2/filteredLinear2V.H

Thanks.

[tiam]

Originally Posted by openfoammaofnepo

Dear Eugene,

Thank you for this suggestions. Do you know any papers or threads in which the LES results from linear and this filteredLinear2V for convection scheme are compared or validated? In the latter, we have two parameters that can be adjusted: "filteredLinear2V 0.2 0". If the first one (0.2 here) is small, this scheme seems equilevant to the linear. The comments from the source file are:

   // Scaling corefficient for the gradient ratio,
        // 0 = linear
        // 1 = fully limited

For the second parameter, in the source files, we have the following comments:

   // Maximum allowed overshoot/undershoot relative to the difference
        // across the face.
        // On input:
        //     0 = no overshoot/undershoot
        //     1 = overshoot/undershoot equal to the difference across the face
        // Note: After input 1 is added to l_

I am not sure I completely understand this usage of the second parameter. Could you please give an example? Thanks.

The link for the source file is here:

https://github.com/OpenFOAM/OpenFOAM-2.1.x/blob/master/src/finiteVolume/interpolation/surfaceInterpolation/limitedSchemes/filteredLinear2/filteredLinear2V.H

Thanks.

This is an old thread but I would like to revive it. I assume much more people have done LES on OF by now. Any developments on convective schemes? As for filteredLinear, we have 5 to choose from (including the V variants) and also a bunch of parameters. Any guidelines/suggestions?