[Nek5000-users] Variable density flows

[nek5000-users at lists.mcs.anl.gov]

These tips are most useful. I am able to run some high Re and high Pe cases
but I still need really large meshes to ensure that the scalar stays
bounded between [0,1].

Is the characteristic time stepping going to be useful for these cases ? I
have read that it allows larger time steps. Since "characteristic" means
some upwind effect, I was wondering does it help to maintain better
monotonicity of advected scalars ?

Thanks
praveen

On Mon, Oct 14, 2013 at 12:37 AM, <nek5000-users at lists.mcs.anl.gov> wrote:

> Hi Praveen,
>
> Your clipping implementation will as you have it -- I normally would
> do it in a single loop:
>
>       n=nx1*ny1*nz1*nelt
>       do i=1,n
>          t(i,1,1,1,2)=min(1.,t(i,1,1,1,2))
>          t(i,1,1,1,2)=max(0.,t(i,1,1,1,2))
>       enddo
>
> You can turn on filtering by setting param(103) in your .rea file.
> Setting p103=.05, say, would reduce the energy in the Nth mode by
> 5% each timestep --- this is usually a good choice.  Sometimes it
> pays to bump this up to, say, .1, but usually a better strategy is
> to add another mode to the filter, by setting p101 to 1, or 2.
> In this case, the filter amplitude for the N-1 mode is alpha/4
> when p101=1 and when p101=2, we have amplitudes as:
>
>    a(N) = alpha := p103
>    a(N-1) = alpha*(2/3)**2 = 4*alpha/9
>    a(N-2) = alpha*(1/3)**2 = alpha/9
>
> which amounts to fitting a parabola through the filter amplitudes so
> that the filter strength decays smoothly to zero.
>
> p103 = .05
> p101 = 1
>
> are my common choices when lx1=8.  If lx1 < 8, I typically set p101=0
> as you're otherwise cutting too deeply into low end of the spectrum.
>
> All this being said, you'll no doubt still need clipping if Sc=100,000.
>
> Paul
>
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