[Nek5000-users] Neek5000 variable viscosity case

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

Hi Francesco,

I coded up the variable viscosity support for the PN-PN formulation
but the low Mach number support is still missing. For low Mach number
flows we do something inconsistent: We assume that nu is identical to
nu_ref (constant in space). Obviously this is not true! This is a
serious kludge rather than an approximation altough the effect is not
known a-priori (and is case dependent)!

Cheers
Stefan

On 5/16/11, nek5000-users at lists.mcs.anl.gov
<nek5000-users at lists.mcs.anl.gov> wrote:
>
>
> Dear Francesco,
>
> For low Mach, if you want var. viscosity you must also set ifexplvis
> to be true.   This will treat the remaining part of the stress tensor
> explicitly, so that you do capture the physics of the full stress tensor.
>
> Now, a couple of comments ---
>
> The original explicit formulation, based on strong form developed by
> Stefan,
> worked well with our dyn. smag. model, but was not in exact agreement
> the PnPn-2 stress formulation until refined to convergence.
>
> I modified this to a weak form in January and it does now agree w/ PnPn-2,
> but the dyn. smag. tests that I've done do not work.
>
> I've been so busy for the past months with travel and proposal writing
> that I've not had a chance to understand why dyn. smag. appears to be
> broken.  I'm hoping to resolve this shortly.
>
> Cheers,
>
> Paul
>
>
> On Mon, 16 May 2011, Francesco Lucci wrote:
>
>> Dear Paul,
>>
>> sorry to bother you but we need a clarification about the variable
>> viscosity
>> simulations.
>>
>> We run a LowMach number case of an heated channel and we see a strong
>> variation
>> changing the viscosity temperature dependence.
>> (in pic we plot the square of the streamwise velocity fluctuation)
>> This is not surprising but we would like to have more insight on how the
>> code
>> treats
>> the variable viscosity in order to valuate the possible assumptions or
>> errors
>> we made.
>>
>> We assume that the code, instead of solving for div( mu S), solves mu
>> div(S)
>> with mu that varies in space.
>> Thus the term (grad muj) S is neglected.
>>
>> Is that correct? Is there anything else we have to know?Have anybody
>> evaluated the effect of this approximation?
>>
>> Thank you a lot for you help.
>> francesco
>>
>>
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