[Nek5000-users] Nek5000 variable viscosity case

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

Dear Francesco,

As Stefan indicated, the current explicit treatment of 
viscosity pertains only to the incompressible formulation
for PnPn.  (My mistake in thinking it would work for low Mach.)

I think the way to proceed is to write down the original
PDE, then set mu = mu_ref + mu'(X,t), and treat the varying
portion explicitly in time.

The rest of the formulation should go through OK, save that
you will have a new forcing term that involves mu'.

If you can come up with the PDE-based form then we should
be able to develop the correct forcing term inside Nek.
Of course, I realize this isn't easy in the full low-Mach
formulation, but by working together we can crack this.

Cheers,

Paul



On Mon, 16 May 2011, nek5000-users at lists.mcs.anl.gov wrote:

> Dear Paul and Stefan,
>
> thank you for the reply.
> I am not sure if i have understood the situation.
>
> We use the LowMach formulation but we did not set ifexplvis to be true.
> we varied the viscosity in uservp by adding :
> ...
>     if (IFLOMACH)      visc= param(2)*TEMP**0.67
> ...
>
> Different cases were run with different exponent for the temperature and we 
> see
> a clear difference between these cases. Thus somehow we have a case of 
> variable viscosity
>
> With 'we assume that nu is identical to nu_ref (constant in space)'
> do you mean that the equations were derived assuming constant nu ( or mu?), 
> but the
> code implementation allows a space variation?  Or that you fix somewhere nu 
> to be constant in the code?
>
> In conclusion, given that i need a variable viscosity,
>
> a)In your opinion, which is the best that we can currently do ?
> set ifexplvis to be true, and add visc= param(2)*TEMP**0.67 ? .
>
> b) to set ifexplvis to be true, do i have to change it manually in connect2 
> and drive2?
>    in only one of the two files? or is there a better way?
>
> thank you a lot
> francesco
>
>
>
> On 05/16/2011 02:45 PM, nek5000-users at lists.mcs.anl.gov wrote:
>> 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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