[Nek5000-users] Nek5000 variable viscosity case

Mon May 16 11:18:41 CDT 2011

Francesco,

you can set vdiff to whatever you like. The implemented formulation
was derived based on the assumption that mue is constant in space. So
no matter what you, (a) or (b) will be wrong!

I don't remember the details but the tricky thing was QTL or the
pressure boundary condition which has change accordingly. Start from
the low Mach number equations and split mue into a constant and
varying part. Then, discretize the eqns using the BDF/EXT scheme and
treat the varying part explicit. Finally you have to derive the
pressure equation and a boundary condition. From this it should be
obvious what needs to be modified. How this could be implemented is
another story.

-Stefan

On 5/16/11, nek5000-users at lists.mcs.anl.gov
<nek5000-users at lists.mcs.anl.gov> wrote:
>
> 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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