[Nek5000-users] Neek5000 variable viscosity case

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

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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