[Nek5000-users] turbulent 3d box

Fri Jan 3 19:40:03 CST 2014

Barak,

I suggest comparing the SIZE files for various examples.  From there you should
be able to see how to properly modify SIZE to change the resolution.   

Of course, you'll need to reduce dt to maintain CFL ~ 0.5

Paul

________________________________________
From: nek5000-users-bounces at lists.mcs.anl.gov [nek5000-users-bounces at lists.mcs.anl.gov] on behalf of nek5000-users at lists.mcs.anl.gov [nek5000-users at lists.mcs.anl.gov]
Sent: Sunday, December 22, 2013 4:25 PM
To: nek5000-users at lists.mcs.anl.gov
Subject: Re: [Nek5000-users] turbulent 3d box

It didn't work (I tried both options). Do you have working example?
(see my previous email and my attachment).
Barak

On 12/22/2013 11:20 PM, nek5000-users at lists.mcs.anl.gov wrote:
>
> Hi Paul,
> On 12/22/2013 11:14 PM, nek5000-users at lists.mcs.anl.gov wrote:
>>
>> Hi Barak,
>>
>> You can either increase lx1,ly1, lz1 in SIZE and re-make, or you
>> can change nelx,nely,nelz in the .box file.
>>
> Thanks.
>> Did you resolve the issue with the initial condition?
>>
> See my previous email. I have inside further questions.
> Best regards,
> Barak
>
>
>> Paul
>> ________________________________________
>> From: nek5000-users-bounces at lists.mcs.anl.gov
>> [nek5000-users-bounces at lists.mcs.anl.gov] on behalf of
>> nek5000-users at lists.mcs.anl.gov [nek5000-users at lists.mcs.anl.gov]
>> Sent: Sunday, December 22, 2013 3:05 PM
>> To: nek5000-users at lists.mcs.anl.gov
>> Subject: Re: [Nek5000-users] turbulent 3d box
>>
>> Hi Paul,
>> Before I run problems with smaller viscosity: how I increase the
>> resolution? and remesh the box?
>> Barak
>>
>> On 12/22/2013 10:24 PM, nek5000-users at lists.mcs.anl.gov wrote:
>>>
>>> Hi Barak,
>>>
>>> When I compute the L2 norm of the function, given as:
>>>
>>>          v2 = [ sum_i  vx_i b_i vx_i + vy_i b_i vy_i + vz_i b_i vz_i
>>> / sum_i b_i ]^(1/2)
>>>
>>> it comes out to ~ 2.23
>>>
>>> Note that there are some places where ux==2, uy==0, uz==1,
>>> which would give an L2 norm of
>>>
>>>        sqrt (2*2 + 0*0 + 1*1) = sqrt(5) > sqrt(3)
>>>
>>> ?
>>>
>>> Paul
>>>
>>> ________________________________________
>>> From: nek5000-users-bounces at lists.mcs.anl.gov
>>> [nek5000-users-bounces at lists.mcs.anl.gov] on behalf of
>>> nek5000-users at lists.mcs.anl.gov [nek5000-users at lists.mcs.anl.gov]
>>> Sent: Sunday, December 22, 2013 11:04 AM
>>> To: nek5000-users at lists.mcs.anl.gov
>>> Subject: Re: [Nek5000-users] turbulent 3d box
>>>
>>> Hi Paul,
>>> My definition for velocity magnitude is:
>>> |v|=sqrt(ux^2+uy^2+uz^2)
>>> so if you see, there are two components in the form:
>>> cos^2(x)+sin^2(x)
>>> etc in each direction
>>> so in fact
>>> |v|=sqrt(3) for any x,y,z.
>>> Isn't correct?
>>> Barak
>>>
>>> On 12/22/2013 05:05 PM, nek5000-users at lists.mcs.anl.gov wrote:
>>>>
>>>> Hi Barak,
>>>>
>>>> If your initial condition is:
>>>>
>>>>          ux=(cos(y)+sin(z))
>>>>          uy=(cos(z)+sin(x))
>>>>          uz=(cos(x)+sin(y))
>>>>
>>>> and your domain is [0,2pi]^3, how is it possible that your velocity
>>>> magnitude
>>>> is less than 2?
>>>>
>>>>
>>>> What is your measure of velocity magnitude?   When I compute the L2
>>>> norm of this initial condition I'm seeing about 2.3+
>>>>
>>>> Paul
>>>>
>>>> ------------------------------------------------------------------------
>>>>
>>>> *From:* nek5000-users-bounces at lists.mcs.anl.gov
>>>> [nek5000-users-bounces at lists.mcs.anl.gov] on behalf of
>>>> nek5000-users at lists.mcs.anl.gov [nek5000-users at lists.mcs.anl.gov]
>>>> *Sent:* Sunday, December 22, 2013 7:53 AM
>>>> *To:* nek5000-users at lists.mcs.anl.gov
>>>> *Subject:* Re: [Nek5000-users] turbulent 3d box
>>>>
>>>> Hi Paul,
>>>> It seems that something does not work, comparing my simulations using a
>>>> standard spectral code (pseudo-spectral). For nu=0.1, I used 16 points
>>>> for each space direction. Now for this case, the nonlinear terms vanish
>>>> (just cancel each other) and the solution is just the ABC flow if you
>>>> set it as initial condition. Then since the forcing is f=nu*ABC the
>>>> dissipation term (Laplacian) dissipation=-nu*ABC and therefore the
>>>> initial value (ABC) remains as it is. If we decrease the viscosity
>>>> above
>>>> the critical value of 1/12 (if I remember correctly) there is a
>>>> bifurcation (Hopf...) and we obtain a new periodic solution (in time),
>>>> where the ABC modes are dominating combined with some high wave numbers
>>>> that are oscillating.
>>>>
>>>>    >From my first tests using NEK, I find that the velocity field (for
>>>> nu>1/12) does not agree with the above characteristics. For example,
>>>> the
>>>> max magnitude of the velocity field should be for the ABC flow sqrt(3),
>>>> whereas here I get a value close to 2.5.
>>>>
>>>> Do you have any idea what's wrong?
>>>> If you wish to read some literature about the ABC flow as a forcing for
>>>> Navier Stokes or as a prescribed velocity for generation of magnetic
>>>> field (i.e., kinematic generation of magnetic field MHD):
>>>>
>>>> 1. D. Galloway and U. Frisch. A numerical investigation of magnetic
>>>> field generation in a flow with chaotic streamlines. Geophysical &
>>>> Astrophysical Fluid Dynamics, 29(1):13–18, 1984.
>>>>
>>>> 2. D. Galloway and U. Frisch. Dynamo action in a family of flows with
>>>> chaotic streamlines.
>>>> Geophysical & Astrophysical Fluid Dynamics, 36(1):53–83, 1986.
>>>>
>>>> 3. Podvigina, Pouquet, On the non-linear stability of the 1:1:1 ABC
>>>> flow,
>>>> Phys. D, Vol. 75, 4, 1994.
>>>>
>>>>
>>>> With my best wishes,
>>>> Barak
>>>>
>>>>
>>>> Quoting nek5000-users at lists.mcs.anl.gov
>>>> <mailto:nek5000-users at lists.mcs.anl.gov>:
>>>>
>>>>> Hi Barak,
>>>>>
>>>>> Thanks for the note about the ABC flow -- that's very useful.
>>>>>
>>>>> What is your target viscosity?
>>>>> If you want it to be 0.1, then you would set param 2 in the .rea file
>>>>> to be 0.1.
>>>>> Setting param 2 = -0.1 would imply that Re=0.1 or equivalently nu=10.
>>>>> Does this resolve the issue for you?
>>>>>
>>>>> Once it's working we could add it as a documented example in the
>>>>> example
>>>>> suite.
>>>>>
>>>>> Thanks!
>>>>>
>>>>> Paul
>>>>> ________________________________________
>>>>> From: nek5000-users-bounces at lists.mcs.anl.gov
>>>>> <mailto:nek5000-users-bounces at lists.mcs.anl.gov>
>>>>> [nek5000-users-bounces at lists.mcs.anl.gov
>>>>> <mailto:nek5000-users-bounces at lists.mcs.anl.gov>] on behalf of
>>>>> nek5000-users at lists.mcs.anl.gov
>>>>> <mailto:nek5000-users at lists.mcs.anl.gov>
>>>>> [nek5000-users at lists.mcs.anl.gov
>>>>> <mailto:nek5000-users at lists.mcs.anl.gov>]
>>>>> Sent: Saturday, December 21, 2013 5:24 PM
>>>>> To: nek5000-users at lists.mcs.anl.gov
>>>>> <mailto:nek5000-users at lists.mcs.anl.gov>
>>>>> Subject: Re: [Nek5000-users] turbulent 3d box
>>>>>
>>>>> Hi Paul,
>>>>> Regarding the 3dbox:
>>>>> The forcing which I propose is known as the ABC flow
>>>>> (Arnold-Beltrami-Childress) and has steady solutions (in the form
>>>>> of ABC
>>>>> flow) with Re=1/nu (nu the viscosity) Re=<12 (in periodic box).
>>>>> The forcing should be in the form:
>>>>> f=Re*ABC in order to balance the dissipation term.
>>>>> This settings can serve as a benchmark for pure hydro/mhd problems.
>>>>>
>>>>> I tried to fix the example which you have sent me, but I don't succeed
>>>>> retreiving the expected values.
>>>>> I attach the relevant files( tarball file).
>>>>> Best regards,
>>>>> Barak
>>>>>
>>>>>
>>>>>
>>>>> On 12/18/2013 03:29 AM, nek5000-users at lists.mcs.anl.gov
>>>>> <mailto:nek5000-users at lists.mcs.anl.gov> wrote:
>>>>>
>>>>>> Hi Barak,
>>>>>>
>>>>>> I added a 3d periodic box under "3dbox" in the examples
>>>>>> directory.
>>>>>>
>>>>>> Paul
>>>>>>
>>>>>>
>>>>>> Dear nek's,
>>>>>> I would like to run the following problem:
>>>>>> a 3D periodic box, small viscosity and the following forcing:
>>>>>>
>>>>>> fx=cos(y)+sin(z)
>>>>>> fy=cos(z)+sin(x)
>>>>>> fz=cos(x)+sin(y)
>>>>>>
>>>>>> Do you have a similar example?
>>>>>>
>>>>>> Many thanks,
>>>>>> Barak
>>>>>>
>>>>>>
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>>>>>>
>>>>> _______________________________________________
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>>>>
>>>>
>>>>
>>>>
>>>>
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