[Nek5000-users] Calculation of Divergence Term
nek5000-users at lists.mcs.anl.gov
nek5000-users at lists.mcs.anl.gov
Sat Nov 12 11:21:36 CST 2011
Hi,
Thanks a lot for your hints and I appreciate your help. Sorry for
interrupting when you were so busy.
Regards,
Alireza Karimi
Quoting nek5000-users at lists.mcs.anl.gov:
>
> Dear Alireza,
>
> You would add this to qvol, since it is the convection-diffusion
> equation -- so, the routine is userq.
>
> You should precompute the term in userchk, store the result in
> a common block, and then dereference in userq. Expanding on Josh's
> suggestion below you would have something like
>
> c-----------------------------------------------------------------------
> subroutine userchk
> :
> :
>
> common /mydiv/ divcv(lx1,ly1,lz1,lelt)
> common /scrns/ dcv1dx(lx1,ly1,lz1,lelt)
> , dcv2dy(lx1,ly1,lz1,lelt)
> , dcv3dz(lx1,ly1,lz1,lelt)
> , work1 (lx1,ly1,lz1,lelt)
> , work2 (lx1,ly1,lz1,lelt)
>
>
> then his calls... be sure to define n
>
> return
> end
> c-----------------------------------------------------------------------
> subroutine userq (ix,iy,iz,eg)
> include 'SIZE'
> include 'TOTAL'
> include 'NEKUSE'
>
> common /mydiv/ divcv(lx1,ly1,lz1,lelt)
>
> integer e,f,eg
> e = gllel(eg)
>
> qvol = divcv(ix,iy,iz,e) Be sure to check the sign of this term
>
> return
> end
> c-----------------------------------------------------------------------
>
>
>
> It's not a priori clear that you need to dealias this term.
> Dealiasing is needed in certain circumstance for stability -- it
> does not materially affect accuracy. Without further context
> I cannot tell you whether you need to dealias or not.
>
> The approach suggested by Josh is perfectly adequate for the
> standard (non-dealiased) approach.
>
>
> Paul
>
>
> On Fri, 11 Nov 2011, nek5000-users at lists.mcs.anl.gov wrote:
>
>> Hi,
>>
>> I'm sending this message again, because I got no answer for my last
>> email. I have some questions regarding the additional term given in
>> my previous message. I don't know where and in which subroutine I
>> should add this term to the right hand side of the
>> advection-diffusion equation. Also, could you please provide me
>> with the best option I can take to dealias this term (if needed at
>> all)? I wonder if you could help me in this regard.
>>
>> Best Regards,
>> Alireza Karimi
>>
>>
>>
>> Quoting nek5000-users at lists.mcs.anl.gov:
>>
>>> Alireza,
>>>
>>> For this, I'll assume that you have split V into V1, V2, V3, and that
>>> they have the structure
>>>
>>> V1(lx1,ly1,lz1,lelv) (and likewise for the other two)
>>>
>>>
>>> First, to multiply c*V1 (V2, V3), you can use col2. It would look like
>>>
>>> call col2(V1,c,n)
>>> call col2(V2,c,n)
>>> call col2(V3,c,n)
>>>
>>> where n = nx1*ny1*nz1*nelv. A couple of things to note here:
>>>
>>> 1) col2 will overwrite V1 (V2, V3). Effectively, the operation is
>>> V1 = c*V1. If this is not desirable, then you could use col3 with
>>> something like
>>>
>>> call col3(cV1,V1,c,n)
>>>
>>> This will store it in cV1 and leave V1 untouched.
>>>
>>> 2) You could run into aliasing issues by multiplying c and V
>>> directly, and so it may be wise to dealias the product before taking
>>> the derivative. Developers, maybe you could suggest the best way of
>>> going about doing this in Nek???
>>>
>>> Moving on, to take the derivatives, there are a few choices. One way
>>> would be do to something like
>>>
>>> call gradm1(dcV1dx,work1 ,work2 ,cV1)
>>> call gradm1(work1 ,dcV2dy,work2 ,cV2)
>>> call gradm1(work1 ,work2 ,dcV3dz,cV3)
>>>
>>> call add4(divcV, dcV1dx, dcV2dy, dcV3dz, n)
>>>
>>> which would give you div(cV) in the array divcV. Note that dcV1dx,
>>> dcV2dy, dcV3dz, work1, work2, and divcV all should have the same
>>> storage structure as V1, c, etc.
>>>
>>> This is probably the simplest way that I can think to do it, although
>>> it is somewhat inefficient since we are computing, for instance,
>>> d(cV1)/dy unnecessarily. To make it more efficient, you can manually
>>> compute the derivatives individually (take a look at the gradm1
>>> subroutine to get an idea of how to do this, located in navier5.f in
>>> the source)
>>>
>>> Hope this helps, and developers, please correct me if I did something
>>> horribly wrong here :).
>>>
>>> Josh
>>>
>>> On Fri, Nov 4, 2011 at 4:59 PM, <nek5000-users at lists.mcs.anl.gov> wrote:
>>>> Hi,
>>>>
>>>> I want to solve an advection-diffusion equation in conjunction with the
>>>> momentum and mass conservation equations. But my equation has an extra
>>>> divergence term which I don't know how to calculate it in Nekton. So,
>>>> denoting the scalar concentration field with c and velocity field with u,
>>>> the equation turns out to be:
>>>>
>>>> dc/dt + u.grad c = Laplacian c - div (c V)
>>>>
>>>> which by d/dt I mean partial derivative with respect to time. V is a known
>>>> vector field calculated from the vorticity tensor. I wonder if you could
>>>> show me how to evaluate this additional term (the last term) in the user
>>>> file.
>>>>
>>>>
>>>> Best Regards,
>>>> Alireza Karimi
>>>>
>>>>
>>>> _______________________________________________
>>>> Nek5000-users mailing list
>>>> Nek5000-users at lists.mcs.anl.gov
>>>> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
>>>>
>>>
>>>
>>>
>>> --
>>> Josh Camp
>>>
>>> "All that is necessary for the triumph of evil is that good men do
>>> nothing" -- Edmund Burke
>>> _______________________________________________
>>> Nek5000-users mailing list
>>> Nek5000-users at lists.mcs.anl.gov
>>> https://lists.mcs.anl.gov/mailman/listinfo/nek5000-users
>>>
>>
>>
>>
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