[Nek5000-users] mesh deformation

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

Dear Sijo,


In the ocyl2 case, the delta is based on a dimension that

is related to the model.  (Here, delta=2*D, where D=1 is

the diameter of the cylinder.)


Such a choice would be reasonable for all flow past a cylinder

cases for that particular geometry, independent of the mesh

resolution.


The boundary-layer thickness criterion is important when there

are multiple objects that are potentially touching.


Keep in mind that any of these choices are moderated by the

fact that we are simply computing a blending function that is

the solution to the (modified) Laplace equation.  It will have

more or less the correct shape --- even the unmodified Laplace

solution is not a terrible blending function.   The modification is

designed simply to help preserve mesh sizes near boundaries.


I paste below a snippet of code that computes the average

thickness of elements near the moving wall.


Paul



         integer e,f


         nxz   = nx1*nz1

         nxyz  = nx1*ny1*nz1

         n     = nxyz*nelv


         srfbl = 0.   ! Surface area of elements in b.l.

         volbl = 0.   ! Volume of elements in boundary layer

         do e=1,nelv

         do f=1,nface

            if (cbc(f,e,1).eq.'mv ') then

               srfbl = srfbl + vlsum(area(1,1,f,e),nxz )

               volbl = volbl + vlsum(bm1 (1,1,1,e),nxyz)

            endif

         enddo

         enddo

         srfbl = glsum(srfbl,1)  ! Sum over all processors

         volbl = glsum(volbl,1)


         delta = volbl / srfbl   ! Avg thickness of b.l. elements


         call rone (h1,n)

         call rzero(h2,n)

         call cheap_dist(d,1,'mv ')


         deltap = 2*delta  ! Protected b.l. thickness

         do i=1,n

            arg   = -(d(i)/deltap)**2

            h1(i) = h1(i) + 9*exp(arg)

         enddo





________________________________
From: Nek5000-users <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: Monday, November 27, 2017 7:52 AM
To: nek5000-users
Subject: [Nek5000-users] mesh deformation

Hello Nek,

 I was trying to validate moving cylinder test cases (ocyl2.usr more specifically). I could validate it properly. Even though I have some doubts regarding the mesh motion. Mesh deformation more precisely. I have seen a function to send the deformation to far field apart from the near wall region (Laplace equation with h1 as blending coefficient).So in order to find h1 in the equation there is a parameter called delta (which is 2 in this case). I have also read a paper regarding this equation. In that paper its clearly states that it is the average thickness of the element which is close to the object. So my question is, is there anyway to calculate it automatically or easily to calculate the thickness of that particular elements from any input files? because if I change the mesh (suppose if I refine the mesh size everything will change right?) how can I find the new average thickness of the elements which is close to the object? or how can I measure the area or volume of the elements which is close to the wall. so could you please help me with this?

Thanks with regards

Sijo George
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