[Nek5000-users] Volume of an element

Tue Feb 27 06:39:46 CST 2018

De: "nek5000-users" <nek5000-users at lists.mcs.anl.gov> 
À: "nek5000-users" <nek5000-users at lists.mcs.anl.gov> 
Envoyé: Mardi 27 Février 2018 13:28:54 
Objet: Re: [Nek5000-users] Volume of an element 

Dear Sijo, 

The volume per grid point is given by the entries in bm1(i,j,k,e) 

For each element e, there are points [i,j,k] with index ranges i=1,...,nx1 

j=1,...,ny1, k=1,...,nz1 

You must reference elements by the 4th index (...,e). 

Fortunately, all the data is contiguous, so, if accessing _all_ points, 

you can use a loop like 

do i=1,n 

volume_of_point_i = bm1(i,1,1,1) 

enddo 

You certainly do not need to do any summing if you only want the 

point wise volume. 

Perhaps it'd be best if you indicate what exactly you are planning to 

do with the volume. 

Finally, I see your use of glmax and glmin. How many processors 

are you using? If more than one, are you intending that they are 

all writing to the file hcoeff.dat? 

hth, 

Paul 

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: Tuesday, February 27, 2018 4:45:30 AM 
To: nek5000-users 
Subject: Re: [Nek5000-users] Volume of an element 
Thanks Paul for your Kind replay. You were right. I want to get the volume associated with each grid point. And I tried the code below, but at each time when I run I get different answers for volume so I think my ocde is not rubust. Could you please verify my code below. 

nxyz = nx1*ny1*nz1 
n = nx1*ny1*nz1*nelv 

vol = vlsum(bm1(1,1,1,1),nxyz) 
a=glmax(vol,1) 
b=glmin(vol,1) 

do e=1,n 
volume = vlsum(bm1(e,1,1,1),nxyz) 
open (unit=10,file="hcoeff.dat") 
write (10,7020) a,b,volume 
7020 format(7f20.14) 
enddo 

Regards 

Sijo GEORGE 

De: "nek5000-users" <nek5000-users at lists.mcs.anl.gov> 
À: "nek5000-users" <nek5000-users at lists.mcs.anl.gov> 
Envoyé: Lundi 26 Février 2018 21:46:02 
Objet: Re: [Nek5000-users] Volume of an element 

Dear Sijo, 

We typically refer to elements as the structures that have (N+1)^3 

points. (Here, N+1 = lx1.) 

They are enumerated in nek5000 by the element number, e: 

integer e 

nxyz = nx1*ny1*nz1 

do e=1,nelt 

volume = volel(e) 

volume = vlsum(bm1(1,1,1,e),nxyz) 

enddo 

would both yield the same value for the volume of element e. 

It appears to me that you are after something different, however. 

It seems like you want to associate a volume with each grid point. 

If that is the case, then the entries of the diagonal mass matrix, bm1, 

are indeed the quantities that you want. 

Each entry of bm1(i,1,1,1) for i=1 to n:=nxyz*nelt, corresponds to the 

integral of the underlying basis function. 

That is, if we think about a function phi_i(x) such that 

u(x) = sum_i=1^n u(i,1,1,1)*phi_i(x) 

then 

bm1(i,1,1,1) = \int phi_i(x) dx 

where the integral (\int) is taken over the entire computational domain. 

[ Note that we really don't have such a global phi_i(x) ... I use it simply 

for illustration, here.] 

hth 

Paul 

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, February 26, 2018 11:59:19 AM 
To: nek5000-users 
Subject: [Nek5000-users] Volume of an element 
Hi Neks, 

How to calculate volume of an element in a 3D mesh? If i write 

n = nx1*ny1*nz1*nelv 
do i=1,n 
v(i) = volume(1,1,1,i) 
enddo 

Can i store the volume of each element into a varibale called v? 

What happens if i use bm1(1,1,1,i)? Instead of volume(1,1,1,i) 

Thanks 
Sijo 

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