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That should have been u''(1)=b for the second "BC"<br>
<br>
<div class="moz-cite-prefix">On 1/10/13 11:18 AM, Sanjay Govindjee
wrote:<br>
</div>
<blockquote cite="mid:50EF13F1.7070209@berkeley.edu" type="cite">
<meta content="text/html; charset=UTF-8" http-equiv="Content-Type">
Perhaps more simply, consider the 1D version of your problem:<br>
<br>
u''=0 on [0,1] with "Boundary Conditions" u'(0)=a and u'(1) =b<br>
<br>
The solution is u(x) = a x + c (for any! value of c).<br>
This is not going to solve happily :(<br>
<br>
<br>
<div class="moz-cite-prefix">On 1/10/13 8:27 AM, Jed Brown wrote:<br>
</div>
<blockquote
cite="mid:CAM9tzSkzob=h85DRjtWQHv=TLNGkchivauj4ef-3-0hibvp0KQ@mail.gmail.com"
type="cite">
<div dir="ltr">... and you describe a "boundary condition" as
"second derivative is zero", which is not a boundary
condition, making your problem ill-posed. (Indeed, consider
the family of problems in which you extend the domain in that
patch and apply _any_ boundary conditions in the extended
domain. All of those solutions are also solutions of your
problem with "second derivative is zero" on your "boundary".)</div>
<div class="gmail_extra"><br>
<br>
<div class="gmail_quote">On Thu, Jan 10, 2013 at 9:44 AM,
David Scott <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:d.scott@ed.ac.uk" target="_blank">d.scott@ed.ac.uk</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex"> All
right, I'll say it differently. I wish to solve<br>
div.grad phi = 0<br>
with the boundary conditions that I have described.
<div class="im"><br>
<br>
On 10/01/2013 15:28, Jed Brown wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div class="im"> Second derivative is not a boundary
condition for Poisson; that is the<br>
equation satisfied in the interior. Unless you are
intentionally<br>
attempting to apply a certain kind of outflow boundary
condition (i.e.,<br>
you're NOT solving Laplace) then there is a problem
with your<br>
formulation. I suggest you revisit the continuum
problem and establish<br>
that it is well-posed before concerning yourself with
implementation<br>
details.<br>
<br>
<br>
On Thu, Jan 10, 2013 at 9:03 AM, David Scott <<a
moz-do-not-send="true"
href="mailto:d.scott@ed.ac.uk" target="_blank">d.scott@ed.ac.uk</a><br>
</div>
<div>
<div class="h5"> <mailto:<a moz-do-not-send="true"
href="mailto:d.scott@ed.ac.uk" target="_blank">d.scott@ed.ac.uk</a>>>
wrote:<br>
<br>
Hello,<br>
<br>
I am solving Poisson's equation (actually
Laplace's equation in this<br>
simple test case) on a 3D structured grid. The
boundary condition in<br>
the first dimension is periodic. In the others
there are Von Neumann<br>
conditions except for one surface where the
second derivative is<br>
zero. I have specified DMDA_BOUNDARY_NONE in
these two dimensions<br>
and deal with the boundary conditions by
constructing an appropriate<br>
matrix. Here is an extract from the Fortran
code:<br>
<br>
if (j==0) then<br>
! Von Neumann boundary
conditions on y=0 boundary.<br>
v(1) = 1<br>
col(MatStencil_i, 1) = i<br>
col(MatStencil_j, 1) = j<br>
col(MatStencil_k, 1) = k<br>
v(2) = -1<br>
col(MatStencil_i, 2) = i<br>
col(MatStencil_j, 2) = j+1<br>
col(MatStencil_k, 2) = k<br>
call MatSetValuesStencil(B, 1,
row, 2, col, v,<br>
INSERT_VALUES, ierr)<br>
else if (j==maxl) then<br>
! Boundary condition on y=maxl
boundary.<br>
v(1) = 1<br>
col(MatStencil_i, 1) = i<br>
col(MatStencil_j, 1) = j<br>
col(MatStencil_k, 1) = k<br>
v(2) = -2<br>
col(MatStencil_i, 2) = i<br>
col(MatStencil_j, 2) = j-1<br>
col(MatStencil_k, 2) = k<br>
v(3) = 1<br>
col(MatStencil_i, 3) = i<br>
col(MatStencil_j, 3) = j-2<br>
col(MatStencil_k, 3) = k<br>
call MatSetValuesStencil(B, 1,
row, 3, col, v,<br>
INSERT_VALUES, ierr)<br>
else if (k==0) then<br>
<br>
<br>
Here the second clause deals with the second
derivative on the boundary.<br>
<br>
In order for this code to work I have to set the
stencil width to 2<br>
even though 'j-2' refers to an interior,
non-halo<br>
point in the grid. This leads to larger halo
swaps than would be<br>
required if a stencil width of 1 could be used.<br>
<br>
Is there a better way to encode the problem?<br>
<br>
David<br>
<br>
--<br>
The University of Edinburgh is a charitable
body, registered in<br>
Scotland, with registration number SC005336.<br>
<br>
<br>
</div>
</div>
</blockquote>
<span class="HOEnZb"><font color="#888888"> <br>
<br>
-- <br>
Dr. D. M. Scott<br>
Applications Consultant<br>
Edinburgh Parallel Computing Centre<br>
Tel. 0131 650 5921</font></span>
<div class="HOEnZb">
<div class="h5"><br>
<br>
The University of Edinburgh is a charitable body,
registered in<br>
Scotland, with registration number SC005336.<br>
</div>
</div>
</blockquote>
</div>
<br>
</div>
</blockquote>
<br>
</blockquote>
<br>
<pre class="moz-signature" cols="72">--
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