
[Sponsors] 
How do I compute the normal vector to a surface with FEM 

LinkBack  Thread Tools  Search this Thread  Display Modes 
July 30, 2009, 08:00 
How do I compute the normal vector to a surface with FEM

#1 
New Member
Join Date: Apr 2009
Posts: 17
Rep Power: 17 
I am trying to compute the normal vector to an inclined plate with the FEM.
Based on theory, the normal vector is given by n=(dz/dx,0,1) This is the normal to the top surface. There is no variation in the ydirection. Consider it as a 2D geometry. The plate has length L, and height H, and it is inclined with a slope equal to PHI, so that it's bottom right corner is located at (a,0) and it's top left corner is located at (b,1). If I use the FEM to compute the dz/dx derivative, I will find that it is equal to zero. Does this make sense? The only way to compute a nonzero value for the dz/dx derivative is to solve the problem where the domain was originally a rectangular which deformed to the one I have now. Is this the only way to compute the normal vector with FEM? 

July 31, 2009, 05:10 

#2 
New Member
Join Date: Apr 2009
Posts: 17
Rep Power: 17 
I am trying to calculate the normal on the outer surface of a circle. The circle lies in the yz plane.
I compute the value of the derivative dz/dy (which is equal to (yy0)/(zz0), if the equation for the circle is (zz0)^2+(yy0)^2=R^2 ), and there is no good agreement between FEM and theory. Any ideas why this is happening? 

July 31, 2009, 06:37 

#3 
New Member
Join Date: Apr 2009
Posts: 17
Rep Power: 17 
My mistake, I can not compute the normal vectors because both x and z are independent variables.


July 31, 2009, 07:59 

#4 
New Member
Join Date: Apr 2009
Posts: 17
Rep Power: 17 
So, how do I calculate the normal vectors in a moving boundary domain.
If we assume that x=f(X,Z) and z=h(X,Z) taking the derivative of dx/dZ will give me the rate of deformation. But, we need to compute quantities such as dx/dz. Does anybody has any idea about how to compute the normal vectors with FEM? 

Thread Tools  Search this Thread 
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
How to get the surface normal vector  ohrmond  CFX  3  February 1, 2018 05:45 
[Gmsh] Error : Self intersecting surface mesh, computing intersections & Error : Impossible  velan  OpenFOAM Meshing & Mesh Conversion  3  October 22, 2015 11:05 
[Gmsh] Problem with Gmsh  nishant_hull  OpenFOAM Meshing & Mesh Conversion  23  August 5, 2015 02:09 
NACA0012 geometry/design software needed  Franny  Main CFD Forum  13  July 7, 2007 15:57 
CFX4.3 build analysis form  Chie Min  CFX  5  July 12, 2001 23:19 