Message Boards Message Boards

0
|
5789 Views
|
4 Replies
|
0 Total Likes
View groups...
Share
Share this post:

Can volume integrals give different results in different coordinate systems

Posted 9 years ago

I am numerically integrating a vector function over a volume, where the function and integral are either in cylindrical coordinates or spherical coordinates. A parametric evaluation produces a different distribution of values between the two integrals. How is this possible?

The likeliest explanation is that I have made an error in one or the other of the coordinate system, but I cannot find that error. Consequently, I am wondering if there is something about numerical integration that may be the issue, since I do get messages about convergence etc.

POSTED BY: Luther Nayhm
4 Replies
Posted 9 years ago

It depends on how you're handling it and precisely what you are doing. Quoting John M. Lee in *Introduction to Smooth Manifolds * he says: "There is no way to integrate real-valued functions in a coordinate independent way on a manifold." The proper way is to use differential forms. Then with a mapping between cylindrical and spherical space, say, you can pull-back the integrand from one space to the other. This involves exterior derivatives. Then if you also pay attention to the respective domains you should get invariant results.

POSTED BY: Luther Nayhm
Posted 9 years ago
POSTED BY: Luther Nayhm
Posted 9 years ago

What do you mean by a "distribution of values" for an integral? In each coordinate system there is a different expression for a volume element, which connects a volume element in the coordinate system to a Euclidean volume element. So the volume pieces being added up are not constant in value for a non-Cartesian coordinate system. But there total is, since the volume itself cannot depend on the coordinate system. Perhaps you could post code in a code block, or attach a notebook.

Edit: My statement above is I think incomplete. There is also the issue of translating the vector function from one coordinate system to the other. As pointed out below, this is a matter for differential geometry.

POSTED BY: David Keith
Posted 9 years ago

Well, I hate to say it, but it all depends.

What I have is a function that depends on the vector between volume elements within two volumes. I am integrating this function over both volumes. The volumes are spheres. I can define the function or the volumes in spherical or cylindrical coordinates. The function essentially "weights" the vector for each volume element pair.

When I set the function to unity, the two integrals produce the same volumes. When I allow the function to take on a value related to the length of the vector between volume elements, the value of this integral differs as the ratio of the sphere radii is varied. Hence, I get a range of values over the range of the radii ratio. When I switch from cylindrical to spherical coordinates, the value of the integral is not the same between the two coordinate systems for a given ratio. Hence, I get a type of scatter plot, but the scatter plot is different between the two coordinate systems.

POSTED BY: Luther Nayhm
Reply to this discussion
Community posts can be styled and formatted using the Markdown syntax.
Reply Preview
Attachments
Remove
or Discard

Group Abstract Group Abstract