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derivative expression
Posted 29 sept. 2010, 09:26 UTC−4 1 Reply
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Hello!
This is probably very simple, but anyway:
how do I write derivative in the expression-windows e.g. I want to do a line integration of a derivative: (du/dx)^2.
I am using version 4.0a
/Emil
This is probably very simple, but anyway:
how do I write derivative in the expression-windows e.g. I want to do a line integration of a derivative: (du/dx)^2.
I am using version 4.0a
/Emil
1 Reply Last Post 29 sept. 2010, 10:54 UTC−4
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Posted:
1 decade ago
Hi
Comsol has a certain number of derivative pre-defined, such as the components of the displacement field _u_ :
ux = du/dx, uy = du/dy, ... vx = dv/dx ...
see the doc (reference guide)
as well as second derivatives
uxx = d(du/dx)/dx ...
and time derivatives if applicable and turned on
ut = du/dt ...
higer derivatives might be obtained with the operator:
"d(var_name,x)"
but note that higher derivatives means also higher noise from the derivation of the meshing variations and the numerical difference of comparable values (binary number representation errors)
Now in V4 you have to take car of which "frame" you are using (physics dependent) as you have the material frame, the spatial and the mesh frame ....
These were there in V3.5 too, but the naming has slightly changed and the convention is more systematic.
Each frame has different coordinate identifiers: i.e. (x,y,z), (X,Y,Z), (Xm,Yn,Zm) ...
i.e. for all items above you also have uX = du/dX etc, and x and X is not necesarily the same (neither ux and uX), as they represent two different frames that might be driven differently, not to talk about the Xm mesh frame elements ...
pls refer to the doc
--
Good luck
Ivar
Comsol has a certain number of derivative pre-defined, such as the components of the displacement field _u_ :
ux = du/dx, uy = du/dy, ... vx = dv/dx ...
see the doc (reference guide)
as well as second derivatives
uxx = d(du/dx)/dx ...
and time derivatives if applicable and turned on
ut = du/dt ...
higer derivatives might be obtained with the operator:
"d(var_name,x)"
but note that higher derivatives means also higher noise from the derivation of the meshing variations and the numerical difference of comparable values (binary number representation errors)
Now in V4 you have to take car of which "frame" you are using (physics dependent) as you have the material frame, the spatial and the mesh frame ....
These were there in V3.5 too, but the naming has slightly changed and the convention is more systematic.
Each frame has different coordinate identifiers: i.e. (x,y,z), (X,Y,Z), (Xm,Yn,Zm) ...
i.e. for all items above you also have uX = du/dX etc, and x and X is not necesarily the same (neither ux and uX), as they represent two different frames that might be driven differently, not to talk about the Xm mesh frame elements ...
pls refer to the doc
--
Good luck
Ivar