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The difference between ux and d(u,x)
Posted 23.09.2015, 08:33 GMT-4 Modeling Tools & Definitions, Parameters, Variables, & Functions, Studies & Solvers Version 4.3 1 Reply
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I am trying to solve a pair of ODE in 1D using comsol. Along with 3 drichlet boundary conditions, one of the boundary conditions is a constraint equating a function of the derivatives of u,v (dependant variables) to zero.
I get a different solution when I use ux and d(u,x) and similiarly vx and d(v,x) in the constraint. Could you please explain why this happens and what is the right symbol to use for du/dx?
But when i plot the solution d(u,x) and ux give me the same graph, then why are they behaving differently when i use it in the constraint,
I get a different solution when I use ux and d(u,x) and similiarly vx and d(v,x) in the constraint. Could you please explain why this happens and what is the right symbol to use for du/dx?
But when i plot the solution d(u,x) and ux give me the same graph, then why are they behaving differently when i use it in the constraint,
1 Reply Last Post 24.09.2015, 04:47 GMT-4
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Posted:
9 years ago
One important thing to note is that variables and expressions can have different meanings on a boundary and on an adjacent domain. If instead of plotting on the interval, you make a Point Evaluation on one of the end points, you will probably see that d(u,x) is zero while ux has a nonzero value.
In general, to get a partial derivative of u on a boundary, it is easiest to use ux. An alternative is to write mean(d(u,x)), since the mean operator makes the evaluation happen on the adjacent domain instead of on the boundary.
In general, to get a partial derivative of u on a boundary, it is easiest to use ux. An alternative is to write mean(d(u,x)), since the mean operator makes the evaluation happen on the adjacent domain instead of on the boundary.