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Unable to modify Laplace equation to satisfaction
Posted 30 sept. 2018, 09:54 UTC−4 Structural & Acoustics, General, Equation-Based Modeling Version 5.1 4 Replies
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Hello, I am trying to find the Pressure distribution within the eye by using the laplace equation.
The equation for this is
K*d(d(Pressure,x))
I have 4 domains inside the eye and they all have different K values.
The equation I have mentioned above is to find the pressure distribution throughout the eye across varying K value in different domains. My problem lies with the fact that COMSOL is not allowing me to break up the whole of eye into 4 different domains and then define the pressure in x direction for the different domains.
For example instead of having the equation K*d(P,x) for pressure in x-direction
I want to use the following equation:
[K1d(P1,x)+K2d(P2,x)+K3d(P3,x)+K4d(P4,x)] as the change in pressure in x direction.
Where K1, K2, K3 and K4 are just constant values for the material and P1, P2, P3, P4 are the pressures in different domains.
I do not have the values of P1-P4. But I have dirichlet boundary conditions for the entire surface of the eye.
If comsol is able to do a simple laplacian like d(P,x) throughout a single domain using boundary conditions, I see no reasons, why cant it do the same thing across multiple domains where:
Kd(P,x)=[K1d(P1,x)+K2d(P2,x)+K3d(P3,x)+K4*d(P4,x)].
How do I resolve this?
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Instead of splitting P into P1-P4 you can define K in different ways in different domains:
Component > Definitions: Add a Variables node.
Change Geometry entity level to Domain.
Select domain 1.
Enter K in the Name column and the value K1 in the Expression column.
Repeat the steps above for domains 2-4. You should end up with four Variables nodes that each provide the definition of K in one domain.
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I just tried that and it doesnt seem to bring any change to the result. The left column in equation view has a warning sign inside it now. I have attached an image showing how I modified the Laplace equation. Thank you for your help
Attachments:
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Hi,
It seems that you are using the Laplace Equation interface, which really just solves 
. To get K into the equation, I would use the Coefficient Form PDE interface with c = K and all other coefficients set to 0. The warning sign indicates that you are changing the built-in definitions of those variables, which you can do, but you should then be aware that you affect the results. If you plot them, you can see that the K values set up as suggested in the previous reply do have an effect, but they will not affect the solution to the Laplace equation.
Best regards, Magnus Ringh, COMSOL
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Hi Magnus that worked wonderfully. Thanks!