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Question on ODE and global variables
Posted 22 nov 2009, 20:07 GMT-5 3 Replies
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hi! I'm trying to define an ODE under "Global Equations" taking in the solution (e.g. X) of each node from a PDE (the Richards equation from the Earth Science Module). However, I get an error "Failed to evaluate variable Jacobian; X; Global Scope" when I tried to solve for the problem. I think this is due to X being dependent also on space as well as time while the ODE accepts only time-dependent variables. I can only solve for the problem when I define an integration coupling variable of X in the domain, and use that in the ODE.
I would like to ask if there is a way to define the ODE such that it accepts a global variable like X, instead of only an integration coupling variable of X within the whole domain? Thank you very much for your advice and help!
I would like to ask if there is a way to define the ODE such that it accepts a global variable like X, instead of only an integration coupling variable of X within the whole domain? Thank you very much for your advice and help!
3 Replies Last Post 23 mag 2011, 13:55 GMT-4
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Posted:
2 decades ago
Hi
I'm not shure I fully follow you, partly because of a confusion for me on the variable name:
Your "X", is it a placeholder for a given variable? or is it the COMSOL "X" defined variable used for the frame coordinates ?
Because I believe your issue could come from
a) a naming conflict, or
b) from the definitions provided in the Global Equation, as you need to give valid derivatives too to have the system to extract the Jacobian correctly (if I'm not fully wrong), and the corrrect Jacobian is needed to get a valid results, (one step at the time we get our way to the final solution...)
Ivar
I'm not shure I fully follow you, partly because of a confusion for me on the variable name:
Your "X", is it a placeholder for a given variable? or is it the COMSOL "X" defined variable used for the frame coordinates ?
Because I believe your issue could come from
a) a naming conflict, or
b) from the definitions provided in the Global Equation, as you need to give valid derivatives too to have the system to extract the Jacobian correctly (if I'm not fully wrong), and the corrrect Jacobian is needed to get a valid results, (one step at the time we get our way to the final solution...)
Ivar
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Posted:
2 decades ago
hi Ivar, thank you so much for your reply! Sorry about the confusion...X is a placeholder for the COMSOL defined variable, "theta" (liquid volume fraction), in the PDE Richards equation.
In the case where we couple an ODE (defined under Global Equations) to the PDE, I'm just confused as to whether we're only allowed to use scalar variables (e.g. integration of theta over the whole domain) in the coupled ODE, and we can't use a variable like theta which varies with x and y. Is this true? Is there a way to write the ODE in terms of "theta", instead of, say, "theta_integral" (integration coupling variable of theta in the domain)?
Thank you very much for your help and advice!
In the case where we couple an ODE (defined under Global Equations) to the PDE, I'm just confused as to whether we're only allowed to use scalar variables (e.g. integration of theta over the whole domain) in the coupled ODE, and we can't use a variable like theta which varies with x and y. Is this true? Is there a way to write the ODE in terms of "theta", instead of, say, "theta_integral" (integration coupling variable of theta in the domain)?
Thank you very much for your help and advice!
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Posted:
1 decade ago
hi! I'm trying to define an ODE under "Global Equations" taking in the solution (e.g. X) of each node from a PDE (the Richards equation from the Earth Science Module). However, I get an error "Failed to evaluate variable Jacobian; X; Global Scope" when I tried to solve for the problem. I think this is due to X being dependent also on space as well as time while the ODE accepts only time-dependent variables. I can only solve for the problem when I define an integration coupling variable of X in the domain, and use that in the ODE.
I would like to ask if there is a way to define the ODE such that it accepts a global variable like X, instead of only an integration coupling variable of X within the whole domain? Thank you very much for your advice and help!
Hi, Ed Low
I encountered the same problem as you, have you known how to fix it? Thank you!