unknown variable after creating it in an analytical function
Posted Jan 2, 2024, 7:28 a.m. EST General, Equation-Based Modeling, User Interface Version 6.0 3 Replies
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Hello,
I have created two variables 't' and 's' using analytic functions namely an1 & an2 respectively. while the first variable 't' seems to be recognised fine , it is the other variable 's' which doesnt seem to be recognised. The expression which has both 't' and 's' shows in orange in comsol . Upon hovering my mouse pointer on this expression i get " unknown variable 's' ". Please help me with this. Been terribly stuck for very long.
I have attached the snapshots of the issue i am facing. I have also attached the mph file ( cleared all solutions to reduce file size).
Thanks, Ashish
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Hi Ashish,
The arguments in the analytic functions are internal to them, used to define the function as a function of one or more variables. When calling the analytic function, you must do so with a variable that is defined in the model. In your case, you can call both an1
and an2
with t
, for example, which is the predefined variable for time in your model. On the other hand, s
is not known outside of an2
(but s1
and s2
would be, as surface parameterizations in 3D, for example; s
is predefined as an edge parameterization in 2D).
Best regards,
Magnus
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Dear Magnus,
Thanks for writing back to me.
However, my issue is little different. In the 'variables' section you should find an variable called 'A'
where, A=(exp(-5.724e-10an2(s)) - 1.0)(0.5/(0.00025an2(s) + 1.0) - 0.5/(0.00025an1(t) + 1.0))^2(1.891e+5an1(t) - 3.782e+8(2.0/(0.00025an1(t) + 1.0) + (0.00025an1(t) + 1.0)^2)(0.00025an1(t) + 1.0)(8.333e-5an1(t) + 0.3333) + 7.564e+8) + (exp(-5.724e-10an2(s)) - 1.0)(0.5/(0.00025an2(s) + 1.0) - 0.5/(0.00025an1(t) + 1.0))^2(1.891e+5an1(t) - 3.782e+8(2.0/(0.00025an1(t) + 1.0) + (0.00025an1(t) + 1.0)^2)((0.00025an1(t) + 1.0)^2 + (0.00025an1(t) + 1.0)(8.333e-5an1(t) + 0.3333)) + 7.564e+8) + (exp(-5.724e-10an2(s)) - 1.0)(0.5/(0.00025an2(s) + 1.0) - 0.5/(0.00025an1(t) + 1.0))(0.5(0.00025an2(s) + 1.0)^2 - 0.5(0.00025an1(t) + 1.0)^2)(94550.0an1(t) + 6.051e+15/(an1(t) + 4000.0)^2 - (2.017e+15(2.0/(0.00025an1(t) + 1.0) + (0.00025an1(t) + 1.0)^2)(0.00025*an1(t) + 1.0))/(an1(t) + 4000.0)^2 + 3.782e+8)
where 't' varies from 0 to 1000 seconds( 1000 seconds is the total simulation time period). The time step (lets say dt) is 1 second. 's' on the other hand is a smaller time step which varies within the confinements of each time step. In other words, when t=1 second , s varies from 0 to 1. when t=2 second , s varies from 1 to 2. when t=3 second , s varies from 2 to 3. .. . . . when t=800 second , s varies from 799 to 800. .. . . when t=1000 second , s varies from 999 to 1000.
To acheive this i used two analytic functions "an1(t)" and "an2(s)" . But the expression for A(shown above) shows orange . This is because 'an2(s)' is not recognised. ( I am pretty sure an1(t) only seems to be recognised as perhaps Comsol perceives it to be time , which is right by the way).
Please help me with this, coz as i said earlier i am terribly stuck with this.
P.S: You may refer to the image and mph files shared earlier in this conversation thread.
Thanks, Ashish
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Hi Ashish,
It is not clear to me what you want to achieve here. The only time variable that the COMSOL Multiphysics software recongnizes is t
, and the time steps for a transient simulation are determined by the solver (but there are many settings that you can use to control the time stepping). The output times (the times for which a solution is computed and stored for further processing) are specified by the user in the Time Dependent (or similar) study step. If you have further questions, I recommend that you contact COMSOL Support: https://www.comsol.com/support/case/.
Best regards, Magnus
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