COMSOL Forums: Internal error in piecewise expression evaluation
http://www.comsol.com/community/forums/general/thread/41355/
Most recent forum messagesSun, 29 Dec 2013 22:16:01 +0000COMSOL Forums: Internal error in piecewise expression evaluationhttp://www.comsol.com/shared/images/logos/comsol_logo.gif
http://www.comsol.com/community/forums/general/thread/41355/
Re: Internal error in piecewise expression evaluation
http://www.comsol.com/community/forums/general/thread/41355/#p111813
Hey Tero,<br />
<br />
thank you that you investigated in my topic. Unfortunately I can't add an initial flow velocity, because the my massflow parameter is fixed. But your advice with "outflow" gave me a hint. I figured out that in the "outlet" condition I selected "laminar outflow to exit pressure" with a too short exit length (although the simulation always succeeded in the past). As soon as I put in a longer exit length, the model is working again. Even without the "outflow" boundary condition.<br />
<br />
Thank you so much that you helped me to find a solution :)<br />
<br />
Have a happy NYE & best regards,<br />
FabianSun, 29 Dec 2013 22:16:01 +000041355.1388355361.111813Re: Internal error in piecewise expression evaluation
http://www.comsol.com/community/forums/general/thread/41355/#p111785
Hi,<br />
<br />
I tested your model and add few things. First I add "Outflow" for heat transfer. Second I add initial value for fluid flow (0.1 m/s). With these I got your model working.<br />
<br />
Best regards<br />
<br />
Tero HietanenSat, 28 Dec 2013 15:42:23 +000041355.1388245343.111785Re: Internal error in piecewise expression evaluation
http://www.comsol.com/community/forums/general/thread/41355/#p111760
I tried to describe the flow consistency index m by an analytic instead of a piecewise function. Now COMSOL states the following error. Maybe it helps to define the problem.. (boundary 3 is the Outlet of the duct)<br />
<br />
"Error in user-defined function.<br />
- Function: m<br />
Failed to evaluate variable.<br />
- Variable: mod1.nitf.m_pow<br />
- Defined as: m((mod1.T/unit_K_cf))<br />
Failed to evaluate variable.<br />
- Variable: mod1.nitf.mu<br />
- Defined as: (mod1.nitf.m_pow*(mod1.nitf.sr^((-1)+mod1.nitf.n_pow)))<br />
Failed to evaluate expression.<br />
- Expression: mod1.nitf.mu<br />
Failed to evaluate operator.<br />
- Operator: mean<br />
- Geometry: 1<br />
- Boundary: 3"Fri, 27 Dec 2013 07:31:55 +000041355.1388129515.111760Internal error in piecewise expression evaluation
http://www.comsol.com/community/forums/general/thread/41355/#p111749
Merry Christmas everybody,<br />
<br />
I was simulating a laminar flow in a heated circular duct with a non-newtonian Fluid. In order to describe the dynamic viscosity I used the non-newtonian power law with the parameters m and n. <br />
I do simulations with heating powers 500-1500W at mass flows 0.25, 0.5, 0.75 and 1 kg/min. For some reason the following simulations abort:<br />
0,75 kg/min: 750W, 1500W<br />
1 kg/min: 500W, 750W, 1000W, 1500W<br />
<br />
The following error is shown:<br />
“Internal error in piecewise expression evaluation.<br />
- Expression: 0.027-0.0050*log(-273.15+T)”<br />
<br />
Which is used in the flow consistency index m. As soon as I raise the parameter border to 273.16K (to prevent log(0) become infinity, but it worked fine earlier) I get the error:<br />
“Failed to find a solution.<br />
Singular matrix.<br />
<br />
There are 84805 equations giving NaN/Inf in the matrix rows for the variable mod1.T.<br />
at coordinates: (0.0039021,0.001), (0.00386713,0.001), (0.00382797,0.001), (0.00378462,0.001), (0.00373706,0.001), ...<br />
<br />
There are 84266 degrees of freedom giving NaN/Inf in the matrix columns for the variable mod1.T.<br />
at coordinates: (0.00382797,0.001), (0.00378462,0.001), (0.00373706,0.001), (0.00368531,0.001), (0.00362937,0.001), ...<br />
There are 42940 degrees of freedom giving NaN/Inf in the matrix columns for the variable mod1.u.<br />
at coordinates: (0.00368531,0.001), (0.00362937,0.001), (0.00382797,0.002), (0.00378462,0.002), (0.00373706,0.002), ...<br />
There are 36019 degrees of freedom giving NaN/Inf in the matrix columns for the variable mod1.w.<br />
at coordinates: (0.00398182,0.035), (0.00395944,0.035), (0.00393287,0.035), (0.00398182,0.036), (0.00395944,0.036), ...<br />
Returned solution is not converged.”<br />
<br />
Do you have any idea what the first error indicates for my equation. Everything worked fine in all the earlier simulations. I just changed the parameter for the heating power :(<br />
<br />
Sincerely, Fabian HolzThu, 26 Dec 2013 11:42:07 +000041355.1388058127.111749