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## Internal error in piecewise expression evaluation

Posted 26/12/2013, 07:42 GMT-05:00 Fluid, Heat Transfer & Phase Change, Computational Fluid Dynamics (CFD), Materials Version 4.2a 3 Replies

Merry Christmas everybody,

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.

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:

0,75 kg/min: 750W, 1500W

1 kg/min: 500W, 750W, 1000W, 1500W

The following error is shown:

“Internal error in piecewise expression evaluation.

- Expression: 0.027-0.0050*log(-273.15+T)”

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:

“Failed to find a solution.

Singular matrix.

There are 84805 equations giving NaN/Inf in the matrix rows for the variable mod1.T.

at coordinates: (0.0039021,0.001), (0.00386713,0.001), (0.00382797,0.001), (0.00378462,0.001), (0.00373706,0.001), ...

There are 84266 degrees of freedom giving NaN/Inf in the matrix columns for the variable mod1.T.

at coordinates: (0.00382797,0.001), (0.00378462,0.001), (0.00373706,0.001), (0.00368531,0.001), (0.00362937,0.001), ...

There are 42940 degrees of freedom giving NaN/Inf in the matrix columns for the variable mod1.u.

at coordinates: (0.00368531,0.001), (0.00362937,0.001), (0.00382797,0.002), (0.00378462,0.002), (0.00373706,0.002), ...

There are 36019 degrees of freedom giving NaN/Inf in the matrix columns for the variable mod1.w.

at coordinates: (0.00398182,0.035), (0.00395944,0.035), (0.00393287,0.035), (0.00398182,0.036), (0.00395944,0.036), ...

Returned solution is not converged.”

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 :(

Sincerely, Fabian Holz

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.

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:

0,75 kg/min: 750W, 1500W

1 kg/min: 500W, 750W, 1000W, 1500W

The following error is shown:

“Internal error in piecewise expression evaluation.

- Expression: 0.027-0.0050*log(-273.15+T)”

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:

“Failed to find a solution.

Singular matrix.

There are 84805 equations giving NaN/Inf in the matrix rows for the variable mod1.T.

at coordinates: (0.0039021,0.001), (0.00386713,0.001), (0.00382797,0.001), (0.00378462,0.001), (0.00373706,0.001), ...

There are 84266 degrees of freedom giving NaN/Inf in the matrix columns for the variable mod1.T.

at coordinates: (0.00382797,0.001), (0.00378462,0.001), (0.00373706,0.001), (0.00368531,0.001), (0.00362937,0.001), ...

There are 42940 degrees of freedom giving NaN/Inf in the matrix columns for the variable mod1.u.

at coordinates: (0.00368531,0.001), (0.00362937,0.001), (0.00382797,0.002), (0.00378462,0.002), (0.00373706,0.002), ...

There are 36019 degrees of freedom giving NaN/Inf in the matrix columns for the variable mod1.w.

at coordinates: (0.00398182,0.035), (0.00395944,0.035), (0.00393287,0.035), (0.00398182,0.036), (0.00395944,0.036), ...

Returned solution is not converged.”

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 :(

Sincerely, Fabian Holz

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3 Replies Last Post 29/12/2013, 18:16 GMT-05:00