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Problems with High Mach Number laminar flow
Posted Aug 9, 2012, 11:52 a.m. EDT Computational Fluid Dynamics (CFD) Version 4.3, Version 5.1 3 Replies
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Hi,
My case is not so common for an application of hmnf that I have not been able to make it work. The main idea consists on modeling the flow physics of an electric arc by considering the arc channel heat as the main energy source for the surrounding gas, using a 2D-axisymmetry model. This gas at the same time will start moving inside an enclosed volume creating shockwaves that will reach the container walls several microsec. after the energy stimulus has been already applied to the one of the solid electrodes in less than 1us.
The problem I've facing is that the simulation does not start and I get the following error message:
Attempt to evaluate real square root of negative number.
- Function: sqrt
I've tried to simplify my model so the initial condition of temperature and pressure (6kK and 0.6MPa at the geometry axis, decreasing to 293K and 0.1MPa within the 0.5mm from the axis) IS the ONLY energy input to the model, on the gas volume (cylinder of r=4mm height=6mm). The materials properties used (copper-electrodes, nitrogen-gas) are mainly constant, avoiding problems with those that might not function for temperatures >1.5kK.
I would really appreciate any comment or suggestion.
Thanks.
My case is not so common for an application of hmnf that I have not been able to make it work. The main idea consists on modeling the flow physics of an electric arc by considering the arc channel heat as the main energy source for the surrounding gas, using a 2D-axisymmetry model. This gas at the same time will start moving inside an enclosed volume creating shockwaves that will reach the container walls several microsec. after the energy stimulus has been already applied to the one of the solid electrodes in less than 1us.
The problem I've facing is that the simulation does not start and I get the following error message:
Attempt to evaluate real square root of negative number.
- Function: sqrt
I've tried to simplify my model so the initial condition of temperature and pressure (6kK and 0.6MPa at the geometry axis, decreasing to 293K and 0.1MPa within the 0.5mm from the axis) IS the ONLY energy input to the model, on the gas volume (cylinder of r=4mm height=6mm). The materials properties used (copper-electrodes, nitrogen-gas) are mainly constant, avoiding problems with those that might not function for temperatures >1.5kK.
I would really appreciate any comment or suggestion.
Thanks.
3 Replies Last Post Mar 24, 2016, 2:34 a.m. EDT
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Posted:
1 decade ago
Hi,
I can only guess. Have you set initial values different from zero (exspecialy velocity field)? Checked the equations not getting zero with the parameters and variables you set? Are you solving pseudo timestepping?
It helped me when I had the same problem.
Bye
I can only guess. Have you set initial values different from zero (exspecialy velocity field)? Checked the equations not getting zero with the parameters and variables you set? Are you solving pseudo timestepping?
It helped me when I had the same problem.
Bye
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Posted:
1 decade ago
hej friedrich,
After several trials, I got to the conclusion that one gas property, the electrical conductivity as a function of temperature is killing the whole simulation and produces the gas temperature (heated up by the resistive losses) diverge drastically after a few time steps during the beginning of the energy injection.
Other properties like thermal conductivity, specific heat, viscosity and NEC radiation are plugged as temperature functions without affecting the convergence.
The biggest problem is that this property changes a lot from 1e-6 @ 293K to 1e4 @ 20kK (begin the changes more notable after 0,4 @ 5kK), which are the expected limits for the lowest and highest temperature at the channel axis.
Thanks for any clue you might give.
mvh,
/Oscar D
After several trials, I got to the conclusion that one gas property, the electrical conductivity as a function of temperature is killing the whole simulation and produces the gas temperature (heated up by the resistive losses) diverge drastically after a few time steps during the beginning of the energy injection.
Other properties like thermal conductivity, specific heat, viscosity and NEC radiation are plugged as temperature functions without affecting the convergence.
The biggest problem is that this property changes a lot from 1e-6 @ 293K to 1e4 @ 20kK (begin the changes more notable after 0,4 @ 5kK), which are the expected limits for the lowest and highest temperature at the channel axis.
Thanks for any clue you might give.
mvh,
/Oscar D
Hi,
I can only guess. Have you set initial values different from zero (exspecialy velocity field)? Checked the equations not getting zero with the parameters and variables you set? Are you solving pseudo timestepping?
It helped me when I had the same problem.
Bye
Please login with a confirmed email address before reporting spam
Posted:
9 years ago
Hi Oscar ;
I guess you already solved the problem since it was in 2012 , I am facing the same problem as i inject the electric arc heat to the hmnf model !! the electric conductivity is a function of temperature which causing all the errors in my model !
Please help
I guess you already solved the problem since it was in 2012 , I am facing the same problem as i inject the electric arc heat to the hmnf model !! the electric conductivity is a function of temperature which causing all the errors in my model !
Please help