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DBD plasma actuator model
Posted Nov 21, 2014, 3:23 p.m. EST Plasma Physics Version 4.3a 3 Replies
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I am trying to study gas discharge at atmospheric pressure using a dielectric barrier discharge plasma actuator (2D geometry) consisting of two electrodes, one exposed to air gas at atmospheric pressure and the other covered by a dielectric material. Air plasma kinetic model was modelled by air cross-sections from file, with N2, O2, N2_1p, O2_1p as species. Volumetric and surface reactions were also considered. N2 species has the largest mass fraction and it was chosen to fulfil the mass constraint. Initial mass fraction of O2 species is equal to 0.22. Initial number density of O2 ions is zero. Electric potential BC is applied on the exposed electrode (Vrf = 5e3[V]*sin(2*pi*1e3*t[1/s])), Ground BC on the other electrode.
I set a time-dependent study step from 0 to 10 ms, but I get the error:
Repeated error test failures. May have reached a singularity.
Time : 1.985019325484047e-005
Undefined value found.
Undefined value found in the stiffness matrix.
There are 2 equations giving NaN/Inf in the matrix rows for the variable mod1.wN2_1p.
at coordinates: (0.00100937,0.00120891), (0.00102875,0.0012092), ...
and similarly for the degrees of freedom, NaN/Inf in the matrix columns.
Last time step is not converged.
The same model was successful solved by applying an electric potential with same freq but lower amplitude (1000V), so I think that the problem is the too much high amplitude of the electric potential Vrf.
I tried to increase the Vrf’s amplitude gradually by using previous solutions (obtained with lower amplitude) as initial values of dependent variables (both variables solved for and variables not solved for), but I tried the same error with amplitudes > 1000 V.
Can someone help me?
Chiara D.P.
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When the error message includes the comment "May have reached a singularity", it is often the case that a dependent variable is tending toward infinity or negative infinity on one of the node points. The software should save the solution at the time of failure, so you can plot the dependent variable that is referenced in the error message (wN2_1p) at this time and check whether a singularity appears anywhere in the solution. If not, check the other dependent variables. If you see a singularity, try refining the mesh in that location and running the model again.
Convergence errors are not uncommon for this type of analysis, and an overly coarse mesh is often the cause. Generally, higher plasma currents require finer meshes. It also appears that you may be using the linear equation formulation. If so, you may want to try the log formulation, which is often more stable.
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Luke Gritter
AltaSim Technologies
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I solved my model by using extremely fine custom meshing.
I thank you so much for your help!
Chiara
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