Non linear term

[lahoussaine BOURRICHE]

Hello community.
I have two equations with two variables :
first depends on u : delta u  = J
second depends on u and T : delta T = sigma * u^2 so u in this equation is
a source
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I have problem with implementation of second term  sigma * u^2
i did something like :
gf.asm('generic',mim,1,"sigma*((abs(u)*abs(u)).Test_T)",region_number,'T',1,mfT,0,'sigma',1,mfs,0',u',0,mfu,0)
but i get values  = nan.
Does the problem come from the non linearity of the term ?