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Deformed geometry model converges until I try to use the lagrange multiplier

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Hi all,

I'm trying to model the growth of a cavity in a block of ice in response to a flow of hot air. I have a model that works fairly well, with conjugate heat transfer and a deformed geometry to track the air / ice interface as melting progresses. At least, it works fine as long as I prescribe the melting rate at a fixed value of 0.001 m/s. In order to simulate melting I followed the approach of the Tin Melting Front example "Stefan problem" model: set the boundary temperature as a weak constraint, and use the lagrange multiplier as input to a prescribed normal deformation on that boundary.

The problem is that whenever I switch from the fixed value to the "Stefan problem" expression for the Prescribed Normal Mesh Velocity, my model no longer converges -- it says:

Nonlinear solver did not converge.
Time: 0
Attempt to evaluate real square root of negative number.

The model is attached. Thanks in advance for any advice!

Aaron

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