FVScalarLagrangeMultiplierInterface
This class should be inherited to create interface constraints in finite volume. A Lagrange multiplier, a scalar variable, is used to enforce the constraint on the interface.
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Lagrange multipliers do not contribute to the diagonal of the residual, which can hurt numerical convergence for some linear solvers. For the lu (sub-)preconditioner, make sure to use a NONZERO (sub_)pc_factor_shift_type.
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kernel_coverage_check, a parameter in the [Problem] block, should be set to false, as the Lagrange multiplier variable is a variable that is not found on any block, only on the interface.