Residuals falling below a threshold do not prove convergence. They measure how well the discretised equations are satisfied, not whether the answer has stopped changing, and in natural convection they can sit at a low value while the flow field is still evolving.
The reliable test is to monitor the quantity you actually care about: the heat transfer rate through a wall, the average outlet temperature, the maximum velocity. When that value stops changing while the residuals stay flat, the solution has converged.
The second check is the energy balance. Whatever enters the domain must leave it or accumulate in it, and a mismatch beyond a small percentage means the case is not converged regardless of what the residual plot shows.
In this tutorial, you will learn how to configure your simulation with the Convection and analyze the convergence criteria to obtain accurate results
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