[thewitcher745]

Hello, I have a project for studying the case of a concentric annulus with a moving core (2 concentric cylinders), through which the fluid flows. There is a gravitational field being exerted perpendicular to the axis of the cylinders. The inner cylinder is spinning at constant speed and there is a temperature difference between the two cylinders (the inner cylinder has a higher temperature.) I want to study the effects of mixed convection on temperature and velocity contours on various lengths in the tube.

The problem arises after I want to reach a solution. Firstly, the solution does not converge very easily. I have turned on gravity, set the density calculation method to Boussinesq, set operating temperatures, set the pressure discretization method to PRESTO! and I have a good structured, consistent mesh. After a while the solution doesn't meet the convergence criteria but rather the residuals reach a point and only fluctuate around one value, and also the result does not contain a clear effect of natural convection as the contours seem very symmetrical around the cylinders' axis except certain points, which go away after a certain length.

How do I fix the convergence and the results? And how do I make sure the effect of convection is clearly seen?