Many natural or free (buoyancy-driven) convection analyses require a large number of iterations to become fully converged. The flow and thermal solutions are tightly coupled, but the temperature distribution in the solids can take a longer time than the flow solution to fully converge.
Quick Natural/Free Convection addresses this problem by automatically running a coupled flow and thermal simulation (in the same manner as a traditional natural convection analysis) followed by mapping the film coefficients to all solids in the model, and then running a conduction-only thermal solution in the solids.
Unlike Quick Forced Convection, Quick Natural/Free uses the full Navier-Stokes solver to arrive at a coupled flow and thermal solution within the model. The time savings occurs after this step is complete (after 200 iterations) in that the thermal solution throughout the solid parts is accelerated. This method will deliver a time savings over a fully coupled natural convection solution of potentially several hundred iterations.
Specify a gravity vector in the two fields below the Natural/Free check-box. If the Gravity Method is set to Earth, then specify a unit vector in the Gravity Direction field. If the Method is set to Components, then enter the components in the Gravity Components line in the analysis length units.
The number of iterations will automatically be set to 200. When the Solve button is pressed, the mesh will generate (in the standard manner), and the coupled flow and thermal solution will progress for 190 iterations. For the final 10 iterations, the flow solution is frozen, and just the thermal solution is computed.
A traditional coupled natural convection analysis can be switched to a Quick Natural/Free solution and continued. If the number of iterations run as a standard analysis is less than 190, then upon restarting the analysis, the solution will run as a coupled solution until 190 iterations have been completed, and then automatically convert to a Quick Natural/Free solution. If the original coupled solution was run more than 190 iterations, then upon restart, the Quick Natural/Free solution will be invoked immediately.
Note that only steady-state solutions are supported for Quick Natural/Free, and that an error will be given in the analysis is set to transient with Quick Natural/Free enabled. Also, mixing models such as scalar, steam, cavitation, and humidity are not supported with Quick Natural/Free.