Flow Freezing in a Permanent Mode

See Figure 1.

At the beginning the entire fluid region is filled with a cold (Tc = 300 K) liquid. A hot (Th = 400 K) liquid enters the vessel through the lower channel (the upper channel is the exit). As a result, a vortex with a cold core is developed in the vessel. The vortex core temperature is changed mainly due to heat diffusion. To measure it, a small body is placed at the vortex center and disabled in the Component Control dialog box, so that it is treated by Flow Simulation as a fluid region. Its minimum temperature (i.e., the minimum fluid temperature in this region) is the Volume Goal of the calculation.

First of all, let us consider Flow Freezing operating in the Permanent mode. The only user-specified parameter in Permanent mode is the starting moment of enabling the Flow Freezing option. Until this moment the calculation runs in a usual manner. After this moment the fluid velocity field becomes frozen, i.e., it is no longer calculated, but is taken from the last iteration performed just before the Flow Freezing Start moment. For the remainder of the run only the equations’ terms concerning heat conduction and diffusion are calculated. As a result, the CPU time required per iteration is reduced.

The starting moment of the Flow Freezing option should be set not too early in order to let the flow field to fully develop. As a rule, an initial period of not less than 0.25 travels is required to satisfy this condition. In most problems the 0.5 travel initial period is sufficient, but there are problems that require a longer initial period.

When first solving the problem under consideration we set the maximum number of travels to 10. The calculation performed without applying the Flow Freezing option then required about 10 travels to reach the convergence of the project Goal (the steady-state minimum fluid temperature in the vortex core). However, the steady-state fluid velocity field was reached in about 0.5 travels, i.e., substantially earlier. So, by applying the Flow Freezing option in the Permanent mode (just after 0.5 travels) the same calculation requires substantially less time on the same computer to reach the convergence of the project Goal. If it is necessary to perform several calculations with the same fluid velocity field, but different temperatures and/or species concentrations, it is expedient to first calculate this fluid velocity field without applying the Flow Freezing option. Then, clone the Flow Simulation project into several projects (including copying the calculation results), make the required changes to these projects, and perform the remaining calculations for these projects using the calculated results as initial conditions and applying the Flow Freezing option in the Permanent mode with a zero Start period.