Flow Freezing in a Periodic Mode

In some problems the flow field depends on temperature (or species concentrations), so both the velocity and the temperature (concentrations) change simultaneously throughout the calculation. Nevertheless, since they change in a different manner, i.e., the velocity field changes faster than the temperature (concentrations) field, therefore approaching its steady state solution earlier, the Flow Freezing option can be used in a Periodic mode to reduce the CPU time required for solving such problems.

The Periodic mode of the Flow Freezing option consists of calculating the velocity field not in each of the iterations (time steps), but periodically for a number of iterations specified in No freezing (iterations) after a period of freezing specified in the Freezing (iterations) (see Figure 1) The temperatures and concentrations are calculated in each iteration. Examples include channel flows with specified mass flow rates and pressures, so the fluid density and, therefore, velocity depend on the fluid temperature, or flows involving free convection, where due to the buoyancy the hot fluid rises, so the velocity field depends on the fluid temperature.

Figure 1. The Periodic mode of the Flow Freezing (V - freezing, X - no freezing).

As an example, let us consider a 3D external problem of an air jet outflow from a body face into still air (see Figure 2, in which the jet outflow face is marked by a red line). Here, the wire frame is the computational domain. The other body seen in this figure is introduced and disabled in the Component Control dialog box (so it is a fluid region) in order to see the air temperature averaged over its face (the project Goal), depending on the air temperature specified at the jet outflow face.

Figure 2. Air jet outflow from a body face into a still air.

This problem is solved in several stages. At the first stage, the calculation is performed for the cold (Tc = 300 K, which is equal to the environment temperature) air jet. Then we clone the project including copying the results. Next, we set the outlet air temperature to Th = 400 K, specify the Periodic mode of the Flow Freezing option by its Start moment of 0.25 travels (in order for the heat to have time to propagate along the jet to the measuring face) and under Duration specify 10 as both the Freezing (iterations) and No freezing (iterations) values.

Then perform the calculation on the same computational mesh with the Take previous results option in the Run box.

As a result, the calculation with flow freezing takes less CPU time than the similar calculation without the Flow Freezing option enabled.