26.6 Setting Solution Limits

In order to keep the solution stable under extreme conditions, ANSYS FLUENT provides limits that keep the solution within an acceptable range. You can control these limits with the Solution Limits dialog box (Figure  26.6.1).

Solution Controls Limits...

Figure 26.6.1: The Solution Limits Dialog Box

ANSYS FLUENT applies limiting values for pressure, static temperature, and turbulence quantities. The purpose of these limits is to keep the absolute pressure or the static temperature from becoming 0, negative, or excessively large during the calculation, and to keep the turbulence quantities from becoming excessive. ANSYS FLUENT also puts a limit on the rate of reduction of static temperature to prevent it from becoming 0 or negative.

Typically, you will not need to change the default solution limits. If pressure, temperature, or turbulence quantities are being reset to the limiting value repeatedly (as indicated by the appropriate warning messages in the console), you should check the dimensions, boundary conditions, and properties to be sure that the problem is set up correctly and try to determine why the variable in question is getting so close to zero or so large. You can use the "marking'' feature (used to mark cells for adaption) to identify which cells have a value equal to the limit. (Use the Iso-Value Adaption dialog box, as described in Section  27.5.) In very rare cases, you may need to change the solution limits, but only do so if you are sure that you understand the reason for the solver's unusual behavior. (For example, you may know that the temperature in your domain will exceed 5000 K. Be sure that any temperature-dependent properties are appropriately defined for high temperatures if you increase the maximum temperature limit.)

For an ideal gas, the absolute pressure and static temperature solution limits are set as described in this section. However, there are no static temperature and absolute pressure solution limits for incompressible flow.

Limiting the Values of Solution Variables

The limiting minimum and maximum values for absolute pressure are shown in the Minimum and Maximum Absolute Pressure fields. If the ANSYS FLUENT calculation predicts a value less than the Minimum Absolute Pressure or greater than the Maximum Absolute Pressure, the corresponding limiting value will be used instead. Similarly, the Minimum and Maximum Temperature are limiting values for energy calculations.

The Maximum Turb. Viscosity Ratio and the Minimum Turb. Kinetic Energy are limiting values for turbulent calculations. If the calculation predicts a $k$ value less than the Minimum Turb. Kinetic Energy, the limiting value will be used instead. For the viscosity ratio limit, ANSYS FLUENT uses the limiting maximum value of turbulent viscosity ( $C_{\mu} k^2 / \epsilon$) in the flow field relative to the laminar viscosity. If the ratio calculated by ANSYS FLUENT exceeds the limiting value, the ratio is set to the limiting value by limiting $\epsilon$ to the necessary value.

Adjusting the Positivity Rate Limit

In ANSYS FLUENT's density-based solver, the rate of reduction of temperature is controlled by the Positivity Rate Limit. The default value of 0.2, for example, means that temperature is not allowed to decrease by more than 20% of its previous value from one iteration to the next. If the temperature change exceeds this limit, the time step in that cell is reduced to bring the change back into range and a "time step reduced'' warning is printed. (This reduced time step will be used for the solution of all variables in the cell, not just for temperature.) Rapid reduction of temperature is an indication that the temperature may become negative. Repeated "time step reduced'' warnings should alert you that something is wrong in your problem setup. (If the warning messages stop appearing, the calculation may have "recovered'' from the time-step reduction.)

For high-speed flow, if your solution is diverging particularly for the energy equation, then lowering this limit to 0.05 or 0.02 might help in overcoming divergence.

Resetting Solution Limits

If you change and save the value of one of the solution limits, but you then want to return to the default limits set by ANSYS FLUENT, you can reopen the Solution Limits dialog box and click the Default button. ANSYS FLUENT will change the values to the defaults and the Default button will become the Reset button. To get your values back again, you can click the Reset button.