Turbulent flow can be hard to stabilize during the first iterations. Fortunately, a solution control feature exists in CFD-ACE+ and CADalyzer that can aid the convergence of turbulent cases. Consider the case study presented in figure 1.

Figure 1: Example case study

This is a small circular jet entering into a rectangular shaped duct.   The inlet conditions for the duct are 30.0 m/s with a turbulence intensity of 0.03 and for the jet are 71.263 m/s with a turbulence intensity of 0.05.  The initial conditions for this case are 30.0 m/s for U velocity, 0.1 for turbulence intensity, and 0.203 for the dissipation rate.  Using default values for relaxation the simulation will actually diverge due to poor initial conditions.  However using the new Turbulent Start Control feature will allow the case to converge.  The residual plots from each of the two cases are shown in figures 2 and 3.

Figure 2: Residuals without turbulence start-up option

 Figure 3: Residuals with turbulence start-up option

So what is this option doing?  It is giving the user the ability to ramp  the values of turbulence calculated by the code.  Sometimes during start-up the effective viscosity computed from the turbulence model can produce values which cause problems with convergence.  This new method holds the value of the effective viscosity to a more reasonable value for a given number of iterations so that the flow field can start to develop.  Once this occurs, the turbulence model will start to produce more reasonable values and a more accurate effective viscosity.  There are three inputs for this feature (see figure 4):

Figure 4: Turbulence start control option in CFD-ACE-GUI

1. Viscosity Ratio: Ratio of effective viscosity to laminar viscosity.
2. Initial Iterations: The effective viscosity is then used over an initial number of iterations (specified in this second field) instead of the values calculated from the turbulence model.
3. Transition Iterations: This gives the user the ability to linearly ramp in the values calculated from the turbulence model.  For instance, if 20 is used, then at the first transition iteration it will use 95% of the value calculated from the viscosity ratio and 5% calculated from the turbulence model.  However if zero is used, then at the end of the Initial Iterations the values from the turbulence model will immediately be used.

If you have any questions about this feature or would like us to discuss some other topic in the future, please let us know.

Regards,
ESI CFD Support Team