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What is the purpose of the Turbulence Start Control option in CFD-ACE+ and CFD-CADalyzer?

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.


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Figure 1.  Example case study


In this example, a small circular jet enters 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 the X-velocity, 0.1 for the turbulence intensity and 0.203 for the dissipation rate. With default values for relaxation, the simulation will diverge due to poor initial conditions. However, using the 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.


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Figure 2.  Residuals without turbulence start-up option


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 Figure 3.  Residuals with turbulence start-up option


So what is this option doing?  It gives the user the ability to ramp up 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. Start-up control 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):


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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 calculated from the ratio is used over an initial number of iterations (specified in this second field) and replaces the values that the turbulence model calculates.
  3. Transition Iterations: This gives the user the ability to linearly ramp up the values calculated from the turbulence model.  For instance, if 20 is used, then at the first transition iteration (iteration #26 in this example) it will use 95% of the value calculated from the viscosity ratio and 5% of the value calculated from the turbulence model. At iteration #27, it will use 90% from viscosity ration and 10% from turbulence model, and so on until the last transition iteration. If zero is used in this field, then at the end of the Initial Iterations the values from the turbulence model will immediately be used.


Consider using this feature if you have problems getting a turbulent case to start-up without diverging.

 

Tested with V2011.0

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Last update: 2012-07-23 12:12
Author: ESI-CFD Support Team
Revision: 1.4

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