If it is unclear if an analysis should be run as laminar or turbulent, try laminar first. If the flow is actually turbulent, the analysis will typically diverge within the first ten to fifteen iterations. Change the setting Turbulent, and start again from iteration 0.
This is the default turbulence model. It is typically more accurate than the constant eddy viscosity model, but more computationally intensive and slightly less robust. It is not as resource intensive as the RNG model, but still gives good results. It is a general purpose model that performs well across a large number of applications.
The constant eddy viscosity model is slightly less rigorous than the k-epsilon model, but more numerically stable. This is a good choice for lower speed turbulent flows and some buoyancy flows. This model is useful if divergence occurs with one of the other models.
The RNG turbulence model is more computational intensive, but sometimes slightly more accurate than the k-epsilon model, particularly for separated flows. This model works best for predicting the reattachment point for separated flows, particularly for flow over a backward-facing step. When using the RNG model, it is often recommended to start with the k-epsilon model and after this model is fairly well converged, enable the RNG model.
The Mixing Length turbulence model is primarily designed for internal natural convection analyses. Use of the mixing length model, in some cases, has been shown to reduce run times and provide better accuracy than the default turbulence model for internal buoyancy-driven flows.
Because this turbulent model does not use wall functions, Mesh Enhancement should be always be enabled. We recommend increasing the number of mesh enhancement layers to 5 (using the Mesh Enhancement controls on the Meshing dialog).
Note that analyses run with this turbulence model may not be as stable as those run with the k-epsilon model. Because of this, the Intelligent Solution Control should be enabled (the switch is located in the Solution Control dialog launched from the Solve task dialog.) Likewise, analyses run with this model may take more iterations to reach a fully converged solution.
High Reynolds flows that are run with the Low Reynolds turbulence model will generally produce the same solution as would the k-epsilon model. Likewise, laminar flows that are run with this model will produce similar results to a solution run as laminar.
This algorithm goes through a number of steps to obtain turbulent flow solutions. The algorithm starts by running 10 iterations using a constant eddy viscosity model, so the k and epsilon equations are not solved. With this solution as an initial guess, the two-equation turbulence model is started. At iteration 10, a spike in the convergence monitoring data will appear for the k and epsilon equations. Other steps are then taken to gradually arrive at the converged result. These steps may involve spikes in the convergence monitoring data at iterations 10, 20 and 50. After 50 iterations, the ATSU is turned off automatically.
If Lock On is selected, the ATSU stays on during the entire analysis until the user manually clicks it off. If there are convergence difficulties after iteration 50 (divergence within 10 iterations), then you should enable Lock On. If the ATSU is turned on, you should run at least 200 iterations to ensure convergence of the turbulent flow solution.
If Extend is selected, an extended version of the ATSU is activated. This method is useful for difficult analyses, particularly compressible analyses. The minimum number of iterations that should be run with this algorithm is 400.
The Turb/Laminar Ratio is the ratio of the effective (turbulent) viscosity to the laminar value. It is used to estimate the effective viscosity at the beginning of the turbulent flow analysis. In most turbulent flow analyses, the effective viscosity is 2-3 orders of magnitude larger than the laminar value. The default value is generally suitable for most flows.
It is often helpful to increase the Turb/Lam Ratio to 1000 or even 10,000 for flows that feature a small, high speed jet shooting into a large plenum. Such flows are typically momentum-driven, and benefit from a larger turbulent viscosity at the beginning of the calculation.
The Turbulence Intensity Factor controls the amount of turbulent kinetic energy in the inlet stream. Its default value is 0.05 and should rarely exceed 0.5. The expression used to calculate turbulent kinetic energy at the inlet is: