Setting relaxation factors for the flow solver

The Relaxation Factors options on the 3D Flow Solver page in the Solver Parameters dialog box control the convergence of the iterative linear solver.

The default values for these options ensure good convergence for most models. In general, you should not increase these values. For hard to converge models, reducing some of these relaxation factors will improve the convergence process but increase solution time. The value of these relaxation factors must be greater than zero and less than or equal to one.

If the convergence gets worse after you have decreased either the Global, Mass or Fluid relaxation factors or a combination of these factors, you probably have a problem with the mesh. You should check the mesh quality.

Steady State Relaxation Time Step

The Steady State Relaxation Time Step may need to be adjusted if there are convergence problems with steady state flow analyses.

Global

The Global relaxation factor relaxes the following quantities:

• Wall shear stress.

• Velocity correction for Moving Frame of Reference boundaries.

• The residuals of the boundary node equations for momentum, energy, and (if applicable), general scalar and K-epsilon equations.

Mass

The Mass relaxation factor applies to the assembled equations of mass at each iteration.

This is probably the most important flow relaxation factor for models with convergence problems and models that oscillate. You can specify very small values for a complex model; however, the solution time will increase.

For a steady state analysis, decreasing the time step in conjunction with the Mass relaxation factor often helps convergence. For example, divide your time step by two and use the default value for Mass.

Models with high head loss or numerous flow blockages leaving very small flow passage are examples of configurations where the Mass relaxation factor is important.

Fluids

The Fluids relaxation factor lets you control the main diagonal terms of the assembled equations of momentum in the iterative solver. Reduce this factor for models that crash with an overflow or a failure of the iterative solver or for models showing linear solver convergence problems.

In the current folder, you can find the LOG file. Convergence information for the linear solver is listed in this LOG file in the Convergence History section under the Linear Solution column. An F or small ok sign in this column indicates that you should reduce the Fluids relaxation factor. The first number listed in the Linear Solution column on the pressure line also indicates a convergence problem with the linear solver. This number represents the work unit. One sweep of the iterative flow solver through the finest mesh, that is the mesh you create, equals a work unit of one. As the linear solver sweeps through coarser meshes created by the multigrid process, fractions of a work unit are computed. A large work unit number means that the linear solver has trouble converging as it sweeps though the meshes. When the work unit reaches 40, the flow solver passes to a new iteration. Work units of 40 are acceptable at the start of an analysis.

Turbulence

The Turbulence relaxation factor applies to the two turbulence equations when modeling turbulence. This relaxation factor applies to the following Turbulence Models:

• Standard K-Epsilon

• RNG K-Epsilon

• Realizable K-Epsilon

• SST — Shear Stress Transport

• SA — Spalart - Allmaras

• K-Omega

You set these turbulence models on the Solution Details page in the Solution dialog box.

The main purpose of this relaxation factor is to prevent wrong values of k or ε or ω in or near the boundary layer from affecting the other variables.

Reduce this value when divergence occurs early in the iterative process soon after the turbulence equations become active. Reducing this value slows down the convergence rate and should be used when convergence cannot be achieved otherwise.

Screen Resistance

The Screen Resistance relaxation factor applies to the pressure drop across screens, such as linear screens, that you model with a Screen simulation object. See Screen for more information.

Decrease the Screen Resistance relaxation factor if you apply a high head loss coefficient to a Screen in the model. Use this setting in conjunction with the Mass relaxation factor to further improve convergence.

Fan Curves (Inlet Flow, Outlet Flow, and Internal Fans)

The Fan Curves (I/O/Internal Fans) relaxation factor applies for volumetric flow rate computed from fan curves at Inlet, Outlet, or Internal Fan type of Flow Boundary Condition simulation object. See Flow Boundary Condition types for more information.

When using fan curves, use this relaxation factor to stabilize the iterative flow solution for the fan curve operating point calculation. If the solution is not converging or is oscillating, reduce the relaxation factor. You may have to use a smaller value if you apply a high head loss coefficient or some heat generation on a fan defined with a fan curve. Use this setting in conjunction with the Mass relaxation factor to further improve convergence.

Fan Curves (Recirculation Fans)

The Fan Curves (Recirculation Fans) relaxation factor applies for volumetric flow rate computed from fan curves defined for the Recirculation Loop type of Flow Boundary Condition simulation object. See Recirculation Loop type for more information.

When using fan curves for a Recirculation Loop, you can decrease this relaxation factor to stabilize the iterative flow solution for the fan curve operating point calculation. For a Recirculation Loop, the elements of the extract and return regions do not have to match in size or number. Convergence at the operating point is harder to achieve. If the solution is not converging or is oscillating, reduce the relaxation factor and try again. You may have to use a smaller value, especially if you define a high head loss coefficient, heat gain or heat loss. Use this setting in conjunction with the Mass relaxation factor to further improve convergence.

How do I

How to choose values for Maximum Normalized Velocity Change and Maximum Normalized Pressure Change

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