CAE-Solver Settings

Initialization

Iterative solution procedures require that all solution variables are assigned initial values before calculating a solution

A good initial guess can reduce the solution time

In some cases a poor initial guess may cause the solver to fail during the first few iterations

The initial values can be set in 3 ways:

1.Solver automatically calculates the initial values

2.Initial values are entered by the user

3.Initial values are obtained from a previous solution

Initial values can be set on a per-domain basis or globally for all domains

Initialization–Setting Initial Values

Insert Global Initialisation from the toolbar or by right-clicking on Flow Analysis 1

Edit each Domain to set initial values on a per-domain basis

-When both are defined the domain settings take precedence

-Solid domain must have initial conditions set on a per-domain basis

The Automatic option means that the CFX-Solver will calculate an initial value for the solved variable unless a previous results file is provided

-Will be based on boundary condition values and domain settings

The Automatic with Value option means that the specified value will be used unless a previous results file is provided

-Can use a constant value or an expression

Initialization–Using a Previous Solution

To use a previous solution as the initial guess enable the Initial Values Specification toggle when launching the Solver

You can provide multiple initial values files

-When simulating a system you can provide previous solutions for each component of the system as the initial guess

-Usually each file would correspond to a separate region of space

-It is best if domains in the Solver Input File do not overlap with multiple initial values files

Solver Control–Editing

Edit the Solver Control object in the Outline tree

Solver Control – Options

The Solver Control panel contains various controls that influence the behavior of the solver

These controls are important for the accuracy of the solution, the stability of the solver and the length of time it takes to obtain a solution

Solver Control–Advection Scheme

The Advection Scheme refers to the way the advection term in the transport equations is modeled numerically

-i.e. the term that accounts for bulk fluid motion

-Often the dominant term

Three schemes are available, High Resolution, Upwind and Specified Blend

-Discussed in more detail next

There is rarely any reason to change from the default High Resolution scheme

Solver Control – Advection Scheme Theory

Solution data is stored at nodes, but variable values are required at the control volume faces to calculate fluxes

Solver Control – Turbulence Numerics

Regardless of the Advection Scheme selection, the Turbulence equations default to the First Order (Upwind) scheme

-Usually this is sufficient

The High Resolution scheme can be selected for additional accuracy

-Can give better accuracy in boundary layers on unstructured meshes

Solver Control – Convergence Control

The Solver will finish when it reaches Max. Iterations unless convergence is achieved sooner

-If Max. Iterations is reached you may not have a converged solution

-Can be useful to set Max. Iterations to a large number

When the Solver finishes you should always check why it finished

Fluid Timescale Control sets the timescale in a steady-state simulation …

Solver Control – Timescale Background

ANSYS CFX employs the so called False Transient Algorithm

-A timescale is used to move the solution towards the final answer

In a steady-state simulation the timescale provides relaxation of the equation non-linearities

A steady-state simulation is a “transient” evolution of the flow from the initial guess to the steady-state conditions

-Converged solution is independent of the timescale used

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