The key to the evolution procedure is to identify the parameters governing the nonlinearity of the problem. Possible choices include the following:
density in problems with inertia
flow rate in a viscoelastic flow
relaxation time in a viscoelastic flow
moving boundaries
power index for a power law fluid
parameter for the temperature dependence of the viscosity
heat conductivity in a heat transfer problem
viscous and wall friction heating
slip coefficient for an extrusion process
surface tension in a free surface problem
Before starting an Ansys Polydata session, you should decide which material
parameters or boundary conditions will be governed by evolution. These parameters
and boundary conditions may concern different sub-tasks, though the evolution in
(for example, 
, 
, 
, the number of iterations, and other numerical parameters) is a
global task attribute. A list of suggested evolution parameters for various types of
problems is given in Table 28.1: Suggestions for Evolution Parameters.
To determine which parameter should be governed by evolution, try increasing (or decreasing) one parameter by a factor of 10 (for example, decrease flow rate, increase the power law index, etc.). Do this until you are able to get the problem to converge (this may require modifying more than one parameter).
Table 28.1: Suggestions for Evolution Parameters
| Problem | Evolution Parameter |
|---|---|
| Large viscous and wall friction heating dissipation | Flowrate 
|
Power law index less than 
|
 , or Picard scheme |
| Bird-Carreau, low index |
|
| Differential viscoelastic fluid | Flowrate  or relaxation time 
|
| 3D free surface without surface tension (extrusion and coextrusion) | Evolution on moving boundaries; alternatively, evolution on the slip coefficient |
| Free surface with surface tension (straight free surface is acceptable) | Surface tension coefficient  ; use quadratic coordinates |
| Free surface with surface tension (recirculations present on the free surface) | Switch to time-dependent mode; use quadratic coordinates |
| Moving contact point | Surface tension coefficient 
|
| Natural convection, low conductivity |
Thermal conductivity |
| Large Péclet number (low conductivity) with temperature dependence of the viscosity | Flowrate  or thermal conductivity 
|
| Integral viscoelastic problem | Evolutive viscosity |
| Radiation | Reference temperature 
|
| Forced convection or inertia |
Flowrate |
| Strong temperature dependence of viscosity |
 or 
|
Interface between two fluids,
 large |
 , with  fixed so that  increases with 
|
| Fiber spinning | Evolution on the pulling velocity |
| Extrusion with intense cooling | Heat transfer coefficient |
Now that you have determined the parameter that is causing your problem to fail to
converge, you should define an evolution problem on that parameter. To do this, you
will assign a value, 
, and a function 
of 
, to this parameter, and select an interval of 
, 
, so that
in the problem you want to solve, this parameter has value
when the parameter is given the value
, a solution can be obtained