Gradients Model Reference

Gradients are used in several places within the finite volume solution methodology. The Gradients model lets you specify the gradient method and limiter method.

Table 1. Gradients Model Reference
Theory	See Gradients.
Provided By	[physics continuum] > Models > Gradient Metrics
Example Node Path	Continua > Physics 1 > Models > Gradients
Requires	Flow: any Energy: any
Properties	Key property: Limiter Method. See Gradients Model Properties.

Gradients Model Properties

Limiter Method

Specifies which limiter method to use.

MinMod
Selects the MinMod method that is given by Eqn. (916).
Venkatakrishnan
Selects the Venkatakrishnan method that is given by Eqn. (915).
Modified Venkatakrishnan
Selects the Modified Venkatakrishnan method that is given by Eqn. (917).

Custom Accuracy Level Selector

If necessary, you can introduce more damping in the numerical scheme and so improve convergence.

This property specifies the accuracy of the numerical scheme at any level between 1.0 (1st-order) and 2.0 (2nd-order). You can use any real value between 1.0 and 2.0. The default value is 2.0, meaning no accuracy changes.

When computing steady-state solutions of complex flow problems using high-order discretization, you can sometimes encounter convergence stall or even lack of convergence. These issues are due to flow instabilities, sometimes local ones, that are generally caused by numerical artifacts.

One solution for these situations is to tune the numerical scheme dissipation to a user-specified level, between the first order and second order of accuracy.

The accuracy level selector is implemented as a global scaling applied to all reconstruction gradients. As such, it affects the convection terms and boundary conditions in all transport equations, and any other calculation where reconstructed values are employed.

Note	Gradients used for diffusion are not affected by this scaling.

Maximum Reconstruction Coefficient

C_{max}^{user}

in Eqn. (910), controlling the amount by which the gradients are limited or scaled. Any non-negative value is allowed. The default value is 1.

Two Pass Velocity Gradient

When On, provides a smoother solution on surfaces with poor geometry resolution and applies a more dissipative scheme for the velocity gradients. The default is Off.

Use TVB Gradient Limiting

TVB (Total Variation Bounded) limiting prevents excessive gradient limiting, which can cause a slow or stalled convergence. See Total Variation Bounded Gradient.

Acceptable Field Variation  (Factor)

Sets the allowed variation of the field due to TVB Gradient Limiting, as a fraction of the largest difference between local maximum and local minimum anywhere in the simulation. Default 0.05.