Appendix: Flow Perturbation

The run control file variable perturbation defines an optional localized random velocity perturbation in the interior of the domain. This feature is typically used to trip a flow into asymmetry (for example, in transition or vortex-shedding studies). It is not an initial condition or a boundary condition.

Run Control File Syntax

The perturbation is specified as an options list in the .vars file:

Example perturbation specification

perturbation: <
  position=[0.25, 0.0, 0.0],
  delta=[0.01, 0.01, 0.01],
  dv=[0.50, 0.00, 0.00],
  nstart=100,
  nstop=500
>

Options for perturbation

Option	Type	Description
position	3-value list	Center of the perturbation region.
delta	Scalar or 3-value list	Half-size of the perturbation region in each coordinate direction. If a scalar is provided, the same value is used in x, y, and z.
dv	3-value list	Maximum perturbation vector magnitude by component.
nstart	Integer	Start time-step index for perturbation activation.
nstop	Integer	Stop time-step index for perturbation activation.

Behavior and Notes

• The perturbation is applied over interior cells near position and bounded by delta (with an additional cell-size buffer based on \(\sqrt[3]{\mathrm{vol}}\)).

• The perturbation added to the velocity is random in sign with component-wise amplitude up to dv.

• position and delta are internally multiplied by the reference length Lref.

• nstart and nstop are optional and default to 0 if omitted.

• If the perturbation variable is not present in the run control file, no perturbation model is activated.

Note

In the current PISO implementation, nstart/nstop gate when the perturbation is applied. In the base non-PISO implementation, these values are currently used to report the active window in the log message, while the perturbation update itself remains spatially gated by position/delta.