# OpenFOAM® v1806: New boundary conditions

## New outlet Mach number condition

This pressure boundary condition maintains a (subsonic) Mach number at an outlet patch by dynamically adjusting the static outlet pressure. It makes it possible, for example, to simulate the flow in a pre-turbine engine exhaust manifold without resolving details of the flow inside the turbine.

The formulation is derived from a simple model of the gas flow through a nozzle with fixed geometry. The nozzle flow is assumed to be quasi-steady, 1D, isentropic and compressible.

The accompanying boundary conditions for velocity should be pressureInletOutletVelocity

- Source code
- $FOAM_SRC/TurbulenceModels/compressible/turbulentFluidThermoModels/derivedFvPatchFields/outletMachNumberPressure
- Examples
- $FOAM_TUTORIALS/combustion/reactingFoam/RAS/chokedNozzle

## New swirl fan velocity condition

This condition can be used in combination with a cyclic pressure jump condition to simulate a fan. The existing fan pressure jump condition implements the fan-normal momentum gain. This new velocity jump condition adds a swirl component.

The velocity jump can be specified in two ways:

- constant velocity
- constant swirl

In the first mode the velocity is calculated as follows:

Where represents the current pressure drop across the cyclic, is the effective radius, is the fan efficiency coefficient and the speed of the fan (in revolutions-per-minute).

In the second mode the inner () and outer () radii are provided instead of , where for and the velocity is given as:

where is the distance from a patch face to the fan axis. Outside and , . The input for this mode is:

rInner 0.005;

rOuter 0.01;

origin (0.0453 0.06 0);

If the is not provided the centroid of the patch is taken.

## New wave modelling: irregular waves

A new irregular wave model based on the frequency-direction spectrum has been added to the suite of available wave models.

The wave height is set according to the equation:

- Source code
- $FOAM_SRC/waveModels/waveGenerationModels/derived/irregularMultiDirectional
- Examples
- $FOAM_TUTORIALS/multiphase/interFoam/laminar/waveExampleIrregularMultiDirection
- Attribution
- These boundary conditions were supplied by the Environmental Hydraulics Institute IHCantabria
- Integration
- The code has been updated by OpenCFD and added to the $FOAM_SRC/waveModels library available to the interFoam family of solvers