On Boundary and Initial Conditions in General
- Boundary conditions are extremely important in OpenFOAM (and any CFD code).
- OpenFOAM uses similar set-up as other CFD codes.
- At the outlet the static pressure is set.
- At the rigid walls velocity is set to zero.
- Multiple Reference Frame (MRF) is used for rotational components.
- For steady-state computations the initial conditions have no influence to the results.
- For steady-state computations the initial conditions just help to make the case run.
The main quantities to be set are following:
The initial and boundary conditions for all quantities are set in files located in directories named by numbers. Typically directory 0 is recommended to start a simulation from. Initial conditions are set in parameter internalField putting the values into the cell centers. At boundaries, initial conditions are set individually by parameter value.
Following table shows recommended model of boundary conditions for computed variables:
The shortcuts from the table above have following meaning:
- fRV – flowRateVelocity prescribes fixed flow rate
- hTP – hydrostaticTotalPressure sets the total pressure at a patch including linear distribution of hydrostatic pressure in the gravity direction with respect to a reference value pInletCenter (e.g. ) in the center of a patch.
- tIKEI – turbulentIntensityKineticEnergyInlet, intensity = 0.05, corresponds to turbulence intensity 5%
- fV – fixedValue, e.g. velocity at the wall (0 0 0), or omega at inlet
- fMV – fixedMeanValue, is the same as fixed value, e.g. for pressure, but certain freedom is allowed to keep the variable average equal to meanValue
- zG – zeroGradient, the flux is zero in direction perpendicular to the surface
- kWF – kqRWallFunction is a standard wall function for k for incompressible flow
- oWF – omegaWallFunction is a standard wall function for omega for incompressible flow
- mI – mixingInterface, averaged variable is mapped from neighbour patch
- iO – inletOutlet is by default zeroGradient, but changes to fixedValue when velocity vector direction points inside the computational domain (backward flow)