On Boundary and Initial Conditions in General
- Boundary conditions are extremely important. See also Chapter (), where the boundary conditions are described.
- OpenFOAM uses similar set-up as other solvers.
- At the inlet (flow inlet) to the computational domain the flow rate is usually prescribed. The rest of variables are reconstructed (extrapolated).
- At the outlet the static pressure is set (its mean value).
- At the rigid walls velocity is zero.
- For rotational components the Multiple Reference Frame (MRF) is used.
- 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 quantities to be set are following:
The initial and boundary conditions for all variables 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.
# cp -r 0.org/* 0
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 quantity is mapped from the neighbour patch
- iO – inletOutlet is by default zeroGradient, but changes to fixedValue when velocity vector direction points inside the computational domain (backward flow)
See also Chapter (), where the boundary conditions are described.