What is the difference between freestream BC and inletoutlet BC
 

Dear all
I'm confused how freestream bc differs from normal inletoulet BC
Based on my readings
Freestream BC

freestream BC has the type inlet/outlet meaning that it looks locally (forevery face of the patch) at the mass flow rate. If the flow is going outside the boundary will be locally zero gradient, if it is going inside the boundary will be locally fixedValue.


This boundary condition provides a free-stream condition. It is a ’mixed’ condition derived from the inletOutlet condition, whereby the mode of operation switches between fixed (free stream) value and zero gradient based on the sign of the flux

inletoutlet
The inletOutlet boundary condition is normally the same as zeroGradient, but it switches to fixedValue if the velocity vector next to the boundary aims inside the domain (backward flow). The value of that fixedValue is inletValue. See figure (https://www.cfdsupport.com/img/crossref.png). (source: https://www.cfdsupport.com/OpenFOAM-...t/node114.html)


Then how Freestream BC differs or modified from conventional inletoutlet BC?

Hi.


I am also wandering the same. Did you get any clear reason for the fundamental difference between these two BC's.


Thanks

not sure
 

In general if I post about this question in other forum also
Researchers try to differentiate between general freestream and inlet outlet
Not specific to openfoam.

What I noticed is, depending on the pressure difference inside the domain either the flow enters or leaves the domain "freestream boundary condition works).
For example in a openchannel water flow modelling, I know only the inlet velocity for water and for air I use freestream

InletOutlet mostly at outlet condition

This was the reply I got it from cfdsupport
Dear Dr. Ijaz Fazil,

the answer is very simple:

freestream and inletOutlet boundary conditions are very the same.
They are switching fixedValue and zeroGradient with respect to the sign of the flux.

freestreamVelocity and freestreamPressure differ a little bit = switching (or exactly blending) between fixedValue and zeroGradient is driven by the velocity orientation.

Have a nice weekend!

Radek






Your CFD Support Helpdesk Team,

Dear Dr. Fazil,
the freestream boundary condition enables you to use field as a freestreamValue, whereas inletOutlet uses inletValues, that is constant for the whole patch. Other than that, they are the same.

Best regards,
Vojtech Prihoda

Your CFD Support Helpdesk Team,

Just to stress the distinction between freestream (or equivalently inletOutlet) and freestreamVelocity/Pressure BC types:

freestream (as well as inletOutlet or outletInlet)
They are either a Dirichlet and Neumann BC based on the flux direction. The condition for freestream and inletOutlet are the same (equating the value of the parameter to the given freestreamValue or inletValue in case of an inflow and zero-grad otherwise) because they are usually considered as output BC. On the contrary, since outletInlet is used mostly as input, it becomes zero-grad when the inflow occurs.


freestreamVelocity and freestreamPressure
They are in fact Robine BC as the openFoam blends the values from one condition to another (again using the same conditions).

This is not clear for me, however. What is the difference between inletValue and freestreamValue in practice?