Boundary conditions over-constrained
 

So, I am trying to use a 1D model input as boundary conditions for my 3D simulation.

I know the velocities at all boundaries from the 1D model, so I input them into Fluent, and left one boundary as a pressure boundary not to over-constrain the physics.

However I tried changing this to a velocity BC and to my surprise the calculation started running with no error message.

What is the meaning of this? Will it calculate but have some continuity imbalance?

Thanks in advance!

So you have only velocity inlet type boundary conditions in your model and none of the outlet types?

For constant density flows it'll probably divergence very quickly. For compressible cases, specifying the velocity everywhere (inlets & outlets) usually results in an under-defined problem rater than an overdefined one. There's always a solution where the flow switches to hyperbolic state (supersonic) and you get a shock at the outlet. So you see it's not theoretically wrong to have velocity specified everywhere.

Well I am modeling a fire inside, where there are energy and mass sources, so buoyancy effect and density changes are calculated.
Perhaps this explains why it allows for calculation? (The continuity residual does not go lower than 5e-2, all the others are lower than that)

The capability to specify velocity at the inlet and outlet (i.e. velocity at all boundaries) was a requested feature in Fluent since its early days. Maybe there should be a warning to prevent newbies from getting stranded in shark-infested waters but it's a feature, not a bug.