# Hex meshing + initial conditions + stability + opening

My CFD problem is simply a tube of hot air flowing into an enclosed yet large space (at room conditions) using RANS Flow.

1)
"Create a set of interconnected volumes each with 6 faces such that Caedium's structured meshing algorithm can then generate high quality hexahedral elements"

I've actually created my geometry from an external 3D modelling software and then imported it into Caedium as a step file since the Builder has limited control.
I've tried multi-block meshing in the past but it is extremely troublesome once there are geometrical complexities.
Will I be able to to multi-block the simple parts (by creating boxes inside my geometry) then while meshing, leave the automatic mesher to create hex elements (with O,C and L-grids) for the multi-blocks but tetrahedral elements for anything else untouched?

2)
"Assign different initial conditions to different volumes within a flow domain to speedup the convergence rate of a simulation."
How is this done? I only have just one whole fluid volume for air to flow through from start to finish since Caedium doesn't support multiple fluids or conjugate heat transfer.
Is assigning different conditions throughout the whole domain the same as the multi-block approach? My biggest worry is disrupting the flow continuum.
Then can I say I specify [u v w] = [0.2 0.3 0.4] for one specified region then [u v w] = [0 0 400] for another?
I've used [0.2 0.3 0.4] because [0 0 0] often doesn't work and I need to accurately portray calm ambient conditions.

3)
Often when I run a steady state problem involving heat transfer, the residuals will oscillate, then spit out errors such as singularity in Ux, Uy or Uz and stop solving. This is much worse when I enable compressibility.
This is usually alleviated when I enable transient simulations but it is very time-consuming in reality.
Another commercial code I used would never have the singularity issue for a similar problem.

4)
I am confused at which boundary condition out of Inlet-Outlet or Outlet-Inlet properly represents an opening in reality (e.g. open window to outside or sky).
In order to avoid not having solutions is to specify Inlet(s) then Outlet(s) for flow rather than Inlet(s) then an Inlet-Outlet/Outlet-Inlet, I want to allow fluid to flow in and out freely and not force it all out.

### Multiple Volumes

Are you assuming incompressible conditions? I ask because you talk about an initial condition of [u v w] = [0 0 400] which for air would be supersonic (Mach = 1 = 340 m/s). In this case you would have to use the Supersonic solver and it is only available as unsteady. If you do have supersonic flow then the incompressible heat transfer setup I discuss below is mute.