Answer For Que-1

1. You can give grading in blockMeshDict (Page 153-157)

2. You can add Layers in snappyHexMesh (Page 168-170)

Both options are explained in the <a href="https://github.com/OpenFOAM/OpenFOAM-7/blob/master/doc/Guides/OpenFOAMUserGuide-A4.pdf" target="" title="">OpenFOAM v7 User Guide</a>.

To answer your queries:

1. Could you be more specific about the object, of which you need to set the thickness?

2. To decide if a flow is subsonic or supersonic, you only need the free stream conditions (velocity and temperatute). Since your question pertains to Hagen-Poiseuille flow, the flow is incompressible. The tutorial uses an incompressible solver to simulate the Hagen-Poiseuille flow. The velocity of the flow can be changed in the u file located in 0 folder. For a compressible flow, say supersonic flow over a wedge, you need a compressible solver. You need to decide on the solver before the start of your simulation. You will need to know before you start your CFD analysis, if the flow is compressible or incompressible. You may refer to the tutorial on Supersonic flow over a wedge to know more about compressible flow simulations. Link to Tutorial: https://spoken-tutorial.org/watch/OpenFOAM/Supersonic+flow+over+a+wedge/English/

3. There is a channel flow tutorial case available with OpenFOAM located at $FOAM_TUTORIALS/incompressible/pimpleFoam/LES/channel395. The case however is for a closed channel, for an open channel, the top wall boundary conditions are replaced with free surface boundary conditions. Assuming that your simulation is single phase, incompressible simulation, you may use zeroGradient pressure and slip velocity as boundary conditions at free surface.

Also, please refrain from posting questions pertaining to different tutorials in one post. Thank you.

Regards,

Ashley