Define boundaries correctly
 

Hi all,

I am new to ANSYS CFX and the forum. Any suggestions or advice are welcome and greatly appreciated. :)

My problem includes an inlet providing both water and air, air part of the nozzle surrounding water part as shown in attachment. Water is of spraying pattern. This inlet will spray downwards at 60 degree to a horizontal surface. Vertical outlets along the side sucks up the water on the surface. I would like to examine the water flow pattern on the surface being sucked up by the outlets.

Question: I am not getting the streamlines that I want and I would like to ask if my domain definitions are suitable?

- My model is enclosed, I define certain surfaces to be at openings, atmospheric pressure.
- I define, in Default Domain, Air at 25C as Continuous fluid, Water as Dispersed fluid (to create spray pattern), Non-buoyant (Q: Should I set them as Buoyant?)
- I define the inlet and outlet by making thin lids and set location to be the other side of the lids
- I keep CFX-Pre's pre-defined Default Domain and Default Fluid Fluid Interface Side (Wall domain and Interface domain). My geometry is imported as Assembly file from Solidworks (saved as .STEP before importing)

With these definition, when I try to Insert 3D Streamline from the inlets, for water I got nothing, for air I got just a few tiny streamlines which stopped halfway as show in attachment.

Pardon for my wall of text and I greatly appreciate any advices and suggestions :)

Lam

you need to give it some flowrate?

Thanks Steffen. Yes I did give velocity to water inlet and static pressure to outlets and air inlet.

thin lid? Do you have a volume?
If there is the same pressure on both sides for air, air won't move?
Check streamlines for velocity.water or whatever you named it.
Maybe try conservative instead of hybrid.
Did the solver actually do somehting or just 1 iteration?

Thanks Steffen. These are based on my latest attempts:

- Thin lid: I have a fluid control volume, it is enclosed. BC for openings are inserted. (maybe never mind about the word "thin lid", I fear it is only more confusing)

- Same air pressure?: I set inlet air pres. to be 6 psi, Default Domain's reference pres. to be 1 atm, outlet pres. to be -500Pa, so there is a difference in pressure.

- Streamline for water.velocity: Streamline only shows as in attachment (Start From the Inlet), does not reach the outlet. Air velocity streamline also shows the same behavior. BC around those parts are all walls.

- Conservative or hybrid?: Both options shows about the same appearance for water, hybrid shows nothing for air.

- Solver: Thanks. I examined it and it shows:

"A wall has been placed at portion(s) of an INLET boundary condition (at 100.0% of the faces, 100.0% of the area) to prevent fluid from flowing out of the domain
The boundary condition name is: InAir.
The fluid name is: Water.
If this situation persists, consider switching to an Opening type boundary condition instead. "

"A wall has been placed at portion(s) of an INLET boundary condition (at 100.0% of the faces, 100.0% of the area) to prevent fluid from flowing out of the domain
The boundary condition name is: InAir.
The fluid name is: Air.
If this situation persists, consider switching to an Opening type boundary condition instead. "

They repeats for 100 iterations. Been googling but I got nothing for this type of error for INLET.
This persists even after I set my inlet further away to prevent backflow. Attached is the graph if it helps. Any suggestions?

Thanks a lot for your help
Lam

there is your answer. 6 PSI are 0.4 atmospheres, the outlet is I guess at 0.5 atmospheres. So everything would flow from outlet to inlet. Thats why there is 100% wall.
try in solver what you get for
massFlowAve(Pressure)@Inlet
massFlowAve(Pressure)@Outlet

Right. Thanks. Will check it out first thing when I get back :)

Lam

if you get streamlines for backward flow from the outlet, then you know

Hi Steffen, sorry for the late reply, we have just got the chance to resume on this project, and I have just managed to get the supposed flow of spray pattern :) Thanks a lot for your advises.

Cheers
Lam