Surface resistant

[Anonymous]

Hi!

I am doing a simulation of the air flow from the diffuser for an HVAC supply. A diffuser for an HVAC supply can be comprised of a lot of very small details (e.g., holes, slots, vanes) with respect to the room that it is supplying, which makes it a good candidate for simplification through a more generic characterization.

My supply diffuser is a simple box with the flow boundary condition on top of it. To simplify the diffuser I made a surface resistance on bottom part of the box with the free ratio of 0.3. The problem is that the velocity magnitude is the same with or without the surface resistance. When I define surface resistance I expect higher velocity because the area through air flows is much smaller.

Did I misunderstood something or something is wrong?!

I attached two screen shots of flow simulation result (meshed and shaded).

I will upload the simulation during the day.

Best regards!

[Anonymous]

And here is the simulation https://www.wetransfer.com/downloads/7d17bc3eec24ac61bcd4c277fb2ff45420150213171559/14c998ef21a8ab48...

[Jon.Wilde]

Hi,

The resistances should provide you with the correct pressure drop based on the the flow rate but we would not show any jetting at all as we average the flow rate out across the surface. They are not really intended to be used in this regard I am afraid.

To capture the flow properly, you may need to mesh the model. Perhaps a small section or a 2D analysis might be useful?

Hope this helps,

Jon

[Anonymous]

To capture the flow properly, you may need to mesh the model

 You mean to design the diffuser with all the details?!

Perhaps a small section or a 2D analysis might be useful?

 Can you elaborate this a bit more?

Best regards!

[OmkarJ]

Yes, probably the first step would be to model the diffuser as shown on details of the plane that cuts it through its symmetrical plane, perhaps the same one that you have shown. This way the walls of the solid portion between the holes will not allow the flow and you will get the jet as you desire. Next step could be to choose a periodic section of the diffuser. 

[Anonymous]

Next step could be to choose a periodic section of the diffuser.

Can you elaborate/explain this a bit more?

[Jon.Wilde]

Check out the help: Periodic Boundary Conditions

[Anonymous]

...any jetting at all as we average the flow rate out across the surface.

 Can you explain what does it mean that simulation CFD average the flow rate across the surface?

I think my best shoot is to simulate diffusers like this http://academy.autodesk.com/library/cfd-aec/diffusers-characterization-cfd

If I have a diffuser with total area of 1 m2 and with effective area of 0,5 m2 (without blades and stuff like that) I will model my diffuser with that area of 0,5 m2 and put the velocity BC on it. Basically what I get then is Q (flow rate) = A (Area=0.5m2) * v (velocity). Do you think that is the right way to do that?

[Jon.Wilde]

Hi,

Give it a go and model one - this will hopefully demonstrate why the resistance approach is less likely to work.

Flow from a surface resistance will exit normal to the surface and any values used in the two directions normal to the flow are not considered with a surface resistance.

I think that the link you provided shows what should be a sensible approach. 

You could also just apply a velocity, which saves calculating the flow rate/area 🙂

Thanks,

Jon