# Boundary conditions for 'wind tunnel'

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November 4, 2013, 13:11
Boundary conditions for 'wind tunnel'
#1
Member

Olie
Join Date: Oct 2013
Posts: 40
Rep Power: 3
Hi all,

I'm trying to set up a simulation that acts as a 'wind tunnel', with a backward facing step in the test section.

I'm using a RANS solver (pisoFoam) with k-epsilon turbulence model.

I've got it all to solve but the results aren't believable - so I think it's to do with my boundary/initial conditions - so here's what I've got at the moment:

Inlet:
U - uniform 30m/s field:
Code:
```inlet
{
type            fixedValue;
value           uniform (30 0 0);
}```
p - allow solver to compute required pressure at inlet in order to produce sufficient pressure gradient across test section:
Code:
``` inlet
{
}```
Outlet:
U - fully developed, so dU/dx = 0:
Code:
```outlet
{
}```
p - fixed at 0 (relative pressure):
Code:
```outlet
{
type            fixedValue;
value           uniform 0;
}```
Fixed walls: U and p, zeroGradient.

Initial k and epsilon for the inlet were calculated, and outlet set to zeroGradient. Fixed walls use k and epsilon wall functions.

Now when the flow reaches steady state I'd expect there to be a higher pressure at the inlet than outlet in order to drive the flow, but a significant drop in pressure just aft of the step in the recirculation region - in the first image I've posted you can see the velocity contours/stream tracer looks as you'd expect - 30m/s at the inlet, about 15m/s at the outlet, and there's the recirculation region just aft of the step. However in the second image the pressure distribution looks all wrong.. The high pressure is at the outlet, not the inlet, and there doesn't seem to be any drop in pressure in the recirculation region.

Thanks a lot,
Olie
Attached Images
 velocity_contours.jpg (25.3 KB, 40 views) pressure_distribution.jpg (15.4 KB, 40 views)

 November 5, 2013, 03:41 #2 Senior Member   Tom Fahner Join Date: Mar 2009 Location: Delft, Netherlands Posts: 222 Rep Power: 10 Hi Odellar, I think you should keep pressure indeed at zeroGradient for the walls, but for the velocity you should use a fixedValue with a velocity of uniform (0 0 0). Otherwise there is no reason why your pressure would drop (no friction). Regards. Tom

 November 5, 2013, 07:10 #3 Member   Olie Join Date: Oct 2013 Posts: 40 Rep Power: 3 [QUOTE=tomf;460581]Hi Odellar, I think you should keep pressure indeed at zeroGradient for the walls, but for the velocity you should use a fixedValue with a velocity of uniform (0 0 0). Otherwise there is no reason why your pressure would drop (no friction). Regards. Tom[/QUOTE Hi Tom, I'd actually made a mistake typing that - yes I have set the walls to U (0 0 0) (no-slip boundary condition). Thanks anyway!!

 November 5, 2013, 08:31 #4 Senior Member   Tom Fahner Join Date: Mar 2009 Location: Delft, Netherlands Posts: 222 Rep Power: 10 Hi, Ah, I've looked at it more closely now. You are plotting static pressure, which will drop with the velocity (Bernoulli's principle). If you calculate total pressure (p+0.5*mag(U)^2) you will probably see the drop you expect. Regards, Tom

November 5, 2013, 09:08
#5
Member

Olie
Join Date: Oct 2013
Posts: 40
Rep Power: 3
Quote:
 Originally Posted by tomf Hi, Ah, I've looked at it more closely now. You are plotting static pressure, which will drop with the velocity (Bernoulli's principle). If you calculate total pressure (p+0.5*mag(U)^2) you will probably see the drop you expect. Regards, Tom
OH I assumed it was plotting total pressure - do you know how I set it to display total pressure?

Thanks,
Olie

 November 5, 2013, 10:32 #6 Senior Member   Tom Fahner Join Date: Mar 2009 Location: Delft, Netherlands Posts: 222 Rep Power: 10 Hi, You can either use the ptot OpenFOAM utility or use the calculator in paraView to get the total pressure. Than you have this as an additional variable in paraView. Regards, Tom

November 5, 2013, 11:31
#7
Member

Olie
Join Date: Oct 2013
Posts: 40
Rep Power: 3
Quote:
 Originally Posted by tomf Hi, You can either use the ptot OpenFOAM utility or use the calculator in paraView to get the total pressure. Than you have this as an additional variable in paraView. Regards, Tom
Oh excellent yes, now getting the results I was after - thanks a lot!!!

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