Wind Tunnel Experiment(pressure problem)

[AspenItIs]

Hey everyone, first time user here!

I'm doing a lab experiment with a small open wind tunnel and I'm having some trouble figuring out how to calculate the airspeed in the test section.

There is a digital pressure gauge displaying what I was told to be the dynamic pressure. The pressure port appears to be mounted flush along the surface wall(perpendicular to the flow) just before the test section in the free stream flow, which makes me think its reading static pressure...?

Anyways, so my thought is that if that is reading dynamic pressure, I would use a barometer to measure the atmospheric pressure then to get the total pressure add those two values(P_atm+ P_dyn = P_total). From there just solve Bernouli's for velocity and substitute the P_total and P_atm values into get velocity.

Does this sound right?

 

[Bob S]

Look at the design and theory of Pitot (wind speed) gauges used on aircraft. See
http://www.efunda.com/designstandards/sensors/pitot_tubes/pitot_tubes_theory.cfm
Bob S

 

[FredGarvin]

If it is a low speed tunnel, you can make the assumption that the static is very close approximation. That being said, is it possible that you are taking statics in 2 places and you are actually seeing the delta between them? That would give you a measure of the velocity in the test section similar to a venturi.

You are correct in your assumption that it is a static port.

 

[rcgldr]

If the port is flush mounted in the wall (perpendicular to flow), it's a static port. Even though air is moving by, there's a small boundary layer that the static port resides inside of, allowing it to "sense" the static pressure of the ambient air flowing by just outside of the boundary layer.

With just a static port there's insufficient information about the air speed in the wind tunnel. If there is a fan blowing air into the upwind side of the tunnel, then pressure will be above ambient. If there's a fan sucking air out of the downwind side of the tunnel, then pressure will be below ambient. If the fan is connected via some type of converging or diverging pipe to the wind tunnel, then venturi effect could allow the pressure in the wind tunnel to be near ambient.

Normally air would accelerate within the wind tunnel unless there is some type of resistance within or at the end of the tunnel opposite the fan in order to keep the flow and pressure near constant within the wind tunnel. Without the resistance, you get a complicated air flow due to the pressure differential at both ends tending to accelerate the air, conflicting with the fact that mass flow is constant, so you end up with turbulent flow as a constricting stream line interacts with the surrounding air in the wind tunnel.

What is needed to sense the air speed is a pitot port that with an opening directed into the air flow. Then the dynamic pressure can be compared with the static pressure to determine the speed.

http://en.wikipedia.org/wiki/Pitot-static_system

 

[AspenItIs]

I am familiar with the way a pitot-static tube works, but unfortunately this is not available to me during the experiment, nor is there one mounted inside the tunnel.

for reference, here is what I am working with...Air is drawn in by a fan mounted after the test section(right side)
http://www.aerolab.com/images/Smoke%20Tunnels/flow-vis-front2.png

When i was messing around with the wind tunnel I saw an increase in the digital resolution of the pressure gauge when i increased the airspeed via the control knob which is even more confusing to me now since I was convinced that it was just reading static pressure. BUT there might have been another vacuum line mounted in the pressure gauge which could be attached to a hidden sensor facing the flow(stagnation pressure) and the resolution is indeed dynamic pressure calculated from the difference of the two pressures.

so if this is indeed the dynamic pressure, I can then get velocity with this pressure. I'll just take the ambient pressure and temperature and use equation of state to get the air density...ah-hah!

Jeff-when you mentioned to the pressure being above/below ambient...which pressure are you referring?

 

[rcgldr]

here is what I am working with...Air is drawn in by a fan mounted after the test section(right side)
http://www.aerolab.com/images/Smoke%20Tunnels/flow-vis-front2.png

What is that box to the left of the wind tunnel?

hidden sensor facing the flow

That would be required to get the total pressure (static + dynamic).

Jeff-when you mentioned to the pressure being above/below ambient...which pressure are you referring?

The pressure of the air in the room. This Nasa article about propellers explains how the pressure is reduced fore of a prop, then experiences a jump in pressure through the prop disk with little change in speed, then continues to accelerate as the now higher than ambient pressure streamline accelerates towards the ambient pressure air downstream of the propeller. The diagram shows the constricting air flow required to achieve constant mass flow within the stream, but doesn't explain the interaction with the surrounding air due to viscosity.

http://www.grc.nasa.gov/WWW/K-12/airplane/propanl.html

Since you're dealing with a proper wind tunnel, that box on the left probably provides some resistance (it might also filter the air), and my guess is that the sections to the left and right of the test section converge/diverge such that the flow and pressure within the test section is nearly constant.