Mythbusters Terminal Velocity Wind Tunnel

[opticaltempest]

Recreating the Mythbusters' Terminal Velocity Wind Tunnel - Need advice on Manometer

Hello,

I am trying to find the terminal velocity of a projectile. I am placing the projectile inside a clear tube and blowing compressed air inside the tube until the projectile is hovering at some height inside the tube. The tube I am using is around 5 feet long. This method works very well! I can get the projectile to perfectly hover at any height inside the tube for as long as I need.

The Mythbusters used this method in two separate episodes where they needed to determine the terminal velocity of a penny and the terminal velocity of a bullet.

I don't believe the height at which the projectile is hovering inside the tube matters. All that matters is the wind velocity at that point inside the tube. Is this correct?

If the above is correct - How did the mythbusters measure the wind velocity inside the tube at that point without disturbing the flow of air? I cannot remember exactly what they did in the episode.

Does anyone have any suggestions as to how I could measure the wind velocity inside a 2" diameter tube at that point? Someone mentioned using a thermistor to measure the temperature difference between the hover point inside the tube and the ambient temperature. This method seems interesting, however, I do not think I will have time to do this.

I put together a homemade wind gauge that will fit on the top of the tube. Should I try balancing the projectile right near the top of the tube, then quickly place the wind gauge on top of the tube?

The wind gauge consists of a modified battery powered fan consisting of a DC motor and fan blades. I am measuring the DC voltage across a 2.2k ohm resistor that is connected to the terminals of the motor. I know what voltage corresponds to which wind speed by driving with it hanging out of the window of my car at varying speeds (that was fun). I then fitted a curve to that data in order to determine wind speed as a function of voltage. Do you think this method is accurate for measuring wind speed?


EDIT:

At this point I have decided to scrap using the fan method and instead measure the total pressure and static pressure inside the tube and connect those across a manometer in order to calculate velocity pressure.

Please see my post at the bottom for my question regarding my setup and this method

 

[russ_watters]

I think they used a pito-static tube and a manometer (or another meter that has a velocity scale directly on it). You can make one yourself with some brass and rubber tubing...

Bend a piece of brass tubing at a 90 degree angle and point it upstream in the tunnel. Put a second piece into the wind tunnel, perpendicular to the flow. Attach a 6-inch U-shaped piece of plastic tubing to a board and fill it halfway with water. Attach the tubes to each other.

Meausre the height distance and use this formula to find velocity in fps: V=4004 sqrt (h)

More info: http://www.dwyer-inst.com/htdocs/airvelocity/AirVelocityIntroduction.cfm The setup I describe is in figure 2, though they use an incline manometer and I described a straight u-tube.

Your method sounds like it should work too, though...

 

[LURCH]

just leave the fan over the tube and take your measurement. True, the fan will impede airflow, but the reading you get will be the accurate reading of how fast the air is flowing including the impedence, and it's that measurement that counts.

 

[opticaltempest]

Hello,

Ok, I have decided to scrap using the fan to measure wind velocity and instead measure the static and total pressure inside the vertical wind tunnel and connect those across a manometer in order to calculate the velocity pressure. Then from the velocity pressure I will calculate velocity.

I made very crude static pressure sensor and velocity pressure sensor hooked up to a manometer that is similar to Figure 2 C at

http://www.dwyer-inst.com/htdocs/airvelocity/AirVelocityIntroduction.cfm

Here is a sketch of my setup:

http://img181.imageshack.us/img181/2163/image0001lc2.jpg In this setup, does the diameter of the tube affect the movement of the water? I don't see any mentioning of the diameter in any equations, but how can it not affect the water height? If I made the diameter of that tubing two feet then how would the water move at all!? So it seems like I must account for the diameter somewhere in the calculations.

 

[russ_watters]

In this setup, does the diameter of the tube affect the movement of the water?

Nope.

I don't see any mentioning of the diameter in any equations, but how can it not affect the water height? If I made the diameter of that tubing two feet then how would the water move at all!? So it seems like I must account for the diameter somewhere in the calculations.

Pressure is pressure. Does the length and width of a swimming pool have any effect on the pressure 10 feet down, at the bottom of the pool?

Remember, air does not flow through a pito/static tube and manometer, it is in static equilibrium, so pipe size effects on velocity do not apply here.

 

[opticaltempest]

Thanks for the quick reply Russ,

I am now going to go test the setup. I will return in a few hours with my results.

Thanks