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opticaltempest
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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
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
Last edited: