Help with a project - Air speed, compressed?

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Discussion Overview

The discussion revolves around the behavior of air speed in a wind tunnel setup with varying tube diameters, specifically focusing on the effects of compression and the relationship between input and output speeds. Participants explore theoretical calculations and practical implications of airflow dynamics.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested, Mathematical reasoning

Main Points Raised

  • Some participants inquire whether the wind speed would increase at the smaller opening due to compression when the larger opening faces the wind.
  • There is a suggestion that at low speeds, such as 20 mph, compression is not a factor, and the area ratio can be used to estimate output speed.
  • One participant proposes a hypothetical scenario with a 1:10 area ratio and 30 mph input speed, questioning if this would yield an output speed of 300 mph.
  • Another participant mentions that compression becomes relevant at around 220 mph, raising questions about how compression might affect output speed.
  • There is confusion regarding the application of the equation of continuity and whether density changes due to compression should be considered in calculations.
  • Participants discuss the need for enclosing airflow to achieve higher output speeds, referencing devices like blow dryers or leaf blowers.

Areas of Agreement / Disagreement

Participants express differing views on the role of compression in airflow dynamics, with some asserting that compression is not a factor at lower speeds while others suggest it becomes significant at higher speeds. The discussion remains unresolved regarding the exact conditions under which compression affects output speed.

Contextual Notes

Limitations include assumptions about airflow behavior, the dependence on specific conditions such as speed and tube dimensions, and unresolved mathematical steps in applying the equations discussed.

bpears
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1.If you have a wind tunnel, a tubing that goes from a large opening, to a smaller opening, with the larger opening facing the wind, will the wind speed be more out the smaller end since its being compressed?

2.Is there a way to calculate/guesstimate the difference in wind speed by the volume or something (you know, a way without a anemometer or something?)

For the sake of explanation: let's say the large opening is 3 times larger than the smaller opposite end. And that the wind speed is 20mph. (Or what ever you prefer for a good example.)
 
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Or would that not make compression?
 
Welcome to PF!

At that low speed, there will be no compression. You can simply ratio the areas of the openings.
 
So if it were 30mph in, and the ratio was 1:10, then it would be 300mph out?

What speed into tube, would cause compression, and then would the compression make the output speed more than the size in/out ratio?
 
At about 220mph input or output, compression starts to be an issue.
 
Well, I actually want compression if it will increase the wind speed out the other end. So would that change the equation with compression? And did I give a correct example of the ratio you describe?

Thanks for your help, I've been looking all over online for something about this, but couldn't find anything.
 
Ok, so I see what you're saying kind of now. I found the equation of continuity.
But are you supposed to square the ratio before multiplying to the input speed?

Here's the equation I have:
V1A1=V2A2.

But I think that is for no change in density? Is change in density meaning compression?

For density, I found this:
p1V1A1=p2V2A2.

I'm having trouble understanding how to use the equations to solve for the other velocity, if someone could work out an example? Or would I just multiply by the ratio like Russ was saying (output size ratio 1:10, 30mph in=300mph out?)?
 
Last edited:
Your example is using a tube open at both ends, so the air in the larger tube is nearly stopped (not flowing at 30 mph), due to the restriction at the transition into the smaller tube.
 
Last edited:
How could I have a higher output speed?
 
  • #10
bpears said:
How could I have a higher output speed?
You would need to enclose the air flow from the fan, such as a blow dryer or a leaf blower.
 

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