Wind scaling and increasing wind speed for wind tunnel problem

In summary, the author is trying to test aerodynamics of 1/24 scale cars using a wind tunnel, but is having trouble increasing the wind speed. He is also trying to measure drag and down force of wings and such.
  • #1
sbalser1
2
0
i am making a wind tunnel in order to test aerodynamics of 1/24 scale cars.
i am having two problem:

1. i am not sure what wind speed at that scale to represent 100mph.

2. i am using a fan that has an outlet wind speed of 19.4 mph. i tried to increase wind speed by decreasing the outlet speed of my tunnel from 144 square inches, to 36 square inches. but in doing this i measured the outlet velosity and it was only 9mph. what am i doing wrong? how do i increase wind speed?
 
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  • #2
The proper scaling parameter you want to use is the Reynolds number. You want the Reynolds number to remain the same regardless of which scale your model is. The Reynolds number is defined as:
[tex]\mathrm{Re} = \frac{\rho V D}{\mu}[/tex]

Where [itex]\rho[/itex] is density, [tex]V[/tex] is velocity, [tex]D[/tex] is some characteristic length scale (often the length of the car or diameter of a pipe etc) and [tex]\mu[/tex] is viscosity. You want to make sure you have the same Reynolds number on your small model as the full sized car. That leaves you with:
[tex]\mathrm{Re_{full}} = \mathrm{Re_{scale}}[/tex]
[tex]\frac{\rho V_{\mathrm{full}} D_{\mathrm{full}}}{\mu} = \frac{\rho V_{\mathrm{scale}} D_{\mathrm{scale}}}{\mu}[/tex]
[tex]V_{\mathrm{full}} D_{\mathrm{full}}= V_{\mathrm{scale}} D_{\mathrm{scale}}[/tex]
[tex]V_{\mathrm{scale}}= V_{\mathrm{full}} \frac{D_{\mathrm{full}}}{D_{\mathrm{scale}}}[/tex]

Now, before you said you wanted a 1/24 scale model, so you can use that to get:
[tex]V_{\mathrm{scale}}= 24V_{\mathrm{full}}[/tex]

In other words, to make a truly scaled model in air, you would need to have the air moving 2400 mph. Of course that doesn't take into account the fact that now you have compressibility effects. What does this mean? it means you can't perfectly scale it. In reality, after a certain Reynolds number most quantities of interest tend to plateau, so you don't always have to match it (and in most cases you can't if you are using a wind tunnel with air as the operating fluid).

So then the question becomes what are you hoping to measure?
 
  • #3
that was the number that i got in doing it but i thought that i was doing it wrong because that makes no sence. it seams that it would be slower because the distance is less from one end of the scale car to the other. so to scale the car to full scale the speed would have to increase to signify the same meters per seconds.

i am trying to measure drag force and down force of wings and such.
 
  • #4
Well if you are just trying to compare shapes and see which is best in terms of drag and lift, then you don't need to match Reynolds number. Otherwise you might consider using a water tunnel?
 

1. How is wind speed increased in a wind tunnel?

Wind speed is increased in a wind tunnel by using a fan or compressor to force air through the tunnel at a higher velocity. This creates a more realistic simulation of wind conditions.

2. What is the purpose of scaling wind in a wind tunnel?

The purpose of scaling wind in a wind tunnel is to simulate real-world conditions on a smaller scale. This allows for more efficient and cost-effective testing of models and prototypes before they are implemented in the real world.

3. How is wind scaling determined in a wind tunnel?

Wind scaling is determined by the Reynolds number, which takes into account variables such as air density, viscosity, and velocity. By adjusting these parameters, the wind speed in a wind tunnel can be scaled to accurately represent different real-world conditions.

4. What is the relationship between wind speed and pressure in a wind tunnel?

In a wind tunnel, as wind speed increases, so does the pressure on the object being tested. This is due to the increased force of the air hitting the object at higher speeds. This relationship is important to consider when testing the aerodynamics of a model.

5. Are there any limitations to wind scaling in a wind tunnel?

While wind scaling in a wind tunnel can provide valuable data and insights, it is important to note that there are limitations. The smaller scale and controlled environment of a wind tunnel may not accurately represent all real-world conditions, and it is important to interpret the results with this in mind.

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