ORIGINAL: iron eagel
ORIGINAL: Allfat
The difference between water and air is pretty much only the density. Other than that, they act very similar.
This being said, to do a test on an airfoil using water, you can use a lot lower velocities of water because the density is a lot higher. This results in the same reynolds number.
Yissht, you may have done the experiment, but it is likely not valid because you did not take into account that the water has a much higher density than air. This will drastically effect the reynolds number and how the airfoil reacts with the environment. To figure out what velocity the water should be at to compare to air, you need to know the velocity you want the airfoil to travel at through the air. Then calculate the reynolds number over the airfoil. Then use that calculated reynolds number to calculate the velocity of water that is needed to obtain the same reynolds number.
Then you need to regulate this velocity and put it over your airfoil and see how it reacts. This will be a valid test to see how your airfoil performs in air using water.
This is a very good method to use when evaluating high speed planes because it is really hard for the average joe to make a wind tunnel that will produce very high speed air. But if you use water, it can move at a much slower speed that is much easier to obtain.
If I may ask...
Could you describe the method that can be used to substitute fluid for air for aerodynamic evaluation of high speed aircraft models?
How do you provide a non a flow with low turbulence for the test and at what scale or size would the test have to be to be valid?
I am just interested in the prospect that there might be a low cost alternative to wind tunnels.
Well, this definitely is a low cost alternative, but there is a lot more calculations to do. To give you a step in the right direction, you are going to need to research all you can on different dimensionless numbers. Reynolds number is one of them, and there are many more. These are numbers that have no unit associated with them at all, so they can be applied to all situations. Keeping that in mind, if you want to compare two completely different situations, (IE using water flowing over a wing instead of air) then you only need to make sure that they dimensionless numbers associated with the two situations are the same and the results will be accurate. They may not be as precise as using the real thing, but they will be close and at the very least give you some relationships that you can compare directly.
With that being said, your second question brings up an interesting point. As for a low turbulence flow, I would make a channel out of a smooth material that is also clear, glass or plastic would work, and it would have to be fairly long. What I would do then is pump water into the top of the channel and then simply raise one end of the channel so there is flow. You can regulate the flow by changing the angle of the channel and then use a pump to circulate the water around to the top of the channel again. Then conduct the tests near the end of the channel so the water has a lot of time to become the least turbulent you can get.
Now with the channel, it would be ideal if it was very large and there was a lot of water moving through it. This is because the water will have a reaction with the wall of the channel and skew your results if you have a small channel that your airfoil barely fits in.
As far as the scale and size, I will again refer you to dimensionless numbers. These are also used for scale tests because, again, they are two different situations. So as long as you keep all the dimensionless numbers the same for the two conditions, you can apply the results of one of the test to the other, untested situation.