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iRemy_y's avatar

Will this experiment work or not?

Asked by iRemy_y (550 points ) November 30th, 2009

My topic is “The aerodynamics of cars”. I think I’m trying to measure “The effect of the air foil of a car, on the drag force”. My plan is to build different car model shapes out of wood. I will get the weight of every shape and place them in a wind tunnel. I will then run the fan at one speed for every car, and find the weight of the car with the wind pressure, using the same scale, inside the tunnel, and calculate the difference. For visual purposes i will run one or two different colored cans of spray paint though the wind tunnel, much like a streamline.

Will this be enough info for a good presentation? What should i add? change? take away? Am i in at least on the right course in aerodynamics?

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10 Answers

timtrueman's avatar

If I’m not mistaken you’ll be measuring the downforce and not the drag. I wish I knew how to measure drag but I don’t. I know computational fluid dynamics software can do it though.

iRemy_y's avatar

=\ im not even taking physics for another 2 years so i dont know the difference between downforce and drag… i can change my hypothesis as long as the rest of them experiment will work.

FiRE_MaN's avatar

you should look up how to find out the drag on google or something. But i am not srue spray paint will work as you hope it will. Hate to be the mast of the obvious but spray paint sprays out and wont give you the nice stream that you want.

iRemy_y's avatar

Oh also another question. Would the car with the most or least drag/down force be the most aerodynamic?

iRemy_y's avatar

@FiRE_MaN yes i know that. the paint is just to show up as a visual on the cars, and on video if i decide to record it. What else would you recommend in place of it? The Tunnel will not be very large and neither will the models.

FiRE_MaN's avatar

the least drag/down force would be the most aerodynamic. Oh so then yes spray paint would work just fine if you want to show that there is wind actually blowing on the car. you could also just tie strands of string to the fan cover or whatever you want to call it so show that its on, thatd be cheaper.

iRemy_y's avatar

So am i measuring the drag or down force in my experiment?!

Flarlarlar's avatar

Try measuring the horizontal force pointing opposite of the driving direction, this is your drag. Put the model on something that provides nearly no friction, because it’ll be much easier to measure drag (you won’t have to account for friction that much then).
Measuring downforce is nice, but it doesnt really tell you anything (unless maybe you’re designing an F1 car).

Since you’ll be measuring multiple models with very different shapes and sizes, its nice to have a dimensionless constant per car (the drag coefficient).

The drag formula is as follows:
D = Cd * ½ * Rho * v^2 * S
in which D = your measured drag in Newtons [N]
Cd = is your dimensionless drag coefficient (which you’ll have to calculate!)
Rho = density of the air [kg/m^3] (or whatever you’re using)
v = your airspeed
S = Either you frontal area or your wetted area (frontal area is easiest to measure, but the wetted area (the surface of the entire “skin” of the car) gives the best results for Cd)

Also, if you want streamlines you should experiment with different types of paint and place a wad of paint on your car. It’ll flow out across the body in lines, places with a lot of paint indicate a “bubble” of air trapped above that (thus indicating a vortex, which increases drag)

Don’t shoot me if there’s some error, it’s kinda late here 12:30 am over here.

I’d really like to help you out if you want, but i’m a bit rusty on this (and i really need some sleep)

Flarlarlar's avatar

oh, and the Cd should be smaller than 1, just so you know what result is usable.
Typically, cars have a Cd between 0.8 and 0.25

Shuttle128's avatar

@Flarlarlar In this case it’s not really necessary to determine a drag coefficient. For the most part his models will be of similar size so scaling will not really be an issue. It will be much simpler to just measure the drag on the cars in force.

@iRemy_y I think the easiest way to measure the drag force would be to place a fairly accurate digital scale on the floor with a single small pulley on a table above it. Tie one end of the string to the car and the other end to a small mass. Place the mass on the scale and find the weight without the fan on. Turn the fan on and for each car record the weight that reads on the scale. The absolute value of the difference in weight will be the total drag (neglecting friction) on the car.

A very good way of doing flow visualization is using smoke. In most smoke streamline wind tunnels mineral oil is burned with a hot electric filament to produce the smoke streams; however, some incense would probably work just fine. You can buy incense at just about any store and it’s on a stick so you can move it around the car to different locations to see how the flow is moving around it.

Another flow visualization technique already mentioned is putting a colored dye or wet paint on the surface; however, this only shows what is happening on the surface of the object and not what is happening further away in the flow. It may help to show where separation occurs on the car though.

Finally you could add small tufts of string or yarn to the surface. This will also help you visualize where separation and turbulence occurs on the model. The only problem with this setup is that at the small scales that I’m expecting you to be using the tufts will probably alter the flow fairly significantly. So take the results with a grain of salt.

In aerodynamics there are several types of drag you have to look out for, but in this case only two will be very important. The ones to look for in this experiment are skin friction drag and separation drag (commonly called pressure drag).

Skin friction drag has to do with the surface conditions of the object being tested. I would suggest testing different surface smoothnesses to find out which of these gives the least drag. Skin friction drag is caused by how much the air flowing over the object tends to “stick” to the surface.

Pressure (or separation) drag is caused by the shape of the object. The reason it is called both pressure or separation drag is because on blunt objects the flow does not flow smoothly over the object and tends to “separate” (separated flow no longer flows smoothly over the object). The separated flow causes a pressure difference between the back part of the object and the front part of the object. This pressure difference causes a force in the drag direction.

Separation can be seen with visualization aids such as smoke or yarn tufts. In normal flow around an object the smoke or tufts will tend to move in a straight line around the object. When separation occurs the smoke will swirl and move erratically, the same with the tufts.

At the scale and speed that I’m anticipating your models to be tested at, downforce will have very little effect on the overall outcome. Downforce is an aerodynamic force caused by the shape of the car and its spoiler (or airfoil, sometimes called a wing). The downforce is used to make sure the rear tires of a race car have enough grip at very high speeds. The downforce does add to the drag on a car (this kind of drag is called induced drag) but it is not very significant until the car is travelling very fast.

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