Wind tunnel problem

You need to fix the model to a surface that somewhat models the road; however, you will quickly run into some simularity problems at your 1:18 scale. If you just want to mess about with things like streamlines over the hood, put it on a sting. At the speed you'll be running, a sting creates essentially no problem.

I was thinking of a gluing the bottom of the car to the head of a thin rigid bolt, and put the model with the bolt attached to it on a platform with a hole in it and tie the bolt with a nut from underneath the platform. DO you get an imagination of what am trying to explain? what do you think about that?

I think that would work OK if you used something like an elevator bolt head. I assume you're putting this sting well back on the car and I assume you're not looking for effects anywhere in the lower third (perhaps - a wild guess) of the car model.

Elevator bolts sounds perfect, concerning stuyding ground effects! well so far we will be forced to disregard ground effects because of the bolt mounting!

You could still arrange a sting so you could see ground effects, but scaling becomes an issue. But, the big issue is that, in your wind tunnel, the air is moving relative to the road whereas, in real life, the air is generally stationary with respect to the road.

What is your level of work? High school? University?

Bsc of Mechanical Engineering graduation project, Senior year. This is our level of work.

Could you please explain to me how the Sting mounting works?

The instrumentation won't be an issue, we have a really good wind tunnel http://www.hampden.com/tortoisecms/uploads/files/Wind%20Tunnel%20Systems.pdf

The only problem we are facing now is the mounting of the models. and yes we do have faculty guidance, the bolts were suggested by a faculty member as well.

By doing this you will never get the correct simulation, as you are loosing the behaviour of the relative wind underneath the car. In my opinion you will have to fix the car onto a simulated road in order to get the real picture.

Wangito.

How about a thin wire cube. Suspend the car with strings or wires from the eight corners of the cube. It is likely that you are going to try sub sonic speeds only.

Consider a moving belt under your model if you are studying ground effects. If the car is supposed to be moving, the ground under it must be moving at that speed, or ground effects won't be the same. That eliminates any thing out the bottom of the car. NASA uses what's called a sting. It is a probe the car is mounted on out the back of the car. It doesn't affect the wind flow. The probe probably mounts inside the car at the center of effort, or some spot like that. Sensors between the probe and the car measure force in all directions.

How can we attach the sting to the car models? Do we have to make a hole inside the body of the model?

If you are only interested in seeing how the air goes around and under the car, you can just solidly mount the sting to the back of the car.

If you are interested in if the car gets lift, down-force, drag, etc, you will need to determine where the center of effort, center of gravity, or whatever, is for the car. It will be somewhere near the center of the car, I presume. Then you have to figure out how to mount strain gauges, or whatever sensors that will do what you want. You may be able to solidly mount the car to the sting, and have the sensors on the base of the sting. Check for an instruction book with the wind tunnel, it should have this info.

PS You will have to counter-balance for the weight of the sting; the car should only be putting correct scale weight downwards if you are measuring lift or downwards force from skirts or wings.

Hi maverick,

You may find a hint here:

model car in wind tunnel test search field

how+to+attach+model+car+in+wind+tunnel+test

1. The Design and Testing Process27 Aug 2001 ... Wind tunnel testing is used extensively when designing a new car. The model is placed in the test section of the wind tunnel where airflow ...
   www.nas.nasa.gov/About/Education/Racecar/testing.html
2. [PDF] Wind Tunnel TestingFile Format: PDF/Adobe Acrobat
   Student teams design their own model car and test it in a simple classroom wind tunnel using a fan and long cardboard box. Next, ...
   www.tryengineering.net/lessons/windtunnels.pdf -

I hope you find what you are looking for.

Good luck and you can always ask again!

If flow under the car is not important, why not fasten it to the platform through the tires?

If it is, you will need to have the tires rolling and the "road" moving with the wind. The sting at the rear would be the best mount in that case.

You could jack up the car on four individual jacks adjascent to each wheel supported on the front and rear axles

Construct a deck that the car will "set" on, with a controllable speed conveyor belt installed just within the width of the tires to simulate a moving road. Install short threaded rods into the contact patch of each of the tires. Drill holes through the deck to allow the rods to extend through and contact load cells mounted below the deck. Be sure to secure the rods to the load cells to measure lift, if any. The height of the car can be adjusted so the tires are at road surface level. You can attach load cells to each side of the rods to measure any sideways deflection. Attach a string (dental floss is pretty strong) and load cell to the front center of pressure to measure drag and keep the rods centered in the holes.