Using Car As Wind Tunnel?

The front. Assuming the air is still.

I have no idea what you're talking about, but mount the damper on the hood.

Be careful!!! As you are carefully watching for the damper to open, a person, tree or building may suddenly appear in front of the car.

Your question isn't clear, what damper are you writing of? is the wind blowing over it or on it?

I am building a manual control volume damper for HVAC. I need to test the plate of the damper which is about 1.405kg with dimension h=157mm and b=816mm , b/h=5.197. The minimum air flow is 25 m/s which is 90 km/h. I need to know whether the pressure drag from the wind blow can lift the plate of the damper. The plate of the damper is subjected to fixed-axis rotation.I don't have wind tunnel , so I wonder I can drive my car at certain speed which is almost equal to the wind speed to test it.

Sure. All you need is an anemometer or a wind speed indicator to adjust the air speed by varying the car's ground speed.

I'd mount a luggage rack on top of the old Hudson and attach the damper to that.

Let us know how it works.

In real life, it will have a drag on the inside and still air on the outside. on the luggage rack it will have drag on both unless the shape of the car forces more air under the damper.

In any case, I don't see it as a valid test, the car is short and air is forced up over it, the duct is continuous.

Wiki has the formula for basic wind pressure, and a factor of 0.7 is usual for outward suction.

Don't get all serious on me!

Thanks all for the comments

If you take some of post #4 and add the duct from post #5 I recon you will get a reasonable test.
Personally I'd rather believe a makeshift test over 3 pages of calculation any day.
Sounds fun. We demand pictures.
_{Pencil in umpteen pages about safety precautions/local laws/sticking you head out the train window etc...Ooooh yes, we've go to have a risk assessment too}
Del
(But then I'm a cat)

Actually, it's a "back of the envelope" calculation. The regulations come in when he's bombing down the interstate, or maybe the M11, and loses the experiment off the top. I agree, a makeshift test is good as long as it has some relevance.

You may not have to use your car. I think you may be able to get close by checking the psi with a gauge at the outlet of one of these hooked up to a compressor. Increase output psi until you are able to open the damper with the air.

Use that number in this equation, backwards, and you should have a close approximation of required wind speed.

People with more time than money have used this type of setup for all kinds of aerodynamic testing. Just try to pick a semi-isolated section of road, and have a good chase driver to pick up any parts that might go flying off. This is not a new idea, just be careful with how you fasten the damper to the car.

And as Del mentioned, please post pictures so us in the peanut gallery can make more rude comments!

Seriously though, this can be a valid test for what you are looking for.

Thanks all.

Reply to #8

What is makeshift test?

Reply to #11

The size of plate is as below. Can the compressor produce the wind blow that will exert equal pressure to the plate?

h=157mm and b=816mm , b/h=5.197

You asked "which part of my car will have the wind speed which is almost same with my car speed".

The obvious answer is any part that is a long distance away from the car body. Any point near the body will have a complicated air flow, if you mount your test on a roof racks but lifted (say) 1 metre above the roof you should be getting airflow at driving speed.

Good luck and don't do anything unsafe.

Thanks for the answer, by the way, I have another question. If let's say I mount the damper on the roof rack about (1 metre) above and I get the wind speed almost same with my car speed, then how should we consider the pressure there? Will it be the same as atmospheric pressure or will there be pressure drop?

If there's nothing there it's at atmospheric pressure, as soon as you mount a test piece (and start diverting the air stream) there will be regions of higher and lower pressure.

How far do you want to go?

You could calculate a rough pressure difference using the Force on the part divided by the apparent cross sectional area.

Or, you could do some serious engineering and look at it as a complex 3D envelope of moving air, with different pressures and speeds at each point, that will change as the orientation of the part changes and probably have chaotic bits as well.

Perhaps if you described what you're are trying to achieve more fully we can help.

make·shift/ˈmākˌSHift/

Google and a dictionary both provide a definition.
Lucky I'm not in a grumpy mood
Del

I would think so, but I'm not a big equation guy. If you were to provide more details, there are probably people here that have computer programs that could run the numbers.

Thanks.

The min velocity of the wind is 25m/s(90km/h), the dimension of the plate is 816mm x 179.4mm x 1.2mm, it is a stainless steel plate. The weight is 1.41kg. The aspect ratio b/h of the plate is 816/157= 5.197. The design pressure is 108.2kPa which is higher than the atmospheric pressure while the design temperature is 37 degree. Thus, the density of the air calculated is 1.21534 kg/m3. The minimum volume flow rate in the inlet of the damper is 22 m3/s. There are 5 plate of the same dimension in the inlet. Thus, one plate will have the volume flow rate of about 22/5 or 22/6.

Local aerodrome, off-road racing circuit, long farm driveway, nearby?

As the object is not large, how about make-up a temp. wind tunnel?

Several farmers have large drying fans, as do many industrial firms,
worth asking around?

Having experienced flying objects departing my own vehicle roof, I can
only advise extreme caution on the public highway! (IF it is even legal!)

I hope you can find a safer way to determine what you need to know.

Varying the pivot point will naturally affect the pressure required?

jt.

My mother said to me, if a stranger in a car pulls up along side you and
they ask you to get in....... Just go!