Wind tunnels - dumb aero question

[rjsa]

It is still nothing like the still air (with wind and turbulence on occasion) that the moving car meets on track.

Is it? I would think it uncommon to encounter perfectly still, completely nonmoving air on a racetrack plied by multiple cars, whereas the flow entering the test section of a wind tunnel is usually fairly laminar.

The air in a wind tunnel is moving at high speed.
The air at the track is stationery or subject to a low wind speed.
Low speed turbulence can be introduced to the air before the car goes through it but it will be low speed.
Where is the similarity?

The air at high speed, the car standing still ovar a rolling road, air standing still, car at high speed over fixed tarmac.

There is something about relative speeds that pretty much makes all that the same, I'm sure about that.

If not just imagine the mess the rough 1,600 KPH we travel around earth's center here in the tropics would make!

[Lycoming]

In both cases, the airflow is moving at a high speed relative to the car. The difference is that they're moving at different speeds relative to the environment, but that's what rolling road is for; the walls should be far enough away that their effect is negligible. As for "low speed turbulence"... it will still be moving at high speed relative to the car. If you think of the intensity of the turbulence in terms of vorticity... in most cases it will be the same in either the wind tunnel of the track.

[rjsa]

I guess I fail at irony in written engish...

[rscsr]

I guess I fail at irony in written engish...

irony is really bad at technical discussions.

[autogyro]

Air flowing fast in a wind tunnel is subject to boundary layer effects before it even reaches the static car.
The part of the fast moving air in a wind tunnel touching the ground displays skin frictional drag and this affects the speed and pressure of the other parts of air up to the top of the moving column.
If you add moving road surfaces into the tunnel the effect is magnified.

This is nothing like the effect on static or slow moving air a moving car has on it.

[rjsa]

You can always control whether the test model is subject or not to the side walls and roof boundary layer in the test section. You just have to make it big enough, or the boundary layer thinner. On the floor, you just add a rolling road and a suction device prior to it.

It's not that complicated. Of course, if you get it wrong you screw your data.

[autogyro]

How do you prevent a high speed column of air from generating a large boundary layer effect when its outer lower surface encounters a rolling road surface travelling equally as fast in the opposite direction?

Ducting lower surface air ahead of the rolling road surface will not prevent this and neither will a tunnel size as large as an aircraft hanger.
The result is the column of air is sucked to the rolling road surface and compromises the data.

[rjsa]

How do you prevent a high speed column of air from generating a large boundary layer effect when its outer lower surface encounters a rolling road surface travelling equally as fast in the opposite direction?

Opposite direction? They roll to the same direction and at the same speed as the air flow, that's exactly the point of the rolling road: no relative movement between the airflow and the floor - mimicking a car going along stationary air and ground.

[Lycoming]

How do you prevent a high speed column of air from generating a large boundary layer effect when its outer lower surface encounters a rolling road surface travelling equally as fast in the opposite direction?

You don't, but it's moot because with rolling road, they travel in the same direction and have 0 velocity relative to each other. I don't understand the point you're trying to make here.

[autogyro]

How do you prevent a high speed column of air from generating a large boundary layer effect when its outer lower surface encounters a rolling road surface travelling equally as fast in the opposite direction?

Opposite direction? They roll to the same direction and at the same speed as the air flow, that's exactly the point of the rolling road: no relative movement between the airflow and the floor - mimicking a car going along stationary air and ground.

Sorry my bad, you are of course right the rolling road surface goes in the same direction as the air flow in the tunnel.
Dont know how I got that wrong unless I was in reverse gear.
However balancing the speed is crucial.

[riff_raff]

I'm not so sure I agree there. You could model the distrubance that the instruments make in CFD as well. We do this all the time - you don't just model the item under test, you model the test rig as well.

That is a good point. Wind tunnel models all use some form of mounting device which is used to measure the aero forces acting on the model. The influence from these blade or sting force balance devices is always considered in the analysis model of the test article.

[Tommy Cookers]

..... Wind tunnel models all use some form of mounting device which is used to measure the aero forces acting on the model. The influence from these blade or sting force balance devices is always considered in the analysis model of the test article.

a sting balance is internal to the model and has a downstream mounting, so there is no influence on the model
the mounting system is angled and curved, hanging first rearwards then 'down' in a shape like a bee sting after use
such that remote operation of the mounting's 2 angled roll axes gives the model all desired combination of Alpha and Beta angles

car work uses internal balances that should not be called sting balances (unless they are downwind mounted)
and external (strut-mounted) balances
these probably would need correction (for aero effects of mountings)

[marcush.]

Trying not to be too nitpicky ,but the fact you are able to correlate a modified device under test says not much about how this all translates into reality .

I´m far from saying tunnel and CFd are useless my Point is you Need to be very sure what is worth correlating and were your tool has reached the outer rim of being useful for the results you are looking for.

Quite often rig testing is giving a lot clearer indication of what is really going on -as you can cancel out influences out of your control.
If the results gathered make a difference in real world can only be tested

[lio007]

Which of the following wind tunnels should be the best, if i only look at the figures.
What is important to a wind tunnel? e.g. a bigger or a smaller Reynold No.?

It's a pity that the datasheets are not available. (the only one I have is about RBR's wind tunnel - from last December)
http://www.ati.org.uk/aerodynamics/faci ... unnel-167/
http://www.ati.org.uk/aerodynamics/faci ... unnel-163/
http://www.ati.org.uk/aerodynamics/faci ... tunnel-73/
http://www.ati.org.uk/aerodynamics/faci ... unnel-1-9/
http://www.ati.org.uk/aerodynamics/faci ... nnel-2-10/

[Tommy Cookers]

What is important to a wind tunnel? e.g. a bigger or a smaller Reynold No.?
http://www.ati.org.uk/aerodynamics/faci ... tunnel-73/]

everything is important
flow quality is hugely important but this fact is buried for good and obvious reasons
it's the dirty secret of wind tunnels
it's the reason why different tunnels give different results
does the existence of a steady uniform flow tunnel imply deficiencies in other tunnels ?
modelling quality is of course Reynolds number dependent

some real-world examples of erroneous (wind tunnel-derived) aircraft performance predictions in the public domain ?
a leading figure publicly said that the cruise drag of the C-17 was underestimated IIRC by 17%
the F/A-18 E/F excessive approach speed in carrier landings was not predictable, claimed its makers in magazine articles
large civil aircraft performance is subject to severe penalty clauses, though