Undertray downforce and heave/pitch frequencies

[marcush.]

typical yaw test in a full scale tunnel with a real vehicle.
note: it´s a 5 belt tunnel and you see the actuators which hold the car in place.
5 belt tunnel = 4 belts to drive the whhels & 1 belt to simulate the moving road.




closeup on the actuators:



typical 5 belt full scale tunnel design used for road/race car development

that 5 belt thing with vehicle suspended on stuts...how close would this come to real world compared to a non moving ground tunnel with decent boundary layer removal?
There is only the middle section of the car under moving belt conditions and the
mix between moving ground /non moving ground is happening under the car in sill
region .. I can imagine this will lead to all new variables to compensate...

I really like the idea of a real tunnel and actually drive the car not the ground..
as some Nascar team is doing.

[747heavy]

Hi Marcus,

Sorry I can´t answer the question, as I´m not into the nuts&bolts of wind tunnel procedures.

What I can say is, that with the results from both tunnels, we found trends (in terms of better/worse) which correlated o.k. with on track performance.

As we have discussed earlier, when talking about test rigs/proccedures (tires/7 post), most of the time you are looking for trends, the absolute numbers are may not 100% correct, but as long a gain is a gain, it is worthwhile using these "inperfect" tools. IMHO

As you see from the pictures, these 5 belt tunnels are mainly used for road car development.
Non of the full scale tunnels I have used, was exclusivly used for race cars.

As with any other test equipment, they all have their pros and cons.
In the tunnel you mentioned, it´s difficult to simulate yaw in believe
You may also get some influence from the stationary tunnel walls, which for some tests (NASCAR) maybe is not a bad thing, and close to reality.

Nothing will do it all - where their is light, their will be shadow as well.

[marcush.]

obviously you are right ... as long as you are aware of the limitations of your tool it still can be used to good effect...
In laymans terms..posessing a hammer does not enable you to put a nail into a board straight ...you still need expertise and experience...try to put the nail into a steel plate ,you will not suceeed the knowing guy would not even try the amateur would bitch about the hammer was crap..

[747heavy]

=D>
well said - hope you have a nice day

[marcush.]

=D>
well said - hope you have a nice day

my fear is some experts choose to blame the hammer to soon instead of pointing towards themselves when things are not going as expected..

[Tim.Wright]

my fear is some experts choose to blame the hammer to soon instead of pointing towards themselves when things are not going as expected..

The truth?

There is no hammer...

[747heavy]

Thankyou very much scarbs!
I found that mag really interesting and want to reproduce a graph here:



As you can see, there is a substantial difference between static and dynamic windtunnel testing.

I wonder if the next step for for optimum aero performance should require shape patterns that comply with the wind waves generated by the heave and pitch.

Hi Belatti,

thanks for your effort
Why I do understand what you want to say, and I agree with that in general, there seems to be something a little bit odd with your scaling of ground clearance.
In your example the dynamic ground clearance changes more then the 4 mm mentioned in the text. It´s more like 7 mm (from about 9.8mm to 17.5 mm).
The fixed body value, looks close to the mean of the moving body value, which makes sense IMO.

[marcush.]

as I understand ,Belatti just copied the graph from the mag ..and the error seems to be amplitude 4mm and should read +/-4mm ?.Then it starts to make sense..

[747heavy]

Sorry, I still don´t get it.
If we talk about the Emmen-Model-Wind Tunnel.pdf file and the 3D map, then
that shows a RH Matrix starting at 30 mm FRH/35 mm RRH going to 52.5 mm FRH/
57.5/60 mm RRH (it´s not 100% clear to see).

So that would leave us with a constant rake of 5 mm and a RH change of 22.5 mm.
Where does the 4 or 8 mm come from?
But maybe I miss something here.

[Belatti]

as I understand ,Belatti just copied the graph from the mag ...

Right, I suggest 747 to re-read the topic, including the scarbs links

[747heavy]

Thanks Belatti - I did

My apologiese, it was not your graph.
Nevertheless, I don´t think that the graph correlates well with what was said in the text. But that does not take away, from the overall statement they wanted to make, with which I agree, and which follows the spirit of the thread.

Hope you have a great weekend.

[bill shoe]

Best topic in a while, thanks much. Bill.

[DaveW]

I don't wish to question the contents of the composite plot extracted by Belatti from scarbs' reference, particularly when little is known about the conditions under which the measurements were taken. However, I do find a couple of things a little odd:

1. The "fixed body" plot suggests that the model was oscillated about a "knee" in the aero map - maximum D/F occurred at a ride height of 13 mm (approx) & reduced when ride height was reduced or increased from that point.

2. If the harmonics are filtered out of the "moving body" plot, then the maximum D/F occurred at minimum ride height, & vice versa. That makes some logical sense, but minimum ride height also coincides with maximum upward acceleration of the "sprung" mass, & vice versa. (Observation, not an accusation.)

I recall compiling and solving an aeroelastic model of a venturi tunnel vehicle complete with skirts using a steady state aero map modified by the Wagner function. The model predicted the critical airspeed with reasonable accuracy (& very little fiddling) & also reproduced both the frequency and the "mode shape" of post-critical sprung mass motion.

I'm reasonably content with the thought that the aerodynamic flow under a flat bottom-diffuser vehicle is (much) more complex than that under an early skirted ground effect vehicle, but I struggle to imagine what might cause the differences between steady & unsteady down-force characteristics as published in scarbs' reference (although I don't admit to being a mobile CFD solver).

[Belatti]

but I struggle to imagine what might cause the differences between steady & unsteady down-force characteristics

Thats why I created this topic for. Windtunnel testing is rather static. One can measure how does DF or drag changes with height and asume those changes are whats going on when the car moves at the track, but there is an oscillatory phenomenon there that, as every oscillatory system, has a natural frequency and its harmonics, contributing for bad or for good.

F1 active suspensions tried to minimize those oscillations amplitudes, but what if you can use them in your favor?

[marcush.]

but I struggle to imagine what might cause the differences between steady & unsteady down-force characteristics

Thats why I created this topic for. Windtunnel testing is rather static. One can measure how does DF or drag changes with height and asume those changes are whats going on when the car moves at the track, but there is an oscillatory phenomenon there that, as every oscillatory system, has a natural frequency and its harmonics, contributing for bad or for good.

F1 active suspensions tried to minimize those oscillations amplitudes, but what if you can use them in your favor?

I imagine you could effectively use the system as a pump having it move more air than was possible under static conditons...
if this activity was creating significant effects could this help to filter out trackside irregularities in df generation?