Water tunnels in F1

[raymondu999]

Hey folks. Here's something that I never quite understood in F1. We always say that the whole system works as a package. But in wind tunnels (at least the videos you see on YouTube) you put some colored smoke in the airflow so you can track where those air particles go. But is this not leaving the rest of the car not tested? I mean, it's looking at one part only, surely?

I'm not an aero person I admit. But I did once work on hydro and submarine propulsion and design. We used to use water tunnels, kind of like a wind tunnel, but the working fluid is water as opposed to air, similar to the one shown in this video:
[youtube]http://www.youtube.com/watch?v=DijdU0rmDdc[/youtube]

We used to put a dye in the water and see how it worked on the hydrofoils and such. Would this not be more effective, given that you would see the airflow (or water flow) in a more complete package?

Again I'm no aero guy, but are the principles of all fluid dynamics (of which aero and hydro dynamics are a subset) the same?

Why does F1 not use this? Any and all enlightenment would be appreciated.

[wunderkind]

We discussed this on another thread some months ago.

I know Mercedes Benz uses water tunnels to fine-tune the aerodynamics of their cars (especially the undercarriage). Some forum members dismissed the idea water tunnels for F1............I wasn't convinced.

[raymondu999]

Do you mean Mercedes Benz the F1 constructor, or the car manufacturer?

[JohnsonsEvilTwin]

With the viscosity being thicker than air, wouldn't it make sense to use water models?

Or are there subtle nuances in the difference of properties at speed?

[raymondu999]

I honestly don't know. They worked fine for me back then, but admittedly my end product would have been going through the water, and as such a water tunnel would have worked a lot better for me.

[n_anirudh]

maybe the sheer size of construction of a water channel that large?

I dont think there is anything that stops a team from building a full scale model of one..as they are limited to testing at 60% in a wind tunnel

also fluroscien and rhodamine dye can be used for flow viz with longer wake trails...

[Richard]

Air is compressible, water is not. That's rather fundamental!

As for the smoke trail, they do move that around to see how the flow works on different parts of the car. However most of the data comes from hundreds of pressure taps in the surface that feed data into a computer to validate the CFD analytical model.

The smoke trail is visual for us humans with our subjective assessment, while pressure taps produce quantitative data for computers.

[raymondu999]

Rich - Is a wind tunnel used for direct observation of airflow characteristics nowadays? Or is it all just to plug into a CFD to calibrate it for better accuracy?

[JohnsonsEvilTwin]

Hmmm

I thought water was compressible, and a quick wiki search indicates it actually is compressible.
Just not to the level that I expected, so I think Richard point stands.

[shelly]

Toyota wind tunnel (now used by Ferrari I think) has the capability for PIV, which is a wind tunnel technique that gives full flow field visualisation which can be put in direct comparison with pictures from cfd.
See here:
http://www.tecplot.com/Community/CaseSt ... veeng.aspx
I think for some time toyota's has been the only F1 wind tunnel with such capability (which needs a lot of extra machinery compared to a normal wind tunnel); I do not know if now some other team can do PIV (except from ferrari using toyota's knowledge, of course)

[noname]

Air is compressible, water is not. That's rather fundamental!

It's just the matter of pressure.

One could be really surprised when faced with, i.e., the compression ratio of the water in high-pressure systems of the nuclear power plants.

[Richard]

OK, fair points about compressibility of water. Nothing in life is perfectly incompressible.

However in comparison to air at ambient temperatures and pressures, water behaves in a hydraulic & relatively incompressible manner.

[Rideway]

I knew them as water channels.

Appart from the fluid compressibility topic, there is another difficulty added: Reynold must remain constant in both real and model in order to obtain fidelity in our simulations.

Re=speed*caracteristic length / viscosity

.... and air kinematic viscosity is 17,4 μPa•s mean while water is 1002 μPa•s!!

that means the following
Re_real(200km/h) = 55,5*L_real/17,4*10^-6 = 4,8333*l_r*10^-6
Re_model(x km/h) = x*l_m/1002*10^-6

Lets say we can achieve a water channel speed of 50 km/h = 13,8m/s then:
l_r/l_m=0,00286 which means that for a length of 3m in the real object, the model must have a length of 1049m!!!

Thats why its so difficult to compare fluids with different densities.

[shelly]

@rideway: you misplaced kinematic and dynamic viscosity so your estimates are wrong

[Rideway]

@rideway: you misplaced kinematic and dynamic viscosity so your estimates are wrong

You are right. Didnt realize that the values belong to kinematic viscosity. Doing the maths again:

air dynamic viscosity is 17,4/1,2*10^-6=14,5*10^-6
water dynamic viscosity is 1002/1000*10^-6 = 1*10^-6

Re_real(200km/h) = 55,5*L_real/14,5*10^-6 = 4,8333*l_r*10^-6
Re_model(x km/h) = x*l_m/1*10^-6

then 4,8333*l_r=x*l_m

Lets say we can achieve a water channel speed of 50 km/h = 13,8m/s then:
l_r/l_m=2,855 which means that for a length of 3m in the real object, the model must have a length of 1,05m.

Even though 50 km/h was too optimistic to expect a laminar flow. With 20 km/h the ratio l_r/l_m=1,15 which means that for a length of 3m in the real object, the model must have a length of 2,60m.

Sorry and thanks for finding out my mistake.