Air Ducts – a down to earth guide for motorsport application

[Kiril Varbanov]

An article from Willem Toet, Motorsport, F1 and Aerodynamics Specialist, ex Sauber:
https://www.linkedin.com/pulse/air-duct ... illem-toet

[Cold Fussion]

We went to a pretty aggressive angle, of something like 15 degrees, in one plane only which was never going to stay attached but we added two splitters to control the expansion in each part of the duct to about 5 degrees

Can someone expand on this? I'm visualising a spike inside the duct to lower the expansion rate but don't know this really beneficial compared to a larger inlet and less expansion. Is it purely because the area inlet of the front of the car was already at a maximum size?

[graham.reeds]

The way I see it is as a wedge with two internal vanes to guide the expansion as much as possible.

The question this raises is for Locost racing. The air filter sticks up out of the bonnet. I was thinking that having it uncovered would mean the majority of air ingested would be from the direction of travel. I was thinking a better was a semicircle which would force air around the back causing an increase in pressure and therefore an increase in chamber? However it seems a horseshoe shape would be better?

Can someone expand on this?

[godlameroso]

Was that pun intended?

[graham.reeds]

No, but once spotted allowed to stand.

[godlameroso]

In simple terms, the expander creates a tendency for vorticity, especially when heat is added. Curious thing about air, no one bothers to account for it's kinematic viscosity, sure it's increase is minimal, and yet, a little can go a long way. Afterall a 747 only has to have the pressure drop under it's wings a relatively minimal amount to lift that heavy thing up in the air.

Anyway back to why an expander causes vorticity, the kinematic viscosity of air rises as it's heated, http://www.engineeringtoolbox.com/air-a ... d_601.html.

What this means is that hot air tends to 'pull' the air around it in the same direction. Now when air moves, it's obvious that it goes from some place to another, but since our atmosphere is pressurized, when you move air, that same space is not going to become a vacuum forever, it gets filled with more air, and if you move it fast enough you'll get a temporary vacuum in a measurable volume. If this is true, then heating this air and increasing it's kinematic viscosity will not only allow you to move the air faster, because it is less dense, but it will also pull more air creating a greater vacuum for a specified volume. Add in an expander and you can create an even greater vacuum as long as your flow remains energized 'ie laminar'. And what happens when the pressure drops enough in relation to energy/speed? Vortecies, just like when you drain the tub.

[trinidefender]

In simple terms, the expander creates a tendency for vorticity, especially when heat is added. Curious thing about air, no one bothers to account for it's kinematic viscosity, sure it's increase is minimal, and yet, a little can go a long way. Afterall a 747 only has to have the pressure drop under it's wings a relatively minimal amount to lift that heavy thing up in the air.

Anyway back to why an expander causes vorticity, the kinematic viscosity of air rises as it's heated, http://www.engineeringtoolbox.com/air-a ... d_601.html.

What this means is that hot air tends to 'pull' the air around it in the same direction. Now when air moves, it's obvious that it goes from some place to another, but since our atmosphere is pressurized, when you move air, that same space is not going to become a vacuum forever, it gets filled with more air, and if you move it fast enough you'll get a temporary vacuum in a measurable volume. If this is true, then heating this air and increasing it's kinematic viscosity will not only allow you to move the air faster, because it is less dense, but it will also pull more air creating a greater vacuum for a specified volume. Add in an expander and you can create an even greater vacuum as long as your flow remains energized 'ie laminar'. And what happens when the pressure drops enough in relation to energy/speed? Vortecies, just like when you drain the tub.

I believe you are way over complicating the workings of the expansion chamber. It is much simpler than that. As the airflow explains in the chamber it will slowdown. This slowing of air increases the static pressure in e chamber and therefore forces more airflow into the brake ducts, driver cooling ducts.

It is a very simple velocity to pressure conversion that goes on in the stator stages of an axial compressor. Airflow slows down therefore it's pressure increases.

Also note that you said a a 747 has a pressure drop below the wing. I believe you mean a pressure drop above the wing.

[godlameroso]

Yes, moment of dyslexia

[Cold Fussion]

What this means is that hot air tends to 'pull' the air around it in the same direction. Now when air moves, it's obvious that it goes from some place to another, but since our atmosphere is pressurized, when you move air, that same space is not going to become a vacuum forever, it gets filled with more air, and if you move it fast enough you'll get a temporary vacuum in a measurable volume. If this is true, then heating this air and increasing it's kinematic viscosity will not only allow you to move the air faster, because it is less dense, but it will also pull more air creating a greater vacuum for a specified volume. Add in an expander and you can create an even greater vacuum as long as your flow remains energized 'ie laminar'. And what happens when the pressure drops enough in relation to energy/speed? Vortecies, just like when you drain the tub.

Is laminar flow really that important of a condition? If the air doesn't detach from the walls from the expanding duct and there is no back flow then I don't think it really matters from a cooling perspective that the flow is laminar. Empirical testing has shown that the convection coefficient is higher for turbulent air over a flat plate compared to laminar (and I believe this extends to tube geometry as well), so your cooling performance will be higher with turbulent flow.

[Tommy Cookers]

most laminar flow has to a greater or lesser extent a turbulent boundary layer

[godlameroso]

Is laminar flow really that important of a condition? If the air doesn't detach from the walls from the expanding duct and there is no back flow then I don't think it really matters from a cooling perspective that the flow is laminar. Empirical testing has shown that the convection coefficient is higher for turbulent air over a flat plate compared to laminar (and I believe this extends to tube geometry as well), so your cooling performance will be higher with turbulent flow.

I guess laminar flow is a bad way to put it, I didn't want to say directional vector because air doesn't really move like that either. I guess we could say the general direction of airflow should try to encompass the whole volume in question, and that by far is the hardest thing to do.

And I agree with you, why else would Ferrari do this?

[olefud]

most laminar flow has to a greater or lesser extent a turbulent boundary layer

Right, as Toet’s reference to tripping the flow would indicate.