Air resistance when driving close to a wall.

[mep]

At Melbourne there was once a sence when a Honda drove
very close to a wall and maybe even toughed it by accident.

One of the TV comentators said that he should not drive so
close to a wall because this would increase air resistance of the car.

Somewhere I heard that, at the american oval races the drivers
drive very close to the walls to decreas air resistance.

So now I wonder what is true and why?

I think the first one sounds more plausibel because the car has
to push the air away and the wall tries to hinder that.
And there should apear a force who tries to push the car to
the outside because of the fast air velocity between car and wall.

It's like a airplane that has more lift, if it flies very close to the ground.

[vis]

Nice topic, hope some expert enlighten us!

[Ranald]

In high speed oval racing then interaction between wall and car certainly is a hugely important factor to consider but I personally don't know enough to say anything specific. In formula 1 or pretty much any other series I would doubt running close to the wall would cause significant negative effects. The car is close to the wall for only a very short amount of time when compared to an oval racing car. I believe that the effect would be negligable in Formula 1 but significant in a series like NASCAR.

[checkered]

I agree that ovals experts will

propably have a well founded and an extensively researched answer for your question. I don't follow ovals racing (and don't have the willpower to go digging in IRL or NASCAR sites, though the latter doesn't correspond well to F1 dynamics), so I can only offer a hypothesis. I'm also thinking that a very simple CFD proof-of-concept model wouldn't be beyond a couple of participants of this forum.

My guess (yes, guess) is that if the effect was a force function of the distance from a reasonably high wall, starting from afar and getting closer the effect would be:

Very far - negligible, unless there's a side wind that creates a horizontal vortex ... and I won't even start to consider that (Or maybe a little bit - depending on wind direction and speed, it might even be advantageous to drive fairly close to a wall if one was downwind from the wall. University of Karlsruhe has studied just such vortices in a larger scale, there's a CFD animation of one example on their webpage ... http://www-ifh.bau-verm.uni-karlsruhe.d ... e/aerodyn/ )

Getting closer - added air resistance to the car on the side of the wall, and hence also better aerodynamical traction that is just as lopsided. A driver might experience a slight force "coasting" the vehicle away from the wall at these distances. Perhaps predictable when driving in a straight line, but could be perilous in situations where the driver changes the dynamic forces of mechanical traction i.e starting a turn or shifting from power to brakes. Just as it is when applying brakes when the tyres on the different sides are on different surfaces.

Very close - suction. The "lateral area" of low pressure by the sides and behind the car is in deep asymmetrical imbalance (the wall side "filling in" much slower) and air moves in a lopsided pattern over the centerline of the vehicle applying a lateral force pushing the car towards the wall. The not-so-nice part of this (if I'm even close in my guesswork) would be that after a certain treshold value, the "push" will become a self-perpetuating exponential force, ever harder to counteract with a sudden correction while the process progressing.

... I've also been taught that in an urban conflict environment, troops taking shelter from light arms fire should keep a few feet from the protecting wall. Don't know if it's a myth or not, but it does make some sense to me on many levels. Anyway, apparently shallow angle ricochets tend not to reflect in the same way as rays of light do (in the same angle) but tend to keep fairly close to the wall (whereas right-angle bullets will just bore in and be subsequently harmless).

But I'm guessing this represents another largely incomparable aero phenomenon, since I take it that a ricochet would also most likely convert part of the hit energy in a rotating movement, the direction of which would in most cases induce lower pressure on the side of the trajectory of the ricochet that is facing the wall. Thus the rotating bullet would be "pushed" towards the wall. That would also account for the rapidly looping variable pitch in the sound of the ricochet, apart from the normal doppler effects.

OK, but surely there is someone here who can be more definite about the answer to the original question than me.

[ginsu]

I don't know for sure what the effects would be. But certainly the shape of the vehicle influences the airflow next to a wall.

I know that when I pass a truck at high speed on the highway, there is a significant push from the standing wave formed at the front of the truck (it pushes me away from the truck). This is similar to the 'bow wave' formed at the front of ship passing through water.





Because the bow wave is of such high dynamic pressure (you can see from the pics), the structure can be likened to a high velocity jet of air. I imagine that if you approach a wall with the bow wave intact, that it will reach a point where it 'chokes' because there isn't enough area for the high pressure air to flow through, so the air starts to build up and would cause alot of drag. Also, maybe the blockage would also create a bit of a 'buffer' of air kind of pushing the car away from the wall.

As far as creating suction, I kind of doubt it would happen, unless the shape of the vehicle allowed the wall and the side of the vehicle to create a venturi shape. But why would you do that? I don't think anybody wants to get sucked into the wall. So, I think they do everything they can to prevent this from happening.

Also, I had an idea about this for Monaco, since the cars run so close to the wall during the whole race. Would it be advantageous for an F1 car to create a larger bow wave, so that it will choke at a decent separation distance from the wall and prevent a car from hitting the wall, thus guiding the car through the corner safely? Hmmm.

[Tp]

Also, I had an idea about this for Monaco, since the cars run so close to the wall during the whole race. Would it be advantageous for an F1 car to create a larger bow wave, so that it will choke at a decent separation distance from the wall and prevent a car from hitting the wall, thus guiding the car through the corner safely? Hmmm

Surely that can only happen if the vehicle is travelling parallel to the barrier, if it was to change direction (not parallel to the barrier) the effect would be lost.

[doodzed]

I remember seeing an article about 15 years ago on the interaction between a stock car and the wall. It turns out there is an optimum distance in terms of aero. I think it was 6 feet from the wall for cars of that era. The article had math and windtunnel data.

Wish I had notes but I bleive it was an ASE journal. I was in 8th grade at the time so forgive the holes in memory.

[syguy]

Advantage CFD(Motor racing CFD consultancy, no longer active) carried out a CFD study on a NASCAR racecar in close proximity to a wall. The study was described in Racecar Engineering, June 2001.

They looked at a NASCAR racecar traveling in a straight line at 150mph (240km/h) 1/4 car width away from a 4'2" (1.28m) high wall, compared to a wall free case. Near the wall the study showed slightly higher overall drag and slightly higher downforce. The most noticeable effect near the wall was a higher pressure near the nose side of the car and a lower pressure near the rear side around the rear wheel. It seems that a venturi effectwas pulling the rear of the car closer to the wall and the nose was being pushed away from the wall. Presumably we should see this effect in a race if a car close to the wall loses traction from the rear tires, the result should be a spin with the rear of the car hitting the wall and the front veering away from the wall in an anti-clockwise spin (assuming a left turn oval).