Aerodynamic Penalty of Highway Dividers and Walls - Fuel Economy, Hypermiling, EcoModding News and Forum

orange4boy · 07-20-2011, 01:03 AM

I've been thinking about this for a while. It occurred to me that being beside a car or truck is a bad thing therefore being beside a wall or divider must also cause additional drag. The aero penalty of tunnels is well known. I have been trying to be in the "clear lane" when possible to avoid this added drag.

Thoughts?

ShadeTreeMech · 07-20-2011, 01:27 AM

not sure what you're talking about...a moving truck would cause problems due to it being a moving object causing turbulence, but a non moving wall? It might increase the effect of the turbulence from your own vehicle by bouncing it back onto you, but can't see how it would cause problems otherwise.....

orange4boy · 07-20-2011, 02:27 AM

Well,

Hucho states that buses can experience up to 6 times the drag in a tunnel compared to the open road and we know that proximity to the ground is a major drag problem so I would think that a wall or divider would have a similar effect.

Six times is a whole bunch of extra drag! Of course there would be a reduced effect for smaller vehicles in the same tunnel. A wall on one side would have a smaller effect as well but depending on the proximity it could be meaningful.

euromodder · 07-20-2011, 05:00 AM

Quote:

Originally Posted by orange4boy

I've been thinking about this for a while. It occurred to me that being beside a car or truck is a bad thing therefore being beside a wall or divider must also cause additional drag. The aero penalty of tunnels is well known.

Depends on the relative tunnel and vehicle size - and on it being uni- or bi-directional.

Biking in a tunnel (4m / 13' wide) is a dream, even on the gentle uphill part.
Tunnels, walls, and dividers shield you from the prevailing wind, which can be very beneficial.

For the walls to have such a huge impact as the "ground effect", it'd have to be a narrow tunnel compared to the vehicle.
There a thread on moving the vehicle's body out of ground effect, that suggests a relative height where the ground effect greatly diminishes.

A uni-directional tunnel will have an air flow through it going in the direction of traffic, so you'll get free tailwind

A tunnel with bi-directional traffic will clearly illustrate the effects of a bow wave.
Even at a modest 50kph / 30mph for both vehicles, the bow wave of a truck can really be felt when going the other way, as it's got nowhere to go but into the opposite lane.

d0sitmatr · 07-20-2011, 10:15 AM

exactly how I was thinking of it.
also, even if you have a bi-directional tunnel, if there is a wall separating the two lanes, even if its only 3' high, I can see it keeping the ground flow directional to the flowing traffic.
I've seen benefits from driving through construction areas on the hwy that have cement dividers for each lane. (north/south-east/west)

graydonengineering · 07-20-2011, 11:52 AM

It makes sense to me that the more side you block off, the more resistance you will have. You have the ground all the time and in the case of a tunnel, you have all sides. If you had an air dam just off the wall surface on the side of your car for example, you would be pushing extra air because that air that could otherwise flow around the side can only go up. If you are driving further from the wall, you will have to plow through side turbulance as the air bounces off the wall and back toward the side of your car. As with all real world cases, there are enless variables you can throw into this but in general it seems to me like driving allong a wall would add resistance, more and more as you get closer to it. Something I never really thought about till now!

mnmarcus · 07-20-2011, 01:12 PM

If you're the first one going through a very narrow tunnel you may experience some additional drag, IMO. 99% of the time I'd think any tunnel or wall would help you, again IMO...

This makes me rethink some of my drafting though... I've always tried to avoid driving next to a vehicle or even slightly behind (and in the other lane.) I've been trying to avoid their "wash". But thinking about it now, would that be better than still air? It's still moving (in some amount) in the direction I'm going. It seems like wind-noise goes up in this "dirty" air which (I think) is why I've tried to avoid it. Extra windnoise seems like extra drag. ???

graydonengineering · 07-20-2011, 01:39 PM

Quote:

Originally Posted by mnmarcus

If you're the first one going through a very narrow tunnel you may experience some additional drag, IMO. 99% of the time I'd think any tunnel or wall would help you, again IMO...

This makes me rethink some of my drafting though... I've always tried to avoid driving next to a vehicle or even slightly behind (and in the other lane.) I've been trying to avoid their "wash". But thinking about it now, would that be better than still air? It's still moving (in some amount) in the direction I'm going. It seems like wind-noise goes up in this "dirty" air which (I think) is why I've tried to avoid it. Extra windnoise seems like extra drag. ???

Next to and slightly behind does give an advantage. Look at the back sweep angle of plane wings and ducks flying in formation as examples. The air is churning and as a result, a low presure zone exsists.

mnmarcus · 07-20-2011, 01:42 PM

Makes sense. On a bicycle you can feel the difference. In a car (I think) the extra noise has made me avoid it.

orange4boy · 07-20-2011, 01:57 PM

Another observation I have made is the pressure wave from a car extends quite far from it's sides. Standing at the side of the road, even a fairly slow moving car will make the grass sway as it passes even when the grass is a few of meters/yards from the car.

I don't think the air is doing any bouncing. It sure feels like bounce when you pass a semi though!

For what it's worth, here is a graph showing the increase in drag with decreasing ground clearance: