question - drag caused by following vehicles?

[SlowSteve]

Hello all.

This isn't an F1 post, but it may have relevance.

My daily runabout car is the exceptionally dull Kia Rio, and more than any car I've had before it seems especially sensitive to sidewinds and turbulance from lorries etc - as it's underpowered, it's very obvious in the foot when I am getting these effects, equally it's simple to feel tow from lorries in front etc.

I have noticed an affect that kicks in at higher speeds - say 60-70mph - where by when a lorry or other large vehicle, like a range rover etc, comes up behind me in a different lane - i.e they are coming up to my left wheel or right wheel , I feel significant drag when it gets to within say 30 feet - enough that I have to actively compensate with more throttle to maintain speed. When they get to about the point where their front wheel is level with my rear wheel, the effect drops away very rapidly.

Can anyone explain what causes this? Does a similar effect happen in F1, and if so, does it make over-taking easier? Could a car be engineered to exploit this effect and improve over-taking as a specific design task?

Thanks

Steve

[Blanchimont]

There's definitely an influence from other cars. I think you should drive on a German Autobahn at 120km/h and wait for a Porsche Cayenne to overtake you with 250km/h in the line next to you. The whole car gets pushed to the side a bit, but i haven't notice an increase in drag from cars overtaking.

When driving with my motorcycle behind trucks, i think i can feel the turbulences in the wake, depending on the distance behind and lateral position. I have the feeling, that the forces on the helmet increase when in these turbulences and make my head shake a little.

[DaveKillens]

When we think of aerodynamics most of us just imagine what happens from the nose going back. But the air in front of the vehicle is also affected. It is a form of bow wave, that the air is compressed, and relative to the motion of a vehicle alongside, the air isn't flowing smoothly anymore. It is shoved sideways, it is accelerated forward relative to the vehicles.

When a large vehicle comes up from the side, that bow wave is suddenly interfering with the design of your car, it is messing up the clean aerodynamics the engineers at Kia have worked so hard at. So if the truck is behind and to the left of you, your rear left side of the car is not experiencing the clean airflow the right side is enjoying, there will be buffeting and an increase in drag.

Answering the second part, a car can take advantage of the air behind another (drafting), and even along the sides. NASCAR drivers practice this at the high speed tracks, by "side-drafting". The driver comes very close to the side of the other car, and picks up additional speed from the effects of the airflow along the side of the car.

[strad]

NASCAR calls it side-drafting.
https://www.youtube.com/watch?v=Gj0mBGYcjdM
.

When two cars travel side by side, their proximity blocks the flow of air between them, forcing that extra air out and around.
.
Two cars traveling side by side are slower because air is blocked from flowing between them, forcing more air around and over both cars, giving them extra air to push. This is know as "interference drag."
.
Side-by-side racing slows cars up terribly. In a single car traveling at 185 mph as soon as another car comes up, you'll run 183 because you're trying to push a bigger column of air.
Eaker said that slowing phenomenon is known as "interference drag." "The sum of the drag of two bodies that are close to each other is higher than their individual drags," he said. "You've essentially added resistance to the flow and pressure to the nose of the car."

guide to NASCAR

[SectorOne]

I have noticed an affect that kicks in at higher speeds - say 60-70mph - where by when a lorry or other large vehicle, like a range rover etc, comes up behind me in a different lane - i.e they are coming up to my left wheel or right wheel , I feel significant drag when it gets to within say 30 feet - enough that I have to actively compensate with more throttle to maintain speed.

Hello Steve,

Oh yes i´ve felt this numerous times. It´s amazing how strong the effect is.
And thanks to the peeps above, i now know exactly what´s going on. Brilliant.

[MadMatt]

I got confused with your question which was also about vehicle coming from behind, and in that case it was found that not only the drag is reduced for the following vehicle (we all know that I hope), but drag was also reduced for the leading vehicle, which is more interesting!

[riff_raff]

Hello all.

This isn't an F1 post, but it may have relevance.

My daily runabout car is the exceptionally dull Kia Rio, and more than any car I've had before it seems especially sensitive to sidewinds and turbulance from lorries etc - as it's underpowered, it's very obvious in the foot when I am getting these effects, equally it's simple to feel tow from lorries in front etc.

I have noticed an affect that kicks in at higher speeds - say 60-70mph - where by when a lorry or other large vehicle, like a range rover etc, comes up behind me in a different lane - i.e they are coming up to my left wheel or right wheel , I feel significant drag when it gets to within say 30 feet - enough that I have to actively compensate with more throttle to maintain speed. When they get to about the point where their front wheel is level with my rear wheel, the effect drops away very rapidly.

Can anyone explain what causes this? Does a similar effect happen in F1, and if so, does it make over-taking easier? Could a car be engineered to exploit this effect and improve over-taking as a specific design task?

Thanks

Steve

I can understand some effects from wake flows interacting with a trailing vehicle in the adjacent lane in close proximity to your vehicle. But when the trailing vehicle is 30ft away at speeds around 60-70mph having a noticeable influence on your car? The trailing vehicle is something like 2-1/2 car lengths behind.

I would chalk it up to you being a considerate driver, and when you see a vehicle in your mirror coming up from behind, your natural instinct is to speed up so as to not hold up that driver. What you feel is not increased drag of your Kia, it's just concern for fellow motorists that I wish every driver had. You're driving 70mph in the right lane and you see a car in your mirrors coming up behind you. It's difficult to gauge closing speed of a vehicle in your mirrors, and as the car gets closer it seems like its speed becomes greater.

[shelly]

A car in isolation develops a region of higher pressure in front, low pressure at the back (wake) and lower pressure on the sides - similar to the picture of 2d pressure distribution around a cylinder.
There is a chance that given this behaviour in isolation, when interacting in side by side lanes the dirver of another car feels the effect of those pressure gradients as drag.

[SectorOne]

I can understand some effects from wake flows interacting with a trailing vehicle in the adjacent lane in close proximity to your vehicle. But when the trailing vehicle is 30ft away at speeds around 60-70mph having a noticeable influence on your car? The trailing vehicle is something like 2-1/2 car lengths behind.

I would chalk it up to you being a considerate driver, and when you see a vehicle in your mirror coming up from behind, your natural instinct is to speed up so as to not hold up that driver. What you feel is not increased drag of your Kia, it's just concern for fellow motorists that I wish every driver had. You're driving 70mph in the right lane and you see a car in your mirrors coming up behind you. It's difficult to gauge closing speed of a vehicle in your mirrors, and as the car gets closer it seems like its speed becomes greater.

Not directly behind. Diagonally behind.

You´re in Lane A doing 80, big truck creeps up in lane B and when it´s just about diagonally from you, you will feel your own car getting pulled back a little bit (i.e. drag)
It´s quite easy to replicate this if you have access to a car.


Edit: like this, once the truck hits that area you will start to notice your own car(red) getting tugged a bit.

[ChrisDanger]

When driving with my motorcycle behind trucks, i think i can feel the turbulences in the wake, depending on the distance behind and lateral position. I have the feeling, that the forces on the helmet increase when in these turbulences and make my head shake a little.

I'm surprised you say "I think I can feel the turbulence in the wake". Maybe it's because I rode a 400cc and am a small guy, but I used to get shaken pretty violently when close behind trucks, and then there's an area of beautiful calmness directly behind, but that's dangerously close to the bumper so you get a bit worried after a while. I learned later this is called a "von Karmann vortex street", and can be seen nicely below. You feel a force from one side, then the next, and these alternate. As I say, for me on a bike this was very noticeable when directly behind, with less of an effect the further away you are.



(Sorry for off topic post.)

[Tommy Cookers]

my 2 cents are with Blanchimont ....
iirc the motorcyclist will feel the bow and stern wakes of a large vehicle he is passing as large discrete changes in drag
yes there can be heavy turbulence behind such a vehicle but .....

it won't be a Von Karmann vortex 'street'
for this you need a body bluffer than a road vehicle's and operating at a lower Reynolds number than the road vehicle at speed
(ok those slow moving short-and-high vans bodied to carry big sheets of glass might be an exception)
at a speed at which there's enough aero drag for the motorcyclist to feel disruption the vehicle wake would already be semi-converged
road riding will be the region above V K but often below the fully converged wake
ie the region of time-variation of the aero vector at a point (turbulence) but not the regular time-variation of V K

V K vortices are associated with flow over a long cylinder ie a relatively tall object (eg they do not occur at an aspect ratio of 1)
iirc the only time I felt V K vortices was riding pillion on a bike with a rather tall single-curvature screen (Moto Guzzi California)
vortices shed by the screen continuously flapping my head side-to-side at about 60 mph, but nothing V K at 40 or 80 mph
bluff body (the screen), very Reynolds number related - and no connection to another vehicle's wake
V K is what rattles the rigging on moored or parked up sail boats all day and all night

btw the drag shield that Guy Martin used for his cycling speed record gave a working zone of forward aero force (ie negative drag)

[riff_raff]

I can understand some effects from wake flows interacting with a trailing vehicle in the adjacent lane in close proximity to your vehicle. But when the trailing vehicle is 30ft away at speeds around 60-70mph having a noticeable influence on your car? The trailing vehicle is something like 2-1/2 car lengths behind.

I would chalk it up to you being a considerate driver, and when you see a vehicle in your mirror coming up from behind, your natural instinct is to speed up so as to not hold up that driver. What you feel is not increased drag of your Kia, it's just concern for fellow motorists that I wish every driver had. You're driving 70mph in the right lane and you see a car in your mirrors coming up behind you. It's difficult to gauge closing speed of a vehicle in your mirrors, and as the car gets closer it seems like its speed becomes greater.

Not directly behind. Diagonally behind.

You´re in Lane A doing 80, big truck creeps up in lane B and when it´s just about diagonally from you, you will feel your own car getting pulled back a little bit (i.e. drag)
It´s quite easy to replicate this if you have access to a car.


Edit: like this, once the truck hits that area you will start to notice your own car(red) getting tugged a bit.

In my post I used the term "adjacent lane" which means the lane immediately to your left or right. And in my post the thing that I was questioning was noticeable drag effects from a large vehicle in the "adjacent lane" trailing some 30ft behind, not "just about diagonally from you".

[OO7]

And in my post the thing that I was questioning was noticeable drag effects from a large vehicle in the "adjacent lane" trailing some 30ft behind, not "just about diagonally from you".

Yes, that the bit I'm also trying to get my head around.

[OO7]

NASCAR calls it side-drafting.
https://www.youtube.com/watch?v=Gj0mBGYcjdM
.

When two cars travel side by side, their proximity blocks the flow of air between them, forcing that extra air out and around.
.
Two cars traveling side by side are slower because air is blocked from flowing between them, forcing more air around and over both cars, giving them extra air to push. This is know as "interference drag."
.
Side-by-side racing slows cars up terribly. In a single car traveling at 185 mph as soon as another car comes up, you'll run 183 because you're trying to push a bigger column of air.
Eaker said that slowing phenomenon is known as "interference drag." "The sum of the drag of two bodies that are close to each other is higher than their individual drags," he said. "You've essentially added resistance to the flow and pressure to the nose of the car."

guide to NASCAR

This is something that fighter aircraft particularly encounter, because of the external payload they carry. A great deal of time, money and effort goes into simulating an testing the effects caused by the stores carried. Due to differing airflow characteristics, a particular store carried under a wing may create more drag than if carried on the fuselage centreline, due to the interference of airflows. Two stores carried under the wing may as create more drag between them, if a suitable gap (if available) is not left between them.

The interference can cause stores to collide when released, it can cause stores to impact the aircraft when released. Because of this certain stores are only permitted in certain locations, because of the collision risk, but sometimes because they haven't been tested in other locations (cost).

[Hobbs04]

When driving with my motorcycle behind trucks, i think i can feel the turbulences in the wake, depending on the distance behind and lateral position. I have the feeling, that the forces on the helmet increase when in these turbulences and make my head shake a little.

I'm surprised you say "I think I can feel the turbulence in the wake". Maybe it's because I rode a 400cc and am a small guy, but I used to get shaken pretty violently when close behind trucks, and then there's an area of beautiful calmness directly behind, but that's dangerously close to the bumper so you get a bit worried after a while. I learned later this is called a "von Karmann vortex street", and can be seen nicely below. You feel a force from one side, then the next, and these alternate. As I say, for me on a bike this was very noticeable when directly behind, with less of an effect the further away you are.

http://mydev.info/Images/Karman2.png

(Sorry for off topic post.)

I've have ridden at over 35 miles per hour behind a semi...

Not me in the video but similar I chickened out after about a mile but I felt like lance on steroids for a bit.

[youtube]http://www.youtube.com/watch?v=lnYp4srEooI[/youtube]