Tailgating a lorry/semi (modelled in CFD)

[tinduck]

Contrary to the color codes in the simulation, you can use drafting to save fuel at longer distances than one car length... at approx. 30-40 ft. distance, I save around 0.5 l/100km with my setup which equals to about 10% fuel savings. Closer in, you may save more, but this cannot be sustained for longer periods of time safely.

Safety will always be an issue. Much depends on how you drive. Burger in one hand, soft drink in the other, cruise control on, foot on the dashboard: yes, you will have a hard time to brake in time. If your eyes, hands and feet are were they belong, your safe driving envelope will grow.

so long,

tinduck

[viio]

Originally Posted by tinduck

If your eyes, hands and feet are were they belong, your safe driving envelope will grow.

I like your style

Originally Posted by Piwoslaw

Viio, could you do another round of simulations, with the truck's wheels removed? There is always a portion of air flowing under the truck which would slightly alter the wake's shape.

Do you mean with just repeated with the wheels removed, or do you want to see the model with zero airflow underneath?

[KamperBob]

Originally Posted by viio

I was just experimenting since I have the modeling software and the skills to use it.

Thank you for doing and sharing. I'd love to hear more about the software you are using and any tips.

If you are game, it might be interesting to repeat the experiment except model from top view.

Rock on!

[Piwoslaw]

Originally Posted by viio

Do you mean with just repeated with the wheels removed, or do you want to see the model with zero airflow underneath?

Just remove the wheels and keep the height the same.

[viio]

Here's the experiment with a top down view. I've changed to flow trajectories instead of the colour map so it's easier to see.

It's 60mph again, and anything that is red is 10mph or less.



I'll do the wheels-off view next.

[Piwoslaw]

Wait! Wait! My 2 dimensional mind just realized that this is in 3D, so the wheels should be on to make it more realistic. Unless the wheels of the model are much wider (the whole width of the truck, for example) than they should.

[viio]

Well I've done a wheels off before reading your post, so we've got one anyway. As you might expect in 3d, the car just piles into high velocity wind anyway, and the wake of the truck is higher up.

[Daox]

Nice simulation pics. Could you post the scales with any new images you post? Its really difficult to know anything without a scale.

Also, is the gradient showing air speed or pressure?

[viio]

It's velocity, with red being the slowest. I'll post scales from now on. I'm glad the images will be useful enough to warrant more info!

[Vekke]

We already know that driving closer the truck saves fuel from mythbusters and other tests done earlier in the books like hucho... What I would like to know what are the savings at proper safety distance like @ 62 MPH 100km/h 3 seconds rule --> 273 feet or 83,4 meters

Also modify the truck lenght so that it is 83,4 meters/4 --> 20,85 meters

Then do simulations also to 62,25, 41,7 meters safety distances. According to Huchos book the drag should decrase with similar steps or how was the example in the book. Someone who has that book can tell the basic laws valid to this drafting.

Lastly make a truck train with 4 trucks with 41,7 meter safety distance and keep the same distance to the last truck with the car 83,4 meters. You should see some nice results this way.

How long does it take to make one simulation like that?

Here is waht I wrote to TDIClub forum on the similar topic:

There are benefits already at that distance about 3 second and more. Savings will come related to the vehicle length in front of you and how large cross section does it have. Also your own car Cd effects and the cross section you have in your car. If I would drive behind small car there would be no gains at those distances. Check book aerodynamic of road vehicles by hucho for hard data. If you like you can also find mythbusters test on the same topic Drafting Semi uploaded by tremorfalcon. 100 tf gave 11% better fuel consumtion.

In europe max length for truck + 2 trailer combination is 25.25 meters. On mythbuster that was only truck with one trailer about 17 meters. So with 8 meters longer combination you can drive that 8 meters more behind and still see the same savings. So that 100 ft or 30 meters would be with 25.25 meter long truck 44.5 meters or 146 feet. and still get 11% fuel savings in that same test truck. If your car smaller you get more savings. So about 3 second behind you can see about 1-6% fuel savigs. If you are driving behind a long truck "convoy" savings will be even greater because the distace needed is then about one convoy not one truck lengt and you can see that 11% at 3 seconds behind the convoy.