Tailgating a lorry/semi (modelled in CFD)

[viio]

Holy crap Vekke, you take this stuff seriously This was just a mock-up model but you make me want to do a precise one now. It'll take a couple of hours - thankfully I have tomorrow off work so I'll produce something then.

Only thing is, I can run the simulation and tell you the air velocity, but I'm not sure how that will translate into fuel savings. I've got no education on aerodynamics as yet, just a bit of modelling skill and plenty of enthusiasm.

[Vekke]

This is my work at the moment so more I know easier it is to sell and also design new products.

That accuracy for the truck is more than enough what you have at the moment. Just make it little bit longer or then use the safety distance what you get if you multiply the trucks current length by 4. It close enough to three second rule. at the moment it looks like its about 16 meters long? Also at the front there is only one front axle at the front between those two you have at the moment, but even that should not effect the big picture.

The catch here is that you should be able to get nice results by first doing three simulations and after that make a simulation about "truck train" and you should be able to compare results easily to wind speed. and also cd when you have the data from huchos book.

[Sven7]

This is in SolidWorks, no? Perhaps I'll have to talk to some product design students to see if we can't figure out a Cd feature or something.

For reference: 60mph/100kph = 88 feet/sec and 27m/s.

So, driving a relatively safe 2 car lengths back, the car should be positioned about 175 feet or 54m back from the rear of the semi. 80m would be about 3 seconds.

If the airflow coloring is different at the car than it is going over the semi we should see some improvement, right?

[viio]

Okay, so here is another model. The colour code is included in my screenshot, windspeed of 25m/s applied (56mph which is the UK speed limit for a truck of this size).

Truck is 15.1m long, 5.1m from top to bottom. The car is 3.7m long. The distance from the rear of the truck to the front of the car is 54.0m as requested.

Seems to me that there's no slipstreaming/drafting effect at this distance. Clicking the picture will open up a 1920x1080 version.

[Sven7]

It's slower air than the blue stuff outside the semi's wake. So instead of hitting 56 mph wind, you're getting 42. I'd say 14 mph less air drag is successful!

[viio]

Oh I missed that, yes I suppose so. Blue air is at full speed, the first green area is about 5 metres per second slower (or 11mph).

[mattbatson]

my conclusion so far is that you can safely draft semi's and see significant improvements in mpg's

seriously though, if you cannot avoid hitting a slowing/braking semi from 100 or 150ft back, then you have no business driving on the highway in the first place.
IMHO

[Vekke]

Yes if the coloring is different than the blue on top of the truck your car will face that 6 m/s slower air and use less fuel. Now It would be also cool to know for how long/ far behind that green 6m/s slower air will go. I have estimated the effect with that 3 second distance to 1-6% savings. If you drive 5m/s slower 18 km/h or 11.1 mph slower 55 MPH -->50MPH your fuel consumtion is about 13% less according aero rolling resistance calculator.

From myhtbusters test they found out the following results in their tests: Wer can now put those distances to that simulation and get following colors
100 ft=30m=11% greenlight and green dark so about 17.5 m/s air speed--> 7.5 m/s saved 11%-->1,46%/(m/s)
50ft=15m=20% yellow air mostly 13.4 m/s
20ft=6m=27% orange air mostly 7,8 m/s
10ft=3m=39% red colored air mostly so 5m/s

If you calculate the difference mean that 6 m/s would mean about 8,8% fuel saved at that distance. For better perpective you need also the upper picture because the truck is narrower and the green is not so long in width section.

[UFO]

At 60mph, 88 feet of distance leaves you with 1 second to react. You are dead if that semi rolls over a 2x4 in the road. On a drag strip, you are looking for that light to drop and the best anyone can do is 0.4 seconds to start. You cannot maintain that level of awareness for 5 minutes let alone a typical commute or road trip.

Personally I will not follow any closer than 3 seconds, and I have always managed to stop when I need to; sometimes I still have had to make a complete stop unexpectedly.

[slowmover]

Hey, great looking stuff!

Originally Posted by Sven7

It's slower air than the blue stuff outside the semi's wake. So instead of hitting 56 mph wind, you're getting 42. I'd say 14 mph less air drag is successful!

Far enough back to see the truck drivers mirrors on both sides at all times, then, yes there is some benefit which makes "drafting" worthwhile. But not any closer.

The second benefit is that other traffic will want to flow around the unit of "you" and the truck as the truck is seen as being the obstacle. The occasional other driver will insert himself, but not often enough to worry over. IOW, let the truck be the front door and be aware he'll slow on grades. . so "you" as the back door will be managing the overtaking & passing traffic on four-laners to the left lane. Done well this will speed them all around faster and make for fewer driver inputs to each vehicle in re traffic concerns.

So that 14-mph reduction is earned without causing the truck driver to feel there's a problem riding his tail on the one hand, and any skill application to move others around more quickly benefits both on the other. This would be win-win, versus the pure-D bull**** of close drafting where there is no benefit to either party worthy of the name.

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