Tailgating a lorry/semi (modelled in CFD)

[Sven7]

[Piwoslaw]

Sven7, was that a video from your car?

Originally Posted by viio

Truck is 15.1m long, 5.1m from top to bottom.

Those proportions look a little off. On the Continent trucks are usually 18m long and 4m tall. I know that UK doesn't have the 4m height limit, but I thought most of them were no more than 4-4.5m tall. I could be wrong, though.
Reducing the height to 4m would reduce the wake's length and make it more symmetric (top-down vs side view).

But even with those proportions in the view of the car 54m back it looks so far behind. Maybe I'm just too used to Polish drivers driving bumper to bumper at highway speeds - not for economy, but because they think it's faster. Maybe I'm wrong about that, too

[viio]

Yeah my proportions are a little squiffy but close enough to learn from. I could model again with a 4m truck. I know Vekke is keen to see a road train so I'm going to model that too.

[viio]

Here is one using a more accurate model of a European HGV, and the proportions are fixed - 18m long, 4.5m high.



Next is a render to see just how far the truck's wake goes, and then the road train.

[Sven7]

Originally Posted by Piwoslaw

Sven7, was that a video from your car?

No

[viio]

Okay, so this is a learning process for me, and I think I've made a bit of a boob.

This time I've set the colour gradients so that it's clearer to see where the air is back at 25m/s, and the wake is much smaller than original expected.

Just after the 40metre indicator on the ruler you can see that the air is back to full speed (where a normal car height would be). That makes me think that the wake is no longer effective 35 metres behind the truck.

Any thoughts?

[theycallmeebryan]

The problem i have with drafting semi's, at least on the Garden State Parkway in New Jersey, is that semi drivers rarely ever follow speed limits, and rarely ever do they drive with constant load. They accelerate hard up upgrades and let off on down grades. They themselves drive very inefficiently, which in turn forces you, the drafter, to drive inefficient. Thus, the potential gains are not as great.

Draft semi's who cruise at a constant speed on flat highway works amazing, though.

[Piwoslaw]

Originally Posted by viio

Any thoughts?

Yeah, I liked the earlier wakes more. I'm not going to draft any your new trucks, viio

No really, what happened? Did you change any of the parameters, or only the coloring? In the previous models it appeared that the 19m/s part of the wake extended quite far, here even the 22m/s vectors end much earlier.

[Daox]

Originally Posted by theycallmeebryan

The problem i have with drafting semi's, at least on the Garden State Parkway in New Jersey, is that semi drivers rarely ever follow speed limits, and rarely ever do they drive with constant load. They accelerate hard up upgrades and let off on down grades. They themselves drive very inefficiently, which in turn forces you, the drafter, to drive inefficient.

I dunno, thats how I drive too and I've hit 100% above EPA driving that way. It surently isn't very inefficient. DWL is not as good as P&G and pulsing up a hill to glide down it is much more efficient as long as you have a manual trans or can keep your torque converter locked.

[theycallmeebryan]

Originally Posted by Daox

I dunno, thats how I drive too and I've hit 100% above EPA driving that way. It surently isn't very inefficient. DWL is not as good as P&G and pulsing up a hill to glide down it is much more efficient as long as you have a manual trans or can keep your torque converter locked.

I understand that. I think what i meant to say is that it's difficult to gauge when a trucker is going to speed up or slow down on small rolling terrain when you are trying to draft 10-15 feet away from it. In these situations, i find myself using the gas and brake more often in order to maintain that distance, more so than i would when driving normally without drafting.

There have been times when a trucker has slowed down considerably and then just floors it and accelerates away from me. For me to have any chance of staying in the wake, I also have to floor it and try to keep up. Then they'll get up past 60mph and i'll have to play catch-up to try to get back in the wake.

Basically what I'm saying is, it really depends on how the trucker is driving whether or not you are going to get a decent amount of benefit out of it, at least around here.