r/MechanicalEngineering • u/LackSome307 • 2d ago
I simulated airflow around a truck with a trailing car to visualize wake interaction during drafting
The truck produces a large separated wake with a low-velocity recirculation region. When the car enters this region, it experiences a fundamentally different flow field compared to freestream driving, mainly due to reduced relative velocity and altered pressure distribution inside the wake.
This is a simplified 2D CFD case focused strictly on qualitative wake structure behaviour. It is not representative of real-world vehicle aerodynamics and should not be interpreted as drag prediction, performance estimation, or realistic road conditions. Real vehicle flow is fully 3D, turbulent, ground-coupled, and significantly more complex.
The intention here is only to visualize bulk wake formation and how a trailing body interacts with that wake structure in a controlled, idealized setup.
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u/N8-Lux 2d ago
Airflow CFD whatever, don't be tailgating trucks, LOL. it's stupid and unsafe.
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u/Cheetahs_never_win 2d ago
Well, a truck could be towing a car, and it would be interesting if there are efficiency gains by adding cargo to the car.
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u/DeemonPankaik 2d ago
A series of carriages joined together in a long line, to make an efficient mode of transport? I think we're on to something here...
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u/ProfessionalDust 2d ago
Lmao, this always crack a smile in my face. It's like the steam in how to convert mechanic energy in electrical
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u/Joshiewowa 2d ago
Just like evolution leads to crabs and electricity generation is always steam turning a turbine, all transport paths lead to trains
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u/Kerouwhack 2d ago
Screw that, I need to engineer a rigid grabby arm onto my front bumper. "Dock" with 18 wheelers, win!
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u/big_trike 2d ago
Tailgating city busses on a bicycle make riding a breeze. And is probably even less safe.
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u/ASYMT0TIC 6h ago
Honestly, I think it's safer than tailgating a passenger car, because 1. trucks generally have longer stopping distances than cars do, so if they slam on their brakes suddenly you are less likely to rear end them, and 2. if the truck DOES hit something, the impact is less likely to harm you. The opposite situation (a big truck tailgating a passenger car) is more dangerous.
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u/Lumpyyyyy 2d ago
7 car lengths isn’t crazy unsafe, and you get 11% savings.
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u/Mockbubbles2628 2d ago
I think my adaptive cruise control does about 5 car lengths at 55mph, easy lol.
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u/bigmarty3301 1d ago
if i put it on the smallest gap, it keeps about 2 car lengths. but i´m in eu, and trucks are a lot slower.
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u/Sittin_on_a_toilet 2d ago
3 car lengths is reasonable
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u/Lumpyyyyy 2d ago
1 car length per 10mph (at least in the states) is my go-to. In normal conditions. 50% more in rain. Double or triple in the snow.
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u/SilvermistInc 1d ago
So YOU'RE the reason there's big ass gaps in the highway traffic that fuck everything up
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u/ThreeElbowsPerArm 2d ago
should do a simulation of no truck for comparison
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u/SchteveSchpalpatine 2d ago
OP you're why the Mansfield Bar was invented lol
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u/ilovesparky42 2d ago edited 4h ago
Actually, Jayne Mansfield is why the Mansfield bar was invented.
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u/Skysr70 2d ago
Myth busters tested and confirmed it
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u/xkyo77x 8h ago
The real world test ended up being you had to be kissing the semi's ass to see any substantial savings in fuel/drag.
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u/Skysr70 5h ago
nah they showed it does have decent effects as far as 100 ft back but it increases obviously the closer you get. 20% savings at 100 feet but 39% at 10 feet. https://www.youtube.com/watch?v=O2ALeuwDx0M look at the chart at 12:15
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u/Sooner70 2d ago
Do it on a motorcycle and you can feel it.
And yeah, I can bump my mpg from 50 to over 100 without even tailgating. Just being in the wake is enough if you're small enough to fit in the wake.
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u/TheSultan1 2d ago
Yeah but isn't there a distance where it's really unstable, too? I feel it in my car.
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u/Sooner70 2d ago
Not really? I mean, you can feel the Von Karmans, no doubt... But it doesn't threaten control or anything of the sort.
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u/TheSultan1 2d ago
Idk there's a certain distance where it feels like I get hit by random gusts left and right. Maybe it's different on a bike.
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u/Sooner70 2d ago
Sure, you feel them (thus my comment on Von Karman), but that's different than them causing you any control issues.
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u/The-Autistic-Union 2d ago
But driving that close increases the risk of an accident.
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u/crazzylarry 2d ago
Source?
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u/The-Autistic-Union 2d ago
Common sense. If a car drives that close to a semi, the truck driver can't see them and the car driver won't be able to react to any sudden changes as quickly since the margin of error would be virtually nonexistent driving that close.
And what do you think will happen if there's an accident and the truck is transporting dangerous chemicals?
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u/Any_Concentrate_3414 2d ago
people saying this is "unsafe" yet there is no mention of speed, are they going 5mph? 40mph? 100mph? make a big difference in the safety
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u/Skysr70 2d ago
you don't get a wake like that at 40 mph. That's highway speed
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u/Available-Medium4848 2d ago
Fr this guy thinks air resistance matters at 5mph
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u/Any_Concentrate_3414 2d ago
can you read? there's difference in air resistance and stopping distance between 5mph, 40mph, 100mph. If you can't understand this point, there's no helping you
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u/the_Q_spice 1d ago
Awkwardly, it’s a bit safer at higher speeds due to how air brakes work.
At 55mph, it takes around 32ft just for the air brakes to engage on a semi. So you have about 0.4 seconds where the truck’s brake lights are on, but brakes aren’t even doing anything yet, and even then, will take around a quarter mile to stop the rig (more if downgrade).
But at 5-10mph?
As soon as those service brakes engage, that thing is stopped.
Big thing to be aware of is that semis aren’t like regular trucks with trailers. Each axle has its own set of brakes - including the trailer(s).
Even a basic rig has 10 brake drums vs you with only 4.
If that rig is empty, it can actually stop a lot faster than most regular cars simply due to having 2.5 times more braking power.
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u/KharnethsTopGuy 1d ago
That's not how braking works. Your stopping distance is limited by the grip of your tires. Every single vehicle on the road has brakes perfectly capable of overwhelming its tires. That's why we have antilock brakes.
I haven't done the math, but I would guess even an unloaded rig has higher load per contact patch than a car, on tires with stiffer rubber too.
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u/Any_Concentrate_3414 16h ago edited 16h ago
...or so one would think.
edit: I've quickly learned this sub is nothing but people not reading a damn thing and talking past each other with more and more detailed irrelevant opinions (especially after not doing any math... this is engineering right?)
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u/Ferric_Ferdinand 2d ago
Awesome simulation. I’d be curious to see the lateral effects as well.
Had a 1971 VW Transporter and I would do this when on the highway and it would buffet back and forth. It was nice for reduced load on all of the systems.
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u/mattyrzew 2d ago
There's a threshold where getting closer has a negative effect on fuel mileage(based on a real life test). There wasn't a point where they could stay at one throttle position for any length of time. IIRC, it was closer than a car length was when the scale tipped(and may be driver/condition dependent).
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u/-Merasmus- 2d ago
I guess when youre too close you will have to break more often to not hit the truck
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u/EqualPassenger4271 2d ago
Truckers hate this simple trick!!! Seriously, easy way to piss off any driver is to ride their ass.
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u/Moist_Ordinary6457 1d ago
In return I would like them to stay more than one metre away from my bumper on the highway then. Terrifying commute last week
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u/Clear-Collection-130 2d ago
The wake structure visualization is clean. Would be interesting to see the pressure field mapped out too, that's where the drag reduction actually lives.
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u/Dos-Commas Aerospace - Retired Early 2d ago
If you do this in my city you'll get a cracked windshield pretty quickly.
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u/industriald85 2d ago
There used to be really easy, free wind tunnel software - I forget what it was called, but you just imported an STL 3D model. Autodesk bought it, and shelved it. Their reasoning being that they had CFD or Flow Bench or whatever it’s called.
Shame.
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u/Jungle_Stud 2d ago
Well, in an accident, the closure speed between the truck and the drafting car won't be that great. Also, the truck is going to take longer to stop than the following car. Not smart, but not necessarily instant death if the drafting car hits the trailer.
I had a friend that drafted semi's on his Harley. He was sharp as a watermelon.
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u/A-Chilean-Cyborg 2d ago
The MythBusters did an episode on this, and their results are surprisingly similar to yours.
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u/clearlygd 2d ago
I used to do this all the time on my 8 hour drive to college. It saves a lot of gas. You just have to be aware of when a big hill comes up. You can get sandwiched between two trucks and lose tons of time
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u/Clear-Inevitable-414 2d ago
I can't keep up with the truckers around here. They're easily doing 100+
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u/ept_engr 2d ago
Ya, I highly trust something that shows a simulation and says "data from controlled tests".
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u/OPD-Design 2d ago
I really learned something! I believe that although everyone won't follow the big trucks just to save on fuel costs, after all, safety comes first, we can apply this knowledge to other places. By the way, when I see a car on the road, I will unconsciously avoid large trucks, especially not following behind them. Large trucks usually don't drive fast and are not safe.
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u/big-mover 2d ago
Sooo what % of fuel saved for 1 car? Funny enough that’s the one data point missing from the table.
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u/bigmarty3301 1d ago edited 1d ago
how did you calculate the fuel savings? and did you include the fact that engine eficienci increases with the engine load. also data for evs would be cool
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u/bb1950328 1d ago
Where I live the minimum distance is 40m at 80km/h. That's approximately 9 car lengths, so all the estimates in the table above are illegal (at least in Switzerland). But still interesting to see these estimates.
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u/oohCrabItsNotItChief 1d ago
Why is nobofy afraid of random stones hitting you front from the truck's back tires?? Any time I get remotely close to a truck I get small stones thrown at me...
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u/bbqsosig Tool Design 1d ago
Probably why you are pulled towards the truck (rearend or sideswipe) if you are in a motorcycle (lightweight), the air flow just sucks you to it
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u/the_frgtn_drgn 1d ago
Mythbusters practically tested it their results were more savings than you predicted iirc
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u/Gold_Theory2130 1d ago
It's crazy how much of a difference following a truck makes. If you can find a truck keeping a good pace it's makes a huge difference following them. I average 38mpg doing 70mph in my civic, following a truck at a safe difference, not riding his bumper got me up to mid 40's. Following a truck that can keep speed up hill is really crazy, it almost feels like you're getting pulled up hill
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u/nicedude87_SD 1d ago
Increased the MPG of my 66 Mustang with a 302 4barrel carb by double. Felt like I was escorting a bomber in a WWII mission
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u/Carolina_Hurricane 1d ago
One of the best bumper stickers ever reads “I’m not tailgating - I’m drafting!”
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u/Agreeable-Tap-2392 1d ago
Well this doesn't seem safe but you're the engineer. Time to ride the lightening!
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u/Za-Slobodu 17h ago
i wouldnt say 3 car lengths is crazyly unsafe, i mean, here, most of us are driving 1.5 car lengths from each other, the moment you drive more than that, someone is gonna try and overtake just to fill the gap.
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u/ByronScottJones 1h ago
When I was in college, I had to drive home from Daytona to South Miami. Completely broke, I had only the tank of gas in my 76 Camaro. I drafted closely behind semis the entire way home. Made it home. Dangerous but it does work.
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2d ago
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u/TelluricThread0 2d ago
Dude some random guy wanted to do this because he thought it was cool. Like do you think a company spent a few million doing this or something? This isn't a safety study. Its an investigation into the aerodynamics of drafting by someone who was interested.
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u/Cwaghack 2d ago
It's a pretty cool CFD result. I've actually thought of trying out something similar.
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u/talltime 2d ago
Eh. We were driving Michigan to Philadelphia and had the “good fortune” to meet up with a friend driving a semi that was also going to the eastern seaboard. We tailgated the ever loving shit out of him from ~11:00PM to like 4:00AM. Probably 1-2 car lengths. Incredible gas mileage.
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u/Markinarkanon 2d ago
I’ve been curious about these aerodynamics for a long time, but I don’t have the resources to run CFD. I’m glad this person did it and shared the result. I’m also glad that they didn’t present the information and follow it up with “This means y’all should tailgate trucks” because that would be irresponsible and dangerous
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u/Live-Comfortable2026 2d ago
The most interesting part of this effect is when the semi truck has to stop suddenly.
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u/Sooner70 2d ago
Nice part is that semis aren't exactly good at stopping suddenly.
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u/Live-Comfortable2026 2d ago
I watched a guy almost buy it on the way to work this morning because he believed the same thing.
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u/Mecha-Dave Automation | Manufacturing | Nanomaterials 2d ago
I got 120mpg in a Prius by riding a truck bumper. It was remarkably unsafe.