It started as a simple Bernoulli question: Why would your car traveling at 45 mph. be pulled toward a passing truck traveling 65 mph. Ok, so the air around the truck is going faster than that around the car, the pressure decreases, and you are pushed toward the truck by the higher pressure around your car. Why is the air around the truck going faster–is it being pushed by the moving truck, or pulled by friction?
Decreased air pressure on one side of you means that the normal air pressure exerting force on all other sides is no longer balanced and you are pushed by it towards the lower pressure region.
Any Bournoulli Effect as a truck actually passes must be pretty negligible on a car with four rubber tyres planted solidly on the road - at least I’ve never noticed it. What I do get is a tug on the steering wheel towards the wrong side of the road, about a second after the truck has go by, as my car encounters the big pocket of low pressure in the truck’s wake.
Right, I understand that. What I am curious about is why the air is moving faster. I have heard three different explainations. The truck is pushing the air, so it is moving faster. Friction pulls the air along, making it go faster. And lastly, that the air is forced into the narrow space between your car and the truck. The air in the narrow space moves faster, decreasing the air pressure. Which of these is correct, or if all are correct, which is the dominant force.
I’m trying to get a mental picture of this…
The truck is motoring along, creating a “bow wave” as it pushes air out of its way. The air kinda goes out to the sides and back past the truck. Then you come tooling along the other way in your Chevy, and you are doing a similar thing. As the two vehicles actually pass, there is a lot of air from both of them being channeled into that small gap between the two. I can’t help thinking that this would create a temporary higher pressure, which would push the vehicles away from one another if anything.
Another factor is that the air pushed out of the way by a vehicle isn’t really going to travel that fast relative to the ground. You’ve seen dust and leaves kicked up by fast trucks - as long as they don’t get caught in that vortex zone directly behind the truck, then they will just do a bit of a dance, but won’t travel far before settling back onto the road. I think then, that there is little or no Bernoulli. If there is movement , it’s probably backwards relative to the truck, and therefor in your direction of travel, so the Bernoulli Effect would be reversed. But my money is on it being too marginal either way to worry about.
For now, I think it’s best not to consider any speed differential between car and truck - just look at steady-state conditions and what the reaction on your car is while you are in different zones with respect to the truck (it also helps sometimes in visualization to consider the vehicles stationary and the air moving past them at 60 or 80 or whatever mph).
Anyway, the region next to the truck has air going faster than the free-steam velocity - since the truck occupies some volume, there is less volume for the air to occupy as it goes by the truck, so it (the air) must speed up. A useful rule of thumb (Bernoulli) is that when velocity goes up, pressure goes down, so the pressure in the air next to the truck is lower than free-stream conditions. When you are in this zone, as Mangetout pointed out, the pressure on your car on the side adjacent the truck is lower than the off-side, and you feel a net force pulling you towards the truck.
As you are in the reqion near the cab, you encounter the bow wake, which is air rushing around the space that the truck occupies - the outward (w.r.t. the truck) rush of air acts to push your car away from the truck. When passing a truck on the highway, I always think that this effect is more pronounced than the pulling-in effect from the previous paragraph.
But what you really notice, IMHO, is the transition between pulling-towards when you are besides the truck and pushing-away when you are near the cab and passing/being-passed. First you are correcting one way, then the other, and the change in correction force on the steering wheel is what you perceive.