truckstop55 said:
Perhaps some of the more science minded contributors can estimate the flow rate needed (making assumptions about contact area etc) to move a 1700kg (including one man) Nissan Leaf with its brakes on!
OK, I'll give some scale numbers for the forces.
First, lets assume a 1.5 metric ton vehicle (about 3300 pounds). I will work in metric units for now, and round Earths gravity to 10 m/s^2 (instead of 9.8). I am in no way saying this is the right number for a LEAF, but it is a good start. With a coefficient of static friction of the tires of about 0.33 (close), this means it takes 1.5 ton * 10 m/s**2 * 0.33 = 5000 Newton of force to slide the car sideways, assuming the tires are not wet (!). The not-wet assumption which gives the 0.33 coefficient of friction is, of course, terrible for a car driving through water, but I don't have numbers for wet tires. We'll get back to that later.
If one starts with 30 cm of water (1 foot) moving 1 m/s, and assume the car is 3.3 meters long, the force from the water is about 1000 Newton. I am assuming that the car stops the water, essentially. The car does let some water underneath but is pretty low-slung, so this won't be a big correction.
So far, one seems relatively safe, since I have found about 1000N of force when it might take 5000 to slide the car. There are many other factors, though, some of which may be in the LEAF's favor.
First is buoyancy. The water tries to float the car. Since a LEAF has a very heavy battery and is a small car, it is very dense, so it doesn't try to float too much. Most cars are, in fact, very low density (much larger footprint on the road for the weight). However, this is a very significant effect, and might make things twice as bad as I assume in the rest of the discussion. In shallow water which goes entirely underneath, it isn't a problem, but as soon as the footprint of the car becomes submerged it is very serious, so water deeper than about 6" is where this can be a killer.
Second is much more problematic. Water piles up on the upstream side of the car, hugely. A 12" deep flow can probably easily end up 24" deep as it runs around the car. This doubles the force to 2000N.
Finally, there is the coefficient of friction, which can be MUCH lower for wet tires than dry. Various web sites indicate that stopping distance doubles on wet pavement, so we are down to only 2500N to displace the car.
The third factor is that the friction coefficients I am using are _static_ friction. When something starts to slip, it falls dramatically, which is why once a car starts to skid, it takes much longer to stop. This can be as much as another factor of two. If turbulence or a wave in the water kicks you loose, or you hit a bump under the water, your holding force may well fall to 1250N.
Now, given the very low speed (1m/s, which is very, very slow water), and the fact that you have 2500N to slide the car (assuming it doesn't get bumped enough to go to sliding friction, in which case it is only 1250), and 2000N from the water, do you want to bet your life on this? I don't. Also, remember the buoyancy correction, which cuts in at about 6" of water and makes things much worse. I was raised in a flood area, and the drownings of people crossing 6" of water were frequent.
DON'T DO IT!
