Why does water splash back onto the hood?

[davidg4781]

That’s an interesting theory but I would like to know what about the grille shutters opening or closing would cause this to happen.
To the OP, I don’t think there is anything to be concerned about here but it would be nice to understand what is happening.

Yeah, I'm curious too. It has only happened twice, that I have seen. And I'm 99% sure the first time happened when I was stationary. I can't remembered if it rained a lot or I sprayed it with a garden hose. This is also the first time I have some type of coating on the car that allows water to collect that way and all dump into the front grill.

When it's time for me to wash it again, I'll spray down the front grill/hood and see what happens.

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[Mark S.]

Well all of this makes sense, except that it isn’t the air that is moving, it’s the car (and the water on the hood).

I sense another discussion coming...

There is potentially a frame of reference argument here but I don’t think it changes the end result.

I think the relative motions of the different elements is precisely the argument. Yes, the hood with the water on it is moving through the air, and from the perspective of the hood/water the air is flowing past them.

Once the water leaves the hood it might rise up some because the air is being forced up by the front of the vehicle but it would not be force back toward the hood, at least not at these speeds (assuming this isn’t happening at higher speeds because of the braking).

The car's movement through the air generates a bow wave of air, just like a boat moving through the water. The front of the Bronco Sport is blunt, which tends to enhance the bow wave effect. The action of forcing the air up and over the hood causes it to accelerate. Once the water is forced upward by the bow wave effect it rapidly decelerates due to friction with the surrounding air while the car continues forward. Some of the decelerated water falls back onto the hood while some of it makes it all the way back to the windshield.

I think that if Mark’s explanation was accurate, this would be a common experience on other vehicles. I haven’t driven an extensive list of other vehicles but I have driven quite a few. I haven’t ever noticed any water on the hood that goes forward when braking being thrown back up into the air and onto the hood.

I don't know how common this phenomenon is, but I've observed the same behavior on a few other vehicles. For example, the 2004 Chevy Silverado 2500 I owned, which also had a relatively flat hood and bluff front end, exhibited the same behavior when braking with water on the hood. And it had no active grill shutters.

I think this would be less common on cars with a more aerodynamic shape (the typical jelly bean) because the relative wind impacts the hood at a steeper angle which decreases the width of the boundary layer and prevents water puddling.

 

[Mark S.]

Active Grille Shutters opening/closing is what causes the water to splash back on to the hood.

Can you explain your theory of the mechanics involved?

 

[davidg4781]

I think this would be less common on cars with a more aerodynamic shape (the typical jelly bean) because the relative wind impacts the hood at a steeper angle which decreases the width of the boundary layer and prevents water puddling.

Maybe this is why I’ve never noticed it. My car history has been curved hoods with a Ford ZX2, Taurus, and a couple of Accords.

 

[Barry S.]

Can you explain your theory of the mechanics involved?

OK. You've done it. I'm perched on a ledge. If this discussion doesn't end immediately, I'm going to jump. Fortunately the ledge is 3 feet above my lawn...but it's the thought that matters.

 

[Mark S.]

OK. You've done it. I'm perched on a ledge. If this discussion doesn't end immediately, I'm going to jump. Fortunately the ledge is 3 feet above my lawn...but it's the thought that matters.

Well, if you can get enough of a bow wave going it will soften your fall. Of course, then you'll get run over...

 

[Barry S.]

Well, if you can get enough of a bow wave going it will soften your fall. Of course, then you'll get run over...

Goodbye cruel world........ See y'all tomorrow.

 

[Meanderthal]

I sense another discussion coming...


I think the relative motions of the different elements is precisely the argument. Yes, the hood with the water on it is moving through the air, and from the perspective of the hood/water the air is flowing past them.


The car's movement through the air generates a bow wave of air, just like a boat moving through the water. The front of the Bronco Sport is blunt, which tends to enhance the bow wave effect. The action of forcing the air up and over the hood causes it to accelerate. Once the water is forced upward by the bow wave effect it rapidly decelerates due to friction with the surrounding air while the car continues forward. Some of the decelerated water falls back onto the hood while some of it makes it all the way back to the windshield.


I don't know how common this phenomenon is, but I've observed the same behavior on a few other vehicles. For example, the 2004 Chevy Silverado 2500 I owned, which also had a relatively flat hood and bluff front end, exhibited the same behavior when braking with water on the hood. And it had no active grill shutters.

I think this would be less common on cars with a more aerodynamic shape (the typical jelly bean) because the relative wind impacts the hood at a steeper angle which decreases the width of the boundary layer and prevents water puddling.

I agree with pretty much all of this, but what does not seem to be addressed is the fact that it happens while braking. At a higher rate of speed, and easy/light braking event would allow the water to remain on the hood because of the airflow, until a lower speed is achieved when the force of the air against the water reduces enough that momentum takes over and carries the water down the hood and off the front edge. If the force of the air on the water is not enough to keep it attached to the hood, then once it is airborne, it would not be enough to push it back onto the hood. Yes, there are boundary level affects as you noted, where the air is moving faster (relative to the hood) as you move away from the hood.

At a lower speed but higher braking force, the momentum of the water is higher and the force of the air is lower, so again not enough to force it back onto the hood.

Aerodynamics is a tricky science. A lot of times things happen that even people that work on it everyday would not predict. If the water were to be able to move forward and off the front edge of the hood (say it builds up enough volume that the air can no longer prevent it from doing so), then I truly believe your explanation holds up. What I think is missing, and thus changes the actual results, is the fact that the vehicle is braking.

I suspect what is happening is that the puddle is moving forward into the area on the hood where the boundary layer is forming. So, it is contained within the boundary layer until it starts to reach the front edge of the hood, where it becomes the thing that is trying to form the boundary layer. Because it is a fluid, it is not very capable of creating a boundary layer in the air and turbulence begins to form. That turbulence is what then starts to break the water up and pull it off the hood and into the air. Maybe this theory is not a huge break from your original explanation but I guess details are important to me when it comes to explaining these phenomena. Overall, I'm thinking this is like white caps forming on a lake in windy conditions.

 

[Cursed Bronco]

It's aerodynamics. When you hit the brakes, the front end dives. The water pours down the hood toward the front end. The bronco sport is relatively flat leading to turbulent air flowing more upwards. The air catches the water and sprays it back up toward the windshield.

 

[Barry S.]

Thanks guys. I was up all night thinking about this. Thanks for the elucidation. Clear as mud...but it covers the ground.

 

[BroncMan]

I tried searching and couldn't find anything. Hopefully I'm not the only one. But this is the first time I've ever seen something like this.

I noticed this first when I bought the BS. It rained sort of heavily about a week later. When I turned it on, water was splashing/ spitting onto the hood from the front. I thought that was odd went to work.

Then a couple of days ago, it rained again, and I had a puddle growing on my hood as I drove, collecting more water as it moved around. I got to a stop sign and braked a little harder because I wanted it to roll off the front. As soon as it went over the edge it splashed back onto the hood.

Is this coming from the fans maybe?

First - you don't say what you turned on ...?
And yes - water beads collect on the hood, as with every vehicle - then they get pushed off more or less depending on the speed and the airflow.
When I brake a bit the droplets move forward, over the front edge, then get blown up/back by
impact of airflow at the front. Completely normal.
As per your description water even comes flying back after you stopped...?? Not possible!

 

[Barry S.]

My solution to the problem required hours of research:
World: Longest Recorded Dry Period
Lane notes that no rainfall has ever been recorded at Calama in the Atacama Desert, Chile

 

[Mark S.]

Aerodynamics is a tricky science. A lot of times things happen that even people that work on it everyday would not predict. If the water were to be able to move forward and off the front edge of the hood (say it builds up enough volume that the air can no longer prevent it from doing so), then I truly believe your explanation holds up. What I think is missing, and thus changes the actual results, is the fact that the vehicle is braking.

I'm not following you here. The ONLY reason this phenomenon occurs is because the vehicle is braking. The vehicle slows while the water wants to keep moving.

That turbulence is what then starts to break the water up and pull it off the hood and into the air. Maybe this theory is not a huge break from your original explanation but I guess details are important to me when it comes to explaining these phenomena. Overall, I'm thinking this is like white caps forming on a lake in windy conditions.

I think we're closer together here. The more acute the angle between relative wind and a surface the thinner the boundary layer, so water nearing the area where the wind impacts the hood at an angle will be affected more by the wind. Additionally, the bow wave of air forced up over the front of the hood with the vehicle in motion creates an area of relatively calm air at the very front. That area gets smaller the slower you go, but if there is any water near the front of the hood in that bubble of calm air it will be less affected by the relative wind, and more prone to continue moving forward under braking.

I don't see much difference between saying water falls off the hood and gets blown back, or gets blown off the hood after rolling forward off the horizontal surface, so view it whichever way helps to most easily visualize it.

Perhaps I'll get a wild hare sometime and mount a camera on the grill to see what's really happening. Not anytime soon, however. I have to go study more aerodynamics to pass the written test before I can obtain my commercial pilot certificate. Wish me luck!

 

[tRex]

I find it amusing and like to point it out to passengers when I can. I've always assumed it was some sort of pressure gradient between the front/leading edge, and the top surface of the hood.

 

[Meanderthal]

I don't see much difference between saying water falls off the hood and gets blown back, or gets blown off the hood after rolling forward off the horizontal surface, so view it whichever way helps to most easily visualize it.

I think the difference is that I don't think the water that does fall off the hood is what is being blown back up. I think what is getting blown back up is water that is still attached to the hood that is entering into the thinnest part of the boundary layer and getting broken up and pulled off the hood by the air in the thinnest part of the boundary layer.

Again, I don't think we are very far apart on this. Probably more of a nitpick to most people. Just interesting conversation, at least to me.

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