Why does water splash back onto the hood?

[Mark S.]

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.

Agreed. Prolly won't be resolved without experimentation.

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[Meanderthal]

Agreed. Prolly won't be resolved without experimentation.

I like the GoPro idea.

 

[davidg4781]

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!

When I turned it on.. It being the Bronco Sport.

And the water didn't splash back after stopping. It splashed when I first turned the ignition on in my Bronco Sport while it was sat stationary in the drive way. That was the first time I noticed, thought it was odd, but I was on my way to work and couldn't be bothered at that exact moment.

Agreed. Prolly won't be resolved without experimentation.

I'll try it again whenever I wash it. The first time, I had rinsed the hood and grill off.

 

[BroncMan]

When I turned it on.. It being the Bronco Sport.

And the water didn't splash back after stopping. It splashed when I first turned the ignition on in my Bronco Sport while it was sat stationary in the drive way. That was the first time I noticed, thought it was odd, but I was on my way to work and couldn't be bothered at that exact moment.


I'll try it again whenever I wash it. The first time, I had rinsed the hood and grill off.

Sorry about the misunderstanding ... anyhow, I cannot think of anything that would make water
splash up/back over the hood when standing/not moving. The front fan ... would rather be the opposite - sucking in water.

 

[Jrl]

Sorry about the misunderstanding ... anyhow, I cannot think of anything that would make water
splash up/back over the hood when standing/not moving. The front fan ... would rather be the opposite - sucking in water.

Front camera cleaning .

 

[Tigger]

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

I’m reading this thread and laughing that I don’t understand a word of it. Then I read this post, had to look up elucidation, felt dumber, laughing stopped. I hope you’re happy @Barry S.!

edit: and just to add to the conversation, we owned 3 ML 350’s and this phenomenon was the same as the Sport

 

[Mark S.]

I’m reading this thread and laughing that I don’t understand a word of it. Then I read this post, had to look up elucidation, felt dumber, laughing stopped. I hope you’re happy @Barry S.!

Just goes to show you can always learn something, even from conversations that hop the line from erudition to pedantry.

 

[Barry S.]

Just goes to show you can always learn something, even from conversations that hop the line from erudition to pedantry.

If you're trying to mess with my head....it's working.

 

[PugDad]

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.

All my life I thought I lacked intelligence. After reading that in depth book report, it is now officially confirmed. Anything I thought I knew, I will now have to reconfirm. My head hurts.

Very well thought out, great answer. Thank you for taking the time to try help peoples issues.

 

[Meanderthal]

All my life I thought I lacked intelligence. After reading that in depth book report, it is now officially confirmed. Anything I thought I knew, I will now have to reconfirm. My head hurts.

Very well thought out, great answer. Thank you for taking the time to try help peoples issues.

You are off to a great start. The most important thing is to never think you truly "know" something and always being open to other possibilities. Of course there are limits, like you probably know how to operate a light switch and shouldn't spend time thinking about it. Though you could get curious about what is happening inside that light switch when you move it from one state to another.

People tend to look at certain people in the world and think they are far more intelligent than they are. Most of the time, the person has just been exposed to different things than you have. They might have a lot of knowledge about certain subjects but not so much on others. I think the best sort of person is the one that is always trying to learn about different subjects and tying them together as best they can.

 

[Mark S.]

Of course there are limits, like you probably know how to operate a light switch and shouldn't spend time thinking about it.

I dunno, according to legend, Mr. Newton and Mr. Galileo spent a great deal of time thinking about what happens when we drop things, and look where that got us.

 

[HomePeace]

Air moving over a surface has a boundary layer. The closer you get to the surface the slower the air moves. In your case, the water on the hood is in the boundary layer, which means the air moving over it is slower than your forward speed. As you brake, you observe Newton's First Law (inertia) in action: the water on the hood wants to continue moving at the same speed it was moving, and its inertia overcomes the relatively slow air movement in the boundary layer and friction of the surface of the hood. Eventually, the water will fall off the front of your hood. When it does it will drop into the slipstream of air moving around your car and out of the boundary layer. Once free of the boundary layer air into the full-speed slipstream, you see Newton's Third Law (action and reaction). While it may appear the water is being blown backward, it is, in fact, the pressure of air from the slipstream forcing the water to move up and slow down while your car continues forward.

If this isn't the most thought out and valid explanation as to what is occurring. While technical I understood everything and was always wondering why this occurs too.

Thank you for this great explanation.

 

[Tigger]

If this isn't the most thought out and valid explanation as to what is occurring. While technical I understood everything and was always wondering why this occurs too.

Thank you for this great explanation.

 

[greengrass47]

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?

I just realized I've always noticed this and never thought of it until I read your post.

 

[Bucko]

Worse, a technical writer who thinks he knows things.

I played this role awhile back. wrote lots of MOPS (method of procedures) when my company (Siemens; telecommunications division) went through ISO 9000 certification, wrote training student guides when I was an instructor then manager. All this for 39 years.

I still make typos when writing. I know things; many such as telephone operation and theory are useless today.

In the case of water puddling on a hood, I determined that it was due to raining outside, as my wipers automatically came on; thanks Ford for taking care of that, as I may not have figured out that the wipers needed to be turned on because I was wasting time pondering why the water on the hood was puddling up.

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