Water Park Fluid Mechanics

[zdas04]

While waiting for my granddaughter to come down a slide at a water park yesterday I noticed an odd configuration (attached). At the bottom of the slide there was a weir and a curb. I see that this is acting as a restriction orifice and that makes sense. What I can't figure out is why the water level on the backside of the weir is nearly (but not quite) as high as the water level on the front of the weir. It seems to me that the friction of the curb would not be great enough to back up the water that much. Anyone have any ideas on the height of water on the curb?

David

 

[patprimmer]

As all as I can say is the restriction of the weir obviously is enough to back up the water that much.

I don't suppose the velocity of the water perpendicular to the wall has to be dissipated against the wall before it can fall into the trough and this dissipation causes a rise. I'm trying to think how conservation of energy and inertia and potential energy from height change all apply together here.

If it where a hose against the wall, some would splash up before it fell down.

Regards
Pat
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[btrueblood]

Dave,

Come on, it's easy! Start at the curb - the water needs to be slightly deeper than the height of the curb to overflow, and the depth over the curb will be more or less proportional to flow rate. Stick the gate upstream, it's just a series resistance, and the water head (hieght) will need to increase slightly more than that over the curb, in order to have the delta-p needed to flow at that rate past the gate's resistance.

Bigger question - what did the people around you think when you whipped out your engineering scale to determine those water depths?

 

[Compositepro]

The plate is a baffle to remove turbulence and waves from the water before it goes over the weir. The water downstream of the plate has upward momentum and will not start to lose that momentum until after it rises above the top of the weir. Even if the top of the weir were a knife edge (essentially zero friction to horizontal flow) the water would rise to close to the same height as the upstream side of the plate.

 

[Latexman]

And the water has to change direction about 3 times to flow through the maze. Head is needed to overcome the loss in momentum.

Good luck,
Latexman

 

[BigInch]

And accelerate the water mass in the other directions.

zdas it would have helped if you had remembered to measure the clear distance below and the thickness of the plate. Can we assume that those dimensions are "to scale" and would therefore be 4" and 1" respectively.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[zdas04]

The teenager running the ride was looking at me very funny as I scoped this out. More measurements might have gotten me a meeting with security.

OK I got it, the height above the curb is a function of the required flow rate, not the friction over the curb.

David

 

[BigInch]

Water below the plate speeds up, head drops to compensate for faster velocity, surface elevation lowers.

Height above the curb is a function of THE flowrate, which is possibly a function of the height of the curb as well.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[Ron]

Friction head loss.

 

[BigInch]

Friction is such a small number, it requires lots of length, hence its effect isn't noticed in this drawing.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[msquared48]

Head loss - frictional and directional

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[btrueblood]

Did they have one of those tippy buckets at the water park, David? The kind that slowly fill with water, then tip to release about 1/4 of it, dousing people below.

Was walking in the woods last hunting season, and heard a strange, intermittent trickle of water. Thought for sure I was hearing a bear doing its business in the woods. Come to find, a piece of flexible tree bark was blocking the entrance to a culvert under the logging road, and acting just like a tipping bucket oscillator.

 

[zdas04]

My son was fasinated by the tippy bucket. Last night over a beer I tried to sketch out the dynamics of the tippyness. He kept being shocked that there wasn't a tipping motor.

David

 

[BigInch]

That's the beginnings of a "tippymotor"

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus