Open Channel Flow: Dynamics & Forces in Irrigation Channels

[aladinlamp]

Hi
what dynamics are moving liquid in open channel flow without slope (irrigation channels)? Are they mostly hydrostatic forces?

imagine 2 scenarios
1. Channels are empty and are being filled for the first time. What forces drive the liquid until it becomes big static lake?
2. Channels are filled but at the end of the channels water is being pumped out for irrigation. What drives the liquid to be replenished in the channels?

 

[russ_watters]

There has to be slope otherwise you just have a big, static lake.

 

[Dale]

what dynamics are moving liquid in open channel flow without slope (irrigation channels)?

Open channel flow is purely gravitational.

 

[aladinlamp]

There has to be slope otherwise you just have a big, static lake.

Ok imagine 2 scenarios
1. Channels are empty and are being filled for the first time. What forces drive the liquid until it becomes big static lake?
2. Channels are filled but at the end of the channel water is being pumped out for irrigation. What drives the liquid to be replenished in the channels?

 

[Dale]

What forces drive the liquid until it becomes big static lake?

Gravity

What drives the liquid to be replenished in the channels?

Gravity

Again, open channel flow is purely gravitational.

 

[Arjan82]

Ok imagine 2 scenarios
1. Channels are empty and are being filled for the first time. What forces drive the liquid until it becomes big static lake?
2. Channels are filled but at the end of the channel water is being pumped out for irrigation. What drives the liquid to be replenished in the channels?

1. At the point you are filling the channel the water surface is raised a bit. Gravity will even this out by moving water away from this point.
2. vice versa.

 

[Lnewqban]

All the molecules of water tend to find a balanced state respect to the neighbour molecules under the action of gravity.
They self-adjust for tiny level differentials caused by wind, evaporation, exfiltration, pumping, drains, etc.
Please, see:
https://en.m.wikipedia.org/wiki/Roman_aqueduct#Conduits_and_gradients

:)

 

[russ_watters]

Back to this question in the OP:

Are they mostly hydrostatic forces?

I'd say no, as hydrostatic force (force due to hydrostatic pressure) is the force on a submerged surface or parcel of water due to the height of the column of water above it (its depth). To drive flow from one place to another, you need a difference in height of one column of water to vs another at another place (not depth). That's called hydraulic or gravitational head:
https://en.wikipedia.org/wiki/Hydra...r piezometric head,or bottom) of a piezometer.

Related, but not quite the same thing. But it's this difference in height (and the resulting pressure difference) that drives the flow.

 

[aladinlamp]

ok little side question
when flow is initiated by height difference, liquid has to overcome viscosity, internal friction.
does it mean, any liquid movement is causing liquid to heat up ?
in perfect heat isolated system, moving liquid(water) will just gain heat and increase its temperature until it will boil ?<

 

[Arjan82]

In principle you are right. Allthough in practice I don't know of anyone having done an experiment with so much friction that the water starts to boil off.

In wind tunnels this can be a problem however. This is a more or less closed system and due to the turbulence generation, which dissipates due to viscosity/friction, the temperature inside the wind tunnel rises. This can cause all sorts of problems, like changing properties of air, instrument calibration issues etc.