flow velocity

[chiddler]

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if the diameter of the nozzle, d, is halved, flow speed does not change. rather, velocity increases to compensate and 0 net change. i'd like to know when this does not apply because i remember milski saying that what i just wrote is not true.

is this only with ideal fluids?

 

[milski]

You got it backwards - velocity does not change, flow is proportional to the area. That's for ideal fluids. For non-ideal fluids the decrease of speed will be slightly larger than the ideal case.

 

[chiddler]

First, one modification. I said half diameter. I should of said half cross sectional area. Flow speed just means the v in Q=Av, right?

There's a TBR question that disagrees with you. Let me quote some of the answer for you:

"Because the nozzle reduces cross sectional area by one half, the flow velocity as the liquid exits the nozzle would be increased. However, the same volume is exiting..."

what do you think?

 

[milski]

First, one modification. I said half diameter. I should of said half cross sectional area. Flow speed just means the v in Q=Av, right?

There's a TBR question that disagrees with you. Let me quote some of the answer for you:

"Because the nozzle reduces cross sectional area by one half, the flow velocity as the liquid exits the nozzle would be increased. However, the same volume is exiting..."

what do you think?

That there's more to it than what you're just quoting. Flow speed is just what you quoted. And the velocity for non-viscous fluid depends only on the pressure difference between the top of the fluid and where the opening is. In a way, that's the definition of viscosity - you cannot transfer momentum from the layers around the edges of the hole to the rest of the flow, which means that the velocity cannot be affected by the shape/size of the hole.

What is the full question? If the flow increases, there must be something else that changes with the diameter.

 

[chiddler]

That there's more to it than what you're just quoting. Flow speed is just what you quoted. And the velocity for non-viscous fluid depends only on the pressure difference between the top of the fluid and where the opening is. In a way, that's the definition of viscosity - you cannot transfer momentum from the layers around the edges of the hole to the rest of the flow, which means that the velocity cannot be affected by the shape/size of the hole.

What is the full question? If the flow increases, there must be something else that changes with the diameter.

Largish image. Real fluid.

And also what comes to mind is exit velocity from the cons. of energy equation which agrees with you. Then maybe i'm missing something from this question.

Waiit. I think I misunderstood this question D:

 

[chiddler]

damn it give me a minute to figure this out.

 

[chiddler]

OK! So I didn't completely misunderstand. It's saying that flow speed at point C does not change which means just as much volume is being squirted out despite a smaller area to leave from. Therefore, more exit velocity.

 

[milski]

Largish image. Real fluid.

And also what comes to mind is exit velocity from the cons. of energy equation which agrees with you. Then maybe i'm missing something from this question.

Waiit. I think I misunderstood this question D:

I think they are comparing the conditions at point C and D when the nozzle is present at point D, not the overal outflow in the situations when the nozzle is/is not there. The former would match with their answer, the latter will contradict every single faucet I've used in my life.

I don't see anything viscosity specific, you can treat it as ideal fluid.

 

[chiddler]

I think they are comparing the conditions at point C and D when the nozzle is present at point D, not the overal outflow in the situations when the nozzle is/is not there. The former would match with their answer, the latter will contradict every single faucet I've used in my life.

I don't see anything viscosity specific, you can treat it as ideal fluid.

I'm not seeing that. Nozzle attached so diameter is half at C. How does flow speed change? Flow speed at C does not change.

no flow speed change at point C where the diameter is 2x the nozzle. this indicates that flow has not changed and exit speed is higher at nozzle.

?

 

[milski]

I'm not seeing that. Nozzle attached so diameter is half at C. How does flow speed change? Flow speed at C does not change.

no flow speed change at point C where the diameter is 2x the nozzle. this indicates that flow has not changed and exit speed is higher at nozzle.

?

With nozzle attached:

Flow speed is cannot be different between C and D - otherwise you'll get cavitation or other ugly problems.

Since the flow speed the same but the area is smaller at D, the velocity has to be higher at D (than at C).

The flow at C with the nozzle will be lower than the flow at C without the nozzle, but it would seem that is not what the question is asking.

 

[chiddler]

With nozzle attached:

Flow speed is cannot be different between C and D - otherwise you'll get cavitation or other ugly problems.

Since the flow speed the same but the area is smaller at D, the velocity has to be higher at D (than at C).

The flow at C with the nozzle will be lower than the flow at C without the nozzle, but it would seem that is not what the question is asking.

oh i see.

thanks again for the generous help <3

 

[Tatiana3325]

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if the diameter of the nozzle, d, is halved, flow speed does not change. rather, velocity increases to compensate and 0 net change. i'd like to know when this does not apply because i remember milski saying that what i just wrote is not true.

is this only with ideal fluids?

Can you please post the entire question?