I Fluid flow through a pipe constriction - what would you feel

[fr3dy]

Hi. Imagine a large pipe 10 m wide through which water is flowing at 10 m/s. You are in the stream flowing at the same speed, two other people are stationary at the entrance and in the middle of the venturi. You flow to the a venturi constriction that is 5 m wide. Apart from the acceleration you would feel, would you feel a decrease or increase or no pressure change?
As I understand it the static pressure would drop but the dynamic goes up and the total p stays the same, but what would that mean for a person flowing in the stream compared to the two stationary in it? (ignoring turbulence etc)
Thanks!

 

[BvU]

Hello fr3dy, (welcome back?)

You need to be accelerated, so there should be a sensible pressure difference. In accordance with the description here
What is dynamic pressure (apart from a calculation instrument) ?

 

[fr3dy]

Hi BvU. Thanks for that. However I think Wikipaedia is wrong: the fluid does does not "expansion and compression of the fluids cause the pressure inside the venturi to change". The streamlines compress but the water does not.
Put it another way: what would happen to a bubble flowing through this pipe entering the venturi??

CHeers!

 

[BvU]

The streamlines compress but the water does not compress

Means that the pressure changes, not the density.

 

[BvU]

what would happen to a bubble

It would be elongated and the volume would increase

( Disclaimer: 'intuitive' answer -- some CFD wiz might be able to correct me if I am wrong ) But I think it's all straightforward Bernoulli

 

[Lord Jestocost]

Put it another way: what would happen to a bubble flowing through this pipe entering the venturi??

You need no bubble entering the venturi constriction in order to understand what's going on. In case the fluid velocity in the venturi tube is high enough, the static pressure in the flow can drop below the vapor pressure of the liquid giving rise to cavitation. Have a look at figures 3 and 4 in [PDF]Physics and Control of Cavitation - (NATO STO).

 

[JRMichler]

Since the OP is apparently mostly interested in getting a feel for what is happening, I suggest an experiment. Find somebody who does whitewater canoeing, and ask for a ride down a chute with a one to two foot drop. Photo of a chute below. It's open channel flow, but the pattern is similar to the OP's description.

If you want a more intensive experience, jump overboard and float down through the chute. Do wear a life jacket and a crash helmet if you try that.

 

[russ_watters]

Hi. Imagine a large pipe 10 m wide through which water is flowing at 10 m/s. You are in the stream flowing at the same speed, two other people are stationary at the entrance and in the middle of the venturi. You flow to the a venturi constriction that is 5 m wide. Apart from the acceleration you would feel, would you feel a decrease or increase or no pressure change?
As I understand it the static pressure would drop but the dynamic goes up and the total p stays the same, but what would that mean for a person flowing in the stream compared to the two stationary in it? (ignoring turbulence etc)
Thanks!

Dynamic pressure isn't something you feel.

 

[russ_watters]

Hi BvU. Thanks for that. However I think Wikipaedia is wrong: the fluid does does not "expansion and compression of the fluids cause the pressure inside the venturi to change". The streamlines compress but the water does not.
Put it another way: what would happen to a bubble flowing through this pipe entering the venturi??

CHeers!

It would get longer and wider.

 

[fr3dy]

Thanks Russ. So you cannot feel dynamic pressure when in the flow, that makes sense, but I presume you do feel it when you are the stationary object in it? (like a pitot tube...)
And as for the bubble, then I also suppose that a person would feel like they were being expanded like in space without a suit, as the air in them would still be a x bar and the water flow has changed the static pressure to <x bar ?

 

[fr3dy]

Means that the pressure changes, not the density.

Yes, that's what I said.
Your link to Wiikipedia says the fluid is compressed. This may be the case for a gas but not for water.

 

[russ_watters]

Thanks Russ. So you cannot feel dynamic pressure when in the flow, that makes sense, but I presume you do feel it when you are the stationary object in it? (like a pitot tube...)

Right. Keeping the analogy of being immersed in the flow, you would only feel the velocity pressure when you slam into an obstruction (like a pitot tube).

And as for the bubble, then I also suppose that a person would feel like they were being expanded like in space without a suit, as the air in them would still be a x bar and the water flow has changed the static pressure to <x bar ?

Right. When traveling with the flow, you are stationary in the only way that matters (with respect to the fluid) and all the normal workings of static pressure apply. If the fluid static pressure drops, bubbles expand.