Venturi effect


by gloo
Tags: effect, venturi
gloo
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#1
Dec29-08, 10:52 AM
P: 75
In the Venturi effect, in the reduction in pressure and increase velocity on the inside of the convergent cone, does the exit of the liquid on the divergent side mean the pressure that is increased (velocity decreased) can only increase to the maximum pressure that was achieved on the inside (before the convergent)? Can the pressure be increased more if somehow there is a drop in velocity?
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LURCH
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#2
Dec29-08, 11:17 AM
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Yes; if the exit (after the convergence) is larger than entrance) before the convergence), velocity will be decreased, and pressure will be increased.
rcgldr
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#3
Dec29-08, 12:33 PM
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The pressure returns to ambient only when the velocity (both linear and angular related (turbulent flow)) returns to back to ambient. However, water based venturi pumps, direct water from a faucet through the cone and out the nozzle output into a chamber that has a hose connection on side, and an exit hole at the far end. This allows the existing flow to remain at it's current velocity and low pressure while also allowing air and/or water to be drawn into the chamber and then travel along with the faucet water, and it makes a pretty good pump.

http://andysworld.org.uk/aquablog/?postid=247

If you click on the Candian patent, you can view the images to see the internal workings. The USA patent also works but requires you install a tiff viewer add-on to your browser.

gloo
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#4
Dec29-08, 02:32 PM
P: 75

Venturi effect


Quote Quote by LURCH View Post
Yes; if the exit (after the convergence) is larger than entrance) before the convergence), velocity will be decreased, and pressure will be increased.
Sorry, I am not sure exactly what you mean by the yes response? So the pressure can be greater than the pressure in the container that exists before the start of the convergence zone? So if the pressure in the container with water is say 130 PSI, it can be greater after it leaves the smallest area point on the nozzle at the exit (like 200 PSI)? What is the factor that magnifies the pressure? Is it the length of the convergence tunnel and how small the exit point is into the outside of the container? Does it mean the container that the water enters is greater in volume than the container in which it came from? Can you clarify what "exit is larger than entrance means?
rcgldr
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#5
Dec29-08, 08:52 PM
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Quote Quote by gloo View Post
So the pressure can be greater than the pressure in the container that exists before the start of the convergence zone?
Only if the fluid had sufficient velocity before the start of the convergence zone. Think of it as a total energy issue, the fluid's initial state includes kinetic energy, pressure energy, gravitational potential energy, and temperature. Ignoring gravity and temperature, the pressure can only be increased if the kinetic energy is decreased. There would have to be some source of power for the flow in the first place, the force would be equal to the pressure times cross sectional area at the source, and the power would be related to this force times the speed of the flow. In the real world, friction with pipe walls and viscosity would cause losses in the system.
LURCH
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#6
Dec30-08, 01:56 PM
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Quote Quote by Jeff Reid View Post
Only if the fluid had sufficient velocity before the start of the convergence zone. Think of it as a total energy issue, the fluid's initial state includes kinetic energy, pressure energy, gravitational potential energy, and temperature. Ignoring gravity and temperature, the pressure can only be increased if the kinetic energy is decreased. There would have to be some source of power for the flow in the first place, the force would be equal to the pressure times cross sectional area at the source, and the power would be related to this force times the speed of the flow. In the real world, friction with pipe walls and viscosity would cause losses in the system.

Right; I was not thinking of the fluid coming from a "container," like a pressurized can, but rather I was assuming a pump moving fluid through a pipe, up to a venturi tube, then out to another pipe. If the second pipe is larger in diameter than the first, then the pressure against the walls of the second pipe will be greater, and the velocity of the fluid will be slower. Of course the rate of flow, in terms of volume/time will remain nearly constant, but the greater volume inside the second pipe will mean that the fluid travels at a slower speed, causing greater pressure against the sides.
gloo
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#7
Jan7-09, 03:35 PM
P: 75
Quote Quote by Jeff Reid View Post
Only if the fluid had sufficient velocity before the start of the convergence zone. Think of it as a total energy issue, the fluid's initial state includes kinetic energy, pressure energy, gravitational potential energy, and temperature. Ignoring gravity and temperature, the pressure can only be increased if the kinetic energy is decreased. There would have to be some source of power for the flow in the first place, the force would be equal to the pressure times cross sectional area at the source, and the power would be related to this force times the speed of the flow. In the real world, friction with pipe walls and viscosity would cause losses in the system.
Jeff, Lurch, thanks so much for helping with this topic. I am a bit of physics idiot so I am not that well versed to grasp some of the ideas thrown at me. I still want to ask a few more questions about this phenomena:

1. What kind of "sufficient velocity" are we talking about? Isn't the velocity in the closed container of water at rest and equal to zero? So how do we choose at what point the velocity is going to be sufficient? Is that a function of the force/pressure applied into the water in the container; i also assume how big the nozzle throat is will affect the velocity at which the fluid leaves the container through the convergence zone? I am wrapping my idea around a piston type set up where a force will applied to the container top.

2. What if the convergence zone is long and tappers off sharply. Won't it mean the velocity starts to increase (and pressure decrease) as it approaches the apex (exit). In other words, the longer convergence zone will give the water more running time to acclerate towards the apex.

3. If the exit and the resulting medium where the venture effect occurs (i.e decrease in speed and increase in pressure) is water as well - does that affect the outcome. What I am wondering is can i ever have a lower pressure (PSI) in the container and have it come out into a higher pressure water medium because it (the exit pressure) is magnified by the venturi effect (by shaping the nozzlen (i.e longer and sharper).

.


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