# Flow inside a venturimeter

1. Jun 11, 2015

### NUCLIDES

When we just insert a venturimeter in a potential flow, the pressure inside the venturimeter is higher than outside(since from Bernoulli's principle pressure decreases when velocity increases) , then how does the flow enter the tube?

2. Jun 12, 2015

### DarioC

Venturi, such as used to create a vacuum for aircraft instruments,develop a low pressure area at the larger area trailing edge of the tapered "funnel" and a higher pressure on the input from the ram pressure of the moving fluid. That produces a pressure gradient from one end of the tube to the other end and is responsible for the flow through the venturi. Not sure why you would think that a vacuum developing device would have a higher pressure "inside."

Did I understand your question correctly?

Last edited: Jun 12, 2015
3. Jun 12, 2015

### NUCLIDES

I am speaking of venturis used to measure the aircraft's speed not create vacuum for the instruments, these donot have vacuum in them , they are at atmospheric pressure.

4. Jun 12, 2015

### DarioC

Aircraft speed, through the air, is measured by a pitot tube. It is simply a tube with an opening at the end facing into the "relative wind." Completely different from any kind of venturi device that I know off. The tube supplies a varying pressure to an airspeed indicator gauge. No "flow" involved except for whatever is required in the tube as the airspeed/pressure varies. Just ram air pressure, no flow, no venturi, no vacuum.

A venturi, when placed in a fluid flow will have a higher pressure area in front of it and a lower pressure are behind it, just as any object impeding the flow of a fluid will have. In the smaller middle area there will be a LOW pressure area relative to the outside "normal."

I Don't understand why you think that Bernoulli's theorem means a high pressure area inside a venturi?

Last edited: Jun 12, 2015
5. Jun 12, 2015

### NUCLIDES

Bernoulli's theorem doesnot mean high pressure inside the venturi, indeed suppose a venturimeter is just placed in a flow, then the pressue inside will be 1 atm, but it will be more than or equal to the outside pressure , so why does the flow enter the tube?? Why will there be a low pressure inside the venturi if we are considering static pressures to determine wether a flow will occur due to pressure difference or not ?

6. Jun 12, 2015

### NUCLIDES

even for a pitot tube ,why does the flow enter the tube if the outside static pressure is lower than the inside pressure which is 1 atm?

7. Jun 12, 2015

### DarioC

What makes you think that the inside pressure on a pitot tube is higher than the ram air pressure at the tip exposed to the high speed airflow past the aircraft? Since pitot tubes don't work by flow, the pressure is equal throughout the system; lower inside to start, equalizing to the same at a constant airspeed.

Back to the original: are you questioning as to why or how air will flow through any open ended tube when initially immersed in a moving liquid? Maybe I still don't understand your question.

8. Jun 12, 2015

### NUCLIDES

The fluid flowing must first enter the pitot tube, otherwise how will the pressure inside increase. And yes I am questioning why will fliud flow in an open ended tube when initially immersed in a moving fluid

9. Jun 12, 2015

### DarioC

Initially you will see a high pressure area on the leading edge of the tube. You see exactly the same affect on the leading edge of an airfoil. The air flows on either side and there is a static high pressure area at the very front. This pressure area will continue even with the air flowing through the tube. At the other end of the tube you will have a low pressure area that is trying to suck the air out of the tube. The fact that there is a restriction in the middle (smaller diameter) doesn't change this.

That is the best I can do in the way of an explanation.

10. Aug 1, 2015

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