# How is it that fluid velocity increases by a low pressure

1. Jan 15, 2016

### travis51

So I've always thought that an increase in velocity of a fluid would decrease pressure. However, I've heard the opposite and that it is an decrease in pressure that increases velocity. All I want to know which one is correct (in terms of something like a venturi pipe) because an decreasing pressure resulting in a increase in velocity makes no sense to me

2. Jan 15, 2016

### Bystander

3. Jan 15, 2016

### travis51

From what I'm reading they go together and neither are really a cause of the other.

4. Jan 15, 2016

### FactChecker

Yes. Think of pressure as being caused by the velocity of billions of molecules hitting the side of a tube that the fluid is going through. That caused the pressure that is measured. When the speed through the tube increases, that is just pointing those molecule velocity vectors more in the direction of flow without making them any larger. So there is a trade-off between pressure (pointing toward the side of the tube) and the flow speed (pointing toward the direction of flow). Neither one causes the other. They go together.

5. Jan 16, 2016

### Shehbaj singh

Check Bernoulli s theorem. Decreasing pressure is wrongly said. You must have known about work-energy principle. Pressure of liquid do work to increase kinetic energy of itself and pressure gets decreased. Check google it's all explained with diagram.

6. Jan 16, 2016

### A.T.

Why wouldn't the velocity vector magnitudes get larger? Why would an increase of the velocity component parallel to the pipe reduce the component perpendicular to it?

7. Jan 16, 2016

### FactChecker

Assuming no energy added, on average.

8. Jan 16, 2016

### A.T.

But the pressure on the walls decreases even if you add kinetic energy to accelerate the fluid. How does you explanation fit into that?

9. Jan 16, 2016

### FactChecker

But you do not add kinetic energy to the system. You just trade off kinetic and potential energy within the system. Same as trading off the velocity components as the velocity vector tilts in the direction of the fluid flow.

The pressure results from randomly oriented velocity vectors. The molecules with a larger component in the direction of flow self-select themselves. They have a correspondingly smaller velocity component in the remaining random directions, so the pressure decreases.

10. Jan 16, 2016

### A.T.

Wouldn't that also imply a decrease in temperature, which is related the average particle KE in the rest frame of the flow?

11. Jan 16, 2016

### mfig

Recall that Bernoulli's principle says that, given the flow conditions for which it applies, the sum of the energy terms associated with a flowing fluid is constant. In a horizontal flow, there are two terms:

Venergy + Penergy = Const

The first term is the kinetic energy (this term is proportional to the square of the velocity) and the second term is the pressure energy (proportional to the pressure). Now think about it for a second. If the sum of two terms is a constant even though they vary individually (but not independently!), then when one energy term increases the other must decrease so that the sum is the same. If the pressure energy term goes down then the velocity energy term must go up.

12. Jan 16, 2016

### FactChecker

Good point! I see what you were getting at. I may have been thinking about this in an over-simplified way for a long time.

The @spamanon post above is still true, but maybe not for the simple reason that I visualized.