# Increasing flow rate makes pressure drops larger

• fysik
In summary, the conversation discusses the relationship between flow rate and pressure drop in venturi meters, orifice plate meters, and other devices with a constriction in a pipe. The Bernoulli equation is mentioned, which states that the pressure difference is proportional to the square of the flow rate. The conversation also touches on the conservation of energy and the dimensions involved in this relationship.
fysik
hello

In venturi meters, orifice plate meters and generally when there is a neck, constriction at a point in a pipe there is, at that point, a drop in pressure and an increase in velocity, so that the whole energy is preserved (theoretically)

my question is, why if we increase the flow rate (which means we increase the pressure in the pipe?) we have a much larger drop in pressure at the constriction?

that drop in pressure is larger, going through the same restricted hole, but why? what is the cause of this?

I could expected the drop in pressure to be the same, as the diameter of the hole is the same

but it seems that MORE energy becomes from pressure energy into kinetic energy with increased flow rate

but why?

thanks

Welcome to PF!

fysik said:
hello

In venturi meters, orifice plate meters and generally when there is a neck, constriction at a point in a pipe there is, at that point, a drop in pressure and an increase in velocity, so that the whole energy is preserved (theoretically)

my question is, why if we increase the flow rate (which means we increase the pressure in the pipe?) we have a much larger drop in pressure at the constriction?

that drop in pressure is larger, going through the same restricted hole, but why? what is the cause of this?

I could expected the drop in pressure to be the same, as the diameter of the hole is the same

but it seems that MORE energy becomes from pressure energy into kinetic energy with increased flow rate

but why?

thanks
At the higher flow rate, the fluid is experiencing greater acceleration (the difference in velocities between upstream and at the contraction is greater) so you need to apply more net force (i.e., a higher pressure difference).

Chet

Well, my take on questions like this is "larger" in what sense? The equation is what it is and is exactly the same for a low flow rate as for a high flow rate. The pressure change in psi or torr may be different at different flow rates, but the equation dictates that the pressure change in % is exactly the same. That's why I'm looking for the OP to dig into the equation.

well, the equation clearly states that the difference in pressure (drop in pressure) is proportional to the flow rate

why fastest water through an orifice causes larger pressure drop through that orifice

russ_watters said:
Well, my take on questions like this is "larger" in what sense? The equation is what it is and is exactly the same for a low flow rate as for a high flow rate. The pressure change in psi or torr may be different at different flow rates, but the equation dictates that the pressure change in % is exactly the same. That's why I'm looking for the OP to dig into the equation.
I'm a little confused. Doesn't the Bernoulli equation say that the pressure difference increases in proportion to the square of the flow rate?

My understanding of Bernoulli is that the work done by the pressure forces is equal to the change in kinetic energy of the fluid (neglecting potential energy changes). Or equivalently, larger pressure differences are required to accelerate the fluid more.

Chet

Chestermiller said:
I'm a little confused. Doesn't the Bernoulli equation say that the pressure difference increases in proportion to the square of the flow rate?
Yes. So a doubling of velocity always yields four times the pressure change, whether the first velocity was 1 or 7 or 200 m/s or ft/sec.

It's just an opinion, but I think it is more useful because it is more specific that way.

fysik said:
Russ and I answered your question in two different, but consistent, ways. I'm just trying to figure out why you still feel that neither of these answers your question.

Chet

fysik said:
well, the equation clearly states that the difference in pressure (drop in pressure) is proportional to the flow rate

why fastest water through an orifice causes larger pressure drop through that orifice
I see two different and perhaps new questions there:
1. Why does pressure change at all? A: Conservation of energy requires it and the pressure is the force that causes the speed change.
2. Why is it a square function? Dimensions: velocity is one dimension (length) and area is two (length and width). So when one affects the other, it must be a square function.

Last edited:

## 1. How does increasing flow rate affect pressure drops?

Increasing flow rate causes a higher velocity of fluid, which results in a larger pressure drop due to the increased friction between the fluid and the walls of the pipe.

## 2. What is the relationship between flow rate and pressure drops?

As flow rate increases, pressure drops also increase. This is due to the fact that a higher flow rate means a larger volume of fluid is passing through a given area, resulting in more resistance and a larger pressure drop.

## 3. Why does increasing flow rate lead to larger pressure drops?

As fluid moves through a pipe, it experiences friction against the walls of the pipe. This friction creates resistance, which results in a pressure drop. As flow rate increases, the velocity of the fluid also increases, leading to more friction and a larger pressure drop.

## 4. Is there a limit to how much increasing flow rate can affect pressure drops?

Yes, there is a limit to the amount of pressure drop that can be caused by increasing flow rate. This limit is determined by the size and shape of the pipe, as well as the properties of the fluid being pumped through it.

## 5. How can I minimize the impact of increasing flow rate on pressure drops?

To minimize the impact of increasing flow rate on pressure drops, you can use larger diameter pipes, smoother pipe materials, and reduce the fluid's viscosity. Additionally, controlling the flow rate and using efficient pumping systems can also help reduce pressure drops.

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