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### Course: Physics library > Unit 9

Lesson 3: Fluid Dynamics- Volume flow rate and equation of continuity
- What is volume flow rate?
- Bernoulli's equation derivation part 1
- Bernoulli's equation derivation part 2
- Finding fluid speed exiting hole
- More on finding fluid speed from hole
- Finding flow rate from Bernoulli's equation
- What is Bernoulli's equation?
- Viscosity and Poiseuille flow
- Turbulence at high velocities and Reynold's number
- Venturi effect and Pitot tubes
- Surface Tension and Adhesion

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# What is volume flow rate?

You know all about the motion of individual objects. Now, let's talk about how to analyze the motion of a fluid.

## What does volume flow rate mean?

You might hear the term $Q$ of a fluid is defined to be the volume of fluid that is passing through a given cross sectional area per unit time. The term

**volume flow rate**and think it sounds boring, but volume flow rate keeps you alive. I'll tell you how in a second, but first we should define volume flow rate. The volume flow rate*cross sectional area*is just a fancy term often used to describe the area through which something is flowing, e.g., the circular area inside the dashed line in the diagram below.Since volume flow rate measures the amount of volume that passes through an area per time, the equation for the volume flow rate looks like this:

In S.I. units (International System of Units), volume flow rate has units of meters cubed per second, $\frac{{\text{m}}^{3}}{\text{s}}$ , since it tells you the number of cubic meters of fluid that flow per second.

So how does volume flow rate keep you alive? Your heart pumps a volume of blood roughly equal to the volume of a can of soda every four seconds.

## Is there another formula for volume flow rate?

It turns out there's a useful alternative to writing the volume flow rate as $Q={\displaystyle \frac{V}{t}}$ .

The volume of a portion of the fluid in a pipe can be written as $V=Ad$ , where $A$ is the cross sectional area of the fluid and $d$ is the width of that portion of fluid, see the diagram below. We can substitute this formula for volume $V$ into the volume flow rate to get the following:

But the term $\frac{d}{t}$ is just the length of the volume of fluid divided by the time it took the fluid to flow through its length, which is just the speed of the fluid. So we can replace $\frac{d}{t}$ with $v$ in the previous equation and get

Be careful though, we're now dealing with two terms that look very similar. The volume is represented with a capital letter $V$ , and the speed is represented with a lowercase letter $v$ . People often mix up the notation for volume, $V$ , and speed, $v$ , since they look so similar.

## Incompressibility of liquids

It turns out that most liquids are nearly incompressible. This means that a gallon of milk can be put into a differently shaped gallon-sized container, but you wouldn't be able to squeeze that entire gallon of milk into a half-gallon-sized container no matter how hard you squeeze.

Because liquids are incompressible, any portion of liquid flowing through a pipe could change shape, but it must maintain the same volume. This is true even if the pipe changes diameter. In the diagram below, the volume, $V$ , of liquid on the left changes shape as it enters a narrow section of pipe, but it maintains the same volume since liquids are incompressible.

## What is the equation of continuity?

Liquids must maintain their volume as they flow in a pipe since they are nearly incompressible. This means that the volume of liquid that flows into a pipe in a given amount of time must equal the volume of liquid that flows out of a pipe in the same amount of time. For instance, if in one hour you pump 2 m${}^{3}$ of water into a pipe that is already full of water, 2 m${}^{3}$ has to flow out of that pipe during that same hour. The only alternatives would be for the liquid to compress inside the pipe—which shouldn't happen—or the pipe balloons in size—which we assume doesn't happen if the pipe is rigid. Remember, you're not confined to considering points only at the beginning or end of the pipe, this argument works just as well for water entering and exiting any two sections of the pipe.

So, the volume flow rate $Q$ for an incompressible fluid at any point along a pipe is the same as the volume flow rate at any other point along a pipe.

This can be represented mathematically with the formula $Q=constant$ , or—choosing any two points in the pipe—we can state mathematically that the volume flow rate is the same at any two points by writing

Now if we substitute the formula $Q={\displaystyle \frac{V}{t}}$ , we get

Alternatively, we could plug in the alternative form of the volume flow rate, $Q=Av$ , into the formula, ${Q}_{1}={Q}_{2}$ , which would give us

This equation is known as the

**equation of continuity**for incompressible fluids—the previous two equations are also sometimes referred to as the equation of continuity. The equation isn't really as mysterious as the name suggests since we found it simply by requiring that volumes be incompressible as they flow through a pipe.The equation is quite useful though, particularly in this form, since it says that the value of $Av$ has a constant value throughout the pipe. In other words, no matter where in the pipe you choose to find $Av$ , the value will always come out to be the same number for a given pipe, if the fluid is incompressible.

So, if the area, $A$ , of a section of pipe decreases, the speed, $v$ , of the liquid there must increase so that the product, $Av$ , remains the same. This means that $A$ . The water must come out with higher speed, $v$ , to ensure the volume flow rate, $Av$ , remains the same. This is why narrow nozzles, which reduce the area ($A$ ), attached to water hoses cause a significant increase in the speed, $v$ , of the fluid at that point.

*fluids speed up when they reach a narrow section of a pipe and slow down when they reach a wider section of a pipe*. This matches everyday experience—think about what happens if you block a portion of the water hose with your thumb, effectively reducing its area,## What do solved examples involving volume flow rate look like?

### Example 1: Mountain Dew dream house

A very wealthy woman who loves soda builds her house with a cylindrical pipe that transports Mountain Dew from downstairs to her upstairs bedroom. The Mountain Dew enters the house downstairs via a pipe with a cross sectional area of 0.0036 m${}^{2}$ where it is traveling with a speed of 0.48 meters per second. At the wealthy lady's bedroom, the faucet pipe through which the Mountain Dew exits has an area of 0.0012 m${}^{2}$ .

**What is the speed of the Mountain Dew as it exits the faucet pipe in the lady's bedroom?**

Note: We could have also solved this problem just by noticing that the area, ${A}_{2}$ , of the pipe in the bedroom was $\frac{1}{3}$ the area of the pipe downstairs, ${A}_{1}$ . This means that the speed of the Mountain Dew has to be going three times as fast in the bedroom pipe, compared to the downstairs pipe, in order for the factor $Av$ to remain the same.

### Example 2: Coconut-milk cupcakes

A chef wants to make sure he always has coconut milk ready for all his cupcake recipes, so he creates a cylindrical pipe that goes from the storeroom to the kitchen. The pipe at the storeroom has a radius of 4 cm where the coconut milk has a speed of 0.25 meters per second. The coconut milk exits the tube in the kitchen with a speed of 1 meter per second.

**What is the radius of the tube at the kitchen through which the coconut milk exits?**

Note: We plugged in our radius, ${r}_{1}=4\text{cm}$ , in units of centimeters, which just means that our answer came out in units of centimeters.

## Want to join the conversation?

- In the paragraph titled, "What is the equation of continuity?" it says that the volume in must equal the volume out. I am a physics novice/dunce so I really need this spelled out for me. It has a "for instance" that I am struggling with. I am hoping it is a mistake, but honestly with my physics knowledge I can't be sure.

"For instance, if in one hour you pump 2 meters cubed of water into a pipe that is already full of water, 3 meters cubed has to flow out of that pipe during that same hour. "

Is this correct? I am trying to understand why the two numbers wouldn't be the same...Please let me know if I am missing something! Thanks!(15 votes)- Hi J

You are correct in thinking this is a typo. The volume flowing in and out of the pipe are the same so the statement should read, "if in one hour you pump 2 meters cubed of water into a pipe that is already full of water,**2**meters cubed has to flow out of that pipe during that same hour."(25 votes)

- "People often mix up the ideas of volume V and speed v since they look so similar." Wait a second, I thought v stood for velocity, not speed! Am I mistaken, or is the text wrong?(8 votes)
- You're correct, but in this case, velocity and speed are the same thing. Speed is a magnitude (in other words, a number representing distance over time) while velocity is a vector - a speed that has a direction. Since liquid through a pipe is flowing in one direction, the direction part doesn't matter. Sal probably used the word speed to make it easier to understand.(31 votes)

- In the previous video, Sal mentioned that the flux or flow rate is denoted by R but here it is being used as Q. Are they still the same thing just being denoted with diferrent letters or are they actually different measurements?(11 votes)
- From what I've seen, volume flow rate is usually denoted by the symbol Q. R is used for resistance in electricity and the gas constant in chemistry (PV = nRT).(1 vote)

- In the Mountain Dew problem, the soda is carried from downstairs to upstairs. Should we consider the pressure that is needed to pump the soda up?(5 votes)
- If you only want to relate the speeds and areas of the pipe, you don't need to consider the pressure. The continuity equation Av=Av has to be true for any incompressible fluid (since the volume flow must be constant) )even if the pressure and height changes. If you wanted to determine the pressure necessary to pump the Mountain Dew, you would need to use Bernoulli's equation as well as the equation of continuity.(9 votes)

- I am answering this question in January of 2023, so I hope you know where I looked for the answer of this question.

The reason why Q is used specifically to represent volume flow rate is rooted in the historical conventions of fluid mechanics. The letter Q is used as an abbreviation for the Latin word "discharge" which is the act of releasing fluid, specifically refers to the flow rate of a fluid. The use of Q for flow rate is convenient because it is a single letter, easy to write and recognize, and is less prone to mistakes than writing out "flow rate" every time it needs to be referenced.

It's common in physics and engineering to use symbols, often letters of the alphabet, as shorthand for certain quantities and concepts to save time and space, especially in mathematical equations and in technical drawings. This can improve the readability of the work, making it more straightforward to interpret and understand the results.(6 votes) - When I open the water tap. I physical increase the the area of the outlet (the tap). However, it seems volume flow rate increase when I open the tap more . The equation of the continuity predicts that I would have the same volume flow rate with different speed depends on the outlet area, clearly this is not the case, I could fill a container in shorter time if I open the tap to its maximum compare if I open it slightly. Where I'm not getting it right?(3 votes)
- as per the continuity equation A1vi = A2v2 it says that the value of Avhas a constant value throughout the pipe. So, if the area, A of a section of pipe decreases, the speed, v of the liquid there must increase so that the product, Av remains the same.

So, if the area, AAA, of a section of pipe decreases, the speed, vvv, of the liquid there must increase so that the product, AvAvA, v, remains the same. This means that fluids speed up when they reach a narrow section of a pipe and slow down when they reach a wider section of a pipe. This matches everyday experience—think about what happens if you block a portion of the water hose with your thumb, effectively reducing its area, AAA. The water must come out with higher speed, v, to ensure the volume flow rate, Av remains the same. This is why narrow nozzles, which reduce the area (A), attached to water hoses cause a significant increase in the speed, v of the fluid at that point.

but i noticed that in my home tap , when i fully open the tap it fills the bucket quickly as compared to when i open it half,(reducing the opening area of tap). but as per law of continuity ,bucket should be filled within same time in both cases.(2 votes)- I hope someone answer this question.

Nicholas:

Can you elaborate more, I don't get how the tap can regulate the (volume flow rate) by reducing the area. This is clearly doesn't explain the equation of the continuity.

Sharma

"Doesn't the Equation of Continuity have nothing to do with time. Because as per A1v1=A2v2, there is no time"

Equation of continuity has everything to do with time, v is speed, speed is meter per second.

Furthermore, A1v1=A2V2 ---> volume flow rate at 1 = volume flow rate at 2

This means, the amount of water passing in 1 sec is the same at both ends.(2 votes)

- I thought Sal said in the video that Flux was V/t? And that Flux was R? Are Vol Flow Rate and Flux the same thing?(2 votes)
- at9:20in "volume flow rate and equation of continuyty" video(1 vote)

- Say you have a hole cut in a pipe and the liquid within was flowing straight down and exiting horizontally out of the hole. How do you measure the width of this flow that would occur from the hole?(2 votes)
- If the fluid is incompressible, the equation of continuity will still apply. The volume flow rate at the exit of the pipe plus the volume flow rate through the role would be equal to the volume flow rate at the entrance of the pipe: Qentrance = Qexit + Qhole.(1 vote)

- what about vertical flow ?

Gravity must have its effect?(2 votes)- While deriving the equation of continuity we cancel the two g's so no matter what the value of g is we will cancel it out.

Yes, gravity will have its effect. We will have to do more work in moving that water column up, nothing else.(1 vote)