Flow Rate: Definition & Equation

Instructor: Aaron Miller

Aaron teaches physics and holds a doctorate in physics.

The subject of this lesson is how we quantify the amount of fluid flowing through a pipe, channel, or other container. We will see that it depends on both the dimensions of the container and the speed of the fluid.

Fluids in Motion

What is the difference between a fluid and a solid? If you think about your everyday experiences with fluids (like water or air) and a solid (like a potato), you may have a sense that one major difference between these two categories of matter is in the way they move. Potatoes are solid because they (mostly) retain their shape when they are thrown. However, the air you are breathing does not retain its shape as it moves into your lungs, so we say it is non-solid. Matter that is non-solid is called fluid. Fluids include liquids and gases. When a fluid moves (for example, as air moves in and out of your lungs during respiration), the matter is said to flow. As a fluid moves, it may change shape and volume (for example, air may be compressed), but, like a moving potato, its total mass stays the same as it moves. Often, the motion of a fluid is quantified by its flow rate instead of its velocity. We will see that flow rate obeys an important law described near the end of this lesson, which is what makes it an important quantity.

The flow rate of water from a hose is one factor that affects how quickly a fire is extinguished.

Definition of Flow Rate for a Pipe

One of the most useful examples where flow rate can be calculated easily is a moving fluid in an enclosed volume (like a pipe), so we will discuss this specific example at length. Many situations in scientific and engineering applications are like fluid flow in a pipe, with the fluid moving uniformly in one direction on average through a contained volume. Some examples are blood flow in your circulatory system and air flow in a building's ventilation system.

Fluid flowing to the right in a circular pipe
Fluid flowing in a circular pipe.

The figure shows a fluid moving in a pipe. Let's assume for simplicity that everywhere within the pipe the fluid has the same velocity (i.e., near the edges of the pipe, the fluid has the same speed and direction of flow as the fluid near the center of the pipe). The direction of the velocity is toward the right, and the speed of the fluid is v. Because the fluid is moving in the same way everywhere in the pipe, this is an example of what is called a uniform flow.

The flow rate within the pipe is defined as the volume of fluid each second that is passing through a cross-sectional slice of the pipe, which is shown in the figure as a dotted circle. It turns out that under the assumption of uniform flow within the pipe, the flow rate, which is often represented by the symbol Q, can be directly related to the fluid speed v and a measure of the size of the slice -- the pipe's cross-sectional area A. In terms of these variables, the flow rate Q can be expressed as

Formula for flow rate
Flow rate formula

where the units of Q are volume per time, or cubic meters per second in Standard International (SI) units. When applying this equation, your answer will only be in SI units if your area and speed are expressed in SI units: square meters and meters per second, respectively.

This formula says the flow rate of a pipe is greater for fluid that is passing more quickly through the pipe, and it is also greater when a pipe's cross-sectional area is larger. In the diagram, the cross-sectional area A is the area of the circle you could measure if you sliced open the pipe vertically, which is perpendicular to the velocity of the fluid. The cross-sectional slice of the pipe does not have to be circular, but in this diagram it is. A pipe carrying fluid can be any shape in general, but the meaning of A in the formula is always the same - it is the area of a slice that the fluid is flowing through. It is important to remember the reason why flow rate depends on A: wider pipes can deliver larger volumes of fluid at a higher rate than narrower pipes.

Non-Uniform Flows

In real-life applications, most of the time the uniform flow assumption we used in the previous example is not entirely true. For example, near the inner surface of the pipe friction forces exist which restrict the fluid's motion. Consequently, fluid near the center of the pipe, where friction forces are weaker, moves with higher speed compared to fluid near the outer edge, so the flow is said to be non-uniform. In a non-uniform flow, the speed of the fluid depends on where exactly in the pipe's cross-section you are measuring. Fortunately, the flow rate formula above still works for non-uniform flows when we use the average speed of the fluid over the cross-section of the pipe for the variable v.

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