# Compressible Fluids vs. Incompressible Fluids: What's the Difference?

Edited by Aimie Carlson || By Janet White || Published on March 17, 2024
Compressible fluids can change volume under pressure, while incompressible fluids have a constant volume regardless of pressure.

## Key Differences

Compressible fluids can undergo significant volume changes when subjected to pressure. In contrast, incompressible fluids exhibit negligible volume change under pressure, maintaining a nearly constant volume.
The density of compressible fluids can vary with pressure and temperature. Incompressible fluids, however, have a density that is essentially constant and independent of pressure variations.
Compressible fluids are often dealt with in aerodynamics and gas dynamics, where changes in pressure and velocity are significant. Incompressible fluids are commonly assumed in hydraulic systems and liquid flow applications where density remains constant.
The speed of sound in compressible fluids is finite and depends on the fluid's properties. In incompressible fluids, the speed of sound is theoretically infinite, meaning pressure waves propagate instantaneously.
The equations governing the motion of compressible fluids are more complex, including terms for density changes. For incompressible fluids, the equations are simplified by assuming constant density.

## Comparison Chart

### Volume Change

Can change significantly
Negligible change

### Density Variation

Varies with pressure/temperature
Nearly constant

### Applications

Aerodynamics, gas dynamics
Hydraulics, liquid flow

### Speed of Sound

Finite, depends on properties
Theoretically infinite

### Equations of Motion

More complex, include density changes
Simplified, constant density

## Compressible Fluids and Incompressible Fluids Definitions

#### Compressible Fluids

Volume changes under pressure.
Air is a compressible fluid that expands when heated.

#### Incompressible Fluids

No significant density variation.
Glycerin is an incompressible fluid used in cosmetics.

#### Compressible Fluids

Significant volume change.
Natural gas is a compressible fluid used for heating.

#### Incompressible Fluids

Constant density.
Oil is an incompressible fluid used in engines.

#### Compressible Fluids

Density varies with conditions.
Steam is a compressible fluid used in turbines.

#### Incompressible Fluids

Used in liquid flow applications.
Mercury is an incompressible fluid used in thermometers.

#### Compressible Fluids

Affected by temperature and pressure.
Carbon dioxide is a compressible fluid used in fire extinguishers.

#### Incompressible Fluids

Negligible volume change.
Water is an incompressible fluid used in hydraulic systems.

#### Compressible Fluids

Used in gas dynamics.
Helium is a compressible fluid used in balloons.

#### Incompressible Fluids

Assumed in hydraulic systems.
Brake fluid is an incompressible fluid used in vehicles.

## FAQs

#### What is an incompressible fluid?

An incompressible fluid is a fluid whose volume remains nearly constant under pressure.

#### Can liquids be compressible?

Liquids are typically considered incompressible, but under extremely high pressures, they can exhibit slight compressibility.

#### Why are air and other gases considered compressible?

Gases are compressible because their molecules are relatively far apart, allowing for significant volume reduction under pressure.

#### Why are hydraulic systems designed with incompressible fluids?

Incompressible fluids are used in hydraulic systems because their constant volume ensures predictable and stable operation.

#### How do compressible fluid equations differ from those for incompressible fluids?

Compressible fluid equations account for density changes, while incompressible fluid equations assume constant density.

#### What is a compressible fluid?

A compressible fluid is a fluid whose volume can change significantly under pressure.

#### What are some applications of incompressible fluids?

Incompressible fluids are used in hydraulic systems, liquid flow applications, and in many engineering designs.

#### Are all gases compressible?

Yes, gases are generally considered compressible fluids.

#### What is the significance of density variation in compressible fluids?

Density variation in compressible fluids is significant because it affects the fluid's behavior under changing pressure and temperature conditions.

#### What are some applications of compressible fluids?

Compressible fluids are used in aerodynamics, gas dynamics, and various industrial processes.

#### Why is the study of compressible fluids important in aerodynamics?

The study of compressible fluids is crucial in aerodynamics to understand the behavior of air at high speeds and varying pressures.

#### How do pressure waves propagate in compressible and incompressible fluids?

In compressible fluids, pressure waves propagate at the speed of sound, while in incompressible fluids, they propagate instantaneously.

#### How does the speed of sound differ in compressible and incompressible fluids?

The speed of sound is finite in compressible fluids and theoretically infinite in incompressible fluids.

#### Can the compressibility of a fluid affect its flow characteristics?

Yes, the compressibility of a fluid can significantly affect its flow characteristics, especially at high speeds or under varying pressure conditions.

#### Are water and oil always considered incompressible?

While generally considered incompressible, water and oil can exhibit slight compressibility under extreme pressures.

#### Are there any fluids that are perfectly incompressible?

No real fluid is perfectly incompressible; the assumption of incompressibility is an approximation used for simplicity.

#### How does temperature affect the compressibility of fluids?

Temperature can affect the compressibility of fluids, with higher temperatures generally increasing the compressibility of gases.

#### Can the assumption of incompressibility lead to errors in calculations?

Yes, assuming incompressibility in situations where the fluid exhibits compressibility can lead to errors in calculations.

#### Can the compressibility of a fluid be measured?

Yes, the compressibility of a fluid can be measured using various experimental techniques and is often expressed as a compressibility factor.

#### Are there any special considerations for handling compressible fluids in engineering?

Yes, engineering designs involving compressible fluids must account for changes in density, pressure, and temperature to ensure safe and efficient operation.