Short Answer:

Subsonic, supersonic, and hypersonic flows are types of fluid motion classified based on the Mach number (M), which is the ratio of the flow velocity to the speed of sound in the medium.

In subsonic flow (M < 1), the flow velocity is less than the speed of sound. In supersonic flow (1 < M < 5), the velocity is greater than the speed of sound, and shock waves are formed. In hypersonic flow (M > 5), the velocity is much higher than the speed of sound, and strong shock waves and high temperatures occur. These classifications help understand the compressibility effects in fluid motion.

Detailed Explanation:

Subsonic, Supersonic, and Hypersonic Flows

The motion of a fluid or an object moving through a fluid can be categorized based on the Mach number (M), which is defined as the ratio of the velocity of flow () to the local speed of sound ():

This dimensionless number helps determine whether the flow is incompressible or compressible, and whether it exhibits phenomena like shock waves, expansion fans, or temperature rise. The three important types of compressible flows are subsonic, supersonic, and hypersonic flows.

These classifications are essential in aerodynamics, propulsion systems, and high-speed flow studies, as the behavior of gases changes dramatically with increasing Mach number.

Subsonic Flow (M < 1)

In subsonic flow, the velocity of the fluid is less than the speed of sound in that medium. This means that any small disturbance, such as a change in pressure or temperature, can move ahead of the object because sound waves travel faster than the object itself.

Characteristics of Subsonic Flow:

Mach number
The density of the fluid remains nearly constant, so compressibility effects are negligible.
Pressure and temperature changes are small compared to supersonic flow.
Streamlines are smooth and continuous.
Examples include airflow around automobiles, ships, and low-speed aircraft.

Engineering Example:
A passenger aircraft like a commercial jet flying below the speed of sound (around 800 km/h) operates in the subsonic range.

Flow Nature:
Since the Mach number is less than 1, the flow is smooth, stable, and nearly incompressible. Therefore, basic fluid mechanics (incompressible flow theory) is often sufficient to analyze such flows.

Supersonic Flow (1 < M < 5)

In supersonic flow, the velocity of the fluid or object is greater than the local speed of sound. In this case, disturbances cannot move ahead of the object, and the flow becomes highly compressible. This results in sudden changes in pressure, density, and temperature, leading to the formation of shock waves.

Characteristics of Supersonic Flow:

Mach number
Density and pressure variations are large.
Flow is compressible, and the behavior of the gas changes significantly.
Shock waves appear ahead or around the object, depending on its shape and speed.
Temperature increases sharply across shock waves.

Engineering Example:
Fighter jets such as the F-16 and missiles operate in the supersonic range.

Flow Nature:
The flow becomes complex due to the appearance of oblique and normal shocks. The air compresses in front of the object, causing drag and temperature rise. Therefore, designs must account for these effects by using sharp-nosed geometries and heat-resistant materials.

Hypersonic Flow (M > 5)

In hypersonic flow, the velocity is much greater than the speed of sound — typically more than five times the speed of sound. This regime involves extreme compressibility effects, very high temperatures, and complex flow phenomena.

Characteristics of Hypersonic Flow:

Mach number
Extremely high temperature due to kinetic energy conversion into heat.
Formation of strong bow shocks and shock layers around the body.
Chemical and thermal reactions, including air dissociation and ionization, can occur.
Material heating becomes a major design concern.

Engineering Example:
Reentry vehicles, space shuttles, and hypersonic missiles (like scramjets) operate in this range.

Flow Nature:
At hypersonic speeds, air molecules near the surface can break apart (dissociation) or become ionized, forming a plasma layer. This causes intense heating and requires the use of special materials and cooling systems to protect the structure.

Comparison of Flow Regimes

Flow Type	Mach Number Range	Main Characteristics	Example
Subsonic	M < 1	Incompressible, smooth flow	Car, low-speed aircraft
Supersonic	1 < M < 5	Shock waves, high compressibility	Fighter jet
Hypersonic	M > 5	Strong shocks, very high temperature	Spacecraft reentry

(Note: The comparison is explained textually for clarity, not presented as a table in the final format.)

Importance of Flow Classification

Understanding these flow regimes helps engineers design and analyze systems involving gases at different velocities. The main areas of importance include:

Aerodynamics:
- Determines lift, drag, and stability of aircraft at different speeds.
Nozzle and Diffuser Design:
- The flow regime decides the nozzle geometry for jet engines and rockets.
Heat Management:
- At high Mach numbers, heat transfer becomes a critical design factor.
Shock Control:
- Helps manage or minimize shock waves for better efficiency.
Material Selection:
- High-speed flows require materials resistant to high temperature and stress.

Example Calculation

If an aircraft is moving at a velocity of and the local speed of sound in air is , then:

This means the aircraft is moving at Mach 3, which corresponds to supersonic flow.

Similarly, if a reentry vehicle travels at :

This represents hypersonic flow.

Conclusion

Subsonic, supersonic, and hypersonic flows are classifications of fluid motion based on the Mach number, which compares flow velocity to the speed of sound. In subsonic flow (M < 1), compressibility is negligible. In supersonic flow (1 < M < 5), density and pressure change rapidly, and shock waves form. In hypersonic flow (M > 5), extreme temperature, strong shocks, and ionization effects dominate. Understanding these regimes is crucial in designing aircraft, rockets, and propulsion systems that operate efficiently and safely at different speeds.