Short Answer:
A shock wave is a sharp and sudden disturbance that occurs in a compressible fluid, such as air, when an object moves faster than the local speed of sound. It is characterized by an almost instantaneous change in pressure, temperature, and density of the fluid.
In a shock wave, the fluid properties change discontinuously, and energy is lost due to irreversible compression and heating of the gas. Shock waves are commonly observed in supersonic aircraft, explosions, and rocket exhausts, where they appear as a thin region separating high-pressure and low-pressure zones.
Detailed Explanation:
Shock Wave
A shock wave is a very thin region in a compressible flow field where the fluid properties such as pressure, density, temperature, and velocity change suddenly. It occurs when a flow undergoes a rapid and intense compression caused by an object or disturbance moving through the medium faster than the speed of sound.
In a normal flow, pressure and velocity vary smoothly. However, when the flow becomes supersonic (Mach number > 1), any disturbance or obstacle cannot transmit pressure information upstream. As a result, the flow compresses abruptly, forming a shock wave.
Definition
A shock wave is defined as a thin region in a compressible fluid where the fluid properties (pressure, temperature, and density) change suddenly and irreversibly due to high-speed compression.
It represents a discontinuity in the flow field where the conservation of mass, momentum, and energy still holds true, but the process is non-isentropic, meaning there is an increase in entropy.
Formation of Shock Waves
When an object moves at subsonic speeds (less than the speed of sound), the pressure disturbances move ahead smoothly in the form of sound waves.
As the object speed increases and approaches the speed of sound (Mach 1), these waves begin to compress and merge together in front of the object. Once the object exceeds the speed of sound, these waves combine into a single, strong shock front, known as a shock wave.
This shock wave moves through the air faster than sound and causes a sudden rise in pressure, temperature, and density across it.
Types of Shock Waves
- Normal Shock Wave:
- Occurs perpendicular to the flow direction.
- The flow changes from supersonic to subsonic across the shock.
- Common in nozzles and diffusers.
- Oblique Shock Wave:
- Inclined to the direction of flow.
- The flow direction changes along with pressure and density.
- Found on the surfaces of supersonic aircraft and wedges.
- Bow Shock Wave:
- A curved shock wave that forms in front of blunt bodies, such as spacecraft or missiles, moving at supersonic speeds.
- Protects the surface from direct impact of high-pressure flow.
- Detached Shock Wave:
- Occurs when the shock is separated from the body’s surface due to high Mach numbers.
- Common in high-speed reentry vehicles.
Properties of Shock Waves
- Thinness:
- The shock wave thickness is extremely small, often a few molecular mean free paths (about 10⁻⁶ meters).
- Irreversibility:
- The process is irreversible due to viscous and thermal effects, resulting in entropy increase.
- Sudden Changes:
- There is an abrupt rise in pressure, temperature, and density and a fall in velocity across the shock.
- Non-Isentropic Process:
- Unlike isentropic flow, shock waves involve energy dissipation and loss due to compression and heat generation.
- Entropy Increase:
- The entropy of the fluid increases significantly as the shock wave passes.
Governing Equations
The properties across a shock wave are governed by the Conservation Laws of:
- Mass:
- Momentum:
- Energy:
Here,
- Subscript 1 = upstream (before shock),
- Subscript 2 = downstream (after shock).
These equations, combined with the perfect gas laws, determine the relationships between temperature, pressure, and density across a shock wave.
Relations for Normal Shock Wave
For a normal shock, the relations between upstream and downstream conditions are as follows:
where:
- = specific heat ratio,
- = Mach number before the shock,
- = Mach number after the shock.
These relations show that after the shock, the flow becomes subsonic and experiences a sharp rise in pressure and temperature.
Physical Effects of Shock Waves
- Temperature Rise:
- The fluid temperature increases significantly due to compression.
- Pressure Rise:
- Static pressure increases suddenly as kinetic energy is converted into internal energy.
- Density Increase:
- Fluid density increases as molecules are compressed.
- Velocity Drop:
- The velocity decreases rapidly after the shock.
- Entropy Increase:
- Entropy increases due to irreversibility and energy dissipation.
Real-World Examples of Shock Waves
- Supersonic Aircraft:
- Airplanes traveling faster than sound generate shock waves that create a sonic boom.
- Rocket and Jet Exhausts:
- High-speed exhaust gases produce oblique shock patterns visible as bright diamond-shaped structures.
- Explosion Waves:
- Explosions in air or other media generate strong spherical shock waves propagating outward.
- Reentry Vehicles:
- Spacecraft reentering Earth’s atmosphere encounter strong bow shocks due to high Mach numbers.
- Shock Tubes:
- Used in laboratories to study high-speed flow and gas dynamic properties.
Difference Between Isentropic and Shock Flow
| Parameter | Isentropic Flow | Shock Wave |
| Heat Transfer | None | None |
| Entropy | Constant | Increases |
| Pressure/Temperature Change | Gradual | Sudden |
| Reversibility | Reversible | Irreversible |
| Flow Type | Smooth and continuous | Discontinuous |
Conclusion
A shock wave is a very thin, high-intensity compression front that forms when a flow transitions from supersonic to subsonic speed. It causes an abrupt increase in pressure, temperature, and density, accompanied by a loss of total energy due to irreversibility. Shock waves are fundamental in high-speed aerodynamics, propulsion systems, and explosions. Understanding their behavior helps engineers design efficient nozzles, diffusers, and aerospace vehicles capable of withstanding extreme flow conditions.