17.8 Shock Waves
3 min read•june 24, 2024
Shock waves are fascinating phenomena that occur when objects move faster than the speed of sound. They create powerful disturbances in the air, causing sonic booms and dramatic changes in , , and .
From aircraft to bullets and explosions, shock waves play a crucial role in many high-speed events. Understanding their formation, properties, and effects is essential for designing faster vehicles and predicting the impact of supersonic travel.
Shock Waves
Formation of sonic booms
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- Object moving faster than speed of sound compresses medium in front of it creates a
- forms cone-shaped region called
- Mach cone angle depends on object's speed relative to speed of sound ()
- Higher Mach numbers result in narrower Mach cones (supersonic aircraft)
- Shock waves from different parts of object merge into single shock wave
- Merging occurs at a specific distance from object determined by its size and speed
- Sonic booms perceived as loud, explosive noise
- Intensity depends on factors such as aircraft size, altitude, and speed (, )
- Can cause vibrations and potentially damage structures (windows, roofs)
Properties of shock waves
- Abrupt changes in pressure, density, and temperature occur when disturbance moves faster than local speed of sound
- Thin, non-linear waves with nearly discontinuous change in properties
- Thickness of shock wave is on the order of of molecules in medium
- Propagate at supersonic speeds relative to undisturbed medium
- Supersonic speed is necessary condition for shock wave formation (bullets, explosions)
- Cause irreversible changes in medium leading to increase
- Irreversibility is due to dissipative processes such as viscosity and heat conduction
- In gases, shock waves compress the gas increasing its pressure, density, and temperature
- Gas properties change according to
- Conservation of mass, momentum, and energy across shock wave ()
- Gas properties change according to
- In liquids and solids, shock waves behave similarly but with some differences due to medium's compressibility ()
- In solids, shock waves can cause and (meteorite impacts, explosives)
Structure of bow wakes
- Bow wakes form when object moves through fluid at speed greater than wave speed in medium
- Wave speed depends on fluid properties such as density and compressibility (water, air)
- Object displaces fluid creating disturbance that propagates outward
- Disturbance forms wave front ahead of object
- Wave front angle depends on object's speed relative to wave speed ()
- Higher Froude numbers result in narrower wave front angles (ships, boats)
- Disturbance forms wave front ahead of object
- consists of series of waves that spread out from object
- Wave pattern depends on object's shape and speed
- Blunt objects create more pronounced than streamlined objects (barges vs. sailboats)
- In shallow water, bow wakes can cause significant wave heights and potentially erode shorelines (coastal areas, rivers)
- Wave pattern depends on object's shape and speed
- Bow wakes can also form in supersonic flows around objects
- In this case, bow wake is part of the shock wave system generated by object (re-entry vehicles)
Advanced shock wave phenomena
- occur when shock waves form at an angle to the flow direction
- are regions of decreasing pressure and density in supersonic flow
- happens when shock waves interact with solid boundaries or other shock waves
- occurs when shock waves encounter obstacles or openings, causing bending of the wave front
Key Terms to Review (31)
Bow wake: A bow wake is a wave pattern created by an object moving through a medium faster than the waves can propagate in that medium. It is commonly observed in water with boats but also occurs in air with supersonic aircraft.
Bow Wake: A bow wake is a disturbance in the water caused by the forward motion of an object, such as a boat or ship, moving through the water. It is a type of shock wave that forms at the front of the moving object and propagates outward from the object's path.
Cerenkov radiation: Cerenkov radiation is the emission of light when a charged particle travels through a dielectric medium at a speed greater than the phase velocity of light in that medium. This results in a characteristic blue glow.
Compressible Flow: Compressible flow refers to the study of fluid dynamics in which the density of the fluid changes significantly as it flows. This is in contrast to incompressible flow, where the fluid's density remains relatively constant. Compressible flow is particularly relevant in the study of shock waves, which are the focus of section 17.8 in the course.
Concorde: Concorde was a turbojet-powered supersonic passenger airliner that operated from 1976 until 2003, known for its ability to travel faster than the speed of sound, which is approximately 343 meters per second or 1,125 kilometers per hour at sea level. This aircraft showcased groundbreaking aerodynamics and engineering, allowing it to create shock waves as it exceeded Mach 1, making it a significant milestone in aviation history.
Density: Density is a fundamental physical property that describes the mass per unit volume of a substance. It is a measure of how much matter is packed into a given space and is a crucial concept in understanding the behavior of fluids, solids, and gases across various physics topics.
Entropy: Entropy is a measure of the disorder or randomness in a system. It represents the amount of energy in a system that is not available for useful work, but instead is dissipated as heat. Entropy is a fundamental concept in thermodynamics that is closely related to the flow and transformation of energy.
Expansion Waves: Expansion waves are disturbances that propagate through a medium, such as a gas or a fluid, in which the pressure and density decrease as the wave travels. These waves are characterized by a gradual decrease in pressure and an increase in velocity, in contrast to shock waves, which involve an abrupt change in these properties.
Froude number: The Froude number is a dimensionless quantity used in fluid mechanics to compare inertial forces to gravitational forces in a fluid flow. It is defined as the ratio of the flow's velocity to the square root of the product of gravitational acceleration and a characteristic length. This number is crucial for understanding wave patterns and flow regimes, especially in open channel flows and ship hydrodynamics.
Linear mass density: Linear mass density is the measure of mass per unit length of a one-dimensional object, such as a string or rod. It is typically denoted by the symbol $\lambda$ and expressed in units of kg/m.
Mach Cone: The Mach cone is a shock wave pattern that forms around an object moving at supersonic speeds through a fluid, such as air. It is named after the Austrian physicist and philosopher Ernst Mach, who studied the properties of these conical shock waves.
Mach number: Mach number is the ratio of the speed of an object to the speed of sound in the surrounding medium. It is a dimensionless quantity used in fluid dynamics and aerodynamics.
Mach Number: The Mach number is a dimensionless quantity that represents the ratio of the speed of an object moving through a fluid (such as air or water) to the speed of sound in that same fluid. It is a fundamental concept in the study of fluid dynamics and is particularly important in the context of shock waves.
Mean Free Path: The mean free path is the average distance a particle, such as a molecule or an atom, travels between successive collisions in a gas or a liquid. It is a fundamental concept in the study of transport phenomena, particularly in the context of shock waves.
Normal Shock Waves: Normal shock waves are sudden changes in pressure, temperature, and density that occur when a supersonic flow encounters an obstacle and transitions to subsonic flow. These shock waves are characterized by their ability to compress the flow, resulting in a sharp increase in pressure and a corresponding decrease in velocity. Understanding normal shock waves is essential for analyzing high-speed aerodynamic phenomena and the behavior of compressible fluids.
Oblique Shock Waves: Oblique shock waves are a type of shock wave that forms at an angle to the flow of a fluid or gas, typically in supersonic or hypersonic flows. These shock waves are characterized by their angled formation, which differs from the normal, perpendicular shock waves that occur in certain flow conditions.
Phase transitions: Phase transitions refer to the changes in the state of matter that occur when a substance alters between different phases, such as solid, liquid, and gas. These transitions are characterized by the energy exchange and the distinct thermodynamic properties that define each phase, influencing how materials behave under varying conditions like temperature and pressure.
Plastic Deformation: Plastic deformation is a permanent change in the shape or size of a material due to the application of external forces, where the material does not return to its original form when the forces are removed. This irreversible alteration of a material's structure is a key concept in understanding the behavior of materials under stress and strain.
Pressure: Pressure is a measure of the force applied per unit area, representing the amount of force exerted on a surface or object. This concept is fundamental in understanding various physical phenomena and principles, including mass and weight, hydraulic systems, fluid dynamics, sound propagation, and shock waves.
Rankine-Hugoniot Conditions: The Rankine-Hugoniot conditions are a set of equations that describe the conservation of mass, momentum, and energy across a shock wave. These conditions are fundamental in the analysis and understanding of shock wave phenomena, which are important in various fields such as fluid dynamics, aerodynamics, and astrophysics.
Schlieren photography: Schlieren photography is an optical technique used to visualize changes in the refractive index of transparent media, which is particularly useful for studying shock waves and other phenomena involving gas flow. This method allows observers to see variations in density, often created by heat or pressure changes, revealing details that are otherwise invisible to the naked eye. In the context of shock waves, schlieren photography becomes essential for capturing the rapid changes in air density that occur when an object travels faster than the speed of sound.
Shock Tube: A shock tube is a device used to generate and study shock waves, which are abrupt changes in pressure, density, and temperature that propagate through a medium at a speed greater than the speed of sound. It is a fundamental tool in the field of fluid dynamics and is particularly important for understanding the behavior of gases under high-pressure and high-temperature conditions.
Shock wave: A shock wave is a type of propagating disturbance that moves faster than the local speed of sound in the medium. It is characterized by an abrupt, nearly discontinuous change in pressure, temperature, and density of the medium.
Shock Wave: A shock wave is a type of propagating disturbance that travels through a medium, such as a gas, liquid, or solid, at a speed greater than the local speed of sound in that medium. It is characterized by an abrupt, nearly discontinuous change in the pressure, density, and temperature of the medium.
Shock Wave Diffraction: Shock wave diffraction is the phenomenon that occurs when a shock wave encounters an obstacle or a change in the medium, causing the shock wave to bend and spread around the obstacle or the boundary. This process is governed by the principles of wave propagation and the conservation of mass, momentum, and energy.
Shock Wave Reflection: Shock wave reflection is the phenomenon that occurs when a shock wave, a type of disturbance that travels faster than the speed of sound, encounters a boundary or surface and is reflected back. This process is an important concept in the study of shock waves and their behavior.
Sonic boom: A sonic boom is a loud explosive noise caused by the shock waves created when an object travels through the air faster than the speed of sound. These shock waves produce a sudden change in pressure, resulting in a characteristic 'boom' sound.
Sonic Boom: A sonic boom is the loud sound caused by the shock waves created when an object, such as an aircraft, travels through the air faster than the speed of sound. It is a phenomenon associated with the Doppler effect and shock waves, and occurs when the object's speed exceeds the speed of sound.
SR-71 Blackbird: The SR-71 Blackbird is a legendary high-altitude, long-range, Mach 3+ strategic reconnaissance aircraft that was operated by the United States Air Force. Its unique design and advanced technologies made it one of the fastest and most capable aircraft ever built, with the ability to outrun and outmaneuver most threats.
Supersonic: Supersonic refers to the speed of an object, such as an aircraft or projectile, that exceeds the speed of sound. This is a significant threshold in fluid dynamics, as it marks the transition from subsonic to transonic and then supersonic flow regimes, each with distinct characteristics and challenges.
Temperature: Temperature is a measure of the average kinetic energy of the particles in a substance, reflecting the degree of hotness or coldness of an object or environment. It is a fundamental concept in physics that is essential for understanding various physical phenomena.