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What is aerodynamics?

By, CoolMotor

• 2023-12-29
• 18 View

Aerodynamics is a branch of fluid mechanics that mainly studies the various forces generated when objects move in air or other gases.

-Aerodynamics is the applied mechanics of fluid mechanics in engineering, especially discussing the flow field situation with Mach number greater than 0.3.

-Aerodynamics: Because the conditions discussed are close to real fluids and the viscosity, compressibility, three-dimensional motion and other characteristics of real fluids are taken into consideration, the calculation equations obtained are relatively complex, usually in the form of nonlinear partial differential equations. In most cases, it is difficult to obtain analytical solutions to this kind of equation. In addition, early computing technology was still relatively backward, so at that time, most of the required data were obtained experimentally.

-With the rapid development of computer technology, it has become possible to use computers to perform a large number of numerical operations to solve aerodynamic equations. Using numerical methods and computational fluid dynamics methods, we can find numerical solutions to nonlinear partial differential equations and obtain various required data, thus saving a lot of experimental costs. Due to the continuous improvement of mathematical models and the continuous improvement of computer computing capabilities, it is now possible to use computer simulations of flow fields to replace some aerodynamic experiments.

Aerodynamics in other fields

In addition to aerospace, aerodynamics also has very important applications in other fields. It is an important factor in the design of all means of transportation, including cars. Large buildings involve wind loads, intraurban aerodynamics studies the microclimate environment of cities, and ambient aerodynamics studies atmospheric circulation and the effects of flight on ecosystems. The heat flow and internal flow involved in engine design are also a very important aspect of aerodynamics.

Continuity assumption

Gases are composed of molecules that are constantly in thermal motion and colliding with each other on a microscopic scale. In aerodynamics, however, gases are assumed to be continuous. This is because the various properties of gases such as density, pressure, temperature, and velocity are well defined at infinitesimal points and change continuously from one point to another. The discreteness and atomicity of gases can be ignored, so from a macro perspective, gases can be regarded as continuous substances. Of course, when the gas is very thin, the continuity assumption no longer holds, and statistical mechanics research is a better choice at this time.

Conservation law

The solution of aerodynamic problems relies on the conservation of gases in three aspects:

Conservation of mass: This law only breaks down when the speed of the gas is high enough that relativistic effects must be taken into account.

Conservation of momentum: derived from Newton's second law.

Conservation of energy: When viscosity is not considered, it is the conservation of mechanical energy; when viscosity must be considered, it is the conservation of mechanical energy and thermal energy.

Boundary layer flow

The boundary layer (also called boundary layer) is a very important concept. In 1904, the famous German scientist Prandtl first proposed the concept of boundary layer. It comes from the basic fact that under normal circumstances, the viscosity or friction of air only plays a major role in a very thin area close to the surface of the object. Leaving this area, the influence of viscosity drops sharply. We call such a small area the boundary layer.

The introduction of the concept of boundary layer makes many previously difficult problems solvable because we only need to consider the influence of viscosity in a small area to solve the Navier-Stokes equations. In other areas, only the potential flow needs to be solved or the Euler equation describing the motion of inviscid fluids needs to be solved. It is well known that the potential flow and Euler equations are much less difficult to solve than the Navier-Stokes equations.

Subsonic aerodynamics

Subsonic aerodynamics are widely used in racing and some commercial vehicle designs

When the fluid flows faster than the speed of sound, we call it subsonic flow. Furthermore, when the Mach number (i.e., the ratio of the fluid speed to the speed of sound) is less than 0.3, the compressibility of the gas is negligible. In the early 20th century, cars were faster than the newly invented airplanes, exceeding 200 kilometers per hour earlier. At that time, the aerodynamics of racing cars were more advanced than those of airplanes.

Transonic aerodynamics

When the fluid velocity approaches or slightly exceeds the speed of sound (that is, when the Mach number is approximately equal to 1), we call it transonic flow. The typical characteristics of transonic flow are shock waves and expansion waves. Within its region, various properties of the fluid change drastically, with such magnitude that we can consider the fluid passing through the shock wave to be discontinuous.

Transonic flow is more complex than simply subsonic and supersonic.

Supersonic aerodynamics

Supersonic aerodynamics studies what happens when flow velocities are greater than the speed of sound. For example, calculating the lift of Concorde in cruise state is a supersonic aerodynamics problem.

There are significant differences between supersonic flow and subsonic flow. At subsonic speeds, pressure fluctuations can be transmitted from the rear to the front of the flow field, but at supersonic speeds, pressure fluctuations cannot be transmitted to the upstream. In this way, the changes in fluid properties are compressed within a very small range, forming a so-called shock wave.

Shock waves convert large amounts of mechanical energy into thermal energy. Accompanied by the compressible characteristics of highly viscous (refer to Reynolds number) fluids, the emergence of shock waves is the basic difference between subsonic and supersonic aerodynamics.