Aerodynamics

Aerodynamics is the science that deals with the study of air movement and the actions it exerts on bodies that move immersed in it.

Summary

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• 1 Definition
• 2 Bernoulli’s theorem
• 3 Aerodynamic profile
• 4 Calculations
• 5 Aerodynamic problems
• 6 Sources

Definition

It is the branch of fluid mechanics that deals with the movement of air and other gaseous fluids, and the forces that act on the bodies that move in these fluids. As an example of the field of aerodynamics we can mention the movement of an airplane through the air, the forces that the wind exerts on a structure or the operation of a windmill, among others. The presence of an object in a gaseous fluid modifies the distribution of pressures and velocities of the particles in the fluid, causing lift and resistance forces. Modifying one of the values ​​(pressure or speed) automatically modifies the other in the opposite way.

Bernoulli’s theorem

The Bernoulli ‘s theorem was formulated in 1738 by the mathematical and physical Daniel Bernoulli and states that a decrease in fluid pressure (liquid or gas) occurs when moving speed increases. The theorem states that the total energy of a fluid system with uniform flow remains constant along the flow path. It can be shown that, as a consequence, the increase in the speed of the fluid must be compensated by a decrease in its pressure.

The theorem applies to flow over surfaces, such as the wings of an airplane or the propellers of a ship . It follows from here that:

PRESSURE + SPEED = CONSTANT This theorem can be easily proved if we take a thin strip of paper, place it next to the lips and blow. At the moment the air movement occurs, the pressure on this flow decreases and below it increases, lifting the paper strip.

Aerodynamic profile

A body that is shaped in such a way that it is possible to take full advantage of the forces caused by the variations in velocities and pressures of an air current is called an aerodynamic profile.

If we make a graphic example taking two particles that move at a speed of 90 Km / h, and with a pressure of 1 Kg / cm2, before the disturbance caused by the introduction of the aerodynamic profile. Between the upper part of the profile and the horizontal upper straight line there is a reduction in space, achieving an increase in air speed, while in the lower part of the profile the path of the particles is horizontal, without modifying the air current .

It can then be seen that the particle (1) increases its speed to 90.3 km / h (Venturi effect) and the pressure decreases to 0.7 kg / cm2 (Bernoulli effect). The particle (2), since it is not modified by the profile, maintains a speed of 90 km / h and a pressure of 1 kg / cm2. Therefore it can be seen that a pressure difference has arisen between the upper and lower face, obtaining as a result an upward force called AERODYNAMIC FORCE (F).

Calculations

By making a model of the fluid field it is possible to calculate, in almost all cases roughly, the forces and moments that act on the body or bodies immersed in the fluid field. The relationship between forces on a body moving within a fluid and speeds is given by the aerodynamic coefficients. There are coefficients that relate speed to forces and coefficients that relate speed to moment. Conceptually the simplest are the first, which give the lift force L, the aerodynamic resistance D and the lateral force Y in terms of the square of the speed (V2), the density of the fluid (ρ) and the cross-sectional area (St):

Lift coefficient

Resistance coefficient

Lateral force coefficient

Due to the complexity of the phenomena that occur and the equations that describe them, both practical tests (for example wind tunnel tests ) and numerical calculations of numerical aerodynamics are extremely useful.

Aerodynamic problems

Several classifications have been established, among which we must highlight:

• depending on your application: aeronautical aerodynamics (or simply aerodynamics) and civil aerodynamics
• depending on the nature of the fluid: compressible and incompressible
• according to the characteristic Mach number of the problem:

or subsonic (M <1: incompressible subsonic M <0.3 and compressible subsonic M <0.8) or transonic (M close to 1) or supersonic (M> 1) or hypersonic (M> 6). [1