When considering air as the aggregate of a large number of molecules, it can be called a continuous medium. In it, individual particles can come into contact with each other. Such an idea can significantly simplify the methods of air research. In aerodynamics, there is such a thing as the reversibility of motion, which is widely used in the field of experiments for wind tunnels and in theoretical studies using the concept of air flow.
The important concept of aerodynamics
According to the principle of reversibility of motion, instead of considering the movement of a body in a stationary medium, one can consider the course of the medium with respect to the stationary body.
The speed of the incident undisturbed flow in reverse motion is equal to the speed of the body itself in still air.
For a body that moves in still air, the aerodynamic forces will be the same as for a motionless (static) body subjected to airflow. This rule works under the condition that the speed of the body in relation to air will be the same.
What is air flow and what basic concepts define it
There are various methods for studying the motion of gas or liquid particles. In one of them, streamlines are studied. With this method, the motion of individual particles must be considered at a given moment in time at a certain point in space. The directional movement of particles that move randomly is air flow (a concept widely used in aerodynamics).
The movement of the air flow will be considered steady if, at any point in the space occupied by it, the density, pressure, direction and magnitude of its velocity remain unchanged over time. If these parameters change, then the motion is considered unsteady.
The streamline is defined as follows: the tangent at each point to it coincides with the velocity vector at the same point. The totality of such streamlines forms an elementary stream. She is enclosed in a certain tube. Each individual trickle can be distinguished and presented in isolation flowing from the total air mass.
When the air flow is divided into trickles, it is possible to visualize its complex flow in space. The basic laws of motion can be applied to each individual jet. It is about saving mass and energy. Using the equations for these laws, a physical analysis of the interactions of air and a solid can be carried out.
Speed and type of movement
Regarding the nature of the flow, the air flow is turbulent and laminar. When streams of air move in one direction and are parallel to each other, this is a laminar flow. If the speed of air particles increases, then they begin to possess, in addition to translational, other rapidly changing speeds. A stream of particles perpendicular to the direction of translational movement is formed. This is an erratic turbulent flow.
The formula used to measure air velocity includes pressure determined in various ways.
The incompressible flow rate is determined using the dependence of the difference between the total and statistical pressure with respect to the air mass density (Bernoulli equation): v = √2 (p 0 -p) / p
This formula works for flows with a speed of no more than 70 m / s.
The density of air is determined by the nomogram of pressure and temperature.
The pressure is usually determined by a liquid pressure gauge.
The air flow rate will not be constant along the length of the pipeline. If the pressure decreases and the air volume increases, then it constantly increases, contributing to an increase in the speed of the particles of the material. If the flow velocity is greater than 5 m / s, then additional noise may appear in the valves, rectangular turns and gratings of the device through which it passes.
Energy indicator
The formula by which the power of the air flow (free) is determined is as follows: N = 0.5SrV³ (W). In this expression, N is the power, r is the air density, S is the area of the wind wheel under the influence of the flow (m²) and V is the wind speed (m / s).
It can be seen from the formula that the output power increases in proportion to the third power of the air flow rate. So, when the speed increases by 2 times, then the power increases by 8 times. Therefore, at low flow rates there will be a small amount of energy.
All the energy from the stream, which is created, for example, by the wind, cannot be extracted. The fact is that passing through a wind wheel between the blades occurs unhindered.
The flow of air has the energy of motion like any moving body. It has a certain supply of kinetic energy, which, as it is converted, passes into mechanical energy.
