When considering air as a combination of a large number of molecules, it can be called a continuous medium. In it, individual particles can come into contact with each other. This representation makes it possible to significantly simplify the methods of studying air. 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.
Important concept of aerodynamics
According to the principle of reversibility of motion, instead of considering the movement of a body in a stationary medium, we can consider the course of the medium in relation to a motionless body.
The speed of the incident unperturbed 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 stationary one(static) body subjected to air flow. This rule works provided that the speed of the body in relation to the air is the same.
What is airflow and what are the basic concepts of it
There are different methods for studying the movement of gas or liquid particles. In one of them, streamlines are investigated. With this method, the motion of individual particles must be considered at a given point in time at a certain point in space. The directed movement of particles that move randomly is an 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 it occupies, the density, pressure, direction and magnitude of its speed remain unchanged over time. If these parameters change, then the movement 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 jet. It is enclosed in a tube. Each individual trickle can be isolated and presented as flowing in isolation from the total air mass.
When the air stream is divided into streams, you can visualize its complex flow in space. The basic laws of motion can be applied to each individual jet. It is about the conservation of mass and energy. Using the equations for these laws, one can conduct a physical analysis of the interactions of air and a solid body.
Speed and type of movement
Regarding the nature of the flow, the air flow is turbulent and laminar. When the streams of air move in the same direction and are parallel to each other, this is a laminar flow. If the speed of air particles increases, then they begin to have, in addition to translational, other rapidly changing speeds. A stream of particles perpendicular to the direction of translational motion is formed. This is the chaotic - turbulent flow.
The formula for measuring airflow includes pressure, which is determined in many ways.
The speed of an incompressible flow is determined using the dependence of the difference between the total and static pressure in relation to the density of the air mass (Bernoulli equation): v=√2(p0-p)/p
This formula works for flows up to 70 m/s.
The air density is determined by the nomogram of pressure and temperature.
Pressure is usually measured with a liquid manometer.
The airflow rate will not be constant along the length of the pipeline. If the pressure decreases and the volume of air 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 occur in the valves, rectangular bends and gratings of the device through which it passes.
Energy indicator
The formula by which power is determinedair flow (free), 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 affected by the flow (m²) and V is the wind speed (m/s).
From the formula, it can be seen 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 flow, which creates, for example, the wind, cannot be extracted. The fact is that the passage through the wind wheel between the blades is unhindered.
The flow of air, like any moving body, has the energy of motion. It has a certain amount of kinetic energy, which, as it transforms, turns into mechanical energy.
Factors affecting airflow volume
The maximum amount of air that can be depends on many factors. These are the parameters of the device itself and the surrounding space. For example, if we are talking about an air conditioner, then the maximum air flow cooled by equipment in one minute depends significantly on the size of the room and the technical characteristics of the device. With large areas, everything is different. For them to be cooled, more intensive air flows are needed.
In fans, the diameter, rotation speed and blade size, rotation speed, material used in its manufacture are important.
BIn nature, we observe such phenomena as tornadoes, typhoons and tornadoes. These are all movements of air, which is known to contain nitrogen, oxygen, carbon dioxide molecules, as well as water, hydrogen and other gases. These are also air flows that obey the laws of aerodynamics. For example, when a vortex is formed, we hear the sounds of a jet engine.
