The Basic Fundamental Concepts and the Law of Conservation in Aerodynamics

About the Study

Aerodynamics is the study of how air interacts with objects in motion, such as aircraft, rockets, and automobiles. It is a branch of fluid dynamics that deals with the motion of air and other gases, and the forces they generate. The rules of aerodynamics explain how an object can achieve lift, which is essential for flying. Studying the motion of air around an object allows engineers and scientists to understand and measure the forces involved in flight or other forms of motion. Aerodynamics also involves the study of internal flow through passages in solid objects, such as the airflow through an engine or a wind tunnel.

Fundamental concepts

Recognizing the velocity of air around an item allows you to calculate the forces and moments acting on it. The forces of importance in many aerodynamics issues are the fundamental forces of flight: Lift, drag, thrust, and weight. Lift and drag are aerodynamic forces, or forces caused by air movement over a solid mass. These variables are frequently calculated on the premise that the flow field acts as a continuum. Flow velocity, pressure, density, and temperature are parameters of continuum flow fields that can vary with place and time. These attributes can be observed directly or indirectly in aerodynamics experiments, or they can be computed using the equations for mass, momentum, and energy conservation in air flows. Flow fields are classified using density, flow velocity, and an extra attribute, viscosity.

Flow classification: Flow velocity is used to characterize flows based on their speed regime. Subsonic flows are flow fields with an air speed field that is always slower than the local speed of sound. Transonic flows encompass both subsonic and supersonic flow zones where the local flow speed exceeds the local sound speed. Supersonic flows are those in which the flow speed exceeds the speed of sound everywhere. A fourth categorization, hypersonic flow, refers to flows with flow speeds that are significantly faster than the speed of sound. Aerodynamicists dispute on how to define hypersonic flow.

Compressible flow takes into consideration the flow's fluctuating density. Subsonic flows are frequently idealized as incompressible, implying that the density is constant. Transonic and supersonic flows are compressible, and computations that ignore density changes in these flow fields will produce incorrect findings. The frictional forces in a flow are connected with viscosity. Viscous effects are so minor in some flow fields that approximate solutions can safely ignore them. These approximations are referred to as inviscid flows. Viscous flows are those in which viscosity is not ignored. Finally, the flow environment may be used to categories aerodynamic difficulties. Internal aerodynamics is the study of flow via channels inside solid objects, whereas external aerodynamics is the study of flow surrounding solid objects of diverse forms.

Continuum assumption: Gases, unlike liquids and solids, are made up of distinct molecules that take up just a small portion of the volume of the gas. Flow fields are formed by the collisions of numerous individual gas molecules with one other and with solid surfaces at the molecular level. The discrete molecular character of gases, however, is overlooked in most aerodynamics applications, and the flow field is supposed to operate as a continuum. Because of this assumption, fluid parameters such as density and flow velocity may be specified anywhere within the flow.

The continuum assumption's validity is determined on the density of the gas and the application in issue. The mean free route length must be substantially less than the length scale of the application in issue for the continuum assumption to be true. Many aerodynamics applications, for example, deal with aircraft flying in atmospheric circumstances with mean free path lengths on the order of micrometers and bodies orders of magnitude bigger. In these circumstances, the aircraft's length scale ranges from a few meters to a few tens of meters, which is significantly greater than the mean free path length. The continuum assumption is suitable for such applications.

For extremely low-density flows, such as those experienced by vehicles at very high altitudes or satellites in Low Earth orbit, the continuity assumption is less applicable. In such instances, statistical mechanics is a more precise way of problem resolution than continuum aerodynamics. The Knudsen number can be used to help decide between statistical mechanics and continuous aerodynamics.

Conservation laws

The assumption of a fluid continuum enables the use of fluid dynamics conservation rules to address issues in aerodynamics. There are three conservation principles that are used:

Conservation of mass: The conservation of mass principle states that mass cannot be generated or destroyed inside a flow; the mass continuity equation is the mathematical articulation of this idea.

Conservation of momentum: This principle's mathematical version might be thought of as an application of Newton's Second Law. External forces, such as viscous forces, and body forces, such as weight, are the only things that can modify the momentum of a flow. The momentum conservation principle can be written as a vector equation or as a combination of three scalar equations.

Conservation of energy: Energy is neither generated nor destroyed inside a flow, according to the energy conservation equation, and any addition or subtraction of energy to a volume in the flow is caused by heat transfer or work into and out of the region of concern.

These equations are known collectively as the Navier-Stokes equations, while other writers define the phrase to include simply the momentum equation. The Navier-Stokes equations have no known analytical solution and are solved computationally in current aerodynamics. Because high-speed computing methods were not previously accessible, and the high computational cost of solving these complicated equations now that they are, simplifications of the Navier-Stokes equations have been and continue to be used. The Euler equations are a collection of comparable conservation equations that ignore viscosity and can be employed when the influence of viscosity is predicted to be minor. Additional reductions result in Laplace's equation and potential flow theory.

Moreover, Bernoulli's equation is a one-dimensional solution to both the momentum and energy conservation equations. The ideal gas law or another type of state equation is frequently used in conjunction with these equations to build a determined system that permits the solution for the unknown variables.