Exploring the Fundamentals of Incompressible and compressible Aerodynamics and it's flow

Introduction

The field of aerodynamics involves studying how air interacts with objects and is split into two branches: incompressible aerodynamics and compressible aerodynamics.This post will delve into the basic principles of both branches, discussing their uses, benefits, and constraints.

Defination of Incompressible Aerodynamics

An incompressible flow is characterized by a constant density. While all real fluids are compressible, a flow problem is often considered incompressible if the density changes in the problem have a small effect on the outputs of interest. This is more likely to be true when the flow speeds are significantly lower than the speed of sound. For higher speeds, the flow would encounter significant compressibility as it comes into contact with surfaces and slows down.

Defination of compressible Aerodynamics

According to the theory of aerodynamics, a flow is considered to be compressible if its change in density with respect to pressure is more than 5%. This means that – unlike incompressible flow – changes in density must be considered. In general, this is the case where the Mach number in part or all of the flow exceeds 0.3. The Mach 0.3 value is rather arbitrary, but it is used because gas flows with a Mach number below 0.3 demonstrate the changes in density with respect to the change in pressure of less than 5%.

Furthermore, a maximum of 5% density change occurs at the stagnation point of an object immersed in the gas flow, and the density changes around the rest of the object will be significantly lower. Transonic, supersonic, and hypersonic flows are all compressible.

We will now observe various forms of flow that are related to aerodynamics.

1.Subsonic Flow

Subsonic aerodynamics is the study of fluid motion that is slower than the speed of sound. There are several branches of subsonic flow, but one special case arises when the flow is inviscid, incompressible, and irrotational. This case is called potential flow. For this case, the differential equations used are simplified version of the governing equations of fluid dynamics, thus making a range of quick and easy solutions available to the aerodynamicist.
In solving a subsonic problem, one decision to be made by the aerodynamicist is whether to incorporate the effects of compressibility. Compressibility is a description of the amount of change of density in the problem. When the effects of compressibility on the solution are small, the aerodynamicist may choose to assume that density is constant. The problem is then an incompressible low-speed aerodynamics problem.
When the density is allowed to vary, the problem is called a compressible problem. In air, compressibility effects are usually ignored when the Mach number in the flow does not exceed 0.3. Above 0.3, the problem should be solved by using compressible aerodynamics.

2.Transonic Flow

The term transonic refers to a range of velocities just below and above the local speed of sound (generally taken as Mach 0.8–1.2). It is defined as the range of speeds between the critical Mach number, when some parts of the airflow over an aircraft become supersonic, and a higher speed,typically near Mach 1.2, when all of the airflow is supersonic. Between these speeds some of the airflow is supersonic, and some is not.

3.Supersonic Flow

Supersonic aerodynamic problems are those involving flow speeds greater than the speed of sound.Calculating the lift on the Concorde during cruise can be an example of a supersonic aerodynamic problem.
Supersonic flow behaves very differently from subsonic flow. Fluids react to differences in pressure; pressure changes in a flow field is ‘informed’ to the flow by the sound waves. It is known that sound is an infinitesimal pressure difference propagating through a fluid; therefore the speed of sound in that fluid can be considered the fastest speed that ‘information’ can travel in the flow. This difference most obviously manifests itself in the case of a fluid striking an object. In front of that object, the fluid builds up a stagnation pressure as impact with the object brings the moving fluid to rest.
In fluid travelling at subsonic speed,this pressure disturbance can propagate upstream,changing the flow pattern ahead of the object and giving the impression that the fluid ‘knows’ the object is there and is avoiding it.However, in a supersonic flow the pressure disturbance cannot propagate upstream.Thus when the fluid finally does strike the object it is forced to change its properties temperature,density,pressure and Mach number in an extremely violent and irreversible fashion across a shock wave.The presence of shock waves along with the compressibility effects of high-velocity fluids is the central difference between supersonic and subsonic aerodynamic problems.

4.Hypersonic Flow

In aerodynamics, hypersonic speeds are speeds that are highly supersonic. In the 1970s, the term generally came to refer to speeds of Mach 5 (five times the speed of sound) and above.The hypersonic regime is a subset of the supersonic regime. Hypersonic flow is characterised by high-temperature flow behind a shock wave, viscous interaction, and chemical dissociation of gas.

Conclusion

In short,incompressible and compressible aerodynamics present unique challenges and solutions in aircraft design and operation. Engineers balance the benefits and constraints of each in pursuit of efficient and safe flight, continuously pushing the boundaries of aerodynamics with advancing technology in the aviation industry.

Exploring the Fundamentals of Incompressible and compressible Aerodynamics and it’s flow