We know that fluid flow is divided into two types of compressible flow and incompressible flow in terms of compressibility. Compressible flow is mainly seen in gases, and liquids are mostly incompressible.
Studying and understanding compressible and incompressible flows in designing complex equipment such as airplanes, missiles, engines, turbines, and compressors is very important and helps engineers design more efficient systems. We will give a preliminary introduction to this type of flow and discuss some CFD applications in the simulation of this type of flow.
Incompressible flow is a flow in which the density remains constant despite changes in pressure or temperature. Most of the flows that we know and face are incompressible. This flow type is usually seen in low-velocity flows, such as water flow in pipes and channels.
Compressible flow is when fluid density changes significantly with increasing or decreasing pressure and temperature. This is a fundamental concept in gas dynamics and aerodynamics. The study of compressible flow includes understanding how density, pressure, temperature, velocity, and other parameters change for each other in a compressible phenomenon. Understanding compressible flow can help us analyze and simulate various physical phenomena such as shock waves, expansion waves, ultrasonic flows, and turbulent flows. And this knowledge can be used to predict the behavior of fluids in different conditions.
This flow type is usually seen in high-speed flows with high dynamic pressure, such as the flow around supersonic jets and the flow from a rocket engine. However, it can also be seen in other phenomena where the fluid faces high static pressure, such as gas turbines and compressors. Analysis and simulation of compressible flow are more complicated than incompressible flow. Because in incompressible flow, there are four unknowns of pressure and velocity components, but in a compressible flow, in addition to these unknowns, two unknowns of density and temperature are also added, and to solve them, we must use the Perfect gas relations, and the energy equation.
On the other hand, there are other definitions to determine the compressibility or incompressibility of the flow according to the fluid speed (Mach number). Mach number is a dimensionless number that expresses the ratio of the speed of an object in a fluid to the speed of sound in the same fluid. If the Mach number is below 0.3, we say that the fluid is incompressible, and if it is above 0.3, we say that the fluid is compressible. In the following, we will introduce some examples of a compressible flow.
The shock wave occurs in compressible flows when the Mach number of the fluid flow reaches one, the shock wave occurs. This wave has destructive effects on the systems because it causes energy loss and temperature increase. In the shock wave, the velocity of the fluid decreases, and the pressure, temperature, and density increase.
During the flight of flying jets, we have seen that when the jet accelerates and breaks the sound barrier, a cone-shaped layer of water vapor is formed around it. This phenomenon is because a shock wave happens at Mach, and the pressure increases, which causes the moisture in the air to evaporate. You can see the picture of this phenomenon below.
The shock wave can be simulated using CFD, and its location can be observed on the aircraft’s body or inside the nozzles. These observations help us to propose solutions to prevent the occurrence of this destructive phenomenon.
Compressors are rotating equipment (of course, there are non-rotating types) to compress air and increase its density. Compressors have many uses in various industries, such as power plant cycles, jet engines, etc. As it was said, the work of the compressor is to condense air or gases in general, so the fluid flow in them is compressible. The compressor draws in high-volume and low-density air at high speed by using mechanical energy supplied by electric motors, and the rotating blades in it, which are arranged in several stages, air compress and condense and increase its density and pressure. During this increase in density, the air temperature also rises sharply.
There are different types of compressors, the most famous of which are centrifugal compressors. CFD can be used to analyze and check the compressible flow in compressors.
Turbines are rotating equipment that converts the energy of the incoming fluid into rotational mechanical energy, which is used to generate electricity in generators. According to their working fluid, turbines are classified into two categories: gas turbines and water turbines. As mentioned above, liquids are generally incompressible, so the flow in water turbines is incompressible. But the flow in gas turbines is compressible, and the fluid’s passage through the turbine expands, and its density decreases.
Usually, in gas turbines, fluid usually enters the compressor before entering the turbine, and its density and pressure increase. Then this high-pressure fluid enters the turbine, causes the turbine blades to rotate, and transfers its energy to the turbine blades. As a result of this energy and power transfer, the fluid expands, and its density decreases.
Compressible flows in gas turbines and incompressible flows in water turbines can be simulated and analyzed using CFD analysis. CFD methods help engineers a lot in designing and optimizing turbines and compressors, because they can check and analyze all kinds of designs without needing to build turbines and compressors.
Another example of compressible flow is the flow inside rocket engines. The geometry of rocket engines is of convergent and divergent nozzle types. There is a very high-pressure difference between the two heads of the rocket engine, which causes the high-speed exit of combustion products from the rocket outlet and provides the propulsion force. The pressure and density of the flow during the movement of the fluid in the rocket change with a significant slope, and the flow is of compressible type.
Also, due to the high density of fluid flow in rockets, there is a possibility of shock wave occurrence, a very destructive phenomenon in rocket engines, and causes a very high drop in the propulsion power the engine provides.