Aerodynamics

Aerodynamics

What is aerodynamics?

(Aerodynamics) can generally be related to the movement of various objects in the air. The laws governing aerodynamics show how an airplane can fly. All things that move in the air are affected by the laws governing aerodynamics. These objects can be moving rockets, flying kites, or racing car. When a moving object is surrounded by air, the science of aerodynamics is applied to it.

Aerodynamics is the science that studies moving objects in the air or moving air around objects. When air passes over an object at a certain speed, it has different effects on that object. Among them, aerodynamic forces, boundary layers, and noise can be mentioned.

It should be noted that the calculation of various parameters in the science of aerodynamics can be done by implementing experimental tests in wind tunnels. Another way to calculate aerodynamic parameters is to numerically study fluid governing equations such as the Navier-Stokes equation in computational fluid dynamics.

As mentioned, many governing concepts and equations in fluid mechanics are also widely used in aerodynamics. Among these equations are the following:

-Continuity and mass conservation equations.

-Navier-Stokes equations.

-Linear momentum.

-Angular momentum.

-Turbulence concept.

-Boundary layer theory.

-Ideal gas assumptions.

Aerodynamic parameters

As mentioned in the previous section, aerodynamics is the science that studies the forces acting on a solid surface that is exposed to airflow. We must define mathematical parameters to investigate and better understand these effects and compare them with different objects and conditions. Among these parameters, drag, lift, “torque,” and “center of pressure” can be mentioned.

drag force

One of the most important parameters in aerodynamics is the fluid’s resistance to an object’s movement, known as drag or drag force. This force enters in the opposite direction of the object’s motion relative to the fluid and generally causes a drop in the efficiency of the device.

The drag force depends on the shape of the object and the relative speed of the object and the fluid.

lift force

Another force acting on a moving object in the air is the lift force. This force is perpendicular to the direction of fluid flow at the object’s entrance. The term lift refers to the use of this force in the aviation industry. This force enables the flight of an object heavier than air. Lift force is also observed in various industrial tools such as propellers, helicopter blades, or wind turbines.

lift and drag

Torque

Aerodynamic torque is produced by using aerodynamic forces applied to the object that tends to rotate the object. This object can be an airplane wing, a car, or a wind turbine blade. The rotation occurs when this force is applied outside the center of pressure or aerodynamic center.

For example, imagine airflow over a moving car. In this case, the airflow distributes forces on different car parts. For instance, at the entrance and the lights, pressure is applied to the car by the air, and this force tends to turn the vehicle.

Center of pressure

The center of pressure is the point where the force produced by the sum of the surface pressures enters. This force can be calculated using the surface integral of the eddy pressure field. Calculating this force is very useful in determining the stability of an aerodynamic object. For example, in the aerodynamic design of a bullet, the distance between the center of pressure and the center of gravity can cause a rotational torque. As a result, the accuracy of this projectile movement will be very low.

Simply, the concept of the center of pressure is similar to the concept of the center of gravity. The center of gravity is the point of the object where the average weight of an object enters. For example, the center of mass of a hammer is located further away from its middle, because the hammer’s handle weighs much less than its head.

Similarly, the center of pressure is where the average aerodynamic forces, such as drag and lift, enter. The center of pressure in aviation applications allows engineers to balance the aircraft. The figure below shows a racket’s center of force and center of gravity.

Center of pressure and gravity

 CFD analysis in aerodynamics and aerospace

Computational fluid dynamics (CFD) is a widely used method for simulating airflow around objects, making it an invaluable asset in aerodynamics and the aerospace industry. CFD is used to analyze and optimize aircraft design, improve engine performance, reduce drag and noise, and analyze turbulence. It can also be used to simulate wind tunnel tests and analyze the effects of air turbulence on aircraft components. Flow analysis is now widely used in all disciplines of aerospace engineering and other engineering disciplines where flow is present.

The application of CFD in aerodynamics enables designers to optimize aircraft designs such as airplanes, helicopters, and rockets. For example, CFD can be used to predict the amount of aircraft drag if changes are made to the airframe design.

It can also be used to reduce noise by predicting the location of the aircraft’s emission and noise level. Furthermore, turbulence analysis is another area where CFD can significantly impact aerospace engineering. Turbulence occurs when air flows around an object and creates miniature and large eddies created randomly and in different dimensions. CFD methods can be used to predict turbulence by calculating the flow field using the Navier-Stokes equations.

turbulent flow

CFD analysis in propulsion

The propulsion subsystem can be considered one of the most challenging areas of CFD usage. The physical diversity of the prevailing flows and the geometrical complexities (especially in the case of jet engines) make the simulation difficult.

Incompressible and compressible flows in the air intake of all types of engines; Two-phase compressible flows with chemical reactions (combustion) in injectors and combustion chambers; Rotating compressible flows in axial and radial compressors and turbines; Compressible flows in the nozzle and exhaust of engines and finally mixed and non-mixed multi-phase flows in fuel tanks, pumps, valves, pipes, and hoses, which are mostly accompanied by turbulence and heat transfer. It shows the extent of flow regimes in the propulsion of flying devices. Turbofan propulsion systems are one of the most complex engines in the aerospace industry in terms of analysis, design, construction, and variety of parts, which includes all the flow regimes described above.

Propulsion

CFD analysis in Buildings and towers

Due to their extensive cross-sectional area, tall buildings and skyscrapers bear a significant wind force. This force is similar to the drag force applied to a solid object in a fluid flow. In addition, when separation is observed around a building, the situation becomes difficult for pedestrians on the surrounding streets.

Also, the boundary layer formed on the surface of the earth in big cities changes due to the presence of tall buildings. This change in the gradient of airspeed near buildings can harm the movement of airplanes that are forced to fly near the surface of the earth and can disrupt their activity. So, during the construction of airports inside the cities, attention should be paid to preventing the possible dangers and crashes of planes flying close to the ground.

Today, these analyzes can be performed using advanced CFD tools. The figure below shows a view of the flow analysis around the Eiffel Tower using CFD tools.

design-building

CFD analysis in Vehicles

The transportation industry is one of the highly competitive industries. Reducing the drag force will improve this industry and reduce the fuel consumption of these devices. Brake cooling and air conditioning also show another application of aerodynamics in these industries.

In addition to the mentioned cases, lift force analysis is very useful in maintaining the stability of these systems. Vehicle stability is one of the most important issues in all vehicles, including those that experience high speeds. With CFD’s help, vehicles’ designs can be optimized for less drag force and fuel consumption.

automotive-cfd_4

By simulating the airflow around the vehicles, it is possible to identify areas of turbulence and pressure that cause sound waves’ propagation. This allows engineers to design more efficient vehicles with less noise and vibration.

Other

In addition to the things mentioned above, there are issues in aerospace sciences that require CFD to solve them. To understand the behavior of operating fluids in pneumatic and hydraulic equipment in almost all sub-systems of flying devices, such as propulsion, landing gear, temperature control (for ground-tracking space systems), and life support, the use of CFD is beneficial. The helipad of equipment and soldiers is another issue that requires solving the unsteady turbulent flow field. Controlling the temperature of satellites before launch and during acceptance tests with the launcher until the moment of separation is also a case requiring simulation using CFD tools. Heat transfer between the engine and other components of other flying devices is also one of the design requirements of aerospace systems.

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