Computational Fluid Dynamics In Aerospace Engineering

The world of aerospace engineering is replete with non-linear partial differential equations that encase the core physics involved in particular physical phenomena. Solving these equations is crucial for assessing the parameters of the flow field, which plays a key role in optimizing aerospace systems. Unfortunately, the analytical solution to these equations does not exist, so obtaining insight into the flow field requires them to be solved numerically. Computational fluid dynamics (CFD) plays a critical role in obtaining the flow field parameters by solving the discretized form of partial differential equations (linear equations) at discrete points inside the flow domain. Several aerospace engineering problems, including those related to aeroacoustics, aerodynamics, aeroelasticity, aerospace propulsion, and others, can be analyzed using Computational Fluid Dynamics.

Airplanes, helicopters, missiles, rockets, and almost all aerospace vehicles generate noise due to their interaction with the surrounding fluid. The study of sound generated by the interaction between the flowing fluid and surrounding objects to determine its origin, estimate its range, and design solutions to reduce or eliminate the noise to comply with sound regulations and improve the user experience is known as aeroacoustics. The noise from the aerodynamic interaction can originate from various sources, including aerodynamic instabilities, shock waves, and turbulent airflows. Computational fluid dynamics could play a significant role in identifying the exact source of the noise by simulating the fluid and aerodynamic structure interaction.

Aerospace propulsion systems are the powerhouse of any aerospace vehicle. The aerospace propulsion systems can be classified as airbreathing and non-airbreathing. Generally, all aerospace vehicles are designed to operate within the earth's atmosphere and are powered by airbreathing engines. On the other hand, non-airbreathing engines are employed to propel the space vehicle. As the name suggests, air is the primary working fluid for all airbreathing engines and usually consists of several components in continuous interaction with the working fluid. The overall efficiency of the propulsion system depends on how these engine components interact with the working fluid to alter the flow field. 

Apart from aerodynamic design optimization of aerospace propulsion systems, Combustion is one of the major areas where high-fidelity CFD models play a significant role in evaluating flame dynamics, heat transfer, thermal wear, etc. These CFD models accurately predict the combustion efficiency of the engines and provide information regarding emissions such as NOx, soot, CO, and others. 

Aerospace vehicles experience aerodynamic lift and drag forces while traversing through the atmosphere due to their interaction with the surrounding air. These aerodynamic forces result from the pressure and shear stress distribution over their surfaces. The optimization of these pressure and shear stress distributions is required to obtain high aerodynamic efficiency.

Computational fluid dynamics compliments the traditional experimental methods by providing flow field data at discrete points inside the flow domain. Aerodynamicists can analyze the flow field around aerospace vehicles by simulating it with CFD and optimizing its design to enhance the flow field. Drag is defined as the force acting in the direction of free-stream and adversely affects the overall aerodynamic performance of aerospace vehicles. Reducing aerodynamic drag is one of the most crucial aspects of aerospace vehicle design. With CFD, engineers can identify the critical regions responsible for higher aerodynamic drag and can adjust design parameters in these regions to obtain a design with higher aerodynamic efficiency.

In high-speed flows, usually at Mach>1, the shock waves are formed inside the flow field due to sudden changes in flow direction caused by the presence of some aerodynamic body. These shock waves move closer and closer to the surface of the aerodynamic body as the Mach number increases and starts interacting with the boundary layer on the surface. These shock-boundary layer interactions cause unsteady oscillations, buffeting, and localized high heating and pressure regions with distinct degrees of thermochemical nonequilibrium. Intense localized heating and pressure load could destroy vehicles, particularly when an external shock impinges on a boundary at hypersonic speed. Therefore, accurate modeling and prediction of shock boundary layer interaction are essential for cost-effective design process purposes of hypersonic vehicles to avoid performance degradation or failure. CFD could provide an accurate prediction regarding the regions where the shock boundary layer interaction is dominant, and their effect can be mitigated or reduced by optimizing the design based on the CFD data. 

In aerospace engineering, the wings of aircraft operating at very high speeds usually have two modes of motion, including bending and twisting. In twisting mode, the wing tends to rotate about its stiffness axis. The effect of twisting is also known as divergence, and the flight speed at which this failure occurs is called divergence speed. While in bending mode, the wing tips flex upwards and downwards relative to the fixed wing root. This bending or low-frequency flapping mode, a.k.a flutter mode, has a synchronized interaction between the twisting and flutter modes, and the energy absorbed from the airflow in one mode increases the amplitude of the other mode. The flight speed at which the flutter becomes so significant as to distort or entirely damage the wings is known as flutter speed. 

The CFD is one of the most crucial parts of aerospace engineering. The advancement in numerical algorithms and computational power has enabled aerospace engineering teams to solve more complex engineering problems involving aeroacoustics, aeroelastic, and aerothermal analysis. CFD provides low-cost solutions for the design and analysis of aerospace vehicles. 

1. Okinawa Institute of Science and Technology, Okinawa, Japan https://www.oist.jp/research/research-units/apde

2. Kowarsch et al. (2015), AEROACOUSTIC SIMULATION OF A COMPLETE H145 HELICOPTER IN DESCENT FLIGHT.

3. Axial compressor https://www.pngwing.com/en/search?q=turbofan

4. Pandey, Krishna Murari and T. Sivasakthivel. “CFD Analysis of Mixing and Combustion of a Hydrogen Fueled Scramjet Combustor with a Strut Injector by Using Fluent Software.” International journal of engineering and technology 3 (2011): n. Pag.

5. Aeroelastic wing shaping (TOP2-251) https://technology.nasa.gov/patent/TOP2-251