Computational fluid dynamics, or CFD, is the use of computers to predict how liquids and gases move, based on key scientific principles like the conservation of mass, momentum, and energy. Fluids play a major role in our everyday lives. For example, when you speak, vibrations in your vocal cords create pressure waves in the air that let others hear your voice. Without fluids, actions like hitting a tennis ball with topspin or flying an airplane would not be possible. CFD software helps us study, understand, and predict how fluids behave in many different situations.
Examples of Computational Fluid Dynamics
CFD is used in many areas where it is important to understand how fluids move and how heat is transferred. It helps engineers see how fluid flow affects a product or system by studying properties such as temperature, pressure, velocity, and density. Here are some examples of how CFD is used in different industries:
Aerospace and Defense:
CFD helps engineers model the airflow around airplanes, which is called external aerodynamics. This helps improve aircraft design for better performance and lower fuel use. CFD is also used to simulate airflow inside the aircraft, such as cabin air circulation, to understand air quality. Common uses include cooling for electronic systems (avionics), studying how light moves through air (aero-optics), airflow around the plane, cabin ventilation (HVAC), and engine performance.
Automotive:
Electric vehicles involve complex systems where mechanical, chemical, and electrical parts work together. CFD helps engineers study how well the motor stays cool and how to reduce battery overheating, which can lead to fires. Important uses in this area include cooling electric motors, studying battery performance, modeling lubrication in gearboxes, testing sensors for self-driving cars, and looking at airflow and noise.
New Energy:
Hydrogen is an important clean fuel that helps reduce pollution. CFD can model every part of how hydrogen is made, stored, moved, and used. It also helps researchers test how hydrogen and other clean fuels work in current engines. Key areas include hydrogen production using electrolysis, storage and transport systems, fuel cell use, and fuel testing.
Healthcare:
CFD is useful in studying how fluids move inside the human body, such as blood in blood vessels or air through the lungs. It can also speed up the development of new medical devices and help test how well medicines might work. Key uses include studying blood flow in the heart and arteries, breathing systems, and making biopharmaceuticals.
How Computational Fluid Dynamics Works
There are several ways to solve fluid flow problems using a computer. Before starting, you must decide what approach to use, including which basic equations will be solved. This decision limits the methods available. If you choose a common method known as the continuum approach, there are three main steps:
- Define the area where the fluid moves (usually using a CAD model).
- Break this area into smaller sections using a grid, called a mesh.
- Use the computer to solve the fluid equations in each section of the mesh.
If high-performance computing (HPC) is available, the work can be divided among many computers to speed up the process through parallel computing.
Challenges of Modeling Fluid Flow
Modeling fluid flow on a computer is difficult because fluid behavior is very complex. Several challenges make this process harder:
Multiphysics interactions:
Fluids often move through or around other objects. For example, trees sway in the wind, but their movement also changes how the wind flows. This kind of interaction between fluids and structures needs a special approach that considers both together. Some software, like Ansys Fluent and LS-Dyna, can handle these situations. Real-world problems often involve more than one fluid or include chemical reactions, such as in an engine or battery. Ansys Fluent is especially good for modeling these kinds of problems.
Nonlinearity:
In fluid dynamics, nonlinearity means that the fluid can affect itself. Many fluid flows are turbulent, meaning they are random and unpredictable. Turbulence also changes how heat and force are transferred, making it even harder to predict. Because of this, fluid equations are too complicated to solve by hand. Computers are needed to solve them using simpler math, even though the answer is just an estimate.
Unsteadiness:
Turbulent flows are also unsteady, meaning they change over time. For example, the air around a car moving on a highway is not constant. If these time-based changes are important, the simulation must include them, which takes more time and computing power. Turbulence is still not fully understood. The well-known physicist Richard Feynman once called it “the most important unsolved problem of classical physics.” While CFD does not solve turbulence from a scientific point of view, it does let engineers create models that include its effects.
History of Computational Fluid Dynamics
CFD began in the early 20th century when scientists first created math models to explain fluid movement. As computers became available in the mid-1900s, CFD advanced quickly because computers could handle the large number of calculations needed.
- Early development (1900s–1940s):
The basic fluid flow equations, known as the Navier-Stokes equations, were developed. These equations form the foundation for fluid dynamics. - Computers emerge (1950s–1960s):
The appearance of computers made it possible to quickly solve fluid problems that were once too complex. - Numerical methods (1960s–1970s):
Scientists began breaking fluid areas into smaller parts using a grid to better understand how fluids behave in each section. - High-Performance Computing (2000s–Present):
With faster computers, large and complex CFD models can now be run in less time. Engineers can simulate complex systems, such as an entire aircraft in flight.
Governing Equations of CFD
Fluid movement is hard to understand because it does not act like solid objects. When you throw a ball, its shape and mass stay the same. Air does not behave that way. The equations used in CFD help us deal with the changing and unpredictable nature of fluids.
The Navier-Stokes equations are at the center of CFD. Named after Claude-Louis Navier and George Gabriel Stokes, they are partial differential equations that explain how fluids move. These equations, created in the 1800s, are used to model all kinds of fluid behavior, like air moving around a plane wing or fuel flowing through an engine. They are based on the conservation of mass, momentum, and energy.
Advancements in CFD
The progress of CFD depends on the power of computing hardware. As computer chips improve and software moves from using traditional processors (CPUs) to faster graphics processors (GPUs), CFD is getting much faster and more accurate. Using multiple GPUs for a single simulation makes it possible to run larger models more quickly. These improvements will help reduce the cost of hardware, lower energy use, and increase the performance of CFD systems.