What Is CFD Analysis?

Fluids are materials that can flow under the influence of a force field, including substances in their liquid or gaseous states. Understanding fluid flow is inherently complex due to the variety of factors at play. To perform fluid flow analysis, we need to examine properties like force, velocity, pressure, density, and temperature, which can vary at every point within the fluid. Studying these properties through experiments or analytical methods is thus impossible in most real-world problems.

The Continuum Assumption in Fluid Mechanics: 

We know that matter is made of atoms and molecules that have empty space between them, but CFD is based on an assumption that says fluid is a continuum ignoring the empty spaces. This means that properties can continuously travel through the medium. For example, let's say we apply a force at some location in a fluid, it will create a deformation which is continuously varying throughout the fluid. 

The core concept of CFD:

In CFD, as each point in the fluid domain may have different properties, we divide the fluid domain into small subdomains, and the properties inside each subdomain are assumed to be constant. The gradients of fluid and flow properties are studied across the walls of these subdomains. 

There are different approaches to performing CFD Analysis, like Finite difference, Finite Volume, and Finite Element which are methods to formulate the fluid problem. 

Finite Volume Method or FVM is most commonly used for CFD Analysis, in this the fluid subdomain is called control volume. The property in the control volume is assumed to be constant considering the size of the control volume is very small to have significant variation in properties. 

Let’s assume we have a fluid element that we have divided into multiple small fluid elements, now when we apply a force on the top layer, the forces (shear forces) are transferred from the top layer to subsequent layers downwards by means of viscosity and hence different layers will move with different velocities.

In CFD though we do not actually move the fluid element, we calculate the change in fluid properties (velocity, pressure, etc.) at the location due to the applied force.

                 Fig. a                                                                            Fig. b                                                                           Fig. c

In Fig. ( c ) we can see the visualization of the velocity profile from the top layer to the bottom layer, with red indicating higher velocity and yellow being low. As the force is applied to the first subdomain, the change in properties is calculated for that subdomain; these properties then affect the neighboring subdomains and so on following the conservation laws (mass, momentum, and energy).

The process of dividing the fluid domain into multiple small subdomains is known as  

Meshing.

As the problem gets bigger and more complex, we have more numbers of subdomains. 

The animation below shows how the velocities will vary with time for the flow around a cyclist. In this, subdomains are smaller and we see a continuous flow field. Though the fluid is passing by the cyclist, we are not concerned about the position of the fluid element but the properties of the fluid at locations around the cyclist. 

Governing equations: 

Navier-Stokes equation which contains 1. continuity equation, 2. momentum equation, and 3. The energy equation defining the conservation of mass, momentum, and energy respectively is the basis of CFD.

Along with it, many other equations can be solved to address the physics involved in the problem such as thermodynamics, chemical reactions, electromagnetism, multiphase, etc.

Continuity Equation

The continuity equation defines the conservation of mass in the control volume which means that the amount of mass going in and coming out of the CV will be equal i.e. the net change in mass inside the control volume considering the density constant, will be zero.  After simplification we get,

Momentum Equation

Momentum equations in all three directions x, y, and z represent the conservation of momentum in those directions.  

Let's consider the momentum equation in the x direction:

The equation is analogous to F = ma, the left side of the equation denotes a change in momentum(~ma), and the right side denotes forces acting on the fluid element (~F).

Similarly, we have momentum equations in the y and z directions,

How are these equations solved?

CFD Analysis is nowadays being used in almost all kinds of industries right from designing the kitchen sinks to spacecraft. 

• Aerospace Engineering:

Optimizing the aircraft wing designs for higher lift-to-drag ratio, improving fuel efficiency, and making flights cheaper, CFD analysis has played a huge role. Experimental labs for hypersonic vehicles are impossible but CFD can help you simulate those conditions in your computers in a cost-effective way.

         CFD in Aerospace Engineering

• Automotive Engineering:

You must have observed the modern cars which have narrowed back, this is to reduce the drag force. Combustion in car engines through optimized air-fuel mixing can be achieved through CFD analysis. 

        CFD in Automotive Engineering

• Manufacturing:

Computational Fluid Dynamics tools are used to design and optimize the manufacturing processes. Recent advancement in the manufacturing industry through additive manufacturing also has the potential application of CFD to model, simulate, and optimize complex processes. 

Additive Manufacturing

• Civil Engineering:

CFD or computational fluid dynamics analysis, finds widespread application in turbine designs, thermal management within power plants, and the optimization of battery thermal management systems. Particularly in the realm of electric vehicle technology, where overheating poses a significant risk, CFD analysis plays a crucial role in enhancing design resilience and ensuring safety. By leveraging CFD simulations, engineers can develop fail-safe systems, mitigating the risks associated with excessive heat generation and enhancing the overall reliability of electric vehicles. 

• Energy and Power Generation:

Turbine designs are one of the most important applications of CFD.  Cooling systems in power plants are optimized with the help of CFD analysis.

• Marine Engineering:

CFD is used in designing ship hulls, and propellers with reduced drag improving fuel efficiency. Marine structure design also has the application of CFD to study the effect of waves on the structure.

CFD in Marine Engineering 

• Biomedical Engineering:

Computational Fluid Dynamics has applications in the biomedical field, such as simulating blood flow through arteries to study hemodynamics or analyzing airflow in the respiratory system. These simulations can help in medical device design.

CFD in Biomedical Engineering

• Environmental Engineering:

In water treatment plants, CFD analysis can be used to optimize the water treatment processes by simulating them. The hydrodynamics of river water in case of dam breaks can be predicted through CFD.

CFD in Environmental Engineering

• Oil and Gas Industry:

Gas flows or oil flow through pipelines can be simulated using the CFD hence, it is useful while designing the transmission network. Similarly, the extraction processes can also be simulated using CFD, this will help in optimizing the processes and avoiding hazards.

• Sports and Athletic Equipment:

You must have seen the cyclist bending to reduce the drag effect, the helmets, and bicycles can be optimized to improve performance. The design of racing cars is also a major application of CFD.

In conclusion, Computational Fluid Dynamics (CFD) analysis stands as a cornerstone in modern engineering and scientific endeavors, offering a virtual window into the intricate behaviors of fluids and gases in diverse environments. From optimizing aerodynamics in aerospace design to safeguarding urban populations against environmental hazards, CFD empowers researchers, engineers, and decision-makers to make informed choices, drive innovation, and foster sustainable solutions.