Introduction To Computational Fluid Dynamics (CFD) For Engineers

What is CFD?

Introduction:- In recent years we have seen a rapid increase in the use of computers for engineers in solving the problems. In the same contrast particularly Computational Fluid Dynamics (CFD) is true subject for the problem solving that involves fluid heat transfer and fluid flow which occur in applications related aerospace, power sector and automobile industry. The various factors that are the reasons for the development of CFD are:-

• Growth in the complexity of the engineering problems that can be unsolved in manual way.
• Need of quick solution with moderate accuracy.
• The expenses that an industry bears during laboratory experiment of physical prototype.
• The absence of analytical solutions.
• Exponential growth in the number crunching abilities and rigorous computer speed and its memory.(N., 2008)

1. Introduction to CFD:-  

Question: What is CFD?

CFD is a method adopted to obtain a discrete solution of problems related to fluid in a real world. The discrete solution is a solution obtained by a finite collection of space points and the level of discrete time. CFD is physical quality of any fluid – flow that is having three governing fundamental principles:-

• Conservation of mass for the fluid.
• Newton’s Second Law: – Which say-s that the rate of the change of momentum is directly pro-portional to the force applied and this rate of change of moment-um is directly proportional to the applied force and in the direction of the applied force.
• Law of conservation of energy which is the first law of thermodynamics and it states that the summation of the rate of change of heat addition must be equal to the work done on the fluid.

In order to continue the understanding of CFD the person must first understand the fluid dynamics and its governing equation.

The Computational Fluid Dynamics equations can be derived in two stages.

• First stage is the numerical discretization.
• Second stage is the specific technique.

The above two stages are used to solve the algebraic equations which is derived from the governing equations.

Steps in CFD Analysis

1. Getting the real world flow problems
2. Mathematical modelling of the given body which includes the general equations, boundary conditions etc.
3. Generating the discrete equations (preferably the algebraic equations).
4. Finding the solution of the above algebraic equations.
5. Post processing and the data analysis and finally the flow visualization.
6. Introduction of CFD in fluid dynamics

Study of motion of the fluid with reference of forces and moments is known as fluid dynamics. In fluid flow there different types of forces occurs in the flow like viscous forces, gravitational forces, pressure forces, surface tension forces, eddy forces (turbulent forces) and different type of other forces.

Fluid dynamics is based upon:

• Conservation of mass.
• Newton’s second law i.e. F=ma.
• Conservation of energy.
• During the vehicle in motion when the fluid flows under adverse pressure condition, fluid loses its momentum. Near the surface the particles have less momentum so they loses their momentum fastly. At the point where momentum of the fluid becomes zero and after which near the surface the fluid surface the fluid moves under adverse pressure condition i.e. in reverse direction that point is known as point of separation. It is not compulsory that separation must be present in adverse pressure condition.

In fluid flow velocity is function of space and time, so the acceleration is the function of space and time. Space component is known as convective acceleration and time component is known as local acceleration. During the ANSYS analysis the above acceleration place an important role and help the engineer to make proper aerodynamic design of the vehicle. This is so because the acceleration and velocity component are responsible for lift and drag of the vehicle. Improper design leads to create serious lift of vehicle and this may results in serious accident. In order to avoid this impact in the absence of physical prototype graphically the prototype is designed and is tested through computer itself by the use of ANSYS and employing Computational Fluid Dynamics theory (Padagannavar P., 2000).

Apart from the above concept some terms need to be described which will helps to validate the whole analysis:-

1. Flow Lines:- Fluid flow can be described by 3 flow lines

• Stream line:- It is an imaginary line or curve drawn in space such that tangent drawn gives velocity vector i.e. velocity vector and stream line vector coincides. The two streamlines never intersect each other as well as stream line also never intersects itself because at the point of intersection there will be two velocity fields which is impossible. So there is no flow across the streamlines(J. & M, 2015).
• Path line:- The line drawn by tracing the path of single fluid particle at different time integrals. It is defined on the basis of ‘Langrangian description’.
• Streak line:- It is an instantaneous picture of all fluid particles passing through single point.

NOTE:- For the steady flow all three lines are identical i.e. all three line coincides.

1. Acceleration in fluid motion:- The velocity is the function of time and spaced in the fluid flow, so the acceleration is the function of space and time. Space component is known as convective acceleration and time component is known as Local acceleration.
2. Contours:- It is an outline representing or bounding the shape or form of something. It is used for representation of typical response system in other words we can say it is typical curve line which describes the response system.
3. Circulation:- It is the line integration of tangential component of velocity along the closed loop. It is the scalar quantity.
4. Vorticity:- It is the mathematical measure of rationality. It is circulation per unit area. It is wice of the rotation. The direction of vorticity is as that of rotation.

To create a computational model of the given box in order to do analyse the box showing its behaviour. .

1. Physical boundary is being defined for the given model and the problem.
2. The box is than divided into discrete particle called mesh. The uniformity of the mesh is arranged according to the situation.
3. The defined boundary conditions have specific fluid behaviour.
4. Starting the simulation and the equations is solved at steady state.
5. Postprocessor is then used for analysis and visualization of the results. (Ravinder, 2014) 

    Applications of CFD:-

1. Aerospace industries.
3. Biomedical
4. Chemical processing.
5. Marines
6. Oil and gas piping industries(J., 2007). 

Analysis Environment: 

Mesh Report

Table 1.  Mesh Information for FFF

Domain Nodes Elements solid surface body 8004 3836

Physics Report 

Table 2.  Domain Physics for FFF

Domain – solid surface body Type cell

Table 3.  Boundary Physics for FFF

Domain Boundaries solid surface body Boundary – inlet Type PRESSURE-INLET Boundary – outlet Type PRESSURE-OUTLET Boundary – symmetry 1 Type SYMMETRY Boundary – symmetry 2 Type SYMMETRY Boundary – wall Type WALL

Results:

Pressure Graphics and animation

Advantages of CFD

1. Eliminates the process of experimentation in laborartories.
2. Provides better details.
3. Provides better predictions in a short peroids of time.
4. Provides better and fast design meeting all the environmental regulations and ensures industry quality.
5. Provides shorter design cycles and supply products faster in the market.
6. Easy to install with minimum downtime.
7. Allows rapid prototyping.
8. More cost effective(P., 2009). 

1. Disadvantages of CFD
2. Initial investment cost is high.
3. Required skilled persons therefore costly in field of professionals.(H. & Malalasekra, 2008)
4. Future of CFD

After the above analysis we have reached a certain platform in understanding of and appreciation for CFD. CFD is the new “third dimension” in fluid dynamics, equally sharing the stage with the other dimension of pure theory and pure experiment.

The major role of CFD is that of research, a tool to enhance our undersytanding of the basic physical nature of fluid dynamics (Jiyuan T., 2018).

References

H., V. & Malalasekra, 2008. An Introduction to Computational Fluid Dynamics The Finite Volume Method. 2nd ed. Noida: Dorling Kinddersley (India) Pvt. Ltd.

J., B., 2007. Computational Fluid Dynamics: Principles and Applications. Waltham: Hayley Gray.

J., C. J. & M, H. M., 2015. Theoretical and Applied Aerodynamics: and Related Numerical Methods. London: Springer Dordrecht Heidelberg.

Jiyuan T., G.-H. Y. L. C., 2018. Computational Fluid Dynamics: A Practical Approach. 3th ed. Kidlington: Katey Birtcher.

N., D. M., 2008. CFD Applications in the Automotive Industry. 22 january, p. 7.

P., N., 2009. Introduction to Computational Fluid Dynamics. Delhi: Dorling Kindersley (India) Pvt. Ltd..

Padagannavar P., B. M., 2000. Automotive computational fluid dynamics simulation of a car using ANSYS. Melbourne: RMIT.

Ravinder, R. P., 2014. THERMAL AND AIR FLOW ANALYSIS OF SHELTER USING CFD. Hyderabad: Chaitanya Bharathi Institute of Technology,Hyderabad.

Zimmerman, W. B., 2000. Process Modelling and. s.l.:World Scientific Publishing co.Pvt. Ltd.