### Computational Fluid Dynamics, the third approach

In previous blog we have talked about interpretation of CFD concepts and we limited our discussions on nature of solution obtained from CFD simulations. In this blog we will talk about the nature of solutions and its interpretations in real-world applications.

As we look into the classical approach of fluid-mechanics, we will see that there were two approaches for determination of fluid phenomenon. The first approach is mostly the theoretical while to validate the first one the second approach (i.e. experiment) is used. We can’t completely rely on theory because of assumed values and simplifications. Similarly, it is not very economical to conduct experiments and to articulate the real life conditions.

Just take an example, if we want to test an aircraft at speed of say M=5, i.e., Mach 5. In this case most of our theoretical calculations fails because of unusual curved geometries and complex structures. The experiments at ground testing facilities requires wind-tunnels of such capacities. We are lacking the wind-tunnels that can simultaneously maintains the higher Mach number and higher flow field temperatures. Also, there is a big question about the accuracy of the results. Hence, it is always better to start with a better estimate of the values. This ‘better estimate’ is provided by ‘computational fluid dynamics (CFD)’.

As we discussed the two approaches, we can consider CFD as the third approach in fluid dynamics. We are not saying that CFD will completely replace the theory or it will completely replace the experiments. Only thing we are saying here this ‘third approach’ will assist both the approaches in reducing the overall effort and cost. Hence, we can more confidently say that CFD is just an engineering tool. A good computer programmer can write a CFD code and solve some problems, but the results he/she will get may not make any physical sense.

The point we are try to make here is, learning some software is not CFD. For using complete potential of CFD, we need to learn the governing principles and ideas driving CFD. Once you are familiar with the theory and algorithms in CFD, you can use any of the software with little practice. It doesn’t matter whether you are using Ansys Fluent, CFX, StarCCM or OpenFOAM, of you are familiar with the concepts any software you can use effectively.

Here in mycadcfd we give more focus on OpenFOAM because, it is free in all respect. It is easy to program, fast and reliable. All the softwares, if used intellectually will give similar, but not exactly same results. This is because of the inherent mathematical approximation in algorithms.

CFD is a developing science and we the mycadcfd team, ensures the proper learning of the students. The current state of CFD is such that we can easily solve the laminar flow problems, but due to extensive computational cost and time required, we need turbulent models for calculating the turbulent flows. Direct Numerical Simulation (DNS) is limited to Reynolds value of few tens of thousands. If nothing new came up recently, then we are correct in mentioning that the achieved DNS till date is for Re=30,000. Still, DNS is not practically useful for most of the engineering problems due to computational limitations.

Grid independence and governing equations are key players in estimation of correctness of the results. Next blog will be dedicated to these topics. For more information, please visit us at: http://www.workshopwale.com/

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