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2 Funny Cases In CFD

Posted October 28, 2015 at 07:17 by adarsh tiwari

In my previous blogs, we had put special emphasis on saying that learning some software is not CFD. You need to have a sound knowledge about the basics of the subject then only you can validate the results.

Today, letís see an example where the two different cases of same geometry and condition will give two different meaning to CFD. But, let me first warn you, this could be little more technical for first few lines afterwards, you can enjoy the meaning.

Case 1: Colorful Fluid Dynamics

The first one obviously abbreviate CFD as Colorful Fluid Dynamics.

Let us take a case of Exhaust Gas Recirculation (EGR) valve of a single cylinder naturally aspirated engine. In this case, we are considering the spring-loaded valve.

The physics of the valve says that it should deliver some fraction of exhaust gases with fresh air during the suction stroke of the engine. The geometry and boundary conditions are such that the temperature and velocity should be in the range of 487K to 298K and 60m/s to 0 m/s respectively. Letís not go much into technical, we assume that the walls are at 298K and with no-slip conditions.

Now, suppose we have our first simulation, done by someone who is very good in operating software, but very poor in basics. For him, the colorful picture is CFD result. Letís see what has he got as a result of 1-wwek of computing.

The key points of the result are:

1. The velocity ranges up to 660 m/s which is nearly equal to M=2. i.e., Mach 2. Now, let me disclose, the engine was single cylinder 400cc naturally aspirated, at no point of time this velocity of M=2 is possible. In other words, you are riding a bike with the speed of a missile. God bless.

2. The temperature range is from 487K to 169K i.e., from about 200degC to -100degC. In other words, you have 3 in one system, generating power from the engine, heating requirements fulfilled and the biggest one, throw-out refrigerators from your home . -100degC, there is no refrigerator at your home which can cool even closer to this temperature.

3. Pressure variations are drastically higher to give enough shock to break the engine body.

In simple words, this results shows that you can not generate enough power with a good quality of heating and start a home production of dry ice . Millionaire in a week!!!

But, unfortunately (or fortunately) this results doesnít have any physical sense. Hence, this simulation abbreviates CFD as ĎColorful Fluid Dynamicsí.

Case 2: Computational Fluid Dynamics

Now, letís take another guy, who knows the governing theories and also knows how to apply it using software tools. The results shown by him are:

The key points of the result are:

1. The velocity ranges up to 60 m/s as well as the temperature is well in range 487K to 298K.

2. Results closely approximate the practical conditions, hence are acceptable.

3. It can be further taken for experimental validation.

4. But unfortunately, this canít make you a millionaire because you can do readymade co-generation of dry-ice, process heat, and power.

This result will abbreviate CFD in its proper sense, i.e., Computational Fluid Dynamics.


Friends, the point I am trying to make here is: the software whether it is Ansys, OpenFOAM, StarCCM or any other, is just a tool. The software is made by us, like any other tool. Hence, it is of prime importance to learn the proper theories before using these tools. In Bangalore and Pune, there are a lot of institutes which train you in CFD, but believe me, they are not training you in CFD. They are just making you end user of some software.

Want to know more about the background working of Softwares, Discretization Techniques, Errors and Error Propagation, Stability Analysis, stay tuned to MYCADCFD

So what say, Guys, have something in mind? Share it and we can discuss together.
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