K-Epsilon VS Shear Stress Transport (SST) on the same model (Comparison)

Today I will write a few words and I will present you an example I've made regarding two methods for turbulence modeling, K-Epsilon and Shear Stress Transport.

For this experiment, I've used a intentionally badly designed diffuser (so I could create as much eddies as possible) with an inlet inner diameter of 30 [mm] and a sharp step driving the inner diameter to 45 [mm].
  

Ok, so let's present the 1st simulation.

The first mesh attempt will be with a generated "fine" discretization,  max element size = 4 [mm]. As for the inflation I've used the following settings: first layer thickness = 0.003 [mm], inflation algorithm = pre (this is making the inflation layers first) and maximum layers = 10.

For the setup of the analysis, I've used as fluid model: "Air Ideal Gas", as I defined my domain (instead of the default domain). In this domain, we must have the Inlet and the Outlet, that are defined as boundaries. Also, the turbulence model was selected as K-Epsilon and after the first simulation it was changed to Shear Stress Transport (It will be seen in the results).

For the inlet, at "Mass and Momentum", Mass Flow Rate was used as boundary condition for the air flow. (0.166 kg/s = 600 kg/h)

 For the outlet, at "Mass and Momentum", we have to pick Avrage Static Pressuve and Relative Pressure = 1 [bar].

The results:

Here are the results (CFD Post) represented with a streamline (starting Inlet) for the analysis with the K-Epsilon method. This simulation ran for 750 iterations, and although the residuals didn't really got above the threshold, it looked very steady.

Here are the results (CFD Post) represented with a streamline (starting Inlet) for the analysis with the Shear Stress Transport method. This simulation ran for 750 iterations, and although the residuals didn't really got above the threshold, it looked very steady.

CONCLUSION:

It is pretty obvious that the flow is very different from a method to another. Keep in mind that the mesh was exactly the same (100%) in the both analysis. 

By comparing the two flows we can see that the K-Epsilon method is showing a more steady flowing line and almost no eddies. 

If we look at the simulation that ran with Shear Stress Transport method, we can observe a lot more eddies and quite larger. 

This means, either that K-Epsilon method is not the best to use when large eddies are expected, or that SST method is giving exaggerated results. We will prove which method is the best by refining the mesh of the model and running the simulations with a lot more iterations.

............................................................................................................

After refining the mesh, we got this. We ran the simulations again and this is what we got as results.

 CONCLUSION:

As I've said, I'm not a professional, I am learning, but from the results, I think the K-Epsilon method should not be used when it is obvious that high turbulence is going to be created. 

Even with the refined mesh, the Shear Stress Transport method showed no big difference, but the K-Epsilon method was totally different.

This is all for now, I posted the videos of the simulation on youtube, you can check them out. Enjoy. Feel free to ask me, tell me I'm wrong or anything. Have a nice day!!

Books that are a must read if you perform FEA Analysis or CFD

Hello, I've decided to give you a few links to some very useful Finite Element and CFD books. It's always a good idea to read the theory and try to understand it. Most of this books have basic knowledge and can give you the know-how if you read them carefully.

• Introduction to the Finite Element Method, Evgeny Barkanov Link to PDF

• Finite Element Procedures for Solids and Structures - Linear Analysis Link to PDF

• Finite Element Analysis of Structures, Ivan Nemec Link to PDF

• A Gentle Introduction to the Finite Element Method, Francisco–Javier Sayas Link to PDF

• The FEMCI Book (NASA) - Large Database of articles Link to Website

• An Introduction to Computational Fluid Dynamics (Finite Volume Method), H.K. Versteeg Link to PDF

• An Introduction to Computational Fluid Dynamics, Dimitri Kuzmin Link to Presentation

• Introduction to Computational Fluid Dyamics, Anil W. Date Link to PDF

Of course, after you get the basics, you could go deeper and read something more specific, because you will be able to find stuff you are interested more easy, by knowing what to search.

I remember my teacher told us at a lecture, that in FEA the most important role is the Engineer (the operator), because the all software have aproximately the same capabilities.

I will write and present you facts while using ANSYS Workbench 16.2. This is the software I use, including the modules: Static Structural, Modal, Harmonic Response and CFX Flow.

All the best, Dan.

Welcome to my FEA-CFD Blog !!

I just want to thank you for entering the blog about my journey as a Mechanical Engineer (currently specializing in the Finite Element Method and Computational Fluid Dynamics).

I consider it a journey because it really is one hell of a journey. Probably many of you know and understand what is like to get a job and start from scratch in this field. This is the reason I want to start writing about it, to let you know the problems I encountered and how I managed to solve them, if I finally did. You all now Finite Element Analysis is a pain in the ass sometimes, not to mention Computational Fluid Dynamics, but hey, we all have to start somewhere.

I'm going to (try at least) write articles that we never find on the world wide web, for example: "This is how I solved my convergence issues", or maybe "This is the difference between a correct structural simulation and a simulation that failed".

It is going to be hard, probably because I'm not a good writer and English is not my best spoken language, hehe, but hopefully this Blog will become interesting for some of you, and that's the only thing I want.

I will try to post 2 articles a week, as a first schedule.

Feel free to contact me anytime you have questions, requests or maybe want a collaboration. 

Dan.