I wanted to open a topic on turbulence models since I often see questions about it but it’s not yet covered in the SimScale documentation (it’s in progress ). Here is a list of the models currently supported by SimScale
- LES Smagorinsky
- LES Spalart-Allmaras
- RANS k-epsilon
- RANS k-omega
- RANS k-omega SST
A couple of questions to get this discussion started:
What are the best resources for understanding these models and how they should be used?
How can we be sure that the turbulence model that has been selected, is in fact within an acceptable range of accuracy? Does anyone have examples?
Very interesting and useful homepage @TobiasHolzmann!
Since I want to learn more about CFD and turbulence in general your post comes along just at the right time. Hope to benifit a lot from your knowledge, experience as well as your videos
since a lot of people are going to be doing a lot of simulations involving walls (eg flow over a car body, flow through pipes etc), it is essential to understand the relevant near wall meshing criteria depending on the turbulence model used.
First and foremost a prism (boundary) layer mesh is essential to get realistic results in all wall-bounded cases (ordinary cartesian meshes are useful only in the simplest of cases). Then, if y is the wall-normal coordinate, it is paramount to determine what value of y1+ ie the distance (non-dimensionalised by the viscous length scale) of the first wall-normal grid cell centre will be. In most cases, the user determines this choice and usually, there are 2 options; either the first cell centre lies in the viscous sublayer of the boundary layer (which in general means y1+ < 5) or it lies in the log-layer of the boundary layer (which means in general y1+ > 30). The choice of y1+ and turbulence model must be made in unison. In general, a y1+ > 30 would be employed with k-epsilon models (with wall-functions calculating the flow for y1+ < 30) whereas y1+ < 5 would be employed with k-omega models. These ideas add to those mentioned @TobiasHolzmann in ‘Which model should be used’.
In case of advanced techniques such LES or DNS, the criteria on near-wall meshing become increasingly strict. In most of these cases, one will see y1+ < 5 and in many cases even < 1. The near-wall meshing criteria are extremely rigorous here as considering the physics, it is important to resolve all the high gradients that are present in the flow close to the wall.
This is a great topic, @AnnaFless, and one that will probably be in discussion forever. Commercial code CFD help documentation is a great starting point, as it is more application based than theory based.
Also, NAFEMS is a wonderful unbiased resource. They have a list of links here: https://www.nafems.org/about/technical-working-groups/cfd/links/ and a list of publications here: https://www.nafems.org/about/technical-working-groups/cfd/publications/
How to ensure the turbulence model is within an acceptable range of accuracy? Start by converging Turbulent Energies/Minimizing Residuals in the computation, and finish by matching your flow results to Test Data!
Is there a public area designated for “Benchmark Projects”? This would be the most direct way to share this knowledge with Simscale users, IMHO.
@mananthakkar87 - thanks for adding onto the topic, great information!
Thanks @fastwayjim, turbulence models seem to be a challenging topic so it’s good to have a place to talk about it! We currently have validation cases in our documentation. However, this is a good idea to create a space in the forum to share this information as well. Maybe you could create a topic to get this started in #using-simscale?
@TobiasHolzmann - I think we need a “Tweet this” button here in the forum. Fantastic post!
I think that the most hard to grasp aspect of turbulence modeling in SimScale is the use of automatic wall functions. I would like to focus on two turbulence models: the RANS k-omega SST and the DES of Spalart-Allmaras. In both it seems that automatic wall functions have been extensively used to promote an almost monotonic error decrease as the meshes are refined near the walls, bringing the adjacent to wall cell centers from y+ > 30 (inertial sublayer where the standard wall function would work) to the y+ < 1 range (laminar sublayer, where these turbulence models could be used without any wall function). And here came my questions (not very good ones, I realized later, see my next post below).
For the Spalart-Allmaras (under the LES label) SimScale uses the Spalding wall function ( http://cpp.openfoam.org/v3/a01670.html#details ) for wall shear stress calculation (when Wall function BC option is selected)? This wall shear stress is used to apply no-slip boundary condition (BC) on the momentum equation while nuTilda = 0 is taken as BC (at walls) for the nuTilda transport equation, right?
For the k-omega SST we need BC’s for momentum, k and omega. SimScale uses the Spalding wall function for obtaining wall shear stress (used in momentum BC) and k at the center of the cell adjacent to the wall (through the third equation in https://www.cfd-online.com/Wiki/Near-wall_treatment_for_k-omega_models ) while the omega value is taken from square root of the sum of squares of the values calculated (as also shown in this same link). Right or wrong?
No-slip is enforced by applying velocity of the wall on wall patches, because flow velocity immediately next to the wall has the velocity of the wall. The Spalding wall function is continuous and therefore allows nut of the wall patches to be consistent with non-dimensional wall units. I don’t know what nuTilda value is applied on wall patches on SimScale - only the staff members would know. But you are right about nuTilda = 0 as wall BC.
Again, I don’t have access to the SimScale code, so not sure what is implemented.
I have just learned that (at least when using one of the OpenFOAM solvers available inside SimScale) we do not need access to the SimScale source code in order to find out what are exactly the boundary conditions used when the “Wall function” option is selected. This information is available within the results compressed file that we can download after each case run (it is at the “0” subdirectory in that compressed file, we can look at the “nuTilda” file to know the BC for Spalart-Allmaras and at the “k” and “omega” files for k-omega SST BC; “alphat” file is also of interest if the problem involves heat transfer). In these files there are keywords and data entries read in OpenFOAM and it is in OpenFOAM source code and documentation that we finally find very detailed and precise information (although somewhat dispersed) about those boundary conditions.
For Spalart-Allmaras it can be seen that the automatic wall function in use in SimScale is based on an if statement that activates the Launder-Spalding* wall function when y+ is greater than a limit value (that can change with the wall roughness parameter E, but for smooth walls is 11.5301). At cells (close to walls) where y+ is below this limit value, no wall function is used (nu_t = 0). * Meaning that y+ = C_mu^0.25k^0.5y/nu_w definition is used. I have made a confusion with the continuous Spalding wall function (that is not used by SimScale) in my previous post.
In k-omega SST it seems that zero gradient BC is used for k. At the cells close to walls, omega is set equal to the square root of the squares of omega_Log and omega_vis, pretty much as explained in https://www.cfd-online.com/Wiki/Near-wall_treatment_for_k-omega_models . The nu_t calculation, however, is performed by the same method used for the Spallart-Allmaras model (in OpenFOAM I did not found any use of the fourth root of fourth powers of friction velocities that appears in the link above). Finally, it should be noticed that the production source term in the k transport equation is modified, when the wall function is used, to deal with the increased shear rate in the boundary layer (recognizing that the shear taking place in the laminar sublayer do not contribute to turbulence production).
Thanks for clarifying those BC’s.
Edit: I don’t think it is possible to tell whether ‘the fourth root of fourth powers of shear velocities’ is used in the computation of wall shear, as wall shear is not used in the turbulence model, ie., calculating wall shear is more likely a utility. The BC’s you have seen apply on the nut boundary field, whereas the blending of friction velocity relies on known yStar values. They are two separate things as far as I understand.
In order to complete my previous post about what I have learned reading the OpenFOAM source code (and did not found clearly stated anywhere else), it is worthy mentioning that OpenFOAM implementation of the Jayatilleke wall function for turbulent thermal boundary layers also uses an if statement, much in the same fashion used in Fluent according to equation (10.8.5) in http://jullio.pe.kr/fluent6.1/help/html/ug/node451.htm (with the difference in notation of yStar, identified as yPlus in OpenFOAM).
I don’t think you have understood what I said. You can’t tell if OpenFOAM uses ‘the fourth root of fourth powers of friction velocities’ to compute wall shear by merely looking at BC’s, because the BC and the wall shear calculation are two separate things.
In terms of yPlus and yStar. I believe OpenFOAM has got both in its wall functions. Depending on whether non-dimensional wall distance is computed by wall-normal velocity gradient or turbulent kinetic energy, the wall function uses the appropriate formulas.
Yes, Dylan, I have been experiencing some difficulty in learning (again or by the first time) what I have not looked at as much as I would like to in these 20 years past since I took a formal course in Turbulence Modeling - where we could not simulate very practical situations using an open source Fortran77 2D code (similar to a famous one used by Patankar in teaching, restricted to cartesian grids) and an under-stimulated access to Fluent. Not to speak about my poor communication in English skills…Thanks for worrying about my difficulties and helping me!
I think that, In some contexts, the phrase “shear stress calculation” may be not just referring to that one used for obtaining a friction drag coefficient, some kind of head loss, or the hydrodinamical load over a surface (at the post-processing stage of a CFD simulation). It may refers also to that of shear stresses implicitly involved in the diffusive terms of the RANS equations and in the production “source” terms appearing in the k and epsilon/omega transport equations (in an internal product by the deformation rate tensor that amounts to power/volume). Since we are talking about subjects scarcely found in a few textbooks I want to praise a recent and comprehensive one, by Moukalled et alii, that I just knew http://www.springer.com/gp/book/9783319168739
About that fourth root of fourth powers of friction velocities, I would like to say that I have nothing against it (is a recommendation of the greats Menter and Esch), but I feel that OpenFOAM developers did not deemed its implementation advantageous.
I have thought what the title of the page https://www.cfd-online.com/Wiki/Near-wall_treatment_for_k-omega_models said exactly - wall treatments of omega-based models. Under no condition, would I know that you also referred to the source terms, and I have explicitly used the term ‘wall shear’. But…thanks for clearing that out.
By the way, I wouldn’t call it implicit if it is a source. Do you mean implicit source as in the source term is solved iteratively?
I have stopped thinking ( and see no point ) why something is not implemented in OF. I simply implement what I believe is right. It is good, though, that you have clarified the BC’s on SimScale.
I have to say I am clearly less experienced ( and have spent less time ) in CFD than you. The only kind of stuff I do is write solvers in OF.
Coming back to the important stuff:
- I wouldn’t solve SA the way it is done on SimScale from what you have described - I wouldn’t use k to predict y+ in this case.
- I would use either the Spalding law or the improved wall function for nut by Allmaras et al (2012)
- I am hardly a SST user, maybe you can point out the advantage of ‘fourth root of fourth powers of friction velocities’.
I have to say that presently my intention is basically to use SimScale to give a taste of CFD to the undergraduate students to whom I give Transport Phenomena classes. Thanks to SimScale I was able to simulate experiments that these students perform in laboratory and I am satisfied by the agreement of these simulations with the measurements (of wall pressure, mean velocity using Pitot tube) and calculations (of Nusselt number) made by them. The question about the automatic wall function was important to give the students correct information about what happens as the grid is refined, especially near walls (and why people worries less about the y+ now than in the past). Precise information will be even more important for reporting this didactic experience to my peers. So I am not doing any ambitious research in turbulence modeling.
I believe that the reason for using the “fourth root of fourth powers of friction velocities” blending formula is essentially the same for using the Spalding continuous wall function or the analytic solution of the SA equation: to obtain smoothly varying (and agreeing with DNS and experimental results) quantities as we refine the grid and have cell centers in the range 1< y+ < 30. My guess is that numerical tests revealed that the simple switching (at the intersection found near y+ = 11.6) between the wall function and the viscous formula for shear stress, is already sufficiently smooth and accurate (agrees satisfactorily with DNS and experiments) when used with k-omega SST or SA turbulence models. Only the omega calculation (in k-omega SST) really demanded the use of a special blending formula. What should be deemed as sufficiently accurate is a rather subjective matter, but it can be argued that the wall function approach is not for those requiring a very high accuracy - they have to go for the full resolution of the boundary layer.
It is interesting that your computations have shown y+ insensitive results on wall pressure, even when the wall unit is in the buffer layer.
I miss some basic information about the respective models.
For instance, I can choose “k-omega”, right, but what model do I actually get then?
Is it the Baseline k-omega model (which I suspect it is) or is it the Wilcox-model?
Good question @bastian!
It is the Wilcox-model, here more information: https://www.cfd-online.com/Wiki/Wilcox’s_k-omega_model