Beginner need advice On External CFD

Hi there,

I am new to the CFD and I have tried to run several tutorial projects but I need some assistance on the projects.

  1. I tried the the " User Guide: External Aerodynamics around a vehicle" I could not find the “Material Point” Option. I could only find “Background Mesh Box”. Where can I find the Material point option?

  2. There are the rule of thumb on mesh box size and several other numbers. How can I find out how each numbers came about? While waiting for Question 1, I am doing “Tutorial: Incompressible turbulent airflow around a front wing” and there are many more parameters that I would like to know how the numbers come about.

  3. How do I do a multiple faces/ box select? in the front wing tutorial, I ended up using right click select all visible parts after hiding the non wanted faces. Is there a drag box or shift-click kind method?

  4. When meshing and simulating, how do I get a preview of estimated time before starting and estimated remaining time? All I see is the percent completed and how long the simulation has been running.

  5. Lastly and perhaps the most newbie question, how do I reverse the rotation direction of my mouse?

Thank you beforehand for your help!

Hi @bna,

  1. If it is an external CFD, Material Point is possibly not available anymore for HEX mesh (automatic at least). Material Point should be set outside the simulated geometry, somewhere in the space around, but I believe now it is not necessary to do it by hand. Perhaps doing parametric HEX mesh you can still manipulate it.

  2. There is new document available now (March 2020) about quality meshing: High quality mesh .

  3. When preparing a mesh, please use ‘Topological entity set’ option, allowing you to group geometry faces. This is manual and your current method is correct. There is no shift-click or drag box.

  4. There is just the percent of work already done. For HEX mesh, you have log file with timestamps, so you can estimate the remaining time. For simulation, you have convergence plot and you see how many steps were already performed. Not direct ETA is provided.

  5. On Windows 10, the reverted scrolling of mouse can be done following that advice: Mouse reverse scrolling




Hi Retsam,

Thanks for the quick reply. I got a few more followups if you don’t mind

  1. Can you explain in short why the material point is not relevant enough anymore for the hex mesh?

  2. Is there a guide that is one step above the tips that were given in the guide? How about setting the domain/base box, is there also a guide?

  3. I couldn’t find the timestamp, Can I read a guide on the content of a mesh log file? For convergence plot, my understanding is that 1e-5 is well converged. the data from my current simulation only shows up to 600 seconds with only omega on a level below that. Is there a tutorial on how to read a convergence plot too?

  4. So there is no simscale setting? that is, I have to change my whole windows scroll setting?

  1. Material point was used to define external CFD (set outside a geometry) or internal CFD (set inside a geometry).

  2. Domain or BMB for external CFD should be 5 - 20 times bigger than your geometry in order to minimise the impact of back pressure on simulation. For HEX mesh, cell aspect should be regular as possible (1:1:1). If your BMB is 10 x 10 x 20 m, it would mean that you need a grid of at least 20 x 20 x 40 elements which will make the level 0 cells. Finer grid could help in convergence, as you know already from guide. When using automatic HEX or TET mesher, you do not have access to those parameters of mesh: mesher will do it for you.

  3. There is possibly no guide on log file. For the moment it may be not necessary: you will only go to that level while in trouble. Convergence is obtained if your curves start to be flat. Pressure, k, omega and others have different levels, so do not wait on all being the same 1e-5 like omega, for instance.

  4. No SimScale setting for mouse scroll, sorry. I thought you wish to switch globally.

  1. For the Cell Aspect, is it possible if I don’t double the BMB and just use the other refinements? That is my box might not be even level 0 but my part would be level 7? And for Feature refinement, how is it treated? is it sequentially or is there a refinement priority or some other way when some areas overlap?

  2. Could you please have a look at my public project? I reduced the refinement but I am getting Cd that is so big. I tried looking at other people’s post but I am not sure how to apply the fix to my project.

  3. The tutorial I followed has a few crucial differences between what is written on the tutorial and the settings that the person actually use on the project file. e.g the K and omega value. Could you also please have a look at that as well and tell me how do I report the discrepancies?

Stay healthy!

  1. Please take a look at documentation provided: Hex mesh main settings

  2. Please paste the project link to the forum.

  3. Tutorials may be outdated: you need to check the matching parts and adjust your project. This is an iterative process, but this is the only way to learn on ‘living’ complex tool like SImScale.

Take care,