Learning Forum

[beeeel]

I'm trying to model fluid dynamics for a biological cell. Although I have some experience with other forms of CAD, this is my first time using Ansys products, so I've probably made all the rookie errors. Most of the issues I've come across so far have been related to the small size of my model, I believe.

I made a workbench project, and yesterday, I started a new Design Modeler geometry. I made a sphere with radius 10um, centred on the origin, and a box that is 1x1x0.5mm, slightly off-centre. Today I load the files, and although the object are all appearing in the Tree Outline, they don't display properly. I've ensured that I'm zoomed to the correct level, but when I select the sphere, nothing appears on screen. Also, when I rotate the view, it's very choppy, and the (x,y,z) orientation display in the corner decided to scale itself to fill the screen for a little while.

Now I have deleted the sphere and made another, centred on the origin, of the same size, and it appeared in the correct place before generating, but I can't see anything after generating it.

Beyond simply "don't make really small models", what can I do to improve my experience?

[Rob]

CAD tools tend to struggle with micron level dimensions as you're building models at the level of the geometry kernel tolerances. You also need to review the system Knudsen Number to ensure the solver will be valid too. 

[beeeel]

Thanks for that advice. Regarding micron level dimensions, is it possible to scale the geometry kernel (or something else in the modelling/meshing settings) to be more appropriate? If not, should I instead create a larger model, and adjust the material properties (e.g. viscosity) of the fluid to compensate?

Regarding Knudsen number, again thanks, I hadn't thought of that. I estimate that for micron-sized objects in water, this will be >100 as free path length is on the order of molecular scale

[Rob]

We can scale in Fluent, I've typically built models in "mm" and scaled down to microns. Be aware that CFD solvers tend not to be designed to run at micron scales so may be missing critical physics.