Capillary forces in Comet
 

I recently tested a simple free surface problem in Comet - rise of water between two plates by capillary force and the code gave an error of about 10%. (The plate distance was 1 mm and the contact angle 45º.) Does anyone have any suggestions as to why this may have occured? So far tech support hasn't been able to solve the problem.

Re: Capillary forces in Comet
 

On what is the error based with this simple test case? 10% is the error on what? Could you be more explicite?

Re: Capillary forces in Comet
 

With a 45º contact angle and a surface tension of 0.07275 N/m water should rise 10.5 mm up between the plates. The best I am getting with Comet is a rise of 9.625 mm and the interface line between the air and water is very diffuse, over 40 cells between 0 and 1 concentrations. (I had to use a cell density of 32 cells/mm to get an answer even this accurate.) Note that using the same conditions with StarCD I get the correct answer of 10.5 mm with a tight air/water interface.

Re: Capillary forces in Comet
 

It sounds to me that you are running a free-surface flow without using the HRIC scheme. What kind of discretisation scheme do you use and with which blending factor? Does it happen that you are running a too big a time-step? Remember that STAR-CD does subcycles if the time step is too large. Finally, 32 cells in the gap is a minimum, if the cells are equidistant. You have to capture the high gradients of concentration at the wall.

Re: Capillary forces in Comet
 

I tried cd, ud and hric. Blending was kept at 1.0. I am using a time step of 1e-4 sec with 50 inner iterations. If >32 cells per mm are required then Comet is unusable for my real model as it is a complex 3D geometry of about 11,000 mm3.