Accuracy drops when using stl instead of geometry


We are working on flow past a sphere. The sphere was created using the geometry class. Results was not too good initially but after some hyperparameter tuning, we got quite reasonable result compared to OpenFOAM.

After tt, we switch to using stl format to define the sphere, everything else being the same. Sampling shows no difference in the results but then the force output was quite bad. We tried different stl files but the result is still bad.

We wonder what could have gone wrong. Anyone has experience in this area? Thanks!