VOF and BED ROUGHNESS

Nial Horton

Dear friends,

i am a research assistant using gambit and fluent to study shallow flow in the river ehen in the lake district.

i have been using a fixed lid approach with the k-epsilon model and have found it very difficult to accurately describe the bed roughness. it says in the user manual for fluent that "it is not physically meaningful to have a mesh size such that the wall-adjacent cell is smaller than the roughness height. For best results, make sure that the distance from the wall to the centroid of the wall-adjacent cell is greater than the roughness height".

My cell size was created from a edge mesh size of 0.1 so i was hoping that you may be able to tel me a range of roughness sizes that would be suitable to investigate.

Secondly, due to the depths being studied it is becoming less and less practical to use the fixed lid method and i have studied the use of the VOF model. i split the inlet and outlet into two by specifying a normal distance from the bed and creating an isovalue region and now have patched the water and air into the respective volumes but i cannot get the simulation to converge between 10 and 20 iterations even when using a very small step size.

anything u can suggest to remedy this problem would be amazingly helpful

thank you very much for your time

Nial