Could someone please explain why hexahedral cells are preferable to tetrahedral cells, I have a complex geometry that I have not been able to mesh in anything other that tets, will this give some error in my results?

You don't necessarily get wrong results, it's just more difficult to get good results with tets since you can never align a tetrahedral element with the flow. It also takes a whole lot more tet cells than hex cells. So you need to take some precautions: 1) Try to keep boundary layers hexahedral 2) Use second order discretization 3) Be extra anal about tet cell skewness

hi Phil, my experience is very bad with tets. Lot of numerical diffusion is expected even with second order discretisation. if you have good experimental data for analysis of results, you go ahead. good luck