Hi all:

I would like to know wether the CFD code captures sudden expnasion and contraction pressure losses in the solver? If so how is it calculated, which is that term. Curious to know.

If the answer to the above question is "yes", then why we are building loss models seperatly for some critical regions such as tip clearance, and so on.

I heard we manually insert some loss terms for a complex turbulence models. Why is it ncecessary, if the solver is capable of doing so (neglecting 100% accurate prediction in wall shear).

In general the answer is yes - CFD codes do capture losses due to sudden expansions and contractions. To compute the loss you consider the pressure before and after the geometry change, either as a value averaged over some normal face or as an instantaneous value following a streamline. In tip regions in turbines CFD can also reasonably compute the losses if the grid resolution is good. However, you bring up another issue that is only indirectly related - turbulence modeling. No solver is 100% capable of predicting wall shear accurately, because turbulence models are just that - models of more complicated physical behavior, and those models are not uniformly accurate.