In starcd why reference cell is needed? If we specify all BCs it is very confusing to give reference cell. I have observed that results change significantly if you change ref cell. How to decide where to give reference cell? kindly guide.

Hi, if you have an inlet boundary where you have specified an specific temperature and pressure, you should put your reference cell in this boundary region. In some calculations, star use reference values e.g. density,temperature,specific heat... taken from this one cell to calculate some average values in the rest of the domain. hope this helps.