Cd is a function of the Reynolds number (Re), and will not be anything like constant at low values of Re so you have to evaluate it at the appropriate Re, but if its highly turb the value will be approximately independent of Re and hence whether its water or gas ( a factor of 15 difference)
Try looking at Levenspiel's "Chemical Reaction Engineering" for some deeper understanding. But in reality it will depend upon the nature of the fluid/suspension, if for example it is a yield stress fluid all you will do is short circuit and never mix the tank.
