Question

why sometime chose (viscosity and velosity)as repeats parameters

Answer 1

u mean viscosity and velocity right? and do u mean repeating the values of parameters?
Of course to get average data (closer to accurate value, though nvr existed physically). In theory we may obtain an accurate value or known as theoretical value.I think this is an essay question right? Then u may give some examples of experiments involving velocities and viscosity.. blah blah. All the best!