Torque calculation in centrifugal pump
 

I am simulating centrifugal pump and I want to calculate the input power to the pump at steady state. So I want to calculate torque on the impeller. Can I presume that it is same as drag*rad_distance on the impeller cells. If so how to calculate drag on rotating machines? in short how to calculate torque exerted on impeller? Please help if anybody knows it. Thanks