I used a pressure inlet and pressure outlet bc for inlet and outlet respectively in a 2d case, i want to use the simulated velocity at inlet to calculate the Reynolds number, so i use coustmer defined function panel, and I chose "select operand field function from (VELOCITY->Velocity Magnitude )*(GRID->Y coordinates)/(PROPERTIES->Molecular viscosity), this means U*D(pipe width)/viscosity, but the results seems not right. any one who konw how to do this? or anyother easier method? thanks you very much

Try the following definition: Re = U*D*density/viscosity

I think you have mixed up dynamic and kinematic viscosity. In your equation "viscosity" represents the kinematic viscosity but Fluent provides the dynamic viscosity.

kinematic visc = dynamic visc / density