Imagine two fluid flow situations: Firstly, a fluid of density ρ_F and viscosity η flows with average speed v in a cylindrical pipe of diameter D. In the second, a particle of diameter d and density ρ_P settles at speed U, in a stationary fluid of density ρ_F and viscosity η contained in a cylindrical vessel of diameter D. Explain how you would define the Reynolds number of the flows in the two situations.