1. The problem statement, all variables and given/known data Which fluid is more viscous at moderate to high Reynolds number if used in this situation? Air or Water? The flow is incompressible and has velocity V and size L. 2. Relevant equations Re = inerial forces/viscous forces = rho*V*L / mu as Re approaches infinity, viscosity approaches zero 3. The attempt at a solution I know that if Re is high, inertial forces dominate. Therefore, viscous forces are small. inertial forces / Re = viscous force => wouldn't water always be more viscous assuming the same Re, V, and L is used for both air and water? I think there is more to this problem that I am missing. Thanks for any help.