1. The problem statement, all variables and given/known data I don't fully understand the Reynolds number and it has arisen in a problem. It says: Fluid with viscosity µ and density ρ fills the gap between two parallel plates at z = 0 and z = h. The upper plate at z = h moves with speed V in the x direction, while the lower plate at z = 0 is stationary. The fluid is also subject to a pressure gradient −G in the x direction. 2. Relevant equations I have solved for the velocity u (no idea if this is needed for the Reynolds number bit at all) to find u(z) = Gz(h-z)/2µ + Vz/h. 3. The attempt at a solution The question then says: What is the Reynolds number for this flow? What are the conditions on V and G for it to be small? Can anybody help at all? It would be much appreciated. All my notes really say is that Re = ρUL/µ or Re = ρL^2/µT but I don't understand the U, L or the T.