# Homework Help: Reynolds Number

1. Feb 12, 2015

### motherh

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.

2. Feb 12, 2015

### DEvens

3. Feb 16, 2015

### motherh

That Wiki helps a little but I'm still confused by this. So the Reynolds number is given by Re = ρUL/µ? As U is the velocity scale it makes to take U as V here. But after this I don't know how G comes into the Reynolds number.