# Reynold's Number

1. Nov 11, 2014

### joshmccraney

Hi PF!

Can you help me out with determining a Reynold's Number over an infinite plate? I know it to be $U L / \nu$ but $L$ isn't exactly defined. Would it be something more like $U \delta / \nu$ where $\delta$ is a vertical distance, say, the distance of the BL?

Thanks!

2. Nov 11, 2014

It depends on your application. Boundary layer thickness is used sometimes. Momentum thickness is used sometimes. The distance along the plate is common. Sizes of small feature can be used if they exist. It just depends the problem at hand.

3. Nov 11, 2014

### joshmccraney

So is it arbitrary or is there a better thought process behind what is happening?

4. Nov 11, 2014

It is a little bit arbitrary. Really it's all about how the governing equations are normalized, and that all depends on the physics you hope to study.

5. Nov 11, 2014

### Staff: Mentor

I think by "normalized,", boneh3ad means "reduced to dimensionless form."

Chet