# Calculating the Reynolds Number (Re) for a non-spherical object?

1. Oct 29, 2009

### vette982

I have the density (ρ) of the particles and the fluid, as well as the complete dimensions of the non-spherical particles. I know the Force of drag (F) and the Coefficient of drag (C). But how do you get the Reynolds number from this? I can't use Stokes' equations because they only apply to spheres.

2. Oct 29, 2009

### FredGarvin

What? Do you know what the Reynolds number is? What are doing that you need to know the Re?

3. Oct 30, 2009

### redargon

4. Oct 30, 2009

### stewartcs

I presume you are referring to the Reynold's number as associated with the determination of drag coefficients - yes?

For non-spheres and cylinders it is common for some shapes to use the length of the body parallel to the flow as the characteristic dimension. So the normal relation one would use in fluid mechanics for pipe flow becomes:

$$N_R = \frac{vL}{\nu}$$

Note that the only change is that the length of the body parallel to the flow(L) is used instead of the pipe diameter (D).

For complex shapes it is always recommended to test them for the proper drag coefficient.

Once you calculate the Reynold's number you can pick off the drag coefficient from a chart for that particular shape and then determine your drag force.

Hope this helps.

CS