Stokes number and gravity
 

Hi,

I did not find an offtopic section, so I hope this place is OK as well.

I am about to validate some simulations of mine with PIV measurements. There are several seeding particles available and I would like to chose the right one.

I chose the Stokes number to evaluate my particles.

The test chamber is approx. 50 cm long and has several obstacles inside it. The flow is horizontal and its velocity ranges from 1 m/s to 10 m/s.

My first calculations looked like this: St = ( (d_p)² * rho_p * u) / (18 * http://upload.wikimedia.org/math/b/7...ff40274044.png * L_char)

d_p is particle diameter 3*10^(-6) m
rho_p is particle density 912 kg/m³
u is velocity 1 m/s
http://upload.wikimedia.org/math/b/7...ff40274044.png is the viscosity of air 17100 kg/(m*s)
L_char is the length of the test chamber 0.5 m

and I get St = 5*10^(-14) for 1 m/s which seems good and St = 1,48*10^(-10) for 10 m/s, which seems good as well.

But some questions remain:
- What do you usually use for the characteristic length? Is the length of the chamber the right choice?

- how do I check for the influence of gravity? As far as I understand my calculation of the Stokes number only looks at changes in direction of the flow.

Thanks for your help.

You want to pick something that has the closest density to the fluid as possible, as small as possible and has the least variation in shape with each particle. Obviously round is preferred because a sphere is the most studied shape.

Yep, at the moment I am evaluating, DEHS particles from 1 to 5 micrometers and hollow glass microspheres around 30 to 50 micrometers in diameter.

But I need to be sure, at least on the theoretical side, that this will work.

I didn't understand what exactly you are trying to do here.

However assuming it is a low Re flow, you might find this useful http://en.wikipedia.org/wiki/Galilei_number

Or try using the good old Pi theorm to find the dimensionless groups.

OK, I just remembered most food coloring has a specific gravity of 1. I noticed your particle density is 912 kg/m3, so I'm assuming your fluid is water?