Stokes number for intake duct bend
 

I am trying to figure out the characteristic length and velocity for a gas turbine intake duct to get the Stokes number for particel injection into that duct. The duct starts off as vertical, rectangular and merges into an annulus duct in a 90 Deg bend. The injection takes place into the annulus duct. Would the length of the annulus and the average fluid velocity be an appropriate measure? Thanks, Friederike