Value of the factor f_hydro (see Eq. (18)) indicative of a reduction of the linear cross section of planetesimals resulting from hydrodynamical effects as a function of dust particle size, for a planetesimal of 1 km radius in an MMSN disk, at an orbital distance of 7.1 au (plain) and 1 au (dashed) and for various levels of turbulence (as labeled). The results are compared to the one obtained by Guillot et al. (2014) which is independent of turbulence amplitude. For the 7.1 au case (which corresponds to Re_p = 400 as in the previous hydrodynamical calculations), the lines corresponding to α = 10^-4 and 10^-6 are hidden behind the laminar case (see text).