Ratio of S to π r ² _g of growing aggregates in the simulations for various impact velocities and critical rolling displacements. The solid lines show the resultant aggregates in our simulations, and the dotted line indicates that of BCCA clusters. We also plot the cross sections obtained from Okuzumi et al.'s expression (Equation (7)) with dashed lines. Okuzumi et al.'s expression reproduces the cross sections well even for compressed aggregates.