Evolution of the Lyα escape fraction f_esc(Lyα) as a function of dust optical depth   for various values of expansion velocity v_exp and mean neutral hydrogen column density . The clumpy medium studied here is built with FF = 0.23, CF = 0.997 (R_min = 49 and R_max = 64 cells), n_ic/n_c = 0.01, and b = 40 km s^-1. The expansion velocities v_exp and H i column densities that we adopt are v_exp = 0, 100, and 250 km s^-1 and = 10¹⁹ cm^-2 and 2 × 10²⁰ cm^-2. It is seen that f_esc(Lyα) increases with increasing v_exp, as well as with decreasing .