Comparison of the density vs. velocity for models r6e4BH and r0e2 at ~5 d after explosion. The dashed lines represent fits to the density distribution using the expression log ρ(V) = log ρ(V = V₀) + N_ρlog (V₀/V), where V₀ = 30 000 km s^-1 and N_ρ = 7.5 for model r6e4BH and V₀ = 20 000 km s^-1 and N_ρ = 9.5 for model r0e2. The greater the energy of the explosion, the more material is present at large velocity. In spherical symmetry, radiation hydrodynamics cannot produce high density regions at both low and high velocity. However, by adopting an axisymmetric configuration it is possible, for example to have dense material at high velocities in the polar direction, and at low velocity in the equatorial regions.