Top panel: horizontal flow divergence of the average supergranule in the non-rotational hydrodynamic simulation. The cyan arrows represent the horizontal flows at the surface. Middle panel: same as the top panel but for the rotational hydrodynamic simulation. Bottom panel: azimuthal averages of the radial velocity ($(x{v}_x+\mathit{y}{v}_{\mathit{y}})/\sqrt{x^2+{\mathit{y}}^2}$) and polar velocity ($(\mathit{y}{v}_x-x{v}_{\mathit{y}})/\sqrt{x^2+{\mathit{y}}^2}$ ) of the flows for the average supergranule. All quantities are measured relative to the centre of the supergranule. The polar velocity is defined as positive in the clockwise direction.