  For vertical velocity field $v_{\rm z} (r,z;R)$ of granular flow through an
aperture of radius $R$, we propose a size scaling form $v_{\rm z}(r,z;R)=v_{\rm
z} (0,0;R)f (r/R_{\rm r}, z/R_{\rm z})$ in the region above the aperture. The
length scales $R_{\rm r}=R- 0.5 d$ and $R_{\rm z}=R+k_2 d$, where $k_2$ is a
parameter to be determined and $d$ is the diameter of granule. The effective
acceleration, which is derived from $v_{\rm z}$, follows also a size scaling
form $a_{\rm eff} = v_{\rm z}^2(0,0;R)R_{\rm z}^{-1} \theta (r/R_{\rm r},
z/R_{\rm z})$. For granular flow under gravity $g$, there is a boundary
condition $a_{\rm eff} (0,0;R)=-g$ which gives rise to $v_{\rm z} (0,0;R)=
\sqrt{ \lambda g R_{\rm z}}$ with $\lambda=-1/\theta (0,0)$. Using the size
scaling form of vertical velocity field and its boundary condition, we can
obtain the flow rate $W =C_2 \rho \sqrt{g } R_{\rm r}^{D-1} R_{\rm z}^{1/2} $,
which agrees with the Beverloo law when $R \gg d$. The vertical velocity fields
$v_z (r,z;R)$ in three-dimensional (3D) and two-dimensional (2D) hoppers have
been simulated using the discrete element method (DEM) and GPU program.
Simulation data confirm the size scaling form of $v_{\rm z} (r,z;R)$ and the
$R$-dependence of $v_{\rm z} (0,0;R)$.
