rho_water 1000[kg/m^3] Density, water
mu_water 1.51e-3[Pa*s] Dynamic viscosity, water
r_grain 1[mm] Radius, grain
V_grain 4/3*pi*r_grain^3 Grain volume
rho_grain 2900[kg/m^3] Grain density
m_grain V_grain*rho_grain Grain mass
F_g -m_grain*g_const Gravitational force on grain