A form of the Navier-Stokes equation which is ρ∂u /∂t = (∂/∂x )(p_xx - ρu²) + (∂/∂y )· (p_xy - ρuv ) + (∂/∂z )(p_xz - ρuw ), where ρ is the fluid density, u, v, and w are the components of the fluid velocity, and p_xx , p_xy , and p_xz are normal and shearing stresses.