Function viscous_diffusion_kernel 

pub fn viscous_diffusion_kernel<R>(
    nu: &KinematicViscosity<R>,
    laplacian_u: &[R; 3],
) -> AccelerationVector<R>
where
    R: RealField,

Viscous diffusion acceleration ν · ∇²u.

Linear in both nu and laplacian_u. Returns the component-wise product scaled by kinematic viscosity. Vanishes in the inviscid limit ν = 0.