Function reynolds_stress_kernel 

pub fn reynolds_stress_kernel<R>(
    u_prime_outer_u_prime: &StrainRateTensor<R>,
) -> ReynoldsStress<R>
where
    R: RealField,

Reynolds-stress tensor R_ij = ⟨u'_i u'_j⟩ packaged as a ReynoldsStress<R>.

The input is the already-averaged outer-product tensor (symmetric by physical construction); the kernel is a typed pass-through that pins the symmetric-tensor invariant in the output. Diagonal entries are non-negative (variances).

The dedicated ReynoldsStress<R> output type distinguishes this from viscous stress at the type level — it cannot be passed into viscous_dissipation_rate_kernel or entropy_production_rate_kernel without an explicit conversion.