Stress tensor divergence terms are incomplete?

[ali-ramadhan]

Maybe I'm misunderstanding how the stress tensor divergence is being computed but looking at the stress tensor components written out explicitly in matrix form (see third posted image), and assuming the volume viscosity μᵥ = 0 and the viscosity is constant, then the x-component of the divergence of the stress tensor should be ∂ⱼτ₁ⱼ = μ∇²u + ⅓μ∂x(∇·u⃗) but the following operators seem to be missing terms (e.g. shouldn't viscous_flux_ux include a ∇·u⃗ term and not sure how the factor of 1/3 ended up in the strain rate tensor).

https://github.com/thabbott/JULES.jl/blob/3dac4ab6bc7d7f3a9f84c6240aa90dd365936ff5/src/Operators/compressible_operators.jl#L170-L196

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Some helpful material from Kundu et al. 6th edition: