# Material Derivative's advective non linear term

I have read this question and the accompanying answers - Linear Vs Non Linear Differential Equation as well as the useful comments and my question is related. I am in fluid dynamics and I deal with the material derivative on a daily basis and I recently read that the advective term

Here is the relevant text from the above link

"Note that the advection of momentum $(V · \nabla)V$ is non-linear and this is the term that leads to much of the interesting behaviour in fluid mechanics."

i.e $$\mathbf u.\nabla A$$

where u is the flow velocity and A is any vector field is non linear and as a result the whole material derivative is non linear due to the presence of a single non linear term.

What exactly is the reason for the non linearity in the advective term of the material derivative ?

Is it because of the product of a tensor ( $\nabla A$) and a vector ?

No, the nonlinearity comes specifically from having the same vector field in both slots in the original advection term $V\cdot(\nabla V)$. Note, in particular, that $(cV)\cdot\nabla(cV) = c^2\,V\cdot(\nabla V)$ (and when you replace $V$ with $V+W$, you likewise get cross-terms in the resulting formula).
• No, homogeneity is violated too. Linearity should pull out $c^1$, not $c^2$. – Ted Shifrin Dec 23 '16 at 0:56