“…Infinite time linear growth of ∇ 2 ω 2 L ∞ and arbitrarily long but finite time exponential growth of ω L ∞ are achieved in the current work for C ∞ c (R 3 )-vorticity. In the presence of a physical boundary, obtaining growth is strictly simpler since the vorticity may not vanish on the boundary, leading to a stable growth mechanism; see [20,19,17,9] and the references therein. When the boundary is absent but if the domain is periodic with respect to an axis, then it is well-known that one can use the 2 1 2 -dimensional flow construction to obtain linear in time growth of the 3D vorticity (see [21,39,4,32]).…”