We consider a model equation for the Navier–Stokes strain equation which has the same
identity for enstrophy growth and a number of the same regularity criteria as the full
Navier–Stokes strain equation, and is also an evolution equation on the same constraint
space. We prove finite-time blowup for this model equation, which shows that the
identity for enstrophy growth and the strain constraint space are not sufficient on their
own to guarantee global regularity for Navier–Stokes. The mechanism for the finite-time
blowup of this model equation is the self-amplification of strain, which is consistent with
recent research suggesting that strain self-amplification, not vortex stretching, is the main
mechanism behind the turbulent energy cascade. Because the strain self-amplification
model equation is obtained by dropping certain terms from the full Navier–Stokes
strain equation, we will also prove a conditional blowup result for the full Navier–Stokes
equation involving a perturbative condition on the terms neglected in the model equation.