Explicit form of spatially linear Navier-Stokes velocity fields
We show that a smooth linear unsteady velocity field $u(x,t)=A(t)x+f(t)$ solves the incompressible Navier--Stokes equation if and only if the matrix $A(t)$ has zero trace, and $\dot{A}(t)+A^{2}(t)$ is symmetric. In two dimensions, these constraints imply that $A(t)$ is the sum of an arbitrary time-dependent traceless symmetric matrix and an arbitrary constant skew-symmetric matrix. One can, therefore, verify by inspection if an unsteady spatially linear vector field is a Navier--Stokes solution. In three dimensions, we obtain a simple ordinary differential equation that $A(t)$ must solve. Our formulas enable the construction of simple yet unsteady and dynamically consistent flows for testing numerical schemes and verifying coherent structure criteria.