Vorticity is central to fluid dynamics. It measures local rotation. It governs turbulence. It structures wakes, boundary layers, mixing. When a fluid dynamicist wants to understand a flow, vorticity is usually the first field they examine.
Abramov studies two-dimensional Poiseuille and Couette flow with van der Waals effects in a periodic channel. The flow is macroscopically laminar — no turbulence — but exhibits complex dynamics in density and velocity. Power spectra show structured decay in density fluctuations, velocity divergence, vorticity, and kinetic energy.
Then the test: fix vorticity at its background value, removing it as a dynamic variable. Keep only density and velocity divergence free. The resulting dynamics and power spectra are qualitatively unchanged. Vorticity was not contributing to the spectral structure. It was present in the full equations but irrelevant to the physics generating the observed behavior.
The power spectra live in density and velocity divergence. Vorticity, the field that every intuition says should matter, is along for the ride. In this flow, the compressible degrees of freedom — the ones usually treated as secondary corrections to the dominant rotational dynamics — are actually primary.
The hierarchy is inverted. The variable that fluid dynamics centers its theory around contributes nothing to the observable dynamics of this system. The dynamics are not where the theory expects them to be.