  In practically all turbulent flows, turbulent energy decay is present and
competes with numerous other phenomena. In Kolmogorov's theory, decay proceeds
by transfer from large energy-containing scales towards small viscous scales
through the "inertial cascade." Yet, this description cannot predict an actual
decay rate, even in the simplest case of homogeneous isotropic turbulence
(HIT). As empirically observed over 50 years, the steepness of the "infrared"
spectrum - at scales larger than energy-containing eddies - determines decay,
but theoretical understanding is still missing. Here, HIT decay laws are
derived from angular momentum invariance at large scales - an approach first
mentioned by Landau in 1944, but unduly dismissed later. This invariance also
predicts the so-far unexplored turbulence decay in layer, tube, and spot zones
in a fluid at rest. Beyond expanded and simplified theoretical descriptions,
these findings suggest new practical modeling strategies for turbulent
dissipation, often deficient in applied simulations.
