  We investigate experimentally the decay of three-dimensional hydrodynamic
turbulence, initially generated by the erratic motions of centimeter-size
magnetic stirrers in a closed container. Such zero-mean-flow homogeneous
isotropic turbulence is well suited to test Saffman's model and Batchelor's
model of freely decaying turbulence. Here, we report a consistent set of
experimental measurements (temporal decay of the turbulent kinetic energy, of
the energy dissipation rate, and growth of the integral scale) strongly
supporting the Saffman model. We also measure the conservation of the Saffman
invariant at early times of the decay and show that the energy spectrum scales
as $k^{2}$ at large scales and keeps its self-similar shape during the decay.
This letter thus presents the first experimental evidence of the validity of
the connection between the Saffman invariant and the $k^{2}$-energy spectrum of
the large scales. The final decay regime closely corresponds to Saffman's model
when the container size is sufficiently large.
