Non-equilibrium length in granular fluids: From experiment to fluctuating hydrodynamics

Velocity correlations in a quasi-2D driven granular fluid are studied in experiments and numerical simulations. The transverse velocity structure factor reveals two well-defined energy scales, associated with the external "bath temperature" T-b and with the internal granular one, T-g < T-b, relevant at large and small wavelengths, respectively. Experimental and numerical data are discussed within a fluctuating hydrodynamics model, which allows one to define and measure a non-equilibrium coherence length, growing with density, that characterizes order in the velocity field.