Abstract: Hydrodynamics is an effective theory which describes the evolution of a near-equilibrium system, which is locally in equilibrium, towards its global-equilibrium state. Local thermal equilibrium corresponds to long wavelength variation of local thermodynamic quantities; So it is possible to express them in terms of "derivative expansions". Taking into account the nonlinear thermal fluctuations, derivative expansion will not truly work at the second order. We discuss the range of validity of second order hydro in the heavy ions collision simulations.