Generalized hydrodynamics and heat waves
By using the evolution equations of generalized hydrodynamics we investigate heat-pulse propagation in a Lennard–Jones liquid contained in the annulus between two concentric cylinders at different temperatures. It is found that the heat pulse propagates as a wave of a finite speed when a composite fluid dynamic number [Formula: see text] that depends on the thermal conductivity and wall temperature ratio is above a critical value, but in the subcritical region the heat pulse propagates diffusively as if predicted by a parabolic differential equation with an infinite speed of propagation. Therefore the question of the hyperbolicity of the system of differential (evolution) equations used is mainly determined by the parameter [Formula: see text]. This implies that the hyperbolicity of evolution equations, i.e., the finiteness of pulse-propagation speed, cannot be the main reason for extending the thermodynamics of irreversible processes as believed by some authors in the literature. This study indicates that for a liquid of high thermal conductivity or a large temperature difference the Fourier law of heat conduction is inadequate for use in the description of the temporal evolution of heat and a suitable generalization of hydrodynamics is necessary. The generalized hydrodynamic equations presented in this and previous papers are examples for such a generalization.