On the influence of the stress tensor and heat flow vector in collisional multispecies fluids

The effective influence of the stress tensor and heat flow vector in collisional multispecies fluids is quite a controversial matter. Suggested by phenomenological laws these parameters are usually evaluated with the simple Newton and Fourier relations. The kinetic theory attributed them to the motion and interactions of particles. The validity of the Navier–Stokes set of equations with Newtonʹs-law of stresses and Fourierʹs law of heat flow has been contested in many cases based on simple traditional models of the collision frequency. Thanks to many recent advances on the evaluation of the Boltzmann collision term, now one can make a more reliable analysis as to proper account for the effect of these parameters. To accomplish this task in this work, the essential aspects of the matter are revised with improved accuracy. It is shown, in particular, that the theory can present now reasonable grounds to properly set appropriate boundaries on the validity of the currently used expressions for modelling stresses and heat flow. The theoretical results presented here are applicable to a wide range of gas and plasma conditions covering the needs from rarefied to dense fluids and also low- (subsonic) and high-speed (supersonic) flows. Thus, they can be used for problems of physics of fluids, geophysics, solar and interplanetary physics.