  Turbulence in fluids is an ubiquitous phenomenon, characterized by
spontaneous transition of a smooth, laminar flow to rapidly changing, chaotic
dynamics. In 1883, Reynolds experimentally demonstrated that, in an initially
laminar flow of water, turbulent motions emerge without any measurable external
disturbance. To this day, turbulence remains a major unresolved phenomenon in
fluid mechanics; in particular, there is a lack of a mathematical model where
turbulent dynamics emerge naturally from a laminar flow. Recently, we proposed
a new theory of turbulence in gases, according to which turbulent motions are
created in an inertial gas flow by the mean field effect of the intermolecular
potential. In the current work, we investigate the effect of viscosity in our
turbulence model, by numerically simulating the air flow at normal conditions
in a straight pipe for different values of the Reynolds number. We find that
the transition between the laminar and turbulent flows in our model occurs
without any deliberate perturbations as the Reynolds number increases from 2000
to 4000. As the simulated flow becomes turbulent, the decay rate of the time
averaged Fourier spectrum of the kinetic energy in our model approaches
Kolmogorov's inverse five-thirds law. Both results are consistent with
experiments and observations.
