This work demonstrates
that an additional resistance term should
be included in the Navier–Stokes equation when fluids and objects
are in relative motion. This is based on an observation that the effect
of the microscopic molecular random velocity component parallel to
the macroscopic flow direction is neglected. The two components of
the random velocity perpendicular to the local mean flow direction
are accounted for by the viscous resistance, e.g., by Stokes’
law for spherical objects. The relationship between the mean- and
the random velocity in the longitudinal direction induces differences
in molecular collision velocities and collision frequency rates on
the up- and downstream surface areas of the object. This asymmetry
therefore induces flow resistance and energy dissipation. The flow
resistance resulting from the longitudinal momentum transfer mode
is referred to as thermal resistance and is quantified by calculating
the net difference in pressure up- and downstream the surface areas
of a sphere using a particle velocity distribution that obeys Boltzmann’s
transport equation. It depends on the relative velocity between the
fluid and the object, the number density and the molecular fluctuation
statistics of the fluid, and the area of the object and the square
root of the absolute temperature. Results show that thermal resistance
is dominant compared to viscous resistance considering water and air
in slow relative motion to spherical objects larger than nanometer-size
at ambient temperature and pressure conditions. Including the thermal
resistance term in the conventional expression for the terminal velocity
of spherical objects falling through liquids, the Stokes–Einstein
relationship and Darcy’s law, corroborates its presence, as
modified versions of these equations fit observed data much more closely
than the conventional expressions. The thermal resistance term can
alternatively resolve d’Alembert’s paradox as a finite
flow resistance is predicted at both low and high relative fluid–object
velocities in the limit of vanishing fluid viscosity.