From Patankar's book
What is, however, the rationale for the main idea underlying the upwind scheme?
More insight will be obtained in the next subsection, but, in the meantime,
a lucid physical picture of the upwind scheme would offer some satisfaction.
The scheme is sometimes said to be based on the "tank-and-tube" model (Gosman,
Pun, Runchal, Spalding, and Wolfshtein, 1969). As shown in Fig. 5.2, the
control volumes can be thought to be stirred tanks that are connected in series
by short tubes. The flow through the tubes represents convection, while the
conduction through the tank walls represents diffusion. Since the tanks are
stirred, each contains a uniform temperature fluid. Then, it is appropriate to
suppose that the fluid flowing in each connecting tube has the temperature that
prevails in the tank on the upstream side. Normally, the fluid in the tube
would not know anything about the tank toward which it is heading, but would
carry the full legacy of the tank from which it has come. This is the essence
of the upwind scheme.