This equation states that the change in A within a given volume, per unit time, is due to gains from external sources, SA, minus the macroscopic and molecular outflow (divergence terms). The macroscopic outflow is due to the velocity field v, and the microscopic transport is described by a vector aA, the components of which give the transport by molecular processes in the directions of the coordinate axes.

Applying the definitions (2.13)-(2.15), the time-averaged version of (2.43) becomes d (p A*) + div(pv * A * +p¥A') + div(s a ) = SA . (2.44)

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