Flow definition in Webster:
a : (1) to issue or move in a stream rivers flow into the sea
(2) : circulate
b : to move with a continual change of place among the constituent particles molasses flows slowly water flowing over the dam
Definition of virtual particle :
It is a mathematical function used in calculating Feynman diagrams for particle interactions, which is identified with the quantum numbers of the exchanged particle, but is off mass shell. Off mass shell means that it is not a real photon in this case, because its mass can be larger or smaller than zero in the calculation, withinthe limits of integration.
The circulate definition ,a2, might be a way to visualize the collective effect of the Cassimir plates, but it is not something that can be measured as any type of flow happens in the probability space of the quantum mechanical model. It is the final effect that can be measured, the forces appearing.