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•  3D and 2D models resulted in different predictions of the shape and size of energy dissipation region after the baffle turning; (Figure 4)
•  3D model predicted a higher maximum energy dissipation rate and a smaller energy dissipation zone; (Figure 4)
• Energy dissipation rate was uniform along the z direction, as expected;(Figure 5)
• There were still non-zero components of velocity in z direction, though insignificant, and not uniform along the z direction.

Discussion:

The difference of the prediction from 3D and 2D models indicates the importance of z component in the flow field, which made the assumption of the equivalence of periodic 3D model and 2D model invalid. When we used 2D model to approximate 3D flows, we were assuming the all the fluid variables only have x,y components and no z components. When applying the periodic boundary conditions to the walls in xy plane, although the assumption that periodic repetition in z direction were equivalent  to "uniform" could be valid, uniformity in z direction alone wasn't equivalent to "no components" in z direction. Thus these two models could not be good approximation to each other, and could generate significantly different predictions. As shown in Figure 6, there were still non-zero components of velocity in z direction, though insignificant, and not uniform along the z direction.

Furthermore, the importance of z components could put the validity of 2D model as an approximation in question: in Case 4,  even small components in z direction could make significant difference in results from 2D model, let alone in the real flow.
However, the above hypothesis must be further investigated, ruling out all other possible causes of differences. Particularly, the effect of the length of the period must be investigated by vary the width of the flocculator. Ideally, periodic repetition with infinitely small period length is equivalent to "uniform".

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