The following figures compare the velocity field in expansion flow and contraction flow. The flow is for micro-fluidic devices; hence, the flow is laminar and in low Reynolds number region.
The difference between expansion flow and contraction flow is that some of the flow in expansion flow is re-circulated as the flow from the small channel empties into the large channel. The re-circulation of the flow was not the case for contraction flows.
Figure1. Typical Expansion Flow Figure2. Typical Contraction Flow
Re =10, 3:1 axisymmetric Re = 10, 3:1 axisymmetric
Mesh Refinement
Re = 10:
Expansion Flow Contraction
Flow
Elements KL
Elements
KL
1760 0.4330 1760 19.2209
7040 0.5402 7040 19.3029
28160 0.5867 28160 19.3440
As compared above, the accuracy is sufficient, and the excess pressure drop in expansion flow is much less than that in contraction flow.