Vortex and other types of “oscillatory” flow meters utilize the behaviour fluid oscillations in order to derive flow rate. This technology works by inducing fluctuations in the fluid properties such as pressure, density or viscosity which can be converted into a flow rate.
When the fluid stream encounters a fixed obstruction, the fluid must divide to pass around the barrier. Because of viscous adhesion, the boundary layer moves slower than the outer layer. At very low flow rates, the viscous forces dominate keeping the fluid attached to the wall of the body and the fluid recombines in a symmetrical fashion. But, as the flow rates increase there comes a point where the flow cannot withstand the adhesion pressure gradient along the surface of the body and boundary layer duly separates from it to form rotating vortices that are carried downstream.
As the flow rates increase even further the vortices become relatively stable and persistent so they line up directly behind the obstruction. The vortex shedding alternates from side to side in sequence and due to the pressure pulse that accompanies the formation of the vortex from the opposite edge. By using this pressure pulse the frequency of this oscillation can be measurement and a flow rate can be derived.
Image courtesy of TÜV NEL