  Air cavities, i.e. air layers developed behind cavitators, are seen as a
promising drag reducing method in the maritime industry. Here we utilize the
Taylor-Couette (TC) geometry, i.e. the flow between two concentric,
independently rotating cylinders, to study the effect of air cavities in this
closed setup, which is well-accessible for drag measurements and optical flow
visualizations. We show that stable air cavities can be formed, and that the
cavity size increases with Reynolds number and void fraction. The streamwise
cavity length strongly depends on the axial position due to buoyancy forces
acting on the air. Strong secondary flows, which are introduced by a
counter-rotating outer cylinder, clearly decrease the stability of the
cavities, as air is captured in the Taylor rolls rather than in the cavity.
Surprisingly, we observed that local air injection is not necessary to sustain
the air cavities; as long as air is present in the system it is found to be
captured in the cavity. We show that the drag is decreased significantly as
compared to the case without air, but with the geometric modifications imposed
on the TC system by the cavitators. As the void fraction increases, the drag of
the system is decreased. However, the cavitators itself significantly increase
the drag due to their hydrodynamic resistance (pressure drag): In fact, a net
drag increase is found when compared to the standard smooth-wall TC case.
Therefore, one must first overcome the added drag created by the cavitators
before one obtains a net drag reduction.
