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

Air cavities at the inner cylinder of turbulent Taylor-Couette flow