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The onset of vortex formation on the free surface intake of hydraulic structures, such as hydropower intakes, pump sumps, and multiphase flow skimmers, is important since it can affect the reliability and operability of a given process.

Intake structures can be positioned in different orientations (vertical, horizontal, inclined) and the critical submergence depth—the minimum depth to avoid air entrainment into the intake—is a function of the intake shape, Froude number, intake Reynolds number (i.e. fluid properties and flow rate), and the approach flow non-uniformity (or the circulation number).

Asymmetry in flow and/or location of the intake are sources of swirl generation in the approach flow and can enhance air-core formation, which in turn leads to air entrainment. For new designs that may operate over a wide range of conditions, the generation and evolution of an air-core vortex between a free surface and the intake needs to be investigated, especially when existing empirical correlations may not be applicable or available (e.g., Swirling Flow Problems at Intakes by Jost Knauss).