Coandă effect in free-surface shallow-water offset jets

The Coandă effect relies on the deviation of a fluid stream, usually a jet, from a straight direction toward a lateral wall. In the past, it has been primarily investigated in the context of air flows to increase lift and optimize propulsion in aeronautical applications, while in this paper, it is considered in the context of shallow-water free-surface flows for potential applications in coastal engineering. Through an extensive experimental campaign, the influence of multiple parameters, including some previously unexplored factors related to the presence of a free surface (i.e., Froude’s number), is investigated by employing time-resolved particle image velocimetry. These parameters are considered individually and simultaneously to provide a comprehensive understanding of their effects and identify the key variables controlling the phenomenon. Results indicate that in this configuration, the Coandă effect is slightly enhanced compared to standard two and three-dimensional air jets, resulting in shorter reattaching lengths. While cross-sectional velocity profiles were measured to be symmetrical and somewhat similar to that of a free jet, it was found that the jet’s axis is not a streamline, implying a significant asymmetry in the entrainment properties on the two sides of the jet. Finally, a simple scaling system based on the predicted reattachment length was proven effective in generalizing the phenomenon’s properties across a wide range of conditions.

Graphical abstract