Jet-Flipping in Scour Hole Downstream of Unsubmerged Weir with Apron

An apron downstream of an unsubmerged weir can cause the main jet to flip up and down cyclically in a local scour hole, significantly influencing the scouring process. This behavior was experimentally investigated using particle image velocimetry (PIV). The flow inside the scour hole is characterized by the presence of coherent structures (i.e., four different types of roller), and the flipping behavior is linked with the cyclical appearance and disappearance of a bed roller beneath the main jet. With the cyclical jet-flipping, the main jet has a pivotal region with significant average kinetic energy (AKE) immediately downstream of the apron, and the turbulence kinetic energy (TKE) distributions attain their peak values around this region. The jet development also exhibited self-similarity along the streamline passing through the peak AKE when scaled by the local length and velocity scales. Compared with the no-apron case, the apron reduced the maximum scour depth by dissipating the jet energy and decreasing the impingement angles of the jet to the bed profile and the water surface.