Unsteady Hydrodynamic Behavior at Piano Key Weirs

Abstract

Piano key weirs (PKWs) are nonlinear hydraulic structures that exhibit unsteady hydraulic phenomena. A study incorporating both physical and numerical modeling was undertaken to identify instabilities that are generated by flow. Several excitations were identified and a portion of their nature is characterized in this paper. These include a system of vortices in the inlet key induced by the breakdown of a free shear layer, oscillations of the overflow nappe, and surging of the air cavity underneath the nappe. Of these, the inlet key vortices were found to be of the most significance as they were observed to occur at nearly all modeled flows (0.02<H/P<0.55) and exposed the sidewall separating the inlet and outlet keys to periodic pressure fluctuations. These fluctuating pressure forces, although small (±10% of the total pressure oscillating at ∼5 Hz), are persistent and may cause structural vibrations. Reinforced concrete PKWs are sufficiently stiff to limit the amplitude of any deflections caused by these vibrations; however, thin, steel-plate PKWs may remain susceptible.