ARCHIVES OF CIVIL AND MECHANICAL ENGINEERING Vol. XI 2011 No. 4 Numerical analysis of hydrodynamic forces due to flow instability at lift gate S.W. KOSTECKI Wrocaw University of Technology, Wybrzee Wyspiaskiego 25, 50-370 Wrocaw, Poland. A numerical method, being a combination of the vortex method and the boundary element method, is used here to predict the two-dimensional flow field in the vicinity of an underflow vertical lift gate. In practice, tunnel-type flat-bottomed lift gates experience strong hydrodynamic loading, due to vortex detachment from the gate bottom edge, and near-wake velocity fluctuations. This paper presents a stream function, and velocity and vorticity distributions for two gate gaps. The vortex detachment mechanism is described and the vortex shed- ding frequency, expressed as a Strouhal number, is presented. The predicted velocity and vorticity fields are then used to calculate the pressure distribution on the gate surface by the boundary element method. The time histories of the lift and drag coefficients are presented. The proposed numerical method has been validated by the measurements of the downpull coefficient for the flow around the lift gate. Keywords: vorticity, vortex method, flow instability, vertical lift gate, boundary element method, hydro- dynamic force 1. Introduction The calculation of hydrodynamic forces on hydraulic gates is a major problem which designers involved in hydroengineering face. When the flow around the gate is unstable, a changeable hydrodynamic load arises, which may result in vibrations of the gate, in- fluencing its operational reliability and safety. This unfavourable phenomenon occurs predominantly at the high head gates of dams and hydropower plants. The most widely used high head gates are vertical lift gates or radial gates, working as regulating (par- tially opened) gates or as emergency and guard gates during opening and closing. In this paper the case of a lift gate with a submerged and pressurized underflow space is inves- tigated. The instability of the stream flowing under the gate depends mainly on the shape of the gate lip from which the shear layer is separated. The mechanism of this instability stems from the changeability of the layer separation point and from the fact that the shear layer impinges on the protruding gate elements. When the stream flowing under the gate is unstable, the resulting pressure fluctua- tions cause the hydrodynamic force to fluctuate too. The latter’s downpull component has been the subject of numerous researches and experiments by among others: Sim- mons [18] who conducted tests (based on the air model) on the flow around the gate,