A numerical study of a liquid sheet sheared on both sides by a high-speed stream is performed in this work, at moderate density and velocity ratio between phases. Near the injection, an interfacial wave develops on both interfaces of the liquid sheet. A vortical detachment of the high-speed stream is observed behind this wave and modifies the pressure field around the sheet. The global flapping mechanism is a consequence of the pressure difference between the two sides of the liquid sheet. The flapping dynamics is characterized and compared to existing correlations available in the literature. A sensitivity study of the flapping dynamics to the high-speed stream boundary layer thickness is performed and a relevant Strouhal number is proposed. (C) 2015 Elsevier Ltd. All rights reserved.