Bending-Torsion Flutter as a Reason of Hydrodynamic Instability of Mast Hoisting Gears

Abstract

This paper examines the reasons of hydrodynamic instability of Mast Hoisting Gears that are related to interactions of bending and torsional oscillations of a structure in the hydraulic flow. A mast hoisting gear loaded by hydrodynamic forces is a non-conservative (self-oscillating) system. Unsteady modes of motion are possible for such systems under certain combinations of the system parameters and speed of external flow (critical speed). Naturally, this brings up the question of the possibility of occurrence of such an event within the limits of operational speeds of real structures and development of respective calculation methods. The paper presents equations of bending-torsion oscillations of Mast Hoisting Gears as a rod of variable cross-section working in bending and in torsion in the hydraulic flow. A calculation algorithm of a critical speed of Mast Hoisting Gear bending-torsion flutter is suggested based on the complex amplitude method. The algorithm extensively uses the capabilities of state-of-the-art computing aids. Taking as an example the actual Mast Hoisting Gear design, complex eigenvalues are determined that allows to assess the Mast Hoisting Gear stability in the hydraulic flow at the specified speed of an underwater object and to determine natural frequencies of oscillations using this algorithm. This paper gives recommendations allowing increase the structure stability threshold to bending-torsion flutter.