Shock structure, mode shape, and geometric considerations for low-pressure turbine flutter suppression
Modern design principles are pushing for lighter aircraft engines and higher tip Mach numbers in steam turbines. Consequently the loading and shock structures seen on aft stages o low-pressure turbines are evolving at a fast pace. Since flutte is a high risk event in these low reduced frequency blades physically understanding and predicting how new operating condition affect aeroelastic behavior proves crucially important Previous numerical studies on the Standard Configuration LPT concluded loading, and in particular the shock structure strongly impacted the aerodynamic damping as a function o mode shape. This paper presents a deeper investigation int the physical explanation behind the interdependency of loading mode shape, reduced frequency, and LPT geometry. Serving a computational test rigs, four unique quasi-2D LPT configuration are exhaustively analyzed using validated linearized unstead CFD. Three primary contributors to this mode-dependen flutter phenomenon are shown to be the passage shocks, blad geometry, and interblade phase angle. Further, the sensitivit of critical reduced frequency as functions of bending mode an loading are divided into four zonal regions exhibiting qualitativel similar behavior. It is shown in some zones, depending o airfoil geometry and shock strength, that loading will always eithe increase or decrease stability monotonically. For pitchin modes, the shock strength, phase, and location with respect t the rigid body center of torsion prove to play a key part in determinin stability. Understanding these shock effects generalize to all LPT geometries yield new design considerations useful fo suppressing flutter.
