Compressible Modal Instability Onset in an Aerodynamically Mistuned Transonic Fan

This paper describes observed modal oscillations arising from a feedback mechanism between an acoustic resonance in the exit flow channel and aerodynamic and aeroelastic disturbances in a transonic fan stage. During tests, the fan suffered from rotating stall and surge which were preceded by low frequency pressure fluctuations. Through a range of aerodynamic and aeromechanical instrumentations, it was possible to determine a clear chain of cause and effect, whereby geometrical asymmetries trigger local instabilities and modal oscillations through an interaction with the system acoustics. To the authors knowledge, this is the first time that modal oscillations occurring before stall are attributed to multiphysical interactions, showing that acoustic characteristics of the system can influence the aerodynamic as well as the aeromechanical stability of fans. This bears implications for the stability assessment of fans and compressors because first, the stability margin may be affected by standing waves generated in bypass ducts or combustion chambers, and second, geometrical variations of the rotor blades which are believed to be beneficial for aeromechanical stability may lead to complex coupling phenomena.

Experimental Evaluation of Numerical Continuation and Bifurcation Methods Applied to Autogyro Rotor Blade Aeromechanical Stability

This paper presents a systematic assessment of the use of continuation and bifurcation techniques, in investigating the nonlinear periodic behaviour of rotor blades in forward autorotation. Our aim is to illustrate the potential of these tools in revealing complex blade dynamics, when used in combination (not necessarily in real time) with physical testing. We show a simple procedure to promote understanding of an existing engineering instability problem when uncertainties in the numerical modelling are present. It is proposed that continuation and bifurcation methods can play a significant role in developing numerical/experimental techniques for studying blade dynamics for both autorotating and powered rotors, which can be applied even at the preliminary design phase.

Aeromechanical Stability Analysis and Control of Smart Composite Rotor Blades

The use of segmented constrained layer damping treatment and closed loop control is investigated for improved rotor aeromechanical stability. The rotor blade load-carrying member is modeled using a composite box beam with arbitrary wall thickness. The ACLs are bonded to the upper and lower surfaces of the box beam to provide active and passive damping in the aeromechanical stability analysis. A finite element model based on a hybrid displacement theory is used to accurately capture the transverse shear effects in the composite primary structure, the viscoelastic and the piezoelectric layers within the ACL. The Pitt-Peters dynamic inflow model is used in the air resonance analysis under hover conditions. Rigid body pitch and roll degrees of freedom and fundamental flap and lead-lag modes are considered in this analysis. A transformation matrix is introduced to transform the time-variant system to a time-invariant system. A LQG controller is designed for the transformed system based on the available measurement output. The control performance is compared with the results of the open loop and the passive control systems. Numerical results indicate that the proposed control system with surface bonded ACL damping treatment significantly increases rotor lead-lag regressive modal damping in the coupled rotor-body system.