AERODYNAMIC DAMPING PREDICTIONS DURING COMPRESSOR SURGE: A NUMERICAL COMPARISON BETWEEN A HALF AND FULL TRANSIENT APPROACH

The prediction of the aerodynamic damping during compressor surge is a challenging task, because the flow is continuously evolving along the four surge cycle phases: Pressurization (PR), Flow-Breakdown (FB), Reversed Flow (RF) and Regeneration (RG) and complex flow conditions like shocks and separations occur. Damping predictions with current existing methods typically consist of two steps. In the first step a modified numerical model is used to simulate transient surge cycles. In the second step, damping analyses are performed for multiple timesteps along the surge cycle phases, which are then assumed as quasi-steady. The damping simulation can be performed using nonlinear or linear approaches. If shocks or separations occur, the latter yields inaccuracies in the flow and thus in the damping predictions. A new approach was developed to take into account and improve these inaccuracies. This new method includes the damping prediction within the transient surge simulation. Thus, all surge cycle phases and the continuously evolving flow conditions are considered and nonlinear simulations are performed to account for shocks and separations. The results of this new method are presented and compared to the former method

Experimental Investigation of the Aeroelastic Stability of an Annular Compressor Cascade at Reverse Flow Conditions

Compressor surge events are unsafe operating regimes yielding highly unsteady flow fields in which complex aeroelastic phenomena occur. If the blade flutter and forced response behaviour (i.e. aeroelastic stability) can be predicted reliably for normal flow conditions, its assessment at severe-off design conditions remains a critical task for compressor development programs. During the flow reversal sequence of a surge cycle, combined aerodynamic phenomena occur which make the accurate prediction of the unsteady forces acting on the blades difficult to assess. The main objective of this investigation is to increase the physical understanding of the unsteady phenomena present during the reverse flow sequence of a typical deep surge cycle. The analysis of the blade surface unsteady pressure distribution enables the identification of the main physical mechanisms present during such extreme flow operating conditions, as well as the evaluation of their contribution on the blade global aerodynamic stability. The approach adopted consists in performing aeroelastic investigations on an annular compressor cascade at established reverse flow conditions. The investigations are carried out at EPFL, in the annular test facility for non-rotating cascades. The cascade is forced to vibrate in a torsional travelling wave mode (controlled vibration). With an upstream swirled flow corresponding to real axial turbomachine conditions, a constant flow can be set in the test section. The steady-state operating conditions are measured upstream and downstream of the test section, using 5-hole aerodynamic probes. Several cascade blades are equipped with pressure taps at 50% span in order to acquire the steady-state and unsteady blade surface pressure distributions. Static pressure taps are also inserted in the casing wall of the test section to assess the steady-state flow field characteristics in the blade tip area. The inlet flow operating conditions are varied in order to determine their influence on the blade unsteady aerodynamic forces. This study presents the measurement results and analyses in details the aerodynamic response of a cascade subjected to controlled vibrations at reverse flow conditions. The data analysis is oriented towards both physical and practical approaches. In particular, the following features are addressed: Identification of the main unsteady physical mechanisms influencing the unsteady aerodynamic forces acting on a blade oscillating and subjected to reverse flow conditions. Determination of the influence of the inlet flow condition variations on these unsteady mechanisms. Evaluation of the contribution of each unsteady phenomenon to the blade aerodynamic stability (in terms of stabilizing or destabilizing impact). Assessment of the key parameters to control in order to minimize flutter risks in case that reverse flow conditions should occur. The data analysis reveals that during the reverse flow sequence of the surge cycle, blade interaction mechanisms play a major role in the blade aerodynamic stability. The global aerodynamic damping coefficient highlights this feature and indicates that aerodynamic instability exists for some operating conditions. A second unsteady phenomenon was detected, generated by the uncommon steady-state flow field characteristics present at reverse flow operating conditions. Within this frame, the presence of a large recirculation zone on the blade suction side was identified, influencing the blade aerodynamic stability. From a more general point of view, this study constitutes a step forward to the understanding of the blade loading processes occurring during a typical deep surge sequence. Results highlight the impact of the steady-state and unsteady phenomena on the blade loading level at reverse flow conditions. For one test case, the measured data was compared with numerical results, performed in parallel to the measurements. The results indicate that even though the agreement is reasonable, the correct prediction of the aerodynamic damping curve re- quires the consideration of a complex blade interaction mechanism. Within this frame, since not many experiments exist at reverse flow conditions, these experimental results are also a precious data source for CFD validation. They enable the improvement of the prediction/simulation accuracy of the compressor performance at off-design flow conditions

AEROELASTIC INVESTIGATION OF AN ANNULAR TRANSONIC COMPRESSOR CASCADE: NUMERICAL SENSITIVITY STUDY FOR VALIDATION PURPOSES

The accuracy of flutter or forced response analyses of turbomachinery blade assemblies strongly depends on the correct prediction of the unsteady aerodynamic loads acting on the vibrating blades. This paper presents the aeroelastic numerical results of an annular transonic compressor cascade subjected to harmonic oscillation conditions. The measurements associated were performed in an annular test facility for non-rotating cascades. The aim of this investigation is to get a deeper understanding of the specific characteristics of this test facility as well as improving the flutter prediction procedure and accuracy. For a subsonic and a transonic flow condition, the steady-state blade surface pressure distributions were predicted with two mesh configurations and results were compared to the experimental results. The first configuration omits the geometrical complexity of the experimental model and only models the blade passage. The second mesh configuration includes the cascade’s detailed geometry and cavities. The presence of leakage flows arisen due to the cascade’s slits and cavities are identified and their impact on the main flow field is analyzed and discussed. For the flutter computations, two mesh resolutions were investigated. The global damping predicted with a fine and a coarse mesh was compared, as well as the local pressure amplitudes and phases predicted with both configurations. Results show that even though similar global damping curves are predicted with both mesh resolution, for some IBPAs, local differences exist on the pressure amplitudes and phases. This highlights that only comparing the global damping coefficient, is not sufficient for code validation

Coupled Mode Flutter Analysis of Turbomachinery Blades using an Adaptation of the P-K Method

Current trends in turbomachinery design significantly reduce the mass ratio of structure to air, making them prone to flutter by aerodynamic coupling between mode shapes, also called coupled-mode flutter. The p-k method, which solves an aeroelastic eigenvalue problem for frequency and damping respectively excitation of the aerodynamically coupled system, was adapted for turbomachinery application using aerodynamic responses computed in the frequency domain. A two-dimensional test case is validated against time-marching fluid-structure coupled simulations for subsonic and transonic conditions. A span of mass ratios is investigated showing that the adapted p-k method is able to predict the transition between aeroelastically stable and unstable cascades depending on the mass ratio. Finally, the p-k method is applied to a low mass ratio fan showing that the flutter-free operating range is significantly reduced when aerodynamic coupling effects are taken into account

On the Effect of Frequency Separation, Mass Ratio, Solidity and Aerodynamic Resonances in Coupled Mode Flutter of a Linear Compressor Cascade

At a low mass ratio of structure to air, the work-per-cycle approach, or better known as the energy method, will lead to non-conservative results as aerodynamic coupling of modeshapes acts destabilizing. Using the p-k method to solve the aeroelastic stability equation, the effects of various structural aspects are investigated for a two-dimensional compressor cascade in subsonic and transonic flow conditions. The investigated key parameters are frequency separation, mass ratio and solidity. Furthermore, the effect of a high frequency dependency of the aerodynamic forces is presented. Such phenomena can happen in case of aerodynamic or acoustic resonances. If the resonance peaks are close to the aeroelastic frequency, a discontinuous behavior of the frequency or damping solution can lead to a rapid destabilization of the system, once the aeroelastic frequency moves from one side to the other of the peak. In these regimes, it is crucial to have a high quality representation of the frequency-dependent generalized aerodynamic forces for an accurate prediction of the flutter onset

Coupled Mode Flutter Analysis of Turbomachinery Blades Using an Adaptation of the p-k Method

Current trends in turbomachinery design significantly reduce the mass ratio of structure to air, making them prone to flutter by aerodynamic coupling between mode shapes, also called coupled-mode flutter. The p-k method, which solves an aeroelastic eigenvalue problem for frequency and damping, respectively, excitation of the aerodynamically coupled system, was adapted for turbomachinery application using aerodynamic responses computed in the frequency domain (FD). A two-dimensional (2D) test case is validated against time-marching fluid-structure coupled simulations for subsonic and transonic conditions. A span of mass ratios is investigated showing that the adapted p-k method is able to predict the transition between aeroelastically stable and unstable cascades depending on the mass ratio. Finally, the p-k method is applied to a low mass ratio fan showing that the flutter-free operating range is significantly reduced when aerodynamic coupling effects are taken into account

Nonlinear aerodynamic damping in a highly loaded vibrating compressor cascade

A numerical aeroelastic study of a 2D linear compressor cascade based on the non-rotating annular compressor cascade with a NACA3506 profile was performed using a one-way coupled technique (prescribed-motion approach). Subsonic and transonic flow conditions with strong shocks in the blade passage were imposed. Using a nonlinear harmonic balance solver, unsteady simulations were performed by enforcing one blade oscillation motion. For each blade structural mode, the blade deflection amplitude was varied and its influence on the blade aerodynamic response was determined in terms of aerodynamic damping. The blade modes investigated consisted of one pitching and two heaving oscillation motion. In transonic flows and for the three structural modes investigated, the cascade shows nonlinear aerodynamic responses depending on the vibration amplitude. The results presented in this work show that nonlinear frequency domain methods are able to capture the blade nonlinear aeroelastic behavior coming from amplitude-dependent aerodynamic responses

Coupled Mode Flutter of a Linear Compressor Cascade in Subsonic and Transonic Flow Conditions

Flutter onset in turbomachinery is typically investigated numerically via decoupled methods due to a high mass ratio of structure to air. The unsteady aerodynamic response of a forced motion vibration is evaluated for a positive or negative work entry. The forced motion simulations assume vacuum mode shape vibrations at certain amplitudes without modal coupling due to aerodynamic forces. This approach, also known as the energy method, is valid for a high blade mass ratio and small logarithmic decrement values. An aeroelastic study of a multi-passage linear compressor cascade was performed. In fluid-structure-coupled time-marching CFD, generic heave and pitch degrees of freedom are allowed to vibrate freely in reaction to any aerodynamic forces. For one subsonic and one transonic flow condition predicted to be stable by the classical energy method approach, aerodynamically coupled-mode flutter is observed. It is shown that variations in the starting conditions, i.e. the initial vibration and inter-blade phase angle of the system, can have a strong influence on the number of CFD iterations required until amplitudes grow. However, if coupled-mode flutter is present in the system, it will ultimately set in at a distinct inter-blade phase angle