Experimental investigations of an oscillating annular compressor cascade at reverse flow conditions

While the advancement of computing hardware now enables accurate predictions of flutter and forced response at normal flow conditions during the compressor design phase, aeroelastic computations at off-design or reverse flow conditions remain a challenging task. During the flow reversal sequence of a surge cycle, complex aerodynamics occur which make the accurate prediction of the unsteady forces acting on the blades difficult to assess. The main objektive of this study is to increase the physical understanding of the unsteady contributions acting on the blades during the reverse flow sequence of a typical deep surge cycle. The approach adopted consisted in performing aeroelastic investigations on an annular compressor cascade at established reverse flow conditions. The cascade blades were equipped with unsteady pressure transducers and were excited to oscillations in travelling wave mode. In this paper, the blade surface fluctuating pressures recorded are analyzed for one flow operating condition. The messurements enable the determination of the blade aerodynamic stability and highlight the unsteady physical mechanisms present during the surge blowdown phase. For the investigated test case, steady-state numerical computations were carried out in parallel to the measurements to enable the comparison between both approaches

COUPLED FLUID STRUCTURE SIMULATION METHOD IN THE FREQUENCY DOMAIN FOR TURBOMACHINERY APPLICATIONS

Turbomachinery components are exposed to unsteady aero- dynamic loads which must be considered during the design pro- cess to ensure the structural mechanical integrity. There are two primary mechanisms which cause structural vibrations and can lead to high-cycle fatigue due to high dynamic stresses: flutter (self-excited vibrations) and forced response (forced excitation, e.g. wakes from upstream blade rows). In this work an emerging numerical frequency-domain method which is designed to effi- ciently simulate coupled fluid-structure interaction (FSI) prob- lems considering nonlinearities in the flow and structure is mod- ified and applied to an academic and a realistic test case. Fur- thermore complex structural eigenmodes are considered instead of purely real modes as was demonstrated in the literature so far. This method is able to predict limit cycle oscillations and forced response amplitudes. The coupled solver uses the Har- monic Balance (HB) method with an alternating frequency time approach to model periodically unsteady flows and structure dy- namics. The resulting nonlinear HB equations of the flow are solved with a pseudo-time stepping method while the nonlinear HB equations of the structure are solved with a Newton method. The dynamics of the involved structure are further simplified by considering only one relevant eigenmode of the structure. The method is applied to a 3D axial turbine configuration with a mod- ified Youngs modulus for the material of the blisk. The standard flutter curve of the blade row shows that at least one eigenmode is aerodynamically unstable at certain nodal diameters. As a first model test case for the harmonic balance solver, the non- linear structural damping is defined as a cubic modal damping term. The results of the frequency-domain method are compared to coupled FSI simulations in the time domain. The analysis shows that the frequency-domain method is very promising in terms of both computational efficiency and accuracy

EVOLUTION OF THE AERODYNAMIC STABILITY OF AN OSCILLATING ANNULAR COMPRESSOR CASCADE WITH INLET REVERSE FLOW CONDITION VARIATIONS

While the advancement of computing hardware now enables accurate predictions of flutter and forced response at normal flow conditions during the compressor design phase, aeroelastic computations at off-design or reverse flow conditions remain a challenging task. During the flow reversal sequence of a surge cycle, complex aerodynamics occur which make the accurate prediction of the unsteady forces acting on the blades difficult to assess. The main objective of this study is to increase the physical understanding of the unsteady contributions acting on the blades during the reverse flow sequence of a typical deep surge cycle. The approach adopted consisted in performing aeroelastic investigations on an annular compressor cascade at established reverse flow conditions. The cascade blades were equipped with unsteady pressure transducers and were excited to controlled oscillations in travelling wave mode. In this paper, the blade surface fluctuating pressures recorded are analyzed for different flow operating conditions. The measurements enable the determination of the blade aerodynamic stability as well as the identification and characterization of the unsteady physical mechanisms present during the surge blow-down phase

FULLY COUPLED AEROELASTIC SIMULATIONS OF LIMIT CYCLE OSCILLATIONS IN THE TIME DOMAIN

In this study a fully coupled aeroelasticity simulation in the time domain of a low pressure turbine (LPT) is demon- strated. The transformation from the unloaded blade geometry to the loaded (consisting of the steady pressure and centrifugal forces) geometry is considered in the initialisation of the cou- pled solver. The fluid-structure interaction (FSI) solver consists of the flow solver TRACE and the structural solver CalculiX. After validation of the FSI solver its performance and behav- ior is evaluated in terms of simulation time and capabilities for limit cycle oscillations

FULLY COUPLED AEROELASTIC SIMULATIONS OF LIMIT CYCLE OSCILLATIONS IN THE TIME DOMAIN

In this study a fully coupled aeroelasticity simulation in the time domain of a low pressure turbine (LPT) is demon- strated. The transformation from the unloaded blade geometry to the loaded (consisting of the steady pressure and centrifugal forces) geometry is considered in the initialisation of the cou- pled solver. The fluid-structure interaction (FSI) solver consists of the flow solver TRACE and the structural solver CalculiX. After validation of the FSI solver its performance and behav- ior is evaluated in terms of simulation time and capabilities for limit cycle oscillations