Aeroelasticity is the branch of physics and engineering studying the interactions between the inertial, elastic, and aerodynamic forces occurring while an elastic body is exposed to a fluid flow. The study of aeroelasticity may be broadly classified into two fields: static aeroelasticity dealing with the static or steady state response of an elastic body to a fluid flow, and dynamic aeroelasticity dealing with the body's dynamic (typically vibrational) response. Aircraft are prone to aeroelastic effects because they need to be lightweight and withstand large aerodynamic loads. Aircraft are designed to avoid the following aeroelastic problems:

divergence where the aerodynamic forces increase the angle of attack of a wing which further increases the force;

control reversal where control activation produces an opposite aerodynamic moment that reduces, or in extreme cases, reverses the control effectiveness; and

flutter which is the uncontained vibration that can lead to the de

Aeroeleasic Investigation of an Annular Transonic Compressor Cascade: Experimental Results

Virginie Anne Chenaux, Peter Ott, Achim Zanker

A reliable determination of the unsteady aerodynamic loads acting on the blades is essential to predict the aeroelastic stability of vibrating compressor cascades with accuracy. At transonic flow conditions, the vibration of the shock may change the blade aeroelastic behavior. Numerical tools still have difficulties to capture the physics associated to this effect. In order to increase the prediction’s accuracy, high quality experimental data at high spatial resolution is therefore required to enable the calibration and validation of these tools. Within the frame of the European project FUTURE, experimental aeroelastic investigations were performed on a transonic compressor cascade in the Non-Rotating Annular Test Facility at EPFL. Associated to the measurements, the numerical flutter prediction procedure was applied. This paper focuses on the experimental results. The experimental database gained during the project is presented and aims at helping the aeroelastic community to develop and improve their flutter prediction capabilities. The test model consists of twenty prismatic blades. Each blade of the cascade assembly was mounted on an elastic spring element enabling harmonic bending vibrations in the twenty possible cascade’s travelling wave modes. Large efforts were made to improve the measuring techniques and to provide high quality data at relatively high spatial resolution. For various sub- and transonic flow conditions, steady-state and unsteady blade surface pressure distributions were measured to evaluate the local contributions to the blade stability in terms of local aerodynamic work. The blade global aerodynamic stability is determined applying an integration of all unsteady pressure signals measured over the airfoil.

2015

Experimental Investigations of the Aerodynamics of an Annular Compressor Cascade at Reversed Flow Conditions

Virginie Anne Chenaux, Peter Ott

Compressor unsafe operating regimes yield unsteady high speed turbulent flows in which complex aeroelastic phenomena occur. If the blade flutter and forced response behavior (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. Due to complex flow fields and highly transient phenomena, reversed flow conditions are still difficult to predict. Within this frame, the understanding of the physical mechanisms of surge onset and events is essential to improve surge occurrence prediction and control. This paper presents steady-state results of an experimental investigation performed on an annular compressor cascade subjected to constant subsonic backflow inlet conditions. The investigations were carried out at EPF Lausanne, in the annular test facility for non-rotating cascades. With an upstream swirled flow corresponding to real axial turbomachine conditions, an axisymmetric flow can be achieved in the test section. This paper constitutes an introduction and a basis to an associated forthcoming experimental study, dedicated to aeroelastic investigations. The steady-state flow conditions are measured upstream and downstream of the test section, with 5-hole aerodynamic probes. The compressor cascade consists of 20 blades, mounted on 20 independent elastic springs and masses to enable the excitation of different IBPA during the aeroelastic measurements. A pair of blades was instrumented with respectively pressure and suction side pressure taps along the airfoil chord length in order to measure the surface steady-state pressure. Static pressure taps were also inserted in the casing of the test section to assess flow characteristics in the blade tip area. To provide a comparison tool to the measurements, associated CFD calculations at offdesign conditions were performed. The comparison between CFD and measurements shows a high level of confidence. Since not many experiments exist at severe off-design conditions, these experimental results are a precious data source for CFD validation.

2011

Experimental and Numerical Assessment of the Aeroelastic Stability of Blade Pair Packages

Peter Ott, Achim Zanker

The stabilizing effect of grouping rotor blades in pairs has been assessed both, numerically and experimentally. The bending and torsion modes of a low aspect ratio high speed turbine cascade tested in the non-rotating test facility at EPFL (Ecole Polytechnique Fédérale de Lausanne) have been chosen as the case study. The controlled vibration of 20 blades in travelling wave form was performed by means of an electromagnetic excitation system, enabling the adjustment of the vibration amplitude and inter blade phase at a given frequency. Unsteady pressure transducers located along the blade mid-section were used to obtain the modulus and phase of the unsteady pressure caused by the airfoil motion. The stabilizing effect of the torsion mode was clearly observed both in the experiments and the simulations, however the effect of grouping the blades in pairs in the minimum damping at the tested frequency was marginal in the bending mode. A numerical tool was validated using the available experimental data and then used to extend the results at lower and more relevant reduced frequencies. It is shown that the stabilizing effect exists for the bending and torsion modes in the frequency range typical of low-pressure turbines. It is concluded that the stabilizing effect of this configuration is due to the shielding effect of the pressure side of the airfoil that defines the passage of the pair on the suction side of the same passage, since the relative motion between both is null. This effect is observed both in the experiments and simulations.

ASME2014

Experimental Investigations of the Aerodynamics of an Annular Compressor Cascade at Reversed Flow Conditions

Virginie Anne Chenaux, Peter Ott

2011