Holger Hennings

Forced Response of Mistuned Bladed Disks in Gas Flow: A Comparative Study of Predictions and Full-Scale Experimental Results

An integrated experimental-numerical study of forced response for a mistuned bladed disk has been performed. A full chain for the predictive forced response analysis has been developed including data exchange between the computational fluid dynamics code and a code for the prediction of the nonlinear forced response for a bladed disk. The experimental measurements are performed at a full-scale single stage test rig with excitation by aerodynamic forces from gas flow. The numerical modeling approaches and the test rig setup are discussed. Comparison of experimentally measured and predicted values of blade resonance frequencies and response levels for a mistuned bladed disk without dampers is performed. A good correspondence between frequencies at which individual blades have maximum response levels is achieved. The effects of structural damping and underplatform damper parameters on amplitudes and resonance frequencies of the bladed disk are explored. It is shown that the underplatform damper significantly reduces scatters in values of the individual blade frequencies and maximum forced response levels.

Forced Response Experiments in a High Pressure Turbine Stage

An experimental investigation was conducted on a single stage high pressure turbine in order to gain a deeper understanding of turbine blade forced response. In particular the main objective of this experiment was to obtain good quality validation data for the prediction methods used by major engine manufactures. The stage investigated consists of an uncooled nozzle guide vane (NGV) and a rotor with 64 blades. To study the complete forced response problem a so called Flexible Rotor was designed and manufactured. This rotor has three modes of interest in the operating range of the stage: first torion, second flap and third edge. The design of experiment was supported by detailed CFD and structural analysis. The mechanical behavior of the Flexible Rotor is well known. In order to identify all interesting modes, all blades are equipped with strain gauges, individually calibrated. To check the unsteady pressures, 18 unsteady pressure transducers were mounted at midspan. This paper deals with experiments only with the Flexible Rotor. Forced response results are presented for the first torsion mode at different pressure ratios. The result obtained, show a large scatter for the maximum response amplitudes at each pressure ratio. This distribution of the amplitudes around the disk is controlled by the mechanical properties of the rotor.

Accurate load prediction by BEM with airfoil data from 3D RANS simulations

In this paper, two methods for the extraction of airfoil coefficients from 3D CFD simulations of a wind turbine rotor are investigated, and these coefficients are used to improve the load prediction of a BEM code. The coefficients are extracted from a number of steady RANS simulations, using either averaging of velocities in annular sections, or an inverse BEM approach for determination of the induction factors in the rotor plane. It is shown that these 3D rotor polars are able to capture the rotational augmentation at the inner part of the blade as well as the load reduction by 3D effects close to the blade tip. They are used as input to a simple BEM code and the results of this BEM with 3D rotor polars are compared to the predictions of BEM with 2D airfoil coefficients plus common empirical corrections for stall delay and tip loss. While BEM with 2D airfoil coefficients produces a very different radial distribution of loads than the RANS simulation, the BEM with 3D rotor polars manages to reproduce the loads from RANS very accurately for a variety of load cases, as long as the blade pitch angle is not too different from the cases from which the polars were extracted.

Experimental Flutter Investigations of an Annular Compressor Cascade: Influence of Reduced Frequency on Stability

Due to the trend of increasing power and reducing weight, the fan and compressor bladings of turbomachinery might be more sensitive to flutter, which must strictly be avoided already in the design process. In order to increase our understanding of the flutter phenomena for fan and compressor cascades, aeroelastic investigations are essential. This paper presents the achievements and results of experimental flutter investigations with a compressor cascade in the test facility of non-rotating annular cascades at EPFL. Flow conditions such as those that occur in rotating cascades are simulated by generating a spiral flow in the upstream. The construction of the cascade which takes into account the structural properties necessary to perform flutter experiments is described. For the simulation of elastic torsional vibrations of a two-dimensional blade section, the cascade consists of 20 blades (NACA3506 profile) mounted on elastic spring suspensions which allows for torsional motion about the midchord. In order to investigate the influence of the reduced frequency on the global stability of the cascade and its local contributions, experiments were performed for two different reduced frequencies. At the higher reduced frequency the cascade remains aerodynamically stable, however, at the lower reduced frequency and transonic flow conditions, some of the interblade phase angles appear to be aerodynamically unstable.

A low Speed Compressor Test Rig for Flutter Investigations

ABSTRACT A test facility for aereolastic investigations has been installed at the chair of Aero Engines at the Technische Universit¨at Berlin.The test rig provides data for tool and code validation and is used for basic aeroelastic experiments. It is a low speed wind tunnel which allows free and controlled flutter testing. The test section contains a linear cascade with eleven compressor blades. Nine of them are elastically suspended. The paper presents a detailed description of the test facility, results to evaluate the overall flow quality and an aeroelastic model to predict the flutter velocity and critical interblade phase angles. Hot-wire anemometry has been applied to examine the inlet flow for several Mach- and Reynolds numbers. The results show small turbulence intensities. The blade surface pressure distribution and the flow field of the blade’s suction and pressure side has been accessed by oil flow visualization.

Experimental investigation of the forcing function and forced pitching blade oscillations of an annular compressor cascade in transonic flow

The interaction between rotor blades and non-rotating stator blades is the most significant blade excitation mechanism in turbomachines. It is well documented in various numerical and experimental investigations for turbine cascades. Like turbine blades, also compressor blades are excited as well by potential fields of the following stator, the downstream flowfield of the stator of the previous stage or struts and incoming flow distortions. In this paper, experimental investigations of the excitation of a transonic compressor cascade due to gust generating struts upstream are presented. The experiments were performed in the test facility of non-rotating annular cascades at EPFL using a compressor cascade, which consists of 20 blades (NACA3506 profile) mounted on elastic spring suspensions for torsional motions at the midchord. For the non-rotating annular cascade, relative flow conditions similar to those present in a rotating cascade are generating by swirling the flow in front of the test test section. The struts are rotating in order to create a periodic excitation upstream of the cascade. The so generated pressure dristribution on the cascade’s profiles as well as the measured vibration response of the blades are presented and compared for a pure subsonic and a transonic flow case.

Forced Response of Mistuned Bladed Discs in Gas Flow: A Comparative Study of Predictions and Full-Scale Experimental Results

An integrated experimental-numerical study of forced response for a mistuned bladed disc has been performed. A full chain for the predictive forced response analysis has been developed including data exchange between the CFD code and a code for the prediction of the nonlinear forced response for a bladed disc. The experimental measurements are performed at a full-scale single stage test rig with excitation by aerodynamic forces from gas flow. Numerical modelling approaches and the test rig setup are discussed. Comparison of experimentally measured and predicted values of blade resonance frequencies and response levels for a mistuned bladed disc without dampers is performed. A good correspondence between frequencies at which individual blades have maximum response levels is achieved. The effects of structural damping and underplatform damper parameters on amplitudes and resonance frequencies of the bladed disc are explored. It is shown that the underplatform damper significantly reduces scatters in values of the individual blade frequencies and maximum forced response levels.Copyright