Analysis of Aeroacoustic Phenomena in Centrifugal Compressors Using a Lattice Boltzmann Flow Simulation Approach

Impeller rotation, vortex shedding, secondary flows or a combination of these phenomena can lead to the generation of acoustic waves in the compressor cascade causing dynamic pressure loading on the impeller. When the eigenfrequency and eigenmode shape of the acoustic mode match with the structural ones of the impeller, high fatigue stresses and vibrations occur, which can lead to structural failure. It is well known that cavities enclosing shrouded impellers may strongly amplify the acoustic excitation of the impeller by means of Tyler-Sofrin modes; however, little knowledge is available about the physics of flow-induced noise and resonance mechanisms. In this research, a Lattice Boltzmann Method based approach is employed to predict the origin and amplitude of pressure loading responsible for the strong impeller trailing edge vibrations measured in experiments. The results reveal that this is caused by the acoustic mode generated from the interaction of upstream vane wakes with the impeller that is reflected by the return channel vanes. This research highlights the importance of accounting for aeroacoustic mechanisms in the design of centrifugal compressor stages and paves the way towards the numerical assessment of unsteady flow and resonance phenomena.

Noise Attenuation Potential Using Helmholtz Absorbers Integrated in Low Pressure Turbine Exit Guide Vanes and Turbine Exit Casing End Walls

To further reduce the noise emitted from modern aircrafts, every possibility has to be taken into account. Acoustic liners are successfully used in the inlet or the bypass duct of aircraft engines to mitigate the noise emitted by the fan. Due to the rough environment (high temperature, flow velocity, higher order duct modes), the exhaust duct is of limited use concerning the application of acoustic liners. It is well known that the last stage low pressure turbine (LPT) has a dominant influence onto the emitted noise of an aircraft engine especially at low load conditions such as approach. A noise reduction in this area could lead to a beneficial result of decreasing the noise content which is directly emitted in the environment. This paper is about noise attenuation using Helmholtz absorbers in various parts of a turbine exit casing (TEC). These single degree of freedom absorbers have been integrated in turbine exit guide vanes (TEGVs), with the openings on the vanes suction side, as well as in the inner and outer duct end walls. Different absorber neck diameters were investigated and combined with different vane designs. The vane designs studied included a state of the art set-up as well as vanes with a lean. Test runs were performed with altered combinations of vanes and end walls under engine relevant operating conditions in a subsonic test turbine facility for aerodynamic, aeroacoustic and aeroelastic investigations (STTF-AAAI) located at the Institute of Thermal Turbomachinery and Machine Dynamics at Graz University of Technology. Comparisons between all these setups and the respective hard wall reference cases were done. The resulting sound pressure levels as well as sound power levels of all investigated combinations are listed and compared concerning each configurations noise attenuation potential. Additionally, the flow field downstream of every setup is analysed if the aerodynamic behaviour is changing. The investigated operating point is the noise certification point Approach (APP) which is of high importance because of the high acoustical impact onto the environment around airports during the landing procedure of an aircraft. The acoustical data has been obtained by using flush mounted condenser microphones located downstream of the TEC. The whole test section was rotated over 360 deg around the flow channel. To detect if the aerodynamical behaviour changes by including openings into the flow channel end walls as well as into the vanes, aerodynamic measurements have been performed downstream of the TEC. The aerodynamical data was obtained by using an aerodynamic five-hole-probe (5HP) as well as a trailing edge probe.

Influences of Turbulence Boundary Conditions on RANS and URANS Simulations for an Inter-Turbine Duct

In recent years, lot of turbine research is focused on the study and optimization of inter-turbine ducts, an aero-engine component for which the design is becoming more challenging due to the turbofan architecture evolution. Starting from the early design phase, the knowledge of the component performance and outlet flow pattern is crucial in the design of the low pressure turbine. To improve prediction, multi-row unsteady simulations are deployed. Unfortunately, some questions arise in the use of these simulations, among others the knowledge of the turbulent boundary conditions and the contribution of the unsteady simulations to the flow solution. In this paper steady and time resolved RANS simulations of a turning inter-turbine duct are investigated. Particularly, two questions are addressed. The first one is the influence of the turbulent quantities boundary conditions in the case of a k–ω Wilcox turbulence model in the flow field solution. The second one is the contribution of the unsteadiness to the mean flow prediction. It will be shown that the mean flow depends on inlet turbulence only if the turbulence length scale is relatively high; otherwise the flow field is almost turbulence-invariant. For the unsteady simulations, unsteadiness modifies the mean flow solution only with low inlet turbulence.

Measurements of Multiple Pure Tone Propagation From a High Bypass Turbofan Rotor in an Internal Flow Facility

An important noise source in modern high bypass ratio turbofans is from multiple pure tones produced by the fan during takeoff. An experiment conducted on a 1.5 pressure ratio fan in an internal flow facility provided dynamic pressure measurements to investigate multiple pure tone generation and propagation. Since multiple pure tones are generated by blade shock variation primarily due to the fan’s blade stagger angle differences, the blade stagger angles were measured with an array of over-the-rotor dynamic pressure transducers. Multiple pure tone measurements were made with 30 wall-mounted dynamic pressure transducers from 0.4 to 1.1 diameters upstream of the rotor. Measured blade stagger angle differences correspond to the the shock amplitude variation measured upstream. The acoustic field was extracted from the dynamic pressure signals using principal component analysis as well as duct mode beamforming. Shocks traveling out the inlet were found to couple to duct modes propagating at similar angles. Over-the-rotor acoustic liners appear to reduce rotor shock variation resulting in a reduction of sub-harmonic multiple pure tone sound pressure levels by 3–4 dB.

Prediction of the Acoustic Reflection in a Realistic Aeroengine Intake With Three Numerical Methods to Analyze Fan Flutter

For a particular range of frequencies, an acoustic coupling between the fan and the air intake can modify fan stability regarding flutter. Previous works have shown that characterizing the reflection on the intake opening might be a crucial element to target operating points for which the risk of acoustic driven flutter is high. To do so, three methodologies are compared in this paper: an aeroelastic CFD simulation, an acoustic potential simulation and an analytical model. Each of them has a different fidelity level and computational cost, what makes their usage more beneficial at some step in the design process. It is shown that results of aeroelastic CFD and acoustic potential simulations are in excellent agreement. Fast acoustic simulations are then a good option in the early design process. The analytical model presents an important error mainly on the phase, and should be adapted before usage.

Towards Multidisciplinary Turbine Blade Tolerance Design Assessment Using Adjoint Methods

This paper shows how to use discrete CFD and FEM adjoint surface sensitivities to derive objective-based tolerances for turbine blades, instead of relying on geometric tolerances. For this purpose a multidisciplinary adjoint evaluation tool chain is introduced to quantify the effect of real manufacturing imperfections on aerodynamic efficiency and probabilistic low cycle fatigue life time. Before the adjoint method is applied, a numerical validation of the CFD and FEM adjoint gradients is performed using 102 heavy duty turbine vane scans. The results show that the relative error for adjoint CFD gradients is below 0.5%, while the FEM life time gradient relative errors are below 5%. The adjoint assessment tool chain further reduces the computational cost by around 85% for the investigated test case compared to non-linear methods. Through the application of the presented tool chain, the definition of specified objective-based tolerances becomes available as a design assessment tool and allows to improve overall turbine efficiency and the accuracy of life time prediction.

Geometrical Variability Modelling of Axial Compressor Blisk Aerofoils and Evaluation of Impact on the Forced Response Problem

The present work focuses on the effect of the manufacturing geometrical variability on the high-pressure compressor of a turbofan engine for civil aviation. The deviations of the geometry over the axial compressor blades are studied and modeled for the representation in the computational models. Such variability is of particular interest for the forced response problem, where small deviations of the geometry from the ideal nominal model can cause significant differences in the vibrational responses. The information regarding the geometrical mistuning is extracted from a set of manufactured components surface scans of a blade integrated disk (blisk) rotor. The optically measured geometries are parameterized, defining a set of opportune variables to describe the deviations. The dimension of the variables domain is reduced using the principal component analysis approach and a reconstruction of the modeled geometries is performed for the implementation in CFD and FEM solvers. The generated model allows a stochastic representation of the variability, providing an optimal set of variables to represent it. The aeroelastic analyses considering geometry based mistuning is carried out on a test-rig case, focusing on how such variability can affect the modal forcing generated on the blades. The force generated by the unsteady pressure field over the selected vibrational mode shapes of the rotor blades is computed through a validated CFD model. The uncertainty quantification of the geometrical variability effect on the modal forcing is performed employing Monte Carlo methods on a reduced model for the CFD solution, based on a single passage multi-blade row setup. The amplitude shift of the unsteady modal forcing is studied for different engine orders. In particular the scatter of the main engine orders forcing amplitudes for the manufactured blades can be compared with the nominal responses to predict the possible amplification due to the geometrical variability. Finally the results are compared to a full assembly computational model to assess the influence of multiple variable blades.

Aero-Thermal Coupled Design Optimization of a Turbine Vane and the Effect on Heat Load of Endwall

The 3D aerodynamic design optimization has been applied in the generation of modern turbine blade profile. However, the traditional design method paid little attention to the decrease of heat transfer coefficients on the blade external surface. In the present work, a typical high load turbine vane, VKI LS89 cascade, was optimized with the decrease of aerodynamic loss and heat load chosen as the optimization objective functions. Numerical simulation methods were validated by the experiment data, and simulations results agreed well with the measured values. Both 2D profiles and stagger curves of the vane were parameterized by no-uniform B-Spline. There were totally seven movable control points for the 2D profiles, and four movable control points for the corresponding stagger curves. And the locations of the B-Spline control points and stagger angles were taken as the design variables. Multi-objective genetic algorithm coupled with surrogate model was adopted to acquire the optimal cases with better aero-thermal performance. The profiles of the vane were firstly optimized in a linear cascade model, and then the stagger curves and sections stagger angle were modified for better overall performance. Mass flow rate of the mainstream and exit flow angle at outlet were constrained by the comprehensive objective functions during the 3D optimization process. The results showed that profiles with high aerodynamic efficiency and low heat load can be obtained by the 2D profiles optimization design. Additionally, the heat load could be decreased by the 3D optimization design. Furthermore, the effects of optimization on the heat load distributions of the endwall were studied, and it can be observed that the 3D optimization obviously modified the heat transfer patterns of the endwall.

Analysis of the Convergence Rate of Turbulence Model Uncertainties for Transonic Axial Compressor Simulation

Numerical experiments were performed to assess the effect of numerical discretization error on the convergence rate of polynomial chaos (PC) approximations for a transonic axial compressor stage. A random variable with a uniform distribution and expected value of one was introduced into the expression for turbulent viscosity of the k-ω SST turbulence model. Model uncertainty was quantified from the expected value and standard deviation estimates obtained via univariate non-intrusive polynomial chaos. Spectral projection and point collocation were both used and their results were compared. The effect of discretization error on convergence of the PC approximation was investigated using a grid refinement study with four grids. The PC expansion was computed for each grid while maintaining the same boundary conditions, basis functions, model evaluations, random variable distribution, and polynomial order. The quantities of interest (QOIs) were total–to–total pressure ratio, total–to–total temperature, and adiabatic efficiency. The grid resolution was found to have an influence on resulting surrogate models and the estimates of expected value and standard deviation for all QOIs. However, the estimates converged towards final values as the mesh was refined. Point collocation provided different estimates from spectral projection and the difference was also found to depend on the mesh size.

Uncertainty Based Optimization Strategy for the Gappy-POD Multi-Fidelity Method

In the surrogate model-based optimization of turbine airfoils, often only the prediction values for objective and constraints are employed, without considering uncertainties in the prediction. This is also the case for multi-fidelity optimization strategies based on e.g. the Gappy-POD approach, in which results from analyses of different fidelities are incorporated. However, the consideration of uncertainties in global optimization has the advantage that a balanced coverage of the design space between unexplored regions and regions close to the current optimum takes place. This means that on the one hand regions are covered in which so far only a few sample points are present and thus a high degree of uncertainty exists (global exploration), and on the other hand regions with promising objective and constraint values are investigated (local exploitation). The genuine new contribution in this work is the quantification of the uncertainty of the multi-fidelity Gappy-POD method and an adapted optimization strategy based on it. The uncertainty quantification is based on the error of linear fitting of low-fidelity values to the POD basis and subsequent forward propagation to the high-fidelity values. The uncertainty quantification is validated for random airfoil designs in a design of experiment. Based on this, a global optimization strategy for constrained problems is presented, which is based on the well-known Efficient Global Optimization (EGO) strategy and the Feasible Expected Improvement criterion. This means that Kriging models are created for both the objective and the constraint values depending on the design variables that consider both the predictions and the uncertainties. This approach offers the advantage that existing and widely used programs or libraries can be used for multi-fidelity optimization that support the (single-fidelity) EGO algorithm. Finally, the method is demonstrated for an industrial test case. A comparison between a single-fidelity optimization and a multi-fidelity optimization is made, each with the EGO strategy. A coupling of 2D/3D simulations is used for multi-fidelity analyses. The proposed method achieves faster feasible members in the optimization, resulting in faster turn-around compared to the single-fidelity strategy.