Imperial College London

DrSinaStapelfeldt

Faculty of EngineeringDepartment of Mechanical Engineering

Advanced Research Fellow
 
 
 
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Contact

 

+44 (0)20 7594 7076s.stapelfeldt

 
 
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Assistant

 

Mr Peter Higgs +44 (0)20 7594 7078

 
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Location

 

557City and Guilds BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

27 results found

Brandstetter C, Ottavy X, Paoletti B, Stapelfeldt Set al., 2021, Interpretation of Stall Precursor Signatures, Journal of Turbomachinery, Vol: 143, ISSN: 0889-504X

<jats:title>Abstract</jats:title> <jats:p>Unsteady aerodynamic phenomena appearing close to the stall boundary in axial fans and compressors have been the subject of extensive investigations. A particular phenomenon known as “rotating instabilities” typically occurs in the tip region at highly loaded conditions and is often linked to blade vibrations. During rig and engine testing, it is usually identified by the characteristic shape of its pressure spectra. In the 1990s, this shape has been explained as the result of a circumferentially propagating disturbance, which is unsteady in a frame of reference rotating with the disturbance. In this analysis, conclusions regarding its propagation speed and frequency have been made based on the analysis of spectral peaks, and this method is still often used to classify unsteady aerodynamic phenomena. However, in high subsonic and transonic machines, where aeroacoustic and aeroelastic phenomena interact with aerodynamic disturbances, the interpretation of measurements using spectra alone is challenging. The present article aims to demonstrate the difficulties and subtleties associated with the analysis of measurement signals, which need to be overcome to correctly interpret stall precursor signatures. At the example of a recent investigation on a composite fan, the consequences of sensor placement and postprocessing techniques are discussed with a focus on spectral averaging, isolation of non-synchronous phenomena, and multisensor cross correlation methods. It is seen that the interpretation of phenomena based solely on spectral peaks and their spacing can be misleading and that the characteristic features of a rotating instability spectrum do not require an unstable pulsating disturbance.</jats:p>

Journal article

Brandstetter C, Stapelfeldt S, 2021, Analysis of a linear model for non-synchronous vibrations near stall, International Journal of Turbomachinery, Propulsion and Power, Vol: 6, Pages: 1-14, ISSN: 2504-186X

Non-synchronous vibrations arising near the stall boundary of compressors are a recurring and potentially safety-critical problem in modern aero-engines. Recent numerical and experimental investigations have shown that these vibrations are caused by the lock-in of circumferentially convected aerodynamic disturbances and structural vibration modes, and that it is possible to predict unstable vibration modes using coupled linear models. This paper aims to further investigate non-synchronous vibrations by casting a reduced model for NSV in the frequency domain and analysing stability for a range of parameters. It is shown how, and why, under certain conditions linear models are able to capture a phenomenon, which has traditionally been associated with aerodynamic non-linearities. The formulation clearly highlights the differences between convective non-synchronous vibrations and flutter and identifies the modifications necessary to make quantitative predictions.

Journal article

Xu Z, Puente R, Stapelfeldt S, 2021, Validation of three-dimensional grid refinement for Lattice Boltzmann methods, AIAA Scitech 2021 Forum, Publisher: American Institute of Aeronautics and Astronautics, Pages: 1-15

The lattice Boltzmann method (LBM) is a method for modeling fluid dynamics which is based on discrete kinetic theory. It discretizes the continuous Boltzmann equation in velocity, spatial and temporal space and solves a transport equation for particle populations. To enable an efficient time-space discretization with discrete velocity sets, common LBM solvers employ uniform grids making them prohibitively expensive for the simulation of high Reynolds number flows. This paper describes the implementation of a vertex-centered grid refinement method with the BGK operator in the Open-source Lattice Boltzmann code, OpenLB. The method is validated for flow around a 3D circular cylinder with Reynolds number varying from 100 to 300 and the 3D lid-driven cavity with Reynolds number equals 3200. The results are in good agreement with experimental measurements and the reference solutions with uniform grid spacing. The simulation with grid refinement allows for remarkable computational cost savings for a given accuracy.

Conference paper

Mitchell SJ, Stapelfeldt S, Puente R, 2021, Evaluation of the single-population lattice Boltzmann method for one-dimensional compressible flows, AIAA Scitech 2021 Forum, Publisher: American Institute of Aeronautics and Astronautics, Pages: 1-16

In this work the stability and accuracy of the one-dimensional single-population Lattice Boltzmann Method (LBM) is studied in its application to compressible flow regimes. A linear stability analysis framework using the von Neumann method is developed, framed in terms of flow regime parameters. In order to validate this analysis and study the behavior of errors with respect to different simulation parameters, two academic test cases are presented. A shock tube configuration is used to assess the shock capturing capabilities. A high subsonic convergent-divergent nozzle is used to evaluate the error dependence on grid size. This test case requires the inclusion of an additional source term to the Boltzmann equation, and a methodology is proposed for the implementation of physically motivated source terms. The proposed numerical method is shown to be accurate, matching theoretical error trends. Through stability analysis, bounds for the simulation parameters resulting in a stable simulation are given, which can inform best practices in the application of LBM to these regimes.

Conference paper

Stapelfeldt S, Brandstetter C, 2020, Non-synchronous vibration in axial compressors: lock-in mechanism and semi-analytical model, Journal of Sound and Vibration, Vol: 488, Pages: 1-20, ISSN: 0022-460X

Non-Synchronous-Vibration (NSV) in high-speed turbomachinery compressors is an aeroelastic phenomenon which can have devastating consequences, including loss of rotor blades. Despite extensive research over the past two decades its underlying mechanisms are not yet understood. This paper aims to explain the physical mechanisms causing NSV in a modern transonic compressor rotor. Referring to previous experimental results and using validated computational fluid dynamics (CFD), a parametric study is performed in order to characterize the aerodynamic disturbance causing NSV, and to understand the lock-in mechanism between the fluid and the structure seen during NSV. The results show that the process is driven by aerodynamics in the tip region. Under highly throttled conditions, the tip leakage flow blocks the passage and causes the disturbance, which is characterised as a vorticity fluctuation, to propagate circumferentially in the leading edge plane. It is found that the propagation speed of the disturbance is determined by the mean flow conditions and only its phase is periodically modulated through interaction with oscillating blades. This is the mechanism facilitating lock-in. Based on these findings a semi-analytic model is developed and calibrated with the numerical results. The model is capable of simulating the lock-in process and correctly predicts unstable vibration modes. It can therefore be used to identify critical operating conditions and develop mitigation measures early in the design.

Journal article

Moreno J, Dodds J, Stapelfeldt SC, Vahdati Met al., 2020, Deficiencies in the SA Turbulence model for the prediction of the stability boundary in highly loaded compressors, Journal of Turbomachinery, Vol: 142, Pages: 121012-1-121012-11, ISSN: 0889-504X

Reynolds-averaged Navier–Stokes (RANS) equations are employed for aerodynamic and aeroelastic modeling in axial compressors. Their solutions are highly dependent on the turbulence models for closure. The main objective of this work is to assess the widely used Spalart–Allmaras model suitability for high-speed compressor flows. For this purpose, an extensive investigation of the sources of uncertainties in a high-speed multi-stage compressor rig was carried out. The grid resolution near the casing end wall, which affects the tip leakage flow and casing boundary layer, was found to have a major effect on the stability limit prediction. Refinements in this region led to a stall margin loss prediction. It was found that this loss was exclusively due to the destruction term in the SA model.

Journal article

Zhang W, Stapelfeldt S, Vahdati M, 2020, Influence of the inlet distortion on fan stall margin at different rotational speeds, AEROSPACE SCIENCE AND TECHNOLOGY, Vol: 98, ISSN: 1270-9638

Journal article

Lu Y, Green J, Stapelfeldt SC, Vahdati Met al., 2019, Effect of Geometric Variability on Running Shape and Performance of a Transonic Fan, JOURNAL OF TURBOMACHINERY-TRANSACTIONS OF THE ASME, Vol: 141, ISSN: 0889-504X

Journal article

Lu Y, Lad B, Green J, Stapelfeldt S, Vahdati Met al., 2019, Effect of geometry variability on transonic fan blade untwist, International Journal of Turbomachinery, Propulsion and Power, Vol: 4

Due to manufacturing tolerance and deterioration during operation, fan blades in the same engine exhibit geometric variability. The absence of symmetry will inevitably exacerbate and contribute to the complexities of running geometry prediction as the blade variability is bound to be amplified by aerodynamic and centrifugal loading. In this study, we aim to address the fan blade untwist related phenomenon known as alternate passage divergence (APD). As the name suggests, APD manifests as alternating passage geometry (and hence alternating tip stagger pattern) when the fan stage is operating close to/at peak efficiency condition. APD can introduce adverse influence on fan performance, aeroacoustics behaviour, and high cycle fatigue characteristics of the blade. The main objective of the study is to identify the parameters contributing to the APD phenomenon. In this study, the APD behaviours of two transonic fan blade designs are compared.

Journal article

Stapelfeldt S, Vahdati M, 2019, Improving the Flutter Margin of an Unstable Fan Blade, JOURNAL OF TURBOMACHINERY-TRANSACTIONS OF THE ASME, Vol: 141, ISSN: 0889-504X

Journal article

Lu Y, Lad B, Vahdati M, Stapelfeldt SCet al., 2019, Nonsynchronous vibration associated with transonic fan blade untwist

Due to manufacturing tolerance and deterioration during operation, fan blades in the aero-engine exhibit geometric variability. This leads to asymmetry in the assembly which will be amplified in the running geometry by centrifugal and aerodynamic loads. This study investigates a phenomenon known as Alternative Passage Divergence (APD), where the blade untwist creates an alternating pattern in passage geometry and stagger angle around the circumference, resulting in two groups of blades. This phenomenon occurs close to, or at, peak efficiency conditions and can significantly reduce overall efficiency. This study focuses on a type of non-integral vibration which occurs during APD. After the formation of alternating tip stagger pattern, APDs unsteady effect can cause the blades from one group to switch to the other, creating a travelling wave pattern around the circumference.It was found from numerical assessment on a randomly mis-staggered assembly that real engines can potentially experience such travelling disturbance and suffer fatigue damage. An idealised case is used to capture the bulk behaviour from the more complex cases in real engines and to decipher the underlying mechanism of this travelling disturbance. The results indicate that the driving force originates from the interaction between passage shock displacement and the passage geometry.

Conference paper

Moreno J, Dodds J, Vahdati M, Stapelfeldt Set al., 2019, DEFICIENCIES IN TURBULENCE MODELLING FOR THE PREDICTION OF THE STABILITY BOUNDARY IN HIGHLY LOADED COMPRESSORS, ASME Turbo Expo: Turbomachinery Technical Conference and Exposition, Publisher: AMER SOC MECHANICAL ENGINEERS

Conference paper

Lu Y, Vahdati M, Stapelfeldt SC, Lad B, Green Jet al., 2019, Effect of geometry variability on transonic fan blade untwist

Due to manufacturing tolerance and deterioration during operation, fan blades in the same engine exhibit geometric variability. The absence of symmetry will inevitably exacerbate and contribute to the complexities of running geometry prediction as the blade variability is bound to be amplified by aerodynamic and centrifugal loading. In this study, we aim to address the fan blade untwist related phenomenon known as Alternate Passage Divergence (APD). As the name suggests, APD manifests as alternating passage geometry (and hence alternating tip stagger pattern) when the fan stage is operating close to/at peak efficiency condition. APD can introduce adverse influence on fan performance, aeroa-coustics behaviour, and high cycle fatigue characteristics of the blade. The main objective of the study is to identify the parameters contributing to the APD phenomenon. In this study, the APD behaviours of two transonic fan blade designs are compared.

Conference paper

Lu Y, Vahdati M, Green J, Stapelfeldt Set al., 2018, Effect of Geometry Variability on Fan Performance and Aeromechanical Characteristics, 15th International Symposium on Unsteady Aerodynamics, Aeroacoustics and Aeroelasticity of Turbomachines

Conference paper

Stapelfeldt SC, Vahdati M, 2018, On the importance of engine-representative models for fan flutter predictions, Journal of Turbomachinery, Vol: 140, Pages: 081005-1-081005-10, ISSN: 0889-504X

Discrepancies between rig tests and numerical predictions of the flutter boundary for fan blades are usually attributed to the deficiency of computational fluid dynamics (CFD) models for resolving flow at off-design conditions. However, as will be demonstrated in this paper, there are a number of other factors, which can influence the flutter stability of fan blades and lead to differences between measurements and numerical predictions. This research was initiated as a result of inconsistencies between the flutter predictions of two rig fan blades. The numerical results agreed well with rig test data in terms of flutter speed and nodal diameter (ND) for both fans. However, they predicted a significantly higher flutter margin for one of the fans, while measured flutter margins were similar for both blades. A new set of flutter computations including the whole low-pressure system was therefore performed. The new set of computations considered the effects of the acoustic liner and mistuning for both blades. The results of this work indicate that the previous discrepancies between CFD and tests were caused by, first, differences in the effectiveness of the acoustic liner in attenuating the pressure wave created by the blade vibration and second, differences in the level of unintentional mistuning of the two fan blades. In the second part of this research, the effects of blade mis-staggering and inlet temperature on aerodynamic damping were investigated. The data presented in this paper clearly show that manufacturing and environmental uncertainties can play an important role in the flutter stability of a fan blade. They demonstrate that aeroelastic similarity is not necessarily achieved if only aerodynamic properties and the traditional aeroelastic parameters, reduced frequency and mass ratio, are maintained. This emphasizes the importance of engine-representative models, in addition to accurate and validated CFD codes, for the reliable prediction of the flutter boundar

Journal article

Stapelfeldt SC, Vahdati M, 2018, Improving the Flutter Margin of an Unstable Fan Blade, ASME Turbo Expo 2018: Turbomachinery Technical Conference and Exposition

Conference paper

Franz D, Salles L, Stapelfeldt S, 2017, Analysis of a Turbine Bladed Disk With Structural and Aerodynamic Mistuning, ASME Turbo Expo 2017: Turbomachinery Technical Conference and Exposition

Conference paper

Stapelfeldt S, Vahdati M, 2017, On the importance of engine-representative models for fan flutter predictions

This paper examines the factors which can result in discrepancies between rig tests and numerical predictions of the flutter boundary for fan blades. Differences are usually attributed to the deficiency of CFD models for resolving the flow at off-design conditions. This work was initiated as a result of inconsistencies between the flutter prediction of two rig fan blades, called here Fan F1 and Fan F2. The numerical results agreed well with the test data in terms of flutter speed and nodal diameter for both fans. However, they predicted a significantly higher flutter margin for F2 than for Fan F1, while rig tests showed that the two blades had similar flutter margins. A new set of flutter computations for both blades using the whole LP domain (intake, fan, OGV and ESS) was therefore performed. The new set of computations considered the effects of the acoustic liner and mistuning for both blades. The results of this work indicate that the previous discrepancies between CFD and tests were due to: 1. Differences in the effectiveness of the acoustic liner in attenuating the pressure wave created by the blade vibration as a result of differences in flutter frequencies between the two fan blades. 2. Differences in the level of unintentional mistuning of the two fan blades due to manufacturing tolerances. In the second part of this research, the effects of blade misstaggering and inlet temperature on aerodynamic damping were investigated. The data presented in this paper clearly show that manufacturing and environmental uncertainties can play an important role in the flutter stability of a fan blade. They demonstrate that aeroelastic similarity is not necessarily achieved if only aerodynamic properties and the traditional aeroelastic parameters, reduced frequency and mass ratio, are maintained. This emphasises the importance of engine-representative models, in addition to an accurate and validated CFD code, for the reliable prediction of the flutter boundary.

Conference paper

Brandstetter C, Holzinger F, Schiffer H, Stapelfeldt S, Vahdati Met al., 2016, Near Stall Behavior of a Transonic Compressor Rotor With Casing Treatment, ASME Turbo Expo 2016: Turbomachinery Technical Conference and Exposition

Conference paper

Stapelfeldt SC, Parry AB, Vahdati M, 2016, Investigation of flutter mechanisms of a contra-rotating open rotor, Journal of Turbomachinery, Vol: 138, Pages: 1-10, ISSN: 0889-504X

The growing pressure to reduce fuel consumption and cut emissions has triggered renewed interest in contra-rotating open rotor (CROR) technologies. One of their potential issues is self-excited or forced vibration of the unducted, light-weight, highly swept blades. This paper presents a numerical study into the flutter behavior of a CROR rig at take-off conditions. The study presented in this paper aimed to validate the numerical approach and provide insights into the flutter mechanisms of the open rotor under investigation. For the initial validation, pressure profiles and thrust coefficients from steady-state mixing plane calculations were compared against rig measurements. A full domain unsteady analysis predicted front rotor instability at low advance ratios. Flutter occurred in the first torsional mode in 0 and 1 nodal diameter (ND) which agreed with experimental observations. Subsequent unsteady computations focused on the isolated front rotor and first torsional mode. The flow field and aerodynamic damping over a range of advance ratios were studied. It was found that minimum aerodynamic damping occurred at low advance ratios when the flow was highly three-dimensional on the suction side. A correlation between the quasi-steady loading on the blade and aeroelastic stability was made and related to the numerical results. The effects of variations in frequency were then investigated by linking local aerodynamic damping to the unsteady pressure on the blade surface.

Journal article

Stapelfeldt SC, Di Mare L, 2015, Reduced passage method for multirow forced response computations, AIAA Journal: devoted to aerospace research and development, Vol: 53, Pages: 3049-3062, ISSN: 0001-1452

This paper presents a time-domain Fourier method for modeling steady and unsteady nonaxisymmetric flows in turbomachinery on a reduced computational domain. The method extends well-established single-passage multirow methods, which efficiently model periodic unsteadiness in single stages, to assemblies with stationary circumferential perturbations with periodicity different from the blade count. Such perturbations are caused, for example, by rotor–rotor/stator–stator interaction or geometric circumferential variations. The method is therefore suitable to study low-engine-order forcing problems, flow past nonuniform assemblies, and clocking problems. The proposed method solves the flow inside several discrete passages, located at different circumferential positions, using a time-accurate scheme. Boundary conditions at the azimuthal and interrow surfaces are approximated via time–space Fourier series and couple the individual passages. The reduced passage model is validated against the whole annulus solution for three test cases: 1) a row of outlet guide vanes with stagger pattern, 2) a high-pressure turbine stage with throat area variation in the stator, and 3) a 1.5-stage compressor. It is demonstrated that the time-domain Fourier method yields results equivalent to the whole annulus model but at a much reduced computational cost.

Journal article

Stapelfeldt S, Parry A, vahdati M, 2015, Investigation of Flutter Mechanisms of a Contra-Rotating Open Rotor, ASME Turbo Expo 2015

Conference paper

Stapelfeldt S, Parry T, Vahdati M, 2015, Validation of time-domain single-passage methods for the unsteady simulation of a contra-rotating open rotor, PROCEEDINGS OF THE INSTITUTION OF MECHANICAL ENGINEERS PART A-JOURNAL OF POWER AND ENERGY, Vol: 229, Pages: 443-453, ISSN: 0957-6509

Journal article

Schrape S, Giersch T, Nipkau J, Stapelfeldt S, Mueck Bet al., 2015, Tyler-Sofrin Modes in Axial High Pressure Compressor Forced Response Analyses, International Symposium on Unsteady Aerodynamics, Aeroacoustics and Aeroelasticity of Turbomachines

Conference paper

Stapelfeldt SC, Parry AB, Vahdati M, 2015, VALIDATION OF TIME-DOMAIN SINGLE-PASSAGE METHODS FOR THE UNSTEADY SIMULATION OF A CONTRA-ROTATING OPEN ROTOR, 11th EUROPEAN CONFERENCE ON TURBOMACHINERY FLUID DYNAMICS AND THERMODYNAMICS, Publisher: EUROPEAN TURBOMACHINERY SOC-EUROTURBO

Conference paper

Stapelfeldt SC, di Mare L, 2013, MODELLING ROTOR-ROTOR INTERACTION ON REDUCED PASSAGE COUNTS USING A TIME-DOMAIN FOURIER APPROACH, 10th European Conference on Turbomachinery: Fluid Dynamics and Thermodynamics, Publisher: EUROPEAN TURBOMACHINERY SOC-EUROTURBO

Conference paper

Stapelfeldt SC, Di Mare L, 2012, A Method For Modelling Flow Past Non-Axisymmetric Configurations On Reduced Passage Counts, 13th International Symposium on Unsteady Aerodynamics, Aeroacoustics and Aeroelasticity of Turbomachines

Conference paper

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