Imperial College London

Professor Mehdi Vahdati

Faculty of EngineeringDepartment of Mechanical Engineering

Principal Research Fellow
 
 
 
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Contact

 

+44 (0)20 7594 7073m.vahdati

 
 
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Location

 

606City and Guilds BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

140 results found

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

Zhang W, Vahdati M, 2020, Stall and Recovery Process of a Transonic Fan With and Without Inlet Distortion, Publisher: ASME, ISSN: 0889-504X

Conference paper

Lu Y, Green J, Stapelfeldt SC, Vahdati Met al., 2019, Effect of Geometric Variability on Running Shape and Performance of a Transonic Fan, 15th International Symposium on Unsteady Aerodynamics, Aeroacoustics and Aeroelasticity of Turbomachinery (ISUAAAT), Publisher: ASME, ISSN: 0889-504X

Conference paper

Rendu Q, Vahdati M, Salles L, 2019, Radial Decomposition of Blade Vibration to Identify a Stall Flutter Source in a Transonic Fan, 15th International Symposium on Unsteady Aerodynamics, Aeroacoustics and Aeroelasticity of Turbomachinery (ISUAAAT), Publisher: ASME, ISSN: 0889-504X

Conference paper

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

© 2019 by the authors. 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

Lee K-B, Wilson M, Vahdati M, 2019, Effects of Inlet Disturbances on Fan Stability, Journal of Engineering for Gas Turbines and Power, Vol: 141, Pages: 051014-051014, ISSN: 0742-4795

Journal article

Lee K-B, Wilson M, Vahdati M, 2019, Effects of Inlet Disturbances on Fan Stability, JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER-TRANSACTIONS OF THE ASME, Vol: 141, ISSN: 0742-4795

Journal article

Venkatesh S, Rendu Q, Vahdati M, Salles Let al., 2019, Effect of manufacturing tolerance in flow past a compressor blade

Copyright © by the Authors. This paper presents the effect of manufacturing tolerance on performance and stability boundaries of a transonic fan using a RANS simulation. The effect of tip gap and stagger angle was analysed through a series of single passage and double passage simulation; based on which an optimal arrangement was proposed for random tip gap and random stagger angle in case of a whole annulus rotor. All simulations were carried out using NASA rotor 67 as a test case and AU3D an in-house CFD solver. Results illustrate that the stagger angle mainly affects efficiency and hence its circumferential variation must be as smooth as possible. Furthermore, the tip gap affects the stability boundaries, pressure ratio and efficiency. Hence its optimal configuration mandates that the blades be configured in a zigzag arrangement around the annulus i.e. larger tip gap between two smaller ones.

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

Zhang W, Vahdati M, Zhao F, 2019, IMPACT OF EXIT DUCT DYNAMIC RESPONSE ON COMPRESSOR STABILITY, ASME Turbo Expo: Turbomachinery Technical Conference and Exposition, Publisher: AMER SOC MECHANICAL ENGINEERS

Conference paper

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

Copyright © 2019 Rolls-Royce plc. 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

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

Copyright © by the Authors. 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

Zhao F, Dodds J, Vahdati M, 2018, Poststall Behavior of a Multistage High Speed Compressor at Off-Design Conditions, JOURNAL OF TURBOMACHINERY-TRANSACTIONS OF THE ASME, Vol: 140, ISSN: 0889-504X

Journal article

Zhang W, Vahdati M, 2018, A Parametric Study of the Effects of Inlet Distortion on Fan Aerodynamic Stability, JOURNAL OF TURBOMACHINERY-TRANSACTIONS OF THE ASME, Vol: 141, ISSN: 0889-504X

Journal article

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, ISSN: 0889-504X

Journal article

Lee K-B, Wilson M, Vahdati M, 2018, Validation of a Numerical Model for Predicting Stalled Flows in a Low-Speed Fan-Part I: Modification of Spalart-Allmaras Turbulence Model, JOURNAL OF TURBOMACHINERY-TRANSACTIONS OF THE ASME, Vol: 140, ISSN: 0889-504X

Journal article

Lee K-B, Dodds J, Wilson M, Vahdati Met al., 2018, Validation of a Numerical Model for Predicting Stalled Flows in a Low-Speed Fan-Part II: Unsteady Analysis, JOURNAL OF TURBOMACHINERY-TRANSACTIONS OF THE ASME, Vol: 140, ISSN: 0889-504X

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

Zhao F, Dodds J, Vahdati M, 2018, POst-stall behaviour of a multi-stage high speed compressor at off-design conditions

Copyright © 2018 Rolls-Royce plc. Stall followed by surge in a high speed compressor can lead to violent disruption of flow, damage to the blade structures and, eventually, engine shutdown. A knowledge of unsteady blade loading during such events is crucial in determining the aeroelastic stability of blade structures, experimental test of such events is however significantly limited by the potential risk and cost associated. Numerical modelling, such as unsteady CFD simulations, can provide a more informative understanding of the flow field and blade forcing during post-stall events, however very limited publications, particularly concerning multi-stage high speed compressors, can be found. The aim of this paper is to demonstrate the possibility of using CFD for modelling full-span rotating stall and surge in a multi-stage high speed compressor, and, where possible, validate the results against experimental measurements. The paper presents an investigation into the onset and transient behaviour of rotating stall and surge in an 8-stage high speed axial compressor at off-design conditions, based on 3D URANS computations, with the ultimate future goal being aeroelastic modelling of blade forcing and response during such events. By assembling the compressor with a small and a large exit plenum volume respectively, a full-span rotating stall and a deep surge were modelled. Transient flow solutions obtained from numerical simulations showed trends matching with experimental measurements. Some insights are gained as to the onset, propagation and merging of stall cells during the development of compressor stall and surge. It is shown that surge is initiated as a result of an increase in the size of the rotating stall disturbance, which grows circumferentially to occupy the full circumference resulting in an axisymmetric flow reversal.

Conference paper

Lee K-B, Wilson M, Vahdati M, 2018, EFFECTS OF INLET DISTURBANCES ON FAN STABILITY, ASME Turbo Expo: Turbomachinery Technical Conference and Exposition, Publisher: AMER SOC MECHANICAL ENGINEERS

Conference paper

Zhang W, Vahdati M, 2018, A parametric study of the effects of inlet distortion on fan aerodynamic stability

Copyright © 2018 ASME. The performance and aerodynamic stability of fan blades operating in a circumferentially non-uniform inlet flow is a key concern in the design of turbofan engines. With the recent trends in the design of civil engines with shorter inlet ducts (such as low-speed fans), or boundary layer ingesting engines, quick and reliable modelling of rotor/distortion interactions is becoming very important. The aim of this paper is to study the effects of inlet distortions on the aerodynamic stability of a fan blade and to identify the parameters that have a major impact on the stability of the blade. NASA rotor 67, for which a significant amount of measured data is available, was used for this study. In the first part of this study, the behavior of the fan with inlet distortion at near stall condition is analyzed, and it is shown how rotating stall is triggered. In the second part of this study, unsteady simulations with inlet distortion were performed to study how parameters, such as exit duct length and distortion strength, influence the stall margin of the blade.

Conference paper

Lee K-B, Wilson M, Vahdati M, 2017, Numerical Study on Aeroelastic Instability for a Low-Speed Fan, JOURNAL OF TURBOMACHINERY-TRANSACTIONS OF THE ASME, Vol: 139, ISSN: 0889-504X

Journal article

Zhao F, Smith N, Vahdati M, 2017, A Simple Model for Identifying the Flutter Bite of Fan Blades, JOURNAL OF TURBOMACHINERY-TRANSACTIONS OF THE ASME, Vol: 139, ISSN: 0889-504X

Journal article

Tebbutt JA, Vahdati M, Carolan D, Dear JPet al., 2017, Numerical investigation on an array of Helmholtz resonators for the reduction of micro-pressure waves in modern and future high-speed rail tunnel systems, Journal of Sound and Vibration, Vol: 400, Pages: 606-625, ISSN: 1095-8568

Previous research has proposed that an array of Helmholtz resonators may be an effective method for suppressing the propagation of pressure and sound waves, generated by a high-speed train entering and moving in a tunnel. The array can be used to counteract environmental noise from tunnel portals and also the emergence of a shock wave in the tunnel. The implementation of an array of Helmholtz resonators in current and future high-speed train-tunnel systems is studied. Wave propagation in the tunnel is modelled using a quasi-one-dimensional formulation, accounting for non-linear effects, wall friction and the diffusivity of sound. A multi-objective genetic algorithm is then used to optimise the design of the array, subject to the geometric constraints of a demonstrative tunnel system and the incident wavefront in order to attenuate the propagation of pressure waves. It is shown that an array of Helmholtz resonators can be an effective countermeasure for various tunnel lengths. In addition, the array can be designed to function effectively over a wide operating envelope, ensuring it will still function effectively as train speeds increase into the future.

Journal article

Sureshkumar P, Vahdati M, Parry A, Bianchi S, Doherty Met al., 2017, Towards a holistic prediction of fan stage tone noise mechanisms, 12th European Conference on Turbomachinery Fluid Dynamics and Thermodynamics, ETC 2017

Copyright © by the Authors. A computed prediction of the tonal noise generated in the fan stage of an aero-engine is presented. This paper focuses in particular on unsteady fan stage aerodynamics, which leads to the generation of noise. It is further shown that there are a variety of sources of tonal noise and that, for a more complete analysis of the fan stage tonal noise balance that is due to rotor and stator interaction, these sources should be evaluated simultaneously. The computational model has been validated against steady state aerodynamic measurements and measured sound pressure levels. Some acoustic theory is also presented to explain the in-duct radiation behaviour of fan stage tonal noise towards the external environment.

Conference paper

Sureshkumar P, Vahdati M, Parry A, Bianchi S, Doherty Met al., 2017, Towards a holistic prediction of fan stage tone noise mechanisms, 12th European Conference on Turbomachinery Fluid Dynamics and Thermodynamics, ETC 2017, ISSN: 2410-4833

Copyright © by the Authors. A computed prediction of the tonal noise generated in the fan stage of an aero-engine is presented. This paper focuses in particular on unsteady fan stage aerodynamics, which leads to the generation of noise. It is further shown that there are a variety of sources of tonal noise and that, for a more complete analysis of the fan stage tonal noise balance that is due to rotor and stator interaction, these sources should be evaluated simultaneously. The computational model has been validated against steady state aerodynamic measurements and measured sound pressure levels. Some acoustic theory is also presented to explain the in-duct radiation behaviour of fan stage tonal noise towards the external environment.

Conference paper

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

© 2017 ASME. 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 bound

Conference paper

Lu Y, Zhao F, Salles L, Vandati Met al., 2017, AEROELASTIC ANALYSIS OF NREL WIND TURBINE, ASME Turbo Expo: Turbine Technical Conference and Exposition, Publisher: AMER SOC MECHANICAL ENGINEERS

Conference paper

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