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

156 results found

He X, Zhao F, Vahdati M, 2021, Uncertainty Quantification of Spalart-Allmaras Turbulence Model Coefficients for Compressor Stall, JOURNAL OF TURBOMACHINERY-TRANSACTIONS OF THE ASME, Vol: 143, ISSN: 0889-504X

Journal article

Lu Y, Lad B, Vahdati M, 2021, Transonic Fan Blade Redesign Approach to Attenuate Nonsynchronous Vibration, JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER-TRANSACTIONS OF THE ASME, Vol: 143, ISSN: 0742-4795

Journal article

Zhao F, Dodds J, Vahdati M, 2021, Flow Physics During Surge and Recovery of a Multi-Stage High-Speed Compressor, JOURNAL OF TURBOMACHINERY-TRANSACTIONS OF THE ASME, Vol: 143, ISSN: 0889-504X

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, Vahdati M, Zhao F, 2020, Impact of Exit Duct Dynamic Response on Compressor Stability, JOURNAL OF TURBOMACHINERY-TRANSACTIONS OF THE ASME, Vol: 142, ISSN: 0889-504X

Journal article

Zhao F, Dodds J, Vahdati M, 2020, Influence of blade vibration on part-span rotating stall, International Journal of Gas Turbine, Propulsion and Power Systems, Vol: 12, Pages: 1-7

This paper presents the interaction between blade vibration and part-span rotating stall in a multi-stage high speed compressor. Unsteady aerodynamic and aeroelastic simulations were conducted using URANS CFD. Steady state computations showed short length scale disturbances formed local to the tip of a front stage rotor. Using a full annulus model, these disturbances were shown to coalesce into flow structures rotating around the annulus at approximately 76% of the shaft rotational speed. Natural evolution of the rotating stall did not result in a coherent spatial pattern. Subsequent analyses carried out with prescribed rotor blade vibration showed a spatial ‘lock-in’ event where the circumferential order of the part-span rotating stall shifted to match that induced by the vibration mode. Moreover, in contrast to its natural form in the absence of vibration, the fully developed rotating stall showed a coherent stall signal.

Journal article

He X, Zhao F, Vahdati M, 2020, Uncertainty Quantification of Spalart-Allmaras Turbulence Model Coefficients for Simplified Compressor Flow Features, JOURNAL OF FLUIDS ENGINEERING-TRANSACTIONS OF THE ASME, Vol: 142, ISSN: 0098-2202

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

Venkatesh S, Suzuki K, Vahdati M, Salles L, Rendu Qet al., 2020, Effect of geometric uncertainty on a one stage transonic compressor of an industrial gas turbine

The geometrical uncertainties can result in flow asymmetry around the annulus of compressor which in turn can detrimentally affect on the compressor stability and performance. Typically these uncertainties arise as a consequence of in-service degradation and/or manufacturing tolerance, both of which have been dealt with in this paper. The paper deals with effects of leading edge damage and tip gap on rotor blades. It was found that the chord-wise damage is more critical than radial damage. It was found that a zigzag pattern of arranging the damaged rotor blades (i.e. most damaged blades between two least damaged blades) would give the best possible performance and stability when performing maintenance and overhauling while a sinusoidal pattern of arrangement had the worst performance and stability. This behaviour of zigzag arrangement of random damaged blades is consonant with the behaviour of zigzag arrangement in random tip gaps. It is also shown in this work that the level of damage has a bigger impact on the compressor performance and stability than the number of damaged blades.

Conference paper

Lu Y, Vahdati M, 2020, Detecting nonsynchronous vibration in transonic fans using machine learning techniques

Due to manufacturing tolerance and deterioration during operation, different blades in a fan assembly exhibit geometric variability. This leads to asymmetry which will be amplified in the running geometry by centrifugal and aerodynamic loads. This study investigates a phenomenon known as Alternate Passage Divergence (APD), where the blade untwist creates an alternating pattern in passage geometry and stagger angle around the circumference. After the formation of alternating tip stagger pattern, APD’s unsteady effect, APD-induced Non-Synchronous Vibration (APD-NSV, abbreviated as NSV), can cause the blades from one group to switch to the other creating a travelling wave pattern around the circumference. Thus, it can potentially lead to high cycle fatigue issues. More importantly, this phenomenon occurs close to, or at, peak efficiency conditions and can significantly reduce overall efficiency. Therefore, it is vital to attenuate the NSV behaviour. The random nature of mis-staggering patterns complicate the evolution of NSV significantly. Thus, machine learning techniques are used to analyse mis-stagger patterns to identify patterns that can lead to NSV and thus help avoid it. Numerical results from 113 numerical cases (1.6 million CPU hours) are used to train and test the classifier. From the results, two parameters contributing to NSV behaviour have been identified with one of them enhancing the understanding found in the previous study.

Conference paper

Suzuki K, Zhao F, Vahdati M, 2020, Numerical analysis of flutter in variable geometry compressors

Aeroelastic behaviour of a transonic rotor in a newly designed 1.5 stage compressor with variable geometry is studied numerically in this paper. The stage is intended to be the front part of a one-shafted large frame industrial gas turbine (IGT) compressor. The compressor was designed using open-source software MULTALL and numerical computations were performed using the three-dimensional aeroelasticity code AU3d, which has been tested and validated for many aeroelastic test cases over the past 25 years. Flutter analysis for the 1F mode was performed at various design and off-design operating conditions which are typically experienced in IGT (varied inlet temperature and inlet guide vane angle). Although in all the cases the rotor remained stable, clear trends in aerodynamic damping were observed, which can be explained by shock position. In the last phase, the effects of increased tip gap size on the flutter stability were studied. The increase in tip clearance did not result in flutter; unsteady computations without blade motion showed a tip rotating instability with 11 cells travelling at 84% of the shaft speed in the stationary frame. Due to the frequency proximity between the rotating instability and blade natural vibration mode, large amplitude displacement driven by lock-in was observed in the fluid-structure coupled simulation. It was concluded that this type of aeroelastic instability which can be mistaken for flutter is the main threat for this IGT compressor.

Conference paper

Lu Y, Lad B, Vahdati M, 2020, Transonic fan blade redesign approach to attenuate nonsynchronous vibration

Due to manufacturing tolerance and deterioration during operation, different blades in a fan assembly exhibit geometric variability. This leads to asymmetry which will be amplified in the running geometry by centrifugal and aerodynamic loads. This study investigates a phenomenon known as Alternate Passage Divergence (APD), where the blade untwist creates an alternating pattern in passage geometry and stagger angle around the circumference. After the formation of alternating tip stagger pattern, APD's unsteady effect, APD-induced Non-Synchronous Vibration (APD-NSV), can cause the blades from one group to switch to the other creating a travelling wave pattern around the circumference. Thus, it can potentially lead to high cycle fatigue issues. More importantly, this phenomenon occurs close to, or at, peak efficiency conditions and can significantly reduce overall efficiency. Therefore, it is vital to attenuate the NSV behaviour. In this study, an redesign approach is investigated.

Conference paper

Zhao F, Dodds J, Vahdati M, 2020, Flow physics during surge and recovery of a multi-stage high-speed compressor

Stall followed by surge in a high-speed compressor can lead to violent disruption of the flow, damage to the blade structures and eventually engine shutdown. Knowledge of unsteady blade loading during surge is crucial for compressor design such as axial gap optimisation. The aim of this paper is to demonstrate the feasibility of using 3D full assembly URANS CFD for modelling surge cycles of an 8-stage high-speed compressor rig. Results from this work show stalling of the mid-stages is the surge trigger. During the flow reversal, a strong acoustic reflection occurs when the convected entropy perturbations reach the intake opening, which increase the blade loading significantly. During recovery, a hysteresis loop was recorded due to hot air re-ingestion, which led to a strong shear at mid-span of the IGV/R1 domain and the formation of rotating helical flow structures. The final phase of recovery was accompanied by a 4-cell multi-row tip rotating stall, which was cleared as the compressor recovered to the forward flow characteristic. It was also shown that the single passage model, despite its limitations and shortcomings in modelling recovery, can provide reasonably accurate transient flow features during surge and thus considerable insight to the flow behaviour, which can be used to obtain a first approximation of casing and blade loading.

Conference paper

He X, Zhao F, Vahdati M, 2020, Uncertainty quantification of spalart-allmaras turbulence model coefficients for compressor stall

The turbulence model in Reynolds-Averaged Navier-Stokes simulations is crucial in the prediction of the compressor stall margin. In this paper, parametric uncertainty of the Spalart-Allmaras turbulence model in predicting two-dimensional airfoil stall and three-dimensional compressor stall has been investigated using a metamodel-based Monte Carlo method. The model coefficients are represented by uniform distributions within physically acceptable ranges. The quantities of interest include characteristic curves, stall limit, blockage size and turbulence magnitude. Results show that the characteristics can be well predicted in the stable flow range, but the inaccuracy and the uncertainty increase when approaching stall. The stall point of the airfoil can be enveloped by the parametric uncertainty range, but that of the rotor cannot. Sensitivity analyses identified the crucial model coefficients to be source-related, where an increase in the predicted turbulence level will delay the onset of stall. Such results imply that implementing new turbulence production terms with respect to the rotor-specific flow features is likely to improve the model accuracy. The findings in this paper not only provide engineering rules of thumb for the model users, but also guide the future implementation of a data-driven turbulence model for the model developers.

Conference paper

Moreno J, Dodds J, Sheaf C, Zhao F, Vahdati Met al., 2020, Aerodynamic loading considerations of three-shaft engine compression system during surge

Compressor surge imposes a limit on aero-engine operability and can compromise integrity because of significant aerodynamic loads imparted on the engine components. The aim of this paper is to use 3D unsteady CFD to predict the surge loadings on a modern three spool engine. The computations are performed using a whole-assembly approach. In this work, the effect of two types of surge initiation on the maximum loading recorded during surge are studied and a physical explanation of the main phenomena which contribute to those loadings is offered. The engine is matched at a high power condition and the surge inception is via throttling of the high pressure compressor (HPC) or turning of the intermediate pressure compressor (IPC) variable stator vanes. It was found that in an aero-engine surge event, the maximum overpressure are caused by a combined effect of the surge shock wave passing and high pressure gas blown towards the front of the engine during depressurisation. The overpressure is dictated by the compression system exit pressure at the moment of the surge inception. The surge initiation via HPC throttling produces larger overpressure and therefore, should be considered for design considerations.

Conference paper

Vahdati M, Lee KB, Sureshkumar P, 2020, A review of computational aeroelasticity of civil fan blades, International Journal of Gas Turbine, Propulsion and Power Systems, Vol: 11, Pages: 22-35

This paper presents a review of aeroelasticity research concerning fan blades in modern civil aircraft engines. It summarises the research carried out at the Rolls-Royce Vibration University Technology Centre (VUTC) at Imperial College over the past 25 years. The purpose of this paper is to gather information on all the aeroelastic issues observed for civil aero-engine fan blades into one document and provide a useful synopsis for other researchers in the field. The results presented here are based on numerical methods but wherever possible data from experiments are used to verify the numerical findings. For cases where such datasets do not exist fundamental principles, engine observations and engineering judgement are used to support the numerical results. Numerical methods offer a cheaper alternative to rig tests, especially in cases of blade failure, and can also provide more information about the nature of instabilities, which can be useful in the design of future civil aircraft engines. In fact, in cases such as crosswind testing that use smaller rig-scale blades, such results can even be more representative of real engine flows.

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

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, 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

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

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, 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

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

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

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

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

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

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