Publications
185 results found
He X, Zhu M, Xia K, et al., 2023, Validation and verification of RANS solvers for TUDa-GLR-OpenStage transonic axial compressor, Journal of the Global Power and Propulsion Society, Vol: 7, Pages: 13-29
This paper presents a comprehensive validation and verification study of turbomachinery Reynolds-averaged Navier-Stokes flow solvers on the tran-sonic axial compressor TUDa-GLR-OpenStage. Two commercial solvers namely Ansys CFX and Numeca FineTurbo are adopted to provide the benchmark solutions, which can be used for verification of other RANS solvers in the future. Based on these solvers, five sets of grids, two advec-tion schemes (i.e., central difference and second-order upwind), four turbulence models (i.e., SA, SA-RC, SST and EARSM) and two rotor-stator interface models (i.e., mixing plane and sliding plane) are investigated to quantify their effects on predicting the performance and the flow field of the compressor stage. Results show that the choices of grid density and turbulence model are most sensitive to the prediction, leading to 5% and 7% variation in compressor performance characteristics, respectively. Regarding the choice of grid density, a method to estimate the grid discret-ization error is demonstrated, which is transferrable to other cases. Regarding the choice of turbulence model, the EARSM model is found overall most accurate among the investigated models, and the limitations and deficiencies of the rest models are discussed in detail based on the analysis of the mean flow fields and the eddy viscosity fields. The grids and the major CFD results presented in this work are open-accessed to the community for further research. The results and discussions presented in this paper provide a useful reference for future practices of RANS simulations for compressors.
He X, Zhao F, Vahdati M, 2022, A Turbo-Oriented Data-Driven Modification to the Spalart-Allmaras Turbulence Model, JOURNAL OF TURBOMACHINERY-TRANSACTIONS OF THE ASME, Vol: 144, ISSN: 0889-504X
He X, Tan J, Rigas G, et al., 2022, On the explainability of machine-learning-assisted turbulence modeling for transonic flows, International Journal of Heat and Fluid Flow, Vol: 97, ISSN: 0142-727X
Machine learning (ML) is a rising and promising tool for Reynolds-Averaged Navier–Stokes (RANS) turbulence model developments, but its application to industrial flows is hindered by the lack of explainability of the ML model. In this paper, two types of methods to improve the explainability are presented, namely the intrinsic methods that reduce the model complexity and the post-hoc methods that explain the correlation between the model inputs and outputs. The investigated ML-assisted turbulence model framework aims to improve the prediction accuracy of the Spalart–Allmaras (SA) turbulence model in transonic bump flows. A random forest model is trained to construct a mapping between the input flow features and the output eddy viscosity difference. Results show that the intrinsic methods, including the hyperparameter study and the input feature selection, can reduce the model complexity at a limited cost of accuracy. The post-hoc Shapley additive explanations (SHAP) method not only provides a ranked list of input flow features based on their global significance, but also unveils the local causal link between the input flow features and the output eddy viscosity difference. Based on the SHAP analysis, the ML model is found to discover: (1) the well-known scaling between eddy viscosity and its source term, which was originally found from dimensional analysis; (2) the well-known rotation and shear effects on the eddy viscosity source term, which was explicitly written in the Reynolds stress transport equations; and (3) the pressure normal stress and normal shear stress effect on the eddy viscosity source term, which has not attracted much attention in previous research. The methods and the knowledge obtained from this work provide useful guidance for data-driven turbulence model developers, and they are transferable to future ML turbulence model developments.
Suriyanarayanan V, Rendu Q, Vahdati M, et al., 2022, Effect of Manufacturing Tolerance in Flow Past a Compressor Blade, Publisher: ASME, ISSN: 0889-504X
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- Citations: 4
Zeng H, Zheng X, Vahdati M, 2022, A Method of Stall and Surge Prediction in Axial Compressors Based on Three-Dimensional Body-Force Model, JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER-TRANSACTIONS OF THE ASME, Vol: 144, ISSN: 0742-4795
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- Citations: 3
Piovesan T, Wenqiang Z, Vahdati M, et al., 2022, INVESTIGATIONS OF THE UNSTEADY AERODYNAMIC CHARACTERISTICS FOR INTAKES AT CROSSWIND
The ground vortex generated in front of an intake operating near the ground and subjected to crosswind is investigated using CFD and compared to the experiments. The flow field of a scale-model intake is numerically simulated with both steady and unsteady approach, with the aim of predicting ground vortex effects and to characterize the vortex unsteady behaviour. The experimental results showed that for an intake near the ground under crosswind the ground vortex that forms under the intake and the in-duct separation, when present, exhibit unsteady behaviour that becomes stronger as the crosswind velocity is increased. The simulations indicate that a steady-state approach only partially reproduces the time-averaged ground vortex characteristics and in-duct distortion losses, while an unsteady approach shows a lower level of unsteadiness compared to the experimental observations. The consequences of the unsteady flow in the intake on the fan aerodynamic and aeroelastic stability are finally discussed to reinforce that these can result in significant non-synchronous vibration (NSV) and loss of stall margin which cannot be adequately assessed if no unsteady component of the inlet distortions is taken into account.
Rauseo M, Zhao F, Vahdati M, et al., 2022, UNCERTAINTY QUANTIFICATION OF COMPUTATIONAL FLUTTER ESTIMATES OF A COMPRESSOR CASCADE
Aeroelastic instabilities such as flutter can greatly limit the operating range and safety of modern aircraft engines. Current computational methods have a central role in the evaluation of turbomachinery blades stability, but can be affected by errors if the investigated flow conditions break model assumptions or are particularly sensitive to small changes in flow variables. In this paper, a machine learning based method is proposed to quantify the uncertainty of computational aerodynamic damping predictions. The test case employed for this study is a two dimensional compressor cascade, which resembles most of the relevant aeroelastic features of modern fan and compressor blades. A random forest based model is trained and tested to construct a mapping between input features and aerodynamic damping, both obtained from linearised CFD computations. The input features concern simple, physically relevant quantities that are available early on in design stage. The results show that the machine learnt model can produce predictions, by interpolating within the range of input features, with a coefficient of determination R2 ≈ 0.94. Moreover, the predictions are enhanced with a measure of uncertainty in terms of confidence intervals. The results show that the confidence intervals can accurately portray the sensitivity of aerodynamic damping with respect to the flow variables. Finally, to underline the relevance of such an approach during design, the model is applied to obtain a conservative flutter boundary on a compressor map, providing a safer operating margin.
He X, Zhao F, Vahdati M, 2022, A TURBO-ORIENTED DATA-DRIVEN MODIFICATION TO THE SPALART-ALLMARAS TURBULENCE MODEL
The Spalart-Allmaras turbulence model is one of the most popular models applied to compressors, but it often over-predicts compressor blockage size and hence under-predicts the compressor stall margin. In this paper, a novel modification to the SA model is proposed to improve the prediction of compressor near-stall flows. The modification is based on the dimensionless vortical pressure gradient, which identifies blockage cells featured by 3D swirling low-momentum flows under adverse pressure gradients. It unblocks the compressor passage by enhancing the eddy viscosity in the identified blockage cells; whereas in canonical 2D flows the modification is automatically switched off. The model coefficients are calibrated via Bayesian inference, which considers the uncertainties involved in experiments and CFD. The rotor exit radial profile data of NASA Rotor 67 at the peak efficiency (PE) and near stall (NS) conditions at the design speed are used for calibration. Validation on the NASA Rotor 67 case shows that the proposed model not only predicts a better agreement with the measured near-tip profiles compared to the original SA model, but also predicts more accurate stall margins at all operating speeds. Further validations are conducted on the TUDa-GLR-OpenStage transonic axial compressor and the low-speed BUAA Stage B rotor, where stall is driven by the rotor tip blockage cell. Results show that the proposed model predicts a more accurate stall margin at all operating speeds of TUDa-GLR-OpenStage and a more accurate tip blockage size of BUAA Stage B rotor. Finally, the proposed model is validated on the LMFA NACA65 cascade featured by corner separation. In combination with the quadratic constitutive relation (QCR), significant improvement in predicting the static pressure rise coefficient, the total pressure loss coefficient, the flow angle and the blade loading is achieved, indicating the model predicts a more accurate corner separation size. The proposed model, terme
He X, Zhao F, Vahdati M, 2022, Detached Eddy Simulation: Recent Development and Application to Compressor Tip Leakage Flow, JOURNAL OF TURBOMACHINERY-TRANSACTIONS OF THE ASME, Vol: 144, ISSN: 0889-504X
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- Citations: 3
Moreno J, Dodds J, Sheaf C, et al., 2021, Aerodynamic Loading Considerations of Three-Shaft Engine Compression System During Surge, JOURNAL OF TURBOMACHINERY-TRANSACTIONS OF THE ASME, Vol: 143, ISSN: 0889-504X
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- Citations: 2
He X, Fang Z, Rigas G, et al., 2021, Spectral proper orthogonal decomposition of compressor tip leakage flow, Physics of Fluids, Vol: 33, ISSN: 1070-6631
To identify the spatiotemporal coherent structure of compressor tip leakage flow, spectral proper orthogonal decomposition (SPOD) is performed on the near-tip flow field and the blade surface pressure of a low-speed compressor rotor. The data used for the SPOD analysis are obtained by delayed-detached eddy simulation, which is validated against the experimental data. The investigated rotor near-tip flow field is governed by two tip leakage vortices (TLV), and the near-tip compressor passage can be divided into four zones: the formation of main TLV (Zone I), the main TLV breakdown (Zone II), the formation of tip blockage cell (Zone III), and the formation of secondary TLV (Zone IV). Modal analysis from SPOD shows that a major part of total disturbance energy comes from the main TLV oscillating mode in Zone I and the main TLV vortex shedding mode in Zone III, both of which are low-frequency and low-rank; on the contrary, modal components in Zones II and IV are broadband and non-low-rank. Unsteady blade forces are mainly generated by the impingement of the main TLV on the blade pressure surface in Zone III, rather than the detachment of the secondary TLV from the blade suction surface in Zone IV. These identified coherent structures provide valuable knowledge for the aerodynamic/aeroelastic effects, turbulence modeling, and reduced-order modeling of compressor tip leakage flow.
Rauseo M, Vahdati M, Zhao F, 2021, Machine Learning Based Sensitivity Analysis of Aeroelastic Stability Parameters in a Compressor Cascade, INTERNATIONAL JOURNAL OF TURBOMACHINERY PROPULSION AND POWER, Vol: 6
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
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- Citations: 2
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
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- Citations: 2
Vahdati M, Gerdooei M, 2021, Numerical Analysis of Hot Deep Drawing Process of the Thick Hemispherical Head Without the Blank-Holder, Transactions of the Indian Institute of Metals, Vol: 74, Pages: 1775-1786, ISSN: 0972-2815
In deep drawing process, if the workpiece is not willing to wrinkle due to small drawing ratio, deep drawing is done without the blank-holder. In present paper, numerical analysis of hot deep drawing for the thick hemispherical head from a plate with the thickness of 63.5 mm which is made of alloy steel HY-100 is presented without using the blank-holder. The drawing process of hemispherical head is studied by using three different geometries of drawing die in three positions as (1) increase in the curvature radius of the die, (2) approximate tractrix and (3) exact tractrix. In this research, Johnson–Cook constitutive model is used in order to explain the flow behavior of the material and the damage model of Johnson–Cook is used for prediction of failure probability in the product. Also, it is assumed that the hot deep drawing is done as isothermal and the strain rate is fixed. The results of numerical analysis show that the probability of damage occurrence in the “exact tractrix” design is lower than the other designs. Also, comparison of the thickness distribution in the wall of hemispherical head shows that the “exact tractrix” design has the least fluctuation of change in the wall thickness. Comparison of drawing force obtained by the numerical analysis show that at the same conditions of blank temperature and friction coefficient, using the “exact tractrix” design can significantly reduce the needed force in order to draw the hemispherical head.
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
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- Citations: 7
Tan J, He X, Rigas G, et al., 2021, TOWARDS EXPLAINABLE MACHINE-LEARNING-ASSISTED TURBULENCE MODELING FOR TRANSONIC FLOWS
A machine-learning-assisted turbulence modeling framework is proposed to improve the prediction accuracy of the Spalart-Allmaras turbulence model. The case studied is the transonic bump flow, which partially resembles the flow physics of a transonic compressor. A random forest model is trained, cross-validated and tested to construct a mapping between the input features and the eddy viscosity discrepancy. These input features concern the physical effects of pressure gradient, strain versus vorticity, flow misalignment, wall proximity and viscosity ratio. Results show that the proposed approach predicts an interpolation and an extrapolation test case with L1-type errors of 11.1% and 16.5%, respectively. The Shapley additive explanations method is employed to investigate the global and local sensitivities of each input feature. The capability of these input features in identifying specific flow features is discussed. The methods and results of this work provide useful guidance for turbulence model developers.
He X, Fang Z, Rigas G, et al., 2021, SPECTRAL PROPER ORTHOGONAL DECOMPOSITION OF COMPRESSOR TIP LEAKAGE FLOW
Spectral proper orthogonal decomposition (SPOD) is performed on the near-stall tip leakage flow of a low-speed compressor rotor. The data used for the SPOD analysis is obtained by delayed-detached eddy simulation (DDES), which is validated against experimental data. The flow quantities of interest include the near-tip axial velocity and the blade surface pressure. Results show that the near-stall flow field of the investigated rotor is governed by two tip leakage vortices (TLV). The main TLV initiated from the leading edge exerts an unsteady force on the blade pressure surface. Its modal component is dominated by the leading modes at low frequencies. The secondary TLV originated from the mid-chord creates a weaker unsteady force on the blade suction surface, and its modal component has more high-frequency components due to its interaction with the suction surface boundary layer flows. These findings improve the understanding of the effects of tip leakage flow on compressor aerodynamic and aeroelastic stability.
Moradi M, Moghadam MK, Shamsborhan M, et al., 2021, Simulation, statistical modeling, and optimization of CO<inf>2</inf> laser cutting process of polycarbonate sheets, Optik, Vol: 225, ISSN: 0030-4026
Laser cutting well-known as a manufacturing process is a rapid, repeatable, and reliable method that is frequently used for cutting various materials such as thermoplastics. Due to their physical and chemical properties such as fatigue resistance, high toughness, and re-melting properties, thermoplastics such as polycarbonate are widely used in automotive parts, electronics, etc. In this study, a numerical simulation of the laser cutting process by a finite element method is developed. The sample simulated in this research is a 3.2 mm thick Polycarbonate sheet that is subjected to the laser cutting process by a low power continuous CO2 laser. The effects of the laser cutting process parameters such as laser power, cutting speed, and laser focal plane position on the top and bottom kerf width, top heat-affected zone, the ratio of upper kerf width to lower kerf width and taper kerf are investigated by statistical techniques of variance analysis. Choosing an appropriate Gaussian distribution is studied as well. The results show that the laser scanning speed has a significant effect on the top kerf width. By choosing a cutting speed of 20 mm/s and a focal length of -3, the taper kerf is minimized. By increasing the laser cutting speed from 4 to 20 mm/s and decreasing the laser power from 50 to 20 W, the heat-affected zone decreases. The developed analysis can predict the depth of kerf in a continuous mode for different values of laser power, speed, and laser focal plane.
He X, Zhao F, Vahdati M, 2021, Detached eddy simulation: Recent development and application to compressor tip leakage flow
Detached Eddy Simulation (DES) and its variants are emerging tools for turbomachinery simulations. In this paper, the state-of-the-art upgrades of DES are reviewed, and their capabilities in predicting compressor tip leakage flow are discussed. The upgrade with the best potential is identified as the Delayed DES (DDES) method with the grid spacing FKHδhyb, which unlocks the physics of the Kelvin-Helmholtz instability in compressor tip leakage flow. The upgraded grid spacing FKHδhyb is compared against the widely used default one δmax in a backward-facing step and a low-speed axial compressor rotor. Results show that the DDES method with FKHδhyb predicts both the main flow field and the turbulence field with reasonably good accuracy. However, the original DDES method with δmax predicts a delayed transition to turbulence, which leads to an inaccurate prediction of the main flow field when using a coarse mesh. The findings in this paper highlight the future opportunities for using the DDES-FKHδhyb method to predict tip-driven compressor stall and generate a turbulence database for turbulence model development.
Zhang W, Vahdati M, 2021, Investigation of the tip injection for stall control in a transonic compressor with inlet distortion, JOURNAL OF THE GLOBAL POWER AND PROPULSION SOCIETY, Vol: 5, Pages: 28-38
Moreno J, Dodds J, Stapelfeldt SC, et 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.
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
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.
Vahdati M, Moradi M, Shamsborhan M, 2020, Modeling and Optimization of the Yield Strength and Tensile Strength of Al7075 Butt Joint Produced by FSW and SFSW Using RSM and Desirability Function Method, Transactions of the Indian Institute of Metals, Vol: 73, Pages: 2587-2600, ISSN: 0972-2815
Friction stir welding (FSW) is introduced as a solid-state welding process. Despite the many benefits of the FSW, the effects of the thermal cycles in this process are causing softening of the joint. This phenomenon generally occurs in heat-treatable aluminum alloys and results in reduced mechanical properties of the joint. To solve this limitation, submerged friction stir welding (SFSW) has been developed which is suitable for welding of heat-sensitive alloys. In this study, 31 butt joints were first produced from Al7075-T6 using the FSW. For this purpose, the response surface methodology was selected as the design of experiments method, and the variables: tool rotational speed, tool feed rate, tool shoulder diameter, and tool tilt angle were determined as the input variables. Then, the statistical analysis of the parameters affecting the yield strength and tensile strength of the joints was investigated. Then, 10 joints were produced using the SFSW based on the optimal values of the tool feed rate and tool tilt angle. Results of the ANOVA and regression analysis of the experimental data confirmed the accuracy and precision of regression equations and showed that the linear, interactional and quadratic terms of tool shoulder diameter and tool rotational speed effect on the yield strength and ultimate tensile strength of submerged joints. Also, the optimal conditions of input variables were determined by the desirability method and confirmed by the verification test.
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
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- Citations: 7
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
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- Citations: 16
Venkatesh S, Suzuki K, Vahdati M, et 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.
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.
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.
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