179 results found
Suriyanarayanan V, Rendu Q, Vahdati M, et al., 2022, Effect of Manufacturing Tolerance in Flow Past a Compressor Blade, Publisher: ASME, ISSN: 0889-504X
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
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
Moreno J, Dodds J, Sheaf C, et al., 2021, Aerodynamic Loading Considerations of Three-Shaft Engine Compression System During Surge, Journal of Turbomachinery, Vol: 143, ISSN: 0889-504X
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 article is to use 3D unsteady computational fluid dynamics (CFD) to predict the surge loadings on a compression system of a modern three-spool engine. The compression system is matched at a high power condition and computations are performed using a whole-assembly approach. In this study, the effect of two types of surge initiation on the maximum loading recorded during surge are studied: throttling of the high pressure compressor (HPC) and turning of the intermediate pressure compressor (IPC) variable stator vanes (VSV). An explanation of the main physical phenomena that contribute to those loadings is offered. It was found that in an aero-engine surge event, the maximum overpressure is driven by a combined effect of the surge shock wave passing and high pressure gas blown toward the front of the engine during depressurization. The amplitude of maximum overpressure is dictated by the compression system exit pressure at the moment of surge inception. The surge initiation via HPC throttling produces larger overpressure and therefore should be considered for design intent.
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
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.
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
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
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, 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.
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.
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
Experimental studies have shown that tip injection upstream of the rotor can extend its operational range when subjected to circumferential inlet distortion. Typically, injectors are placed uniformly around the annulus. However, such arrangement consumes a large amount of high-pressure air and decreases the overall efficiency of the compression system. The aim of this paper is to minimise the amount of the injected air by determining the most effective circumferential location for the injector. In this study, NASA stage 35 was used as the test case. The experiment was conducted with a circumferential total pressure distortion of 120 degrees. In the first part of this paper, numerical simulations were compared against the experimental data and good match was obtained. In the second part, tip injection at three different positions were tested: the clean flow region (Position 1), the distorted region (Position 2) and the border between the clean and distorted regions (Position 3). It was found that a mild injection (0.66% of the main flow) at Position 2 and Position 3 can extend the stall margin by 1.8% and 2.7%, respectively. No obvious improvement was observed for the injection at Position 1. With a larger injection of 1.5% of main flow at Position 3, the stall margin improved further with no efficiency loss.
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
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
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.
Moreno J, Dodds J, Sheaf C, et 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.
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.
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.
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.
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.
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.
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.
Zhang W, Vahdati M, 2020, Stall and Recovery Process of a Transonic Fan With and Without Inlet Distortion, Publisher: ASME, ISSN: 0889-504X
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