Publications
197 results found
Chennuru VYT, Zhao F, Vahdati M, 2024, An Optimal Numerical Strategy for Intake in Crosswind Conditions, ISSN: 0742-4795
Crosswind can reduce the operability of an aeroengine significantly at static or near static operating conditions. Computational fluid dynamics predictions of the flow at crosswind conditions will play an important part in future designs, however, accurate numerical predictions of the flow within the intake remain challenging even for simulations of cases with intake only. The main objective of this paper is to demonstrate the importance of numerical setup and to determine an optimal computational model for crosswind investigations that can be used by other researchers. By considering the flow to be inherently unsteady, the influence of inlet and exit boundary conditions, and grid sensitivity is studied by using unsteady Reynolds-average Navier-Stokes (URANS) simulations. Numerical predictions of the time-averaged intake pressure recovery (IPR) and the motion of the vortex on the ground are compared against the existing experimental data. The results show that for an intake under crosswind, the ground vortex that forms under the intake and the in-duct separation, when present, exhibit unsteady behavior that become stronger as the crosswind velocity is increased. The steady-state simulation is only representative at lower crosswinds. The intake flow separation and ground vortex predictions are influenced by the inlet boundary layer profiles. Moreover, acoustic reflection was observed at the intake exit boundary which propagates upstream, creating artificial unsteady frequencies in IPR. The reflection is generated from the use of uniform boundary conditions at the intake exit and is not dissipated in the grid due to the long wavelength; this can be mitigated by using a choked nozzle at the intake exit. Acoustic reflection was also observed at the far-field exit boundary. These reflections are caused from interaction of trailing vortex and far-field boundary.
Yan C, Wang B, He X, et al., 2024, Extension and Validation of the Turbomachinery Capabilities of SU2 Open Source Computational Fluid Dynamic Code, Journal of Turbomachinery, Vol: 146, ISSN: 0889-504X
Computational Fluid Dynamic (CFD) tools have revolutionized the way to design engineering systems, but most established codes are proprietary and closed-source, making it difficult, if not impossible, to modify, debug, or add new features to the code. To provide a freely available open-source CFD code for turbomachinery aerodynamics and aeroelasticity, this paper enhances the turbomachinery capabilities of the open-source SU2 code and demonstrates its capabilities of single-passage steady simulation, full-annulus unsteady simulation, and aeroelasticity analysis in two high-speed compressors, namely NASA Stage 35 and TUDa-GLR-OpenStage, and a linear cascade SC1. For the single-passage steady simulation of NASA Stage 35, the SU2 results are validated against the measured data and verified against the commercial solver ANSYS CFX, and the performance characteristics results are in reasonably good agreement with each other. For the single-passage steady simulation of TUDa-GLR-OpenStage, grid and turbulence model sensitivity studies are performed and results are validated against the measured data, and SU2 can predict both the performance characteristics and the radial profiles with sufficient accuracy. For the full-annulus unsteady simulation of NASA Stage 35, it is demonstrated that SU2 can predict the propagation of inlet distortion equally well as ANSYS CFX. For the linear cascade, SU2 can predict the unsteady pressure and aerodynamic damping coefficient accurately. The presented results demonstrate the turbomachinery aerodynamics and aeroelasticity capabilities of SU2. The major modifications of SU2 made in this work will be shared with the code maintainer and the community in the future.
Ba D, Du J, Vahdati M, et al., 2024, Design optimization of a hybrid casing treatment based on axial momentum budget analysis in the tip flow region, Physics of Fluids, Vol: 36, ISSN: 1070-6631
In this paper, a novel slot-groove hybrid casing treatment is designed and optimized to improve the stall margin of a low-speed axial compressor. A combination of the axial slot and circumferential groove casing treatments is utilized to increase the stall margin without incurring efficiency loss. The slot meridional profile is described with 2 B-spline curves. Circumferential grooves are parametrically described with groove height and width. An in-house optimization design platform is constructed based on the nondominated sorting genetic algorithm II and Kriging surrogate model. The optimization objectives are the stall margin and the peak efficiency of the compressor at the design rotating speed. To avoid the large number of unsteady simulations that are required to predict the stall margin, a stall margin improvement indicator is proposed based on the axial momentum budget analysis at the rotor tip region. The performance of the optimal slot-groove hybrid casing treatment design is tested and simulated. The experimental data show that the optimal slot-groove hybrid casing treatment improves the stall margin by 8.42% without generating efficiency loss. The flow details are captured by unsteady simulations and analyzed in depth. The application of the optimal casing treatment enhances the blade tip axial momentum and the interface between tip leakage flow and incoming main flow is pushed downstream. Consequently, the stability of the compressor is improved.
Zhao F, Moreno J, Dodds J, et al., 2023, Methodology and Validation of Surge Modeling in a Three-Shaft Compression System, Journal of Turbomachinery, Vol: 145, ISSN: 0889-504X
In this paper an efficient numerical strategy for computational fluid dynamics (CFD) simulation of surge events in a three-shaft engine compression system is presented. Numerical results are compared against measured data and the sources of discrepancies and uncertainties are addressed. It is discovered that the engine bleed system has a major impact in reducing the aerodynamic loading during surge in the core system. To the best knowledge of the authors, this is the first time that such a complex CFD computation is attempted and compared against real engine measured data and will provide valuable information to other CFD users as well as gas turbine manufacturers.
Zhao F, Chen D, Liu J, et al., 2023, A framework for simulating snow accumulation and ice accretion on high-speed trains, PROCEEDINGS OF THE INSTITUTION OF MECHANICAL ENGINEERS PART F-JOURNAL OF RAIL AND RAPID TRANSIT, ISSN: 0954-4097
Yan C, Wang B, He X, et al., 2023, EXTENSION AND VALIDATION OF THE TURBOMACHINERY CAPABILITIES OF SU2 OPEN SOURCE CFD CODE
Computational Fluid Dynamic (CFD) tools have revolutionized the way to design engineering systems, but most established codes are proprietary and closed-source, making it difficult, if not impossible, to modify, debug or add new features to the code. To provide a freely available open-source CFD code for turbomachinery aerodynamics and aeroelasticity, this paper enhances the turbomachinery capabilities of the open-source SU2 code and demonstrates its capabilities of single-passage steady simulation, full-annulus unsteady simulation and aeroelasticity analysis in two high-speed compressors, namely NASA Stage 35 and TUDa-GLR-OpenStage, and a linear cascade SC1. For the single-passage steady simulation of NASA Stage 35, the SU2 results are validated against the measured data and verified against the commercial solver Ansys CFX, and the performance characteristics results are in reasonably good agreement with each other. For the single-passage steady simulation of TUDa-GLR-OpenStage, grid and turbulence model sensitivity studies are performed and results are validated against the measured data, and SU2 can predict both the performance characteristics and the radial profiles with sufficient accuracy. For the full-annulus unsteady simulation of NASA Stage 35, it is demonstrated that SU2 can predict the propagation of inlet distortion equally well as Ansys CFX. For the linear cascade, SU2 can predict the unsteady pressure and aerodynamic damping coefficient accurately. The presented results demonstrate the turbomachinery aerodynamics and aeroelasticity capabilities of SU2. The major modifications of SU2 made in this work will be shared with the code maintainer and the community in the future.
Chennuru VYT, Zhao F, Vahdati M, 2023, AN OPTIMAL CFD STRATEGY FOR INTAKE IN CROSSWIND CONDITIONS
Crosswind can reduce the operability of an aeroengine significantly at static or near static operating conditions. CFD predictions of the flow at crosswind conditions will play an important part in future designs, however, accurate numerical predictions of the flow within the intake remain challenging even for simulations of cases with intake only. The main objective of this paper is to demonstrate the importance of numerical setup and to determine an optimal computational model for crosswind investigations that can be used by other researchers. By considering the flow to be inherently unsteady, the influence of inlet and exit boundary conditions, and grid sensitivity is studied by using URANS simulations. Numerical predictions of the time-averaged intake pressure recovery (IPR) and the motion of the vortex on the ground are compared against the existing experimental data. The results show that for an intake under crosswind, the ground vortex that forms under the intake and the in-duct separation, when present, exhibit unsteady behaviour that become stronger as the crosswind velocity is increased. The steady-state simulation is only representative at lower crosswinds. The intake flow separation and ground vortex predictions are influenced by the inlet boundary layer profiles. Moreover, acoustic reflection was observed at the intake exit boundary which propagates upstream, creating artificial unsteady frequencies in IPR. The reflection is generated from the use of uniform boundary conditions at the intake exit and is not dissipated in the grid due to the long wavelength; this can be mitigated by using a choked nozzle at the intake exit. Acoustic reflection was also observed at the far-field exit boundary. These reflections are caused from interaction of trailing vortex and far-field boundary.
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
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- Citations: 3
Xia K, He X, Zhu M, et al., 2023, Endwall geometric uncertainty and error on the performance of TUDA-GLR-OpenStage transonic axial compressor, JOURNAL OF THE GLOBAL POWER AND PROPULSION SOCIETY, Vol: 7, Pages: 113-126
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, Pages: 1-16, 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: 8
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: 5
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: 9
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: 3
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: 3
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: 8
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, Fang Z, Rigas G, et al., 2021, SPECTRAL PROPER ORTHOGONAL DECOMPOSITION OF COMPRESSOR TIP LEAKAGE FLOW, ISSN: 2313-0067
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.
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.
Tan J, He X, Rigas G, et al., 2021, TOWARDS EXPLAINABLE MACHINE-LEARNING-ASSISTED TURBULENCE MODELING FOR TRANSONIC FLOWS, ISSN: 2313-0067
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.
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