Wall-resolved Large Eddy Simulation Research Articles

Guideline for Large-Scale Analysis of Centrifugal Blower Using Wall-Resolved Large Eddy Simulation

Abstract To reduce the number of prototypes during product design and accurately predict unsteady phenomena occurring at off-design points, a method for accurately predicting the performance of centrifugal blowers through numerical analysis is required. This article presents a guideline for accurately predicting the performance of centrifugal blowers using compressible flow analysis with large eddy simulation (LES). In LES analysis, it is important to have a grid resolution that resolves the minimum vortex scale near the wall (referred to as wall-resolved LES) and to consider detailed geometry such as the length of the suction pipe. The calculations in this study used a model blower, which is a scale model of a single-stage centrifugal blower for use in industrial plants. The model blower was experimentally measured for various parameters such as the blower pressure coefficient, the static-pressure-rise coefficients of the impeller and vane-less diffuser, the shaft power, and the pressure fluctuations at the inlet of the impeller and the inlet of the vane-less diffuser. The results of these measurements were compared with those obtained from the wall-resolved LES. The study confirmed that the accuracy of performance prediction can be improved to less than a 4.0% error in the blower pressure coefficient at both design and off-design operating points by resolving the minimum vortex scale with 14.6 billion-grid elements and considering the detailed geometry.

Characterizing disturbance growth and transition of laminar separation bubbles subjected to varying freestream turbulence

The spatial manifestation of a boundary layer subjected to an adverse pressure gradient for varying freestream turbulence (FST) is discussed here through experiments and a wall-resolved large eddy simulation (LES). Three levels of FST, i.e., 1.02%, 2.1%, and 3.5%, are considered at a Reynolds number of 0.2 × 106 based on the inlet velocity and plate length. A laminar separation bubble (LSB) appears for FST levels of 1.02%, where the shear layer becomes unstable via the Kelvin-Helmholtz (K-H) mechanism. Bubble suppression is apparent, where transition occurs via the Klebanoff mode (K-mode), bypassing the K-H instability as the FST level is increased to 3.5%. The streamwise evolution of fluctuations reveals an exponential growth at a low FST of 1.02%, whereas it changes to algebraic at a higher FST level of 3.5%. Notably, a dual growth of velocity fluctuations, i.e., an algebraic growth in the initial part of LSB followed by an exponential growth, illustrates the coexistence of K-H instability and K-mode at an FST level of 2.1%: a finding corroborated by LES. Additionally, local linear stability analysis and the evolution of intermittency provide valuable insights into the growth of modal and non-modal instabilities and their interactions.

Boundary layer stability on a rotating wind turbine blade section

Wall-resolved large eddy simulations of the flow on a rotating wind turbine blade section are conducted to study the rotation effects on laminar-turbulent transition on the suction surface. A chord Reynolds number of 1×105 and angles of attack (AoA) of 12.8°, 4.2°, and 1.2° are considered. Simulations with and without rotation are performed for each AoA. For AoA=12.8°, rotation increases the reverse flow from 7% of the free-stream velocity in the non-rotating case to 16% of it in the rotating case in the laminar separation bubble (LSB), triggering an oblique instability mechanism in the latter, leading to a faster breakdown to small-scale turbulence. However, rotation delays transition and reattachment in 3%–4% of the chord due to the acceleration of the boundary layer upstream of the LSB, which is subject to a strong adverse pressure gradient (APG), stabilizing Tollmien–Schlichting (TS) waves. Regarding AoA=4.2° and 1.2°, rotation slightly decelerates the attached boundary layer since the APG is very mild but accelerates the separated flow downstream, stabilizing Kelvin–Helmholtz (KH) modes. This mitigates the oblique instability mechanism and slows down the breakdown of KH vortices in the rotating case. In these cases, the transition location is little affected by rotation, possibly due to a rotation-independent absolute instability. Rotation also generates a spanwise tip-flow in the LSB for AoA=4.2° and 1.2°, which is highly unstable and triggers stationary and traveling crossflow modes. Nevertheless, the amplitudes of these modes remain too low to trigger transition.

A critical assessment of Navier–Stokes and lattice Boltzmann frameworks applied to high-lift configuration through a multiresolution approach

The present work focuses on a thorough assessment of the influence of two very different numerical approaches, namely, Navier–Stokes (NS) and the lattice Boltzmann method (LBM), to simulate the flow past a three-element airfoil through zonal detached eddy simulation (ZDES). Both computations (ZDES-NS and ZDES-LBM) are compared to the reference results, namely, a wall-resolved large eddy simulation (WRLES) as well as the experimental data. It is shown that despite very different numerical modeling, the two ZDES provide very consistent results, with the first- and second-order statistics obtained with equivalent accuracy in the impingement region. In light of present results, the ZDES mode 2 (2020) turbulence model within an LBM framework appears as a judicious combination for high-lift flow applications owing to its robustness regarding the use of very fine isotropic Cartesian grids. In addition, ZDES-NS exhibits a very good agreement with both references, especially WRLES despite having 40 times less nodes.

Role of very large-scale motions in shock wave/turbulent boundary layer interactions

The present study investigates the cause of low-frequency unsteadiness in shock wave/turbulent boundary layer (TBL) interactions. A supersonic turbulent flow over a compression ramp is studied using wall-resolved large eddy simulation (LES) with a freestream Mach number of 2.95 and a Reynolds number (based on δ0: the thickness of the incoming TBL) of 63 560. From the view of stability analysis, the effect of intrinsic instability on such low-frequency unsteadiness is excluded from the flow system by designing a ramp angle of 15°, and our attention is paid to the convective instability contributed by the incoming TBL. The LES results are analyzed by linear and nonlinear disambiguation optimization (LANDO), spectral proper orthogonal decomposition (SPOD), and resolvent analysis. The LANDO results reveal a streamwise scale-frequency relation of coherent structures in a very long (around 60δ0) TBL, which indicates that the dynamics of very large-scale motions (VLSMs) in the TBL are featured by a low frequency. The SPOD results reveal that the most energetic SPOD mode features a low frequency that is identical to the dominant low frequency of the wall-pressure spectrum. Additionally, coherent structures of the mode resemble the VLSMs in the incoming TBL. These consistencies imply that the dynamics of VLSMs contribute to the low-frequency unsteadiness of the present flow. A resolvent analysis then further suggests that the origins of low-frequency dynamics of the present flow are from the VLSMs, which can be optimally amplified by the forcing in the turbulent flow.

Compressibility effect on flow characteristics over a circular cylinder at Reynolds number of 3900

The compressibility effect of flow over a circular cylinder has been investigated using wall-resolved large eddy simulation. The Reynolds number in terms of the freestream quantities and the cylinder diameter is fixed at 3900 while varying the freestream Mach number from 0.01 to 0.5. Mesh quality, numerical scheme, and subgrid-scale model are carefully verified prior to the detailed flow study. Results such as Mach number effects on the drag coefficient, the shape of the mean streamwise velocity profile in the near-wake, the length of the recirculation zone, and shear layer instability are provided. Although compressibility suppresses the Kelvin–Helmholtz instability and mixing process within the shear layer, it increases the velocity fluctuation in the boundary layer and the pressure difference between the freestream and recirculation zone, which intensifies the wake oscillation, shedding amplitude, thus shortens the recirculation zone significantly with the Mach number up to 0.5. This phenomenon is different from that found in lower Reynolds number cases where the length of the recirculation zone elongates as the Mach number increases. The deficit of the velocity profile shape in the near-wake depends on the length of the recirculation zone. Furthermore, inside the wake zone and near the cylinder back wall, two pairs of recirculation bubbles emerge as the Mach number increases.

Large-eddy simulation of vortex interaction in pitching-fixed tandem airfoils

In this study, the interaction of vortices generated from an oscillating airfoil with a hindfoil placed downstream of the oscillating forefoil at low-Reynolds-number flow was investigated numerically. The forefoil entered a deep dynamic stall induced by large-amplitude pitching oscillation. The dynamic stall process is characterized by unsteady separation and the formation of a strong clockwise vortex. A wall-resolved large-eddy simulation approach was applied to compute the flowfield. The numerical measurements were performed for an incompressible flow at a Reynolds number of Re = 30 000 based on chord length with a pitching reduced frequency of K= 0.5, and amplitude of A = 14.1° over Selig–Donovan 7003 airfoils. A single-airfoil case was validated against numerical and experimental measurements. In the present study, we investigated the flowfield and aerodynamic coefficients resulting from the deep dynamic stall of the pitching forefoil and the vortex interaction in tandem-airfoil configuration related to micro-air vehicle applications by employing large-eddy simulation approach. Large-eddy simulation was also compared to two-dimensional unsteady Reynolds-averaged Navier–Stokes simulation to determine the accuracy and validity of the low-fidelity approach in prediction of deep dynamic stall and vortex interaction at low-Reynolds-number flow.

Low-frequency dynamics of turbulent recirculation bubbles

We revisit the origin of low-frequency unsteadiness in turbulent recirculation bubbles (TRBs), and, in particular, the hypothesis of a dynamic feedback mechanism between unconstrained separation and reattachment locations. To this end, we conduct wall-resolved large-eddy simulations of a novel experimental configuration where a shock-induced TRB forms over a backward-facing step that is intended to intercept the hypothesized dynamic feedback. Our results demonstrate, for the first time, effective suppression of the low-frequency characteristics of the TRB without reducing its size, strongly supporting our hypothesis.

Towards Wall-Resolved Large-Eddy Simulation of the High-Lift Common Research Model

In the present study, we performed a sixth-order wall-resolved large-eddy simulation (WRLES) of the high-lift Common Research Model (CRM-HL) on the Leadership Class Computing Cluster Summit. During the 4th High-Lift Prediction Workshop (HLPW-4), wall-modeled LES (WMLES) and hybrid Reynolds-averaged Navier–Stokes (RANS)/LES approaches showed promise in predicting the maximum lift and large flow separations near stall. In those simulations, however, laminar and transition regions were not resolved since the flow was assumed to be fully turbulent. If one wants to resolve these regions, the only viable approach is WRLES. The main purpose of the present study is to demonstrate the feasibility of WRLES for a complex real-world configuration at the wind tunnel Reynolds number. A high-order unstructured mesh LES solver called hpMusic was employed in the study. The simulation at solution polynomial order P=5 has more than 14 billion degrees of freedom per equation. Computational results are compared to experimental data in the form of surface oil flows and pressure coefficient profiles. We highlight lessons learned from attempting a grand-challenge type large-scale simulation: 1) such a simulation is feasible but very expensive, estimated to be at least three orders more expensive than WMLES, 2) oil flows produced with WRLES agree very well with experimental oil flows in fine detail, not observed in those produced with WMLES, and 3) flow visualization is a severe bottleneck, and parallel postprocessing capability is needed.

URANOS-2.0: Improved performance, enhanced portability, and model extension towards exascale computing of high-speed engineering flows

We present URANOS-2.0, the second major release of our massively parallel, GPU-accelerated solver for compressible wall flow applications. This latest version represents a significant leap forward in our initial tool, which was launched in 2023 (De Vanna et al. [1]), and has been specifically optimized to take full advantage of the opportunities offered by the cutting-edge pre-exascale architectures available within the EuroHPC JU. In particular, URANOS-2.0 emphasizes portability and compatibility improvements with the two top-ranked supercomputing architectures in Europe: LUMI and Leonardo. These systems utilize different GPU architectures, AMD and NVIDIA, respectively, which necessitates extensive efforts to ensure seamless usability across their distinct structures. In pursuit of this objective, the current release adheres to the OpenACC standard. This choice not only facilitates efficient utilization of the full potential inherent in these extensive GPU-based architectures but also upholds the principles of vendor neutrality, a distinctive characteristic of URANOS solvers in the CFD solvers' panorama. However, the URANOS-2.0 version goes beyond the goals of improving usability and portability; it introduces performance enhancements and restructures the most demanding computational kernels. This translates into a 2× speedup over the same architecture. In addition to its enhanced single-GPU performance, the present solver release demonstrates very good scalability in multi-GPU environments. URANOS-2.0, in fact, achieves strong scaling efficiencies of over 80% across 64 compute nodes (256 GPUs) for both LUMI and Leonardo. Furthermore, its weak scaling efficiencies reach approximately 95% and 90% on LUMI and Leonardo, respectively, when up to 256 nodes (1024 GPUs) are considered. These significant performance advancements position URANOS-2.0 as a state-of-the-art supercomputing platform tailored for compressible wall turbulence applications, establishing the solver as an integrated tool for various aerospace and energy engineering applications, which can span from direct numerical simulations, wall-resolved large eddy simulations, up to most recent wall-modeled large eddy simulations. Program summaryProgram title: Unsteady Robust All-around Navier-StOkes Solver (URANOS)CPC Library link to program files:https://doi.org/10.17632/pw5hshn9k6.2Developer's repository link:https://github.com/uranos-gpu/uranos-gpu, https://github.com/uranos-gpu/uranos-gpu/tree/v2.0Licensing provisions: BSD License 2.0Programming language: Modern Fortran, OpenACC, MPINature of problem: Solving the compressible Navier-Stokes equations in a three-dimensional Cartesian framework.Solution method: Convective terms are treated with high-resolution shock-capturing schemes. The system dynamics is advanced in time with a three-stage Runge-Kutta method. Parallelization adopts MPI+OpenACC.

Large Eddy Simulation of Laminar and Turbulent Shock/Boundary Layer Interactions in a Transonic Passage

Abstract Shock/boundary layer interactions (SBLI) are a fundamental fluid mechanics problem relevant in a wide range of applications including transonic rotors in turbomachinery. This paper uses wall-resolved large eddy simulation (LES) to examine the interaction of normal shocks with laminar and turbulent inflow boundary layers in transonic flow. The calculations were performed using GENESIS, a high-order, unstructured LES solver. The geometry created for this study is a transonic passage with a convergent-divergent nozzle that expands the flow to the desired Mach number upstream of the shock and then introduces constant radius curvature to simulate local airfoil camber. The Mach numbers in the divergent section of the transonic passage simulate single stage commercial fan blades. The results predicted with the LES calculations show significant differences between laminar and turbulent SBLI in terms of shock structure, boundary layer separation and transition, and aerodynamic losses. For laminar flow into the shock, significant flow separation and low-frequency unsteadiness occur, while for turbulent flow into the shock, both the boundary layer loss and the low-frequency unsteadiness are reduced. The time period of the unsteadiness was hypothesized to align with the time it takes for turbulent structures to convect from the shock to the trailing edge and acoustic disturbances to travel back from the trailing edge to the shock, and this is supported by the LES results.

Large-Eddy Simulations of Flow over a Backward-Facing Ramp with a Wall-Mounted Cube

Passive flow control devices, such as vortex generators (VGs), can effectively modulate the turbulent boundary layer flow near regions of adverse pressure gradients, but the interactions between the salient flow structures produced by VGs and those of the separated flow are not fully understood. In this study, a spatially evolving turbulent boundary layer interacting with a wall-mounted cube ahead of a backward-facing ramp is investigated using wall-resolved large-eddy simulations for a Reynolds number of 19,600, based on the inlet boundary layer thickness and freestream velocity. Different cube configurations are examined to isolate the effects of cube height and streamwise position. The large counter-rotating flow produced by the larger cubes generated more turbulent kinetic energy, thereby resulting in smaller separated regions over the ramp. Although significantly lower levels of turbulent kinetic energy dissipation than production are observed in the outer flow regions of the horseshoe vortex and in the induced counter-rotating flow, the spatial distribution of dissipation is similar to that of the turbulent kinetic energy. When the upstream position of an isolated single cube, relative to the leading ramp edge, is more than three cube heights, the streamwise decay of the counter-rotating flow results in lower levels of turbulent kinetic energy.

Characterization of near-wall flow and in-cylinder free-stream turbulence during the intake phase of a direct-injection engine: A flow bench study

This study investigates the characteristics of the near-wall flow and pressure-induced wall-influenced turbulence inside a direct-injection engine during the intake phase by utilizing a wall-resolved Large Eddy Simulation. An engine flow bench operating under stationary conditions is used to reduce the complexity of engine flows and to shed light on the in-cylinder flow during the intake phase. The investigation focuses on in-cylinder turbulence on symmetry (SP) and valve planes (VP), particularly emphasizing the flow close to the intake valves and the impingement region on the liner wall. An experimental dataset acquired through 2D high-speed particle image velocimetry (PIV) facilitates validation of the simulation of both time-averaged velocity field and associated turbulence structure. Turbulence anisotropy analysis shows that, in complex wall-bounded flows, the IC engine-related in-cylinder turbulence structure is characterized by the anisotropic states that cope with both axisymmetric expansion and contraction straining events. Due to the strong influence of the intake flow, the turbulence anisotropy level, expressed in terms of the two-componentality parameter F, retains its value of less than 0.85, and no turbulence occurs in accordance with the fully three-component isotropic state, characterized by the F-parameter value of 1. In the vicinity of the intake valve near wall area, the turbulence anisotropy state transitions from that resembling a canonical channel flow characteristic to curvature-induced acceleration after impingement, which aligns with axisymmetric expansion in the anisotropy invariant map. The presence of intake flow sustains turbulence anisotropy. Limitations in applying wall-function treatments for near-wall flow in engine contexts due to simplifications, such as the zero-pressure gradient assumption, are highlighted. In this respect, the interplay of non-equilibrium effects in the near-wall region needed to be further explored. Accordingly, analysis of the viscosity-affected layer near the intake valve and the flow impingement area of the liner wall underscores the importance of the wall-tangential pressure gradient in shaping the near-wall behavior. A budget analysis of the turbulent kinetic energy equation at the intake valve emphasizes the dominance of fluctuating pressure-induced diffusive transport in turbulence generation.

Modulation of flow, heat, and mass transfer in turbulent double-diffusive convection

The present study addresses numerical simulations of the double-diffusive convection in a turbulent regime. The characteristic concentration and temperature Prandtl numbers (PrC = 700 and PrT = 7) correspond to the typical seawater properties. Here, we applied the Large-Eddy Simulation (LES) approach, with relatively simple subgrid turbulence closures of momentum, concentration, and temperature for the unresolved scales. The wall-resolved LES approach proved to work well on significantly coarser numerical mesh than used in recent Direct Numerical Simulation (DNS) studies of Yang et al. (2016) in the intermediate range of working parameters, 107 ≤ RaC ≤ 109, 0 ≤ RaT ≤ 106, which covers the quasi-Rayleigh-Bénard, fingering, and damping flow regimes. A good agreement between concentration and temperature Nusselt numbers is obtained. The instantaneous Nusselt numbers distribution revealed a significant impact of the imposed strong thermal stratification (RaT = 106) in comparison to the neutral case (RaT = 0).

Large-Eddy Simulation and Aeroacoustic Prediction of Supercritical Airfoil Side-Edge Noise

Airfoil self-noise is investigated on a cantilever wing with a supercritical profile with a compressible wall-resolved large-eddy simulation. The Reynolds number based on the chord is 620,000, and the angle of attack is 5°. The aerodynamic results reveal the development of a complex vortex system at the side edge, including primary, secondary, and tertiary vortices that govern aerodynamic noise production. The wall-pressure coefficient and root-mean-square pressure coefficient contours highlight the side-edge shear layer and flow impingement of the primary vortex at the pressure-side edge as important noise-generation mechanisms. Dynamic mode decomposition shows that the dominant pressure modes on the airfoil exist along primary and secondary vortex impingement lines. Far-field acoustic predictions based on both solid and porous Ffowcs-Willliams and Hawkings’ analogies demonstrate good agreement with experimental results between 1.5 and 9 kHz. Low-frequency tonal humps observed in the spectra result from duct acoustic modes excited by airfoil self-noise, a consequence of the installation effect. For this configuration, airfoil side-edge noise did not dominate over airfoil surface noise levels, suggesting that at low to moderate angles of attack, airfoil side-edge noise is not a significant contributor to the overall acoustic emissions.

Embedded shear layers in turbulent boundary layers of a NACA0012 airfoil at high angles of attack

An investigation of turbulent boundary layers (TBLs) is presented for a NACA0012 airfoil at angles of attack 9 and 12 deg. Wall-resolved large eddy simulations (LES) are conducted for a freestream Mach number M=0.2 and chord-based Reynolds number Re=4×105, where the boundary layers are tripped near the airfoil leading edge on the suction side. For the angles of attack analyzed, mild, moderate and strong adverse pressure gradients (APGs) develop over the airfoil. Despite the strong APGs, the mean flow remains attached along the entire airfoil suction side. Similarly to other APG-TBLs investigated in the literature, a secondary peak appears in the Reynolds stress and turbulence production profiles. This secondary peak arises in the outer layer and, for strong APGs, it may overcome the first peak typically observed in the inner layer. The analysis of the turbulence production shows that other components of the production tensor become important in the outer layer besides the shear term. For moderate and strong APGs, the mean velocity profiles depict three inflexion points, the third being unstable under inviscid stability criteria. In this context, an embedded shear layer develops along the outer region of the TBL leading to the formation of two-dimensional rollers typical of a Kelvin–Helmholtz instability which are captured by a spectral proper orthogonal decomposition (SPOD) analysis. The most energetic SPOD spatial modes of the tangential velocity show that streaks form along the airfoil suction side and, as the APG becomes stronger, they grow along the spanwise and wall-normal directions, having a spatial support along the entire boundary layer.

Continuous Eddy Simulation (CES) of Transonic Shock-Induced Flow Separation

Reynolds-averaged Navier–Stokes (RANS), large eddy simulation (LES), and hybrid RANS-LES, first of all wall-modeled LES (WMLES) and detached eddy simulation (DES) methods, are regularly applied for wall-bounded turbulent flow simulations. Their characteristic advantages and disadvantages are well known: significant challenges arise from simulation performance, computational cost, and functionality issues. This paper describes the application of a new simulation approach: continuous eddy simulation (CES). CES is based on exact mathematics, and it is a minimal error method. Its functionality is different from currently applied simulation concepts. Knowledge of the actual amount of flow resolution enables the model to properly adjust to simulations by increasing or decreasing its contribution. The flow considered is a high Reynolds number complex flow, the Bachalo–Johnson axisymmetric transonic bump flow, which is often applied to evaluate the performance of turbulence models. A thorough analysis of simulation performance, computational cost, and functionality features of the CES model applied is presented in comparison with corresponding features of RANS, DES, WMLES, and wall-resolved LES (WRLES). We conclude that CES performs better than RANS, DES, WMLES, and even WRLES at a little fraction of computational cost applied for the latter methods. CES is independent of usual functionality requirements of other methods, which offers relevant additional advantages.

Sensitivity analysis of wall-modeled large-eddy simulation for separated turbulent flow

In this study, we conduct a parametric analysis to evaluate the sensitivities of wall-modeled large-eddy simulation (LES) with respect to subgrid-scale (SGS) models, mesh resolution, wall boundary conditions and mesh anisotropy. While such investigations have been conducted for attached/flat-plate flow configurations, systematic studies specifically targeting turbulent flows with separation are notably sparse. To bridge this gap, our study focuses on the flow over a two-dimensional Gaussian-shaped bump at a moderately high Reynolds number, which involves smooth-body separation of a turbulent boundary layer under pressure-gradient and surface-curvature effects. In the simulations, the no-slip condition at the wall is replaced by three different forms of boundary condition based on the thin boundary layer equations and the mean wall-shear stress from high-fidelity numerical simulation to avoid the additional complexity of modeling the wall-shear stress. Various statistics, including the mean separation bubble size, mean velocity profile, and dissipation from SGS model, are compared and analyzed. The results reveal that capturing the separation bubble strongly depends on the choice of SGS model. While simulations approach grid convergence with resolutions nearing those of wall-resolved LES meshes, above this limit, the LES predictions exhibit intricate sensitivities to mesh resolution. Furthermore, both wall boundary conditions and the anisotropy of mesh cells exert discernible impacts on the turbulent flow predictions, yet the magnitudes of these impacts vary based on the specific SGS model chosen for the simulation.

Improved self-adaptive turbulence eddy simulation for complex flows and stall prediction using high-order schemes

It is challenging to apply numerical simulations to accurately predict the stall behavior of aircraft equipped with high-lift devices. Simulations with Reynolds-Averaged NavierStokes (RANS) models suffer from lack of the reliability at high angles of attack with separated and reattached boundary layers, whereas wall-resolved Large Eddy Simulations (LES) of wall-bounded flows at high Reynolds numbers currently costs too much computational resources. A new unified hybrid turbulence modeling approach, denoted the Self-Adaptive Turbulence Eddy Simulation (SATES), is proposed and applied for complex turbulent flows combining with high-order numerical scheme of the Weighted Compact Nonlinear Scheme (WCNS) in the present study. It enables a seamless evolution from unsteady RANS to LES and finally approaches Direct Numerical Simulation (DNS) depending on the turbulent scales. In the framework of SATES, a new SATES-σ model with an adaptive model coefficient is developed by extending the underlying LES mode based on an enhanced sub-grid-scale model of the σ-model. The new SATES-σ is first examined in two benchmark cases of channel flow and flow past a square cylinder. Then, it is validated in supercritical flow past a circular cylinder to assess the performance of turbulent models. The results show significant improvements over the previous SATES and IDDES in the predictions of boundary layer flow. Finally, successful application is achieved in the accurate prediction of the stall of the MD-30P30N airfoil at a Reynolds number of 9×106 with wide angles of attack. The simulation results show good agreement with experimental results for surface pressure even for the challenging cases of 21 and 23 deg angles of attack. Again, the SATES-σ shows better results than the previous SATES and IDDES. The presented method has considerable potential for the challenging stall predictions.

Assessment of Machine Learning Wall Modeling Approaches for Large Eddy Simulation of Gas Turbine Film Cooling Flows: An a Priori Study

Abstract In this work, a priori analysis of machine learning (ML) strategies is carried out with the goal of data-driven wall modeling for large eddy simulation (LES) of gas turbine film cooling flows. High-fidelity flow datasets are extracted from wall-resolved LES (WRLES) of flow over a flat plate interacting with the coolant flow supplied by a single row of 7-7-7 shaped cooling holes inclined at 30 degrees with the flat plate at different blowing ratios (BR). The WRLES are performed using the high-order Nek5000 spectral element computational fluid dynamics (CFD) solver. Light gradient boosting machine (LightGBM) is employed as the ML algorithm for the data-driven wall model. Parametric tests are conducted to systematically assess the influence of a wide range of input flow features (velocity components, velocity gradients, pressure gradients, and fluid properties) on the accuracy of ML wall model with respect to prediction of wall shear stress. In addition, the use of spatial stencil and time delay is also explored within the ML wall modeling framework. It is shown that features associated with gradients of the streamwise and spanwise velocity components have a major impact on the prediction fidelity of wall model, while the effect of gradients of wall-normal velocity component is found to be negligible. Moreover, adding flow feature information from an x-y-z spatial stencil significantly improves the ML model accuracy and generalizability compared to just using local flow features from the matching location. Overall, highest prediction accuracy is achieved when both spatial stencil and time delay features are incorporated within the data-driven wall modeling paradigm.