Wall-modeled Large Eddy Simulation Research Articles

Time-resolved flow field and thermal loads in a single-element GOx/GCH4 rocket combustor

This work presents Large Eddy Simulations of a single-element GOx-GCH4 combustion chamber at elevated pressure. A robust and efficient flamelet model accounting for heat losses is applied to represent the hydrocarbon combustion process at affordable computational effort. Three different near-wall treatments are used: wall-resolved LES, wall-modeled LES and a hybrid RANS/LES approach. The results are compared to experimental data in terms of wall heat fluxes, chamber pressure profile and optical OH∗emission images. Generally, a good agreement with respect to the measurements is observed. In addition, different features of the flow field, like, e.g., the near-injector region and the axial development of the flame, are thoroughly discussed. Finally, a detailed a priori assessment of the different modeling assumptions – regarding the combustion model and relevant transport processes – is carried out to derive possible areas for future improvements.

Effect of Local Grid Refinement on Performance of Scale-Resolving Models for Simulation of Complex External Flows

Numerical simulations are crucial for fast and accurate estimations of the flow characteristics in many engineering applications such as atmospheric boundary layers around buildings, external aerodynamics around vehicles, and pollutant dispersion. In the simulation of flow over urban-like obstacles, it is crucial to accurately resolve the flow characteristics with reasonable computational cost. Therefore, Large Eddy Simulations on non-uniform grids are usually employed. However, an undesirable accumulation of energy at grid-refinement interfaces was observed in previous studies using non-uniform grids. This phenomenon induced oscillations in the spanwise velocity component, mainly on fine-to-coarse grid interfaces. In this study, the two challenging test cases of flow over urban-like cubes and flow over a 3-D circular cylinder were simulated using three different scale-resolving turbulence models. Simulations were performed on uniform coarse and fine grids on one hand, and a non-uniform grid on the other, to assess the effect of mesh density and mesh interfaces on the models’ performance. Overall, the proposed One-Equation Scale-Adaptive Simulation (One-Equation SAS) showed the least deviation from the experimental results in both tested cases and on all grid sizes and types when compared to the Shear Stress Transport-Improved Delayed Detached Eddy Simulation (IDDES) and the Algebraic Wall-Modeled Large Eddy Simulation (WMLES).

High-Reynolds-number wall-modelled large eddy simulations of turbulent pipe flows using explicit and implicit subgrid stress treatments within a spectral element solver

We present explicit and implicit large eddy simulations for fully developed turbulent pipe flows using a continuous-Galerkin spectral element solver. On the one hand, the explicit stretched-vortex model (by Misra and Pullin [45] and Chung and Pullin [14]), accounts for an explicit treatment of unresolved stresses and is adapted to the high-order solver. On the other hand, an implicit approach based on a spectral vanishing viscosity technique is implemented. The latter implicit technique is modified to incorporate Chung & Pullin virtual-wall model instead of relying on implicit dissipative mechanisms near walls. This near-wall model is derived by averaging in the wall-normal direction and relying in local inner scaling to treat the time-dependence of the filtered wall-parallel velocity. The model requires space-time varying Dirichlet and Neumann boundary conditions for velocity and pressure respectively. We provide results and comparisons for the explicit and implicit subgrid treatments and show that both provide favourable results for pipe flows at Reτ=2×103 and Reτ=1.8×105 in terms of turbulence statistics. Additionally, we conclude that implicit simulations are enhanced when including the wall model and provide the correct statistics near walls.

Wall-modelled large-eddy simulation of turbulent flow past airfoils

We present large-eddy simulation (LES) of flow past different airfoils with $Re_{c}$, based on the free-stream velocity and airfoil chord length, ranging from $10^{4}$ to $2.1\times 10^{6}$. To avoid the challenging resolution requirements of the near-wall region, we develop a virtual wall model in generalized curvilinear coordinates and incorporate the non-equilibrium effects via proper treatment of the momentum equations. It is demonstrated that the wall model dynamically captures the instantaneous skin-friction vector field on arbitrary curved surfaces at the resolved scale. By combining the present wall model with the stretched-vortex subgrid-scale model, we apply the wall-modelled LES approach to three different airfoil cases, spanning different geometrical parameters, different attack angles and low to high $Re_{c}$. The numerical results are verified with direct numerical simulation (DNS) at low $Re_{c}$, and validated with experiment data at higher $Re_{c}$, including typical aerodynamic properties such as pressure coefficient distributions, velocity components and also more challenging measurements such as skin-friction coefficient and Reynolds stresses. All comparisons show reasonable agreement, providing a measure of validity that enables us to further probe simulation results into aspects of flow physics that are not available from experiments. Two techniques to quantify hitherto unexplored physics of flows past airfoils are employed: one is the construction of the anisotropy invariant map, and the second is skin-friction portraits with emphasis on flow transition and unsteady separation along the airfoil surface. The anisotropy maps for all three $Re_{c}$ cases, show clearly that a portion of the flow field is aligned along the axisymmetric expansion line, corresponding to the turbulent boundary layer log-law behaviour and the appearance of turbulent transition. The instantaneous skin-friction portraits reveal a monotonic shrinking of the near wall structure scale. At $Re_{c}=10^{4}$, the interaction between the primary separation bubble and the secondary separation bubble contributes to turbulent transition, similar to the case of flow past a cylinder. At higher $Re_{c}=10^{5}$, the primary separation breaks into several small separation bubbles. At even higher $Re_{c}=2.1\times 10^{6}$, near the turbulent separation, the skin-friction lines show small-scale reversal flows that are similar to those observed in DNS of the flat plate turbulent separation. A notable feature of turbulent separation in flow past an airfoil is the appearance of turbulence structures and small-scale reversal flows in the spanwise direction due to the vortex shedding behaviour.

Using a PANS Simulation Approach for the Transient Flow Around the Japan Bulk Carrier

In this study, a partially averaged Navier-Stokes (PANS) modeling approach developed based on the k-ω turbulence model has been applied to the flow around the Japan Bulk Carrier. Three different meshes have been used for a fixed physical resolution for the PANS modeling. The results are encouraging, with small-scale flow dynamics being allowed to develop on reasonably small mesh sizes, but more studies are required before reliable predictive simulations can be performed. 1. Introduction Steady resistance and wake simulations in ship hydrodynamics are currently being performed with relatively high reliability using the Reynolds averaged Navier-Stokes (RANS) simulation approach. There are, however, a number of cases where RANS is not sufficient and a scale-resolving transient simulation approach is needed. This is well established in cases of flow around bluff bodies, e.g., vortex-induced vibrations and flow across cylinders (Pereira 2018), or for ship to ship interaction (Arslan et al. 2015), where the main interest is on global parameters, such as shedding frequency or force variation, but the transient separating flow is not well predicted by RANS simulation. It could also be that details of the transient flow behavior is part of the study, e.g. relating to propeller dynamics (Jang & Mahesh 2013; Bensow 2015; Guilmineau et al. 2017), vortex dynamics (Fureby et al. 2016; Visonneau et al. 2018), or in cavitation (Bensow 2011); in these cases, RANS modeling is inherently not able to capture the details of the governing flow physics because of the underlying averaging. However, it may also be the case where the flow is smoothly separating, or close to, as is the case for the Japan Bulk Carrier (JBC), see Visonneau et al. (2016) and further the discussion below. Using a wall-resolved large eddy simulation (LES) approach is computationally very expensive, with an unfavorable scaling toward full-scale Re numbers (Liefvendahl & Fureby 2017), thus the attention has rather turned toward hybrid RANS/LES approaches or wall-modeled LES; this was used in many of the studies referenced previously, e.g., Guilmineau et al. (2017); Pereira (2018); Visonneau et al. (2018). Grid size estimates for different modeling approaches for simulating the JBC in model scale are given by Liefvendahl and Johansson (2018), amounting to around 2•109 for the wall-resolved LES, 150•106 for the wall-modeled LES, 50•106 for the hybrid RANS/LES, and finally 30•106 for the RANS approach, where the distance between wall-resolved LES and the others will greatly increase with the Re number. In terms of computational effort, one needs to remember that the transient flow approaches, compared with RANS, further needs to include long time-series for sampling statistics.

Numerical analysis of simulant effect on natural convection characteristics in corium pools

In-vessel retention (IVR) is considered as an effective mitigation strategy in realms of nuclear safety. Extensive experimental investigations have been done on the natural convection characteristics of corium pools according to the IVR concept, using various simulant materials. Based on the known COPRA (Corium Pool Research Apparatus) molten salt experiments, in which the eutectic salt (50 mol% NaNO3-50 mol% KNO3) and the non-eutectic salt (20 mol% NaNO3-80 mol% KNO3) were used as simulant materials, this paper evaluates extensively the simulant effect of simulant material on natural convection behaviors inside the corium pools through numerical simulation. The Wall-Modeled LES (WMLES) method was employed for the turbulence calculation. The distributions of corium pool temperature and heat flux as well as the crust thickness from the numerical simulations were validated with experimental data. Through numerical simulations, this work provides detailed temperature fields and velocity fields inside a large internally heated corium pool, and further discusses in depth the interplay between the heat transfer characteristics and the natural convection behaviors. This work can provide references for the selection of simulant material in the future experimental research.

Characteristics of hydrogen jet combustion in a high-enthalpy supersonic crossflow

The facilitation of a stable combustion process is of utmost importance for the realizability and performance of hypersonic propulsion systems. To elucidate the turbulent combustion characteristics, wall-modeled large eddy simulations of a transverse jet injection into a heated supersonic flow are conducted employing a detailed reaction mechanism. The computation framework utilizes an adaptive central-upwind weighted essentially nonoscillatory (WENO-CU) scheme to achieve the sixth-order accuracy in smooth flowfields, while keeping a good shock-capturing ability. The reacting zones agree well with experimental measurements in terms of the instantaneous distribution of OH radicals. And the flame penetration height has been predicted with an error of less than 17%. It is found that the turbulent reacting flow is dominated by nonpremixed combustion mainly taking place in the near-wall region and jet windward shear-layer. Moreover, the autoignition process, which plays a critical role in stabilizing supersonic combustion, shows to favor a fuel-lean or not very fuel-rich environment of a high enthalpy. Local scalar dissipation induced by turbulence gives rise to a rapid fuel mixing with the surrounding air. However, this effect may also lead to the decrease in local temperature.

Systematic study of accuracy of wall-modeled large eddy simulation using uncertainty quantification techniques

The predictive accuracy of wall-modeled large eddy simulation is studied by systematic simulation campaigns of turbulent channel flow. The effect of wall model, grid resolution and anisotropy, numerical convective scheme and subgrid-scale modeling is investigated. All of these factors affect the resulting accuracy, and their action is to a large extent intertwined. The wall model is of the wall-stress type, and its sensitivity to location of velocity sampling, as well as law of the wall’s parameters is assessed. For efficient exploration of the model parameter space (anisotropic grid resolution and wall model parameter values), generalized polynomial chaos expansions are used to construct metamodels for the responses which are taken to be measures of the predictive error in quantities of interest (QoIs). The QoIs include the mean wall shear stress and profiles of the mean velocity, the turbulent kinetic energy, and the Reynolds shear stress. DNS data is used as reference. Within the tested framework, a particular second-order accurate CFD code (OpenFOAM), the results provide ample support for grid and method parameters recommendations which are proposed in the present paper, and which provide good results for the QoIs. Notably, good results are obtained with a grid with isotropic (cubic) hexahedral cells, with 15 000 cells per δ3, where δ is the channel half-height (or thickness of the turbulent boundary layer). The importance of providing enough numerical dissipation to obtain accurate QoIs is demonstrated. The main channel flow case investigated is Reτ=5200, but extension to a wide range of Re-numbers is considered. Use of other numerical methods and software would likely modify these recommendations, at least slightly, but the proposed framework is fully applicable to investigate this as well.

A library for wall-modelled large-eddy simulation based on OpenFOAM technology

This work presents a feature-rich open-source library for wall-modelled large-eddy simulation (WMLES), which is a turbulence modelling approach that reduces the computational cost of standard (wall-resolved) LES by introducing special treatment of the inner region of turbulent boundary layers (TBLs). The library is based on OpenFOAM and enhances the general-purpose LES solvers provided by this software with state-of-the-art wall modelling capability. The included wall models belong to the class of wall-stress models that account for the under-resolved turbulent structures by predicting and enforcing the correct local value of the wall shear stress. A review of this approach is given, followed by a detailed description of the library, discussing its functionality and extensible design. The included wall-stress models are presented, based on both algebraic and ordinary differential equations. To demonstrate the capabilities of the library, it was used for WMLES of turbulent channel flow and the flow over a backward-facing step (BFS). For each flow, a systematic simulation campaign was performed, in order to find a combination of numerical schemes, grid resolution and wall model type that would yield a good predictive accuracy for both the mean velocity field in the outer layer of the TBLs and the mean wall shear stress. The best result, ≈1% error in the above quantities, was achieved for channel flow using a mildly dissipative second-order accurate scheme for the convective fluxes applied on an isotropic grid with 27000 cells per δ3-cube, where δ is the channel half-height. In the case of flow over a BFS, this combination led to the best agreement with experimental data. An algebraic model based on Spalding’s law of the wall was found to perform well for both flows. On the other hand, the tested more complicated models, which incorporate the pressure gradient in the wall shear stress prediction, led to less accurate results. Program SummaryProgram Title: libWallModelledLESProgram Files doi:http://dx.doi.org/10.17632/m8dnsnp4nd.1Licensing provisions: GPLv3Programming language: C++Nature of problem: Large-eddy simulation (LES) is a scale-resolving turbulence modelling approach providing a high level of predictive accuracy. However, LES of high Reynolds number wall-bounded flows is prohibitively computationally expensive due to the need for resolving the inner region of turbulent boundary layers (TBLs) [1]. This inhibits the application of LES to many industrially relevant flows [2] and prompts for the development of novel modelling techniques that would modify the LES approach in a way that allows it to retain its accuracy (at least away from walls) yet significantly lower its computational cost. Solution method: Wall-modelled LES (WMLES) is an approach that is based on complementing LES with special near-wall modelling that allows to leave the inner layer of TBLs unresolved by the computational grid. Many types of wall models have been proposed [1,3], commonly tested within the framework of in-house research codes. Here, an open-source library implementing several wall models is presented. The library is based on OpenFOAM, which is currently the most widely used general-purpose open-source software for computational fluid dynamics. The developed library can be directly applied to both academic and industrial flow cases, leading to a wider adoption of wall modelling and better understanding of its strengths and limitations.[1] J. Larsson, S. Kawai, J. Bodart, and I. Bermejo-Moreno. Large eddy simulation with modeled wall-stress: recent progress and future directions. Mechanical Engineering Reviews, 3(1):1-23, 2016.[2] J. Slotnick, A. Khodadoust, J. Alonso, D. Darmofal, W. Gropp, E. Lurie, D. Mavriplis. CFD vision 2030 study: A path to revolutionary computational aerosciences, Tech. rep., NASA, 2014.[3] S. T. Bose and G. I. Park. Wall-modeled large-eddy simulation for complex turbulent flows. Annual Review of Fluid Mechanics, 50(1):535–561, 2018.

Wall-modeling for large-eddy simulation of flows around an axisymmetric body using the diffuse-interface immersed boundary method

A novel method is proposed to combine the wall-modeled large-eddy simulation (LES) with the diffuse-interface direct-forcing immersed boundary (IB) method. The new developments in this method include: (i) the momentum equation is integrated along the wall-normal direction to link the tangential component of the effective body force for the IB method to the wall shear stress predicted by the wall model; (ii) a set of Lagrangian points near the wall are introduced to compute the normal component of the effective body force for the IB method by reconstructing the normal component of the velocity. This novel method will be a classical direct-forcing IB method if the grid is fine enough to resolve the flow near the wall. The method is used to simulate the flows around the DARPA SUBOFF model. The results obtained are well comparable to the measured experimental data and wall-resolved LES results.

Turbomachinery Noise Predictions: Present and Future

In future Ultra-High By-Pass Ratio turboengines, the turbomachinery noise (fan and turbine stages mainly) is expected to increase significantly. A review of analytical models and numerical methods to yield both tonal and broadband contributions of such noise sources is presented. The former rely on hybrid methods coupling gust response over very thin flat plates of finite chord length, either isolated or in cascade, and acoustic analogies in free-field and in a duct. The latter yields tonal noise with unsteady Reynolds-Averaged Navier–Stokes (u-RANS) simulations, and broadband noise with Large Eddy Simulations (LES). The analytical models are shown to provide good and fast first sound estimates at pre-design stages, and to easily separate the different noise sources. The u-RANS simulations are now able to give accurate estimates of tonal noise of the most complex asymmetric, heterogeneous fan-Outlet Guiding Vane (OGV) configurations. Wall-modeled LES on rescaled stage configurations have now been achieved on all components: a low-pressure compressor stage, a transonic high-pressure turbine stage and a fan-OGV configuration with good overall sound power level predictions for the latter. In this case, hybrid Lattice–Boltzmann/very large-eddy simulations also appear to be an excellent alternative to yield both contributions accurately at once.

THE INFLUENCE OF GEOMETRICAL AND OPERATIONAL PARAMETERS ON INTERNAL FLOW CHARACTERISTICS OF INTERNALLY MIXING TWIN-FLUID Y-JET ATOMIZERS

Internally mixing twin-fluid Y-jet atomizers are widely used in coal fired thermal power plants for start-up, oil-fired thermal power plants and industrial boilers. The flow through internally mixing Y-jet atomizers is numerically modeled using the compressible Navier-Stokes equations; Wall Modeled Large Eddy Simulations (WMLES) is used to resolve the turbulence with Large Eddy Simulations whereas the Prandtl Mixing Length Model is used for modeling the subgrid scale structures, which are affected by geometric and operational parameters. Moreover, the Volume-of-Fluid (VOF) method is used to capture the development and fragmentation of the liquid-gas interface within the Y-jet atomizer. The numerical results are compared with correlations available in open literature for the pressure drop; further results are presented for the multiphase flow regime maps available for vertical pipes. The results show that the mixing point pressure is strongly dependent on the mixing port diameter to airport diameter ratio, specifically for gas to liquid mass flowrate ratio (GLR) in the range 0.1 < GLR < 0.4; the mixing port length moderately affects the mixing point pressure while the angle between mixing and liquid ports is found not to have an appreciable effect. Moreover, it is found that the vertical pipe multiphase flow regime maps in the literature could be applied to the flow through the mixing port of the twin-fluid Y-jet atomizer. The main flow regimes found under the studied operational conditions are annular and wispy annular flow.

Development and validation of a hybrid temporal LES model in the perspective of applications to internal combustion engines

CFD simulation tools are increasingly used nowadays to design more fuel-efficient and clean Internal Combustion Engines (ICE). Within this framework, there is a need to benefit from a turbulence model which offers the best compromise between prediction capabilities and computational cost. The Hybrid Temporal LES (HTLES) approach is here retained within the perspective of an application to ICE configurations. HTLES is a hybrid Reynolds-Averaged Navier Stokes/Large Eddy Simulation (RANS/LES) model based on a solid theoretical framework using temporal filtering. The concept is to model the near-wall region in RANS and to solve the turbulent structures in the core region if the temporal and spatial resolutions are fine enough. In this study, a dedicated sub-model called Elliptic Shielding (ES) is added to HTLES in order to ensure RANS in the near-wall region, regardless of the mesh resolution. A modification of the computation of the total kinetic energy and the dissipation rate was introduced as first adaptions of HTLES towards non-stationary ICE configurations. HTLES is a recent approach, which has not been validated in a wide range of applications. The present study intends to further validate HTLES implemented in CONVERGE code by examining three stationary test cases. The first validation consists of the periodic hill case, which is a standard benchmark case to assess hybrid turbulence models. Then, in order to come closer to real ICE simulations, i.e., with larger Reynolds numbers and coarser near-wall resolutions, the method is validated in the case of a channel flow using wall functions and in the steady flow rig case consisting in an open valve at a fixed lift. HTLES results are compared to RANS k-ω SST and wall-modeled LES σ simulations performed with the same grid and the same temporal resolution. Unlike RANS, satisfactory reproduction of the flow recirculation has been observed with HTLES in the case of periodic hills. The channel flow configuration has underlined the capability of HTLES to predict the wall friction properly. The steady flow rig shows that HTLES combines advantages of RANS and LES in one simulation. On the one hand, HTLES yields mean and rms velocities as accurate as LES since the scale-resolving simulation is triggered in the core region. On the other hand, hybrid RANS/LES at the wall provides accurate pressure drop in contrast with LES performed on the same mesh. Future work will be dedicated to the extension of HTLES to non-stationary flows with moving walls in order to be able to tackle realistic ICE flow configurations.

Effects of numerical dissipation on the predictive accuracy of wall-modelled large-eddy simulation

The effect of numerical dissipation on the predictive accuracy of wall-modelled large-eddy simulation is investigated via systematic simulations of fully-developed turbulent channel flow. A total of 16 simulations are conducted using the open-source computational fluid dynamics software OpenFOAM®. Four densities of the computational mesh are considered, with four simulations performed on each, in turn varying in the amount of numerical dissipation introduced by the numerical scheme used for interpolating the convective fluxes. The results are compared to publicly-available data from direct numerical simulation of the same flow. Computed error profiles of all the considered flow quantities are shown to vary monotonically with the amount of dissipation introduced by the numerical schemes. As expected, increased dissipation leads to damping of high-frequency motions, which is clearly observed in the computed energy spectra. But it also results in increased energy of the large-scale motions, and a significant over-prediction of the turbulent kinetic energy in the inner region of the boundary layer. On the other hand, dissipation benefits the accuracy of the mean velocity profile, which in turn improves the prediction of the wall shear stress given by the wall model. Thus, in the current framework, the optimal choice for the dissipation of the numerical schemes may depend on the primary quantity of interest for the conducted simulation. With respect to the resolution of the grid, the change in the accuracy is much less predictable, and the optimal resolution depends on the considered quantity and the amount of dissipation introduced by the numerical schemes.

Numerical investigation of geometric size effect on the internal heated water pool natural convection behavior

The internal heated melt pool natural convection behavior is one of the important phenomena in the nuclear reactor severe accident investigation aspect. Based on the previous research, the Nusselt number is the function of the internal Rayleigh number, and the geometric size could affect the internal Rayleigh number in the order of 5. According to the previous experimental and simulation results, the relationship of Nusselt number and internal Rayleigh number varies with different tests and conditions, and the local heat transfer coefficient is also varied with the vessel polar angle. Therefore, it is important to investigate how geometric size could affect the internal heated melt pool natural convection behavior. In the present work, the validation of numerical simulation method is conducted through comparison with the COPRA water test experimental data. Then the geometric size effect on the internal heated water pool natural convection behavior is investigated by using ANSYS FLUENT 18.2 and the wall modeled large eddy simulation (WMLES) turbulence model is applied in the current simulation. The water pool temperature distribution, water pool velocity distribution, and local heat flux along the curved wall are obtained in this study. The mechanism of geometric size effect on the internal heated water pool natural convection behavior is investigated. Besides, both the upward and downward Nusselt number with internal Rayleigh number correlations are proposed based on the current simulation results. Based on the current simulation, the cases with 0.1 m, 0.3 m and 0.5 m water pool radius show the similar natural circulation behavior in the temperature distribution and heat flux distribution aspects, and the cases with 1.0 m, 1.5 m, 2.0 m and 2.5 m water pool radius show another type of natural circulation behavior. However, the cases with 0.1 m, 0.3 m, 0.5 m and 1.0 m water pool radius show the similar Nusselt number with internal Rayleigh number correlations, and the cases with 1.5 m, 2.0 m and 2.5 m water pool radius show another type of Nusselt number with internal Rayleigh number correlations. The present study could provide useful references for the severe accident management, and further improve the understanding of how geometric size could affect the internal heated water pool natural convection behavior.

Dynamic slip wall model for large-eddy simulation.

Wall modelling in large-eddy simulation (LES) is necessary to overcome the prohibitive near-wall resolution requirements in high-Reynolds-number turbulent flows. Most existing wall models rely on assumptions about the state of the boundary layer and require a priori prescription of tunable coefficients. They also impose the predicted wall stress by replacing the no-slip boundary condition at the wall with a Neumann boundary condition in the wall-parallel directions while maintaining the no-transpiration condition in the wall-normal direction. In the present study, we first motivate and analyse the Robin (slip) boundary condition with transpiration (non-zero wall-normal velocity) in the context of wall-modelled LES. The effect of the slip boundary condition on the one-point statistics of the flow is investigated in LES of turbulent channel flow and a flat-plate turbulent boundary layer. It is shown that the slip condition provides a framework to compensate for the deficit or excess of mean momentum at the wall. Moreover, the resulting non-zero stress at the wall alleviates the well-known problem of the wall-stress under-estimation by current subgrid-scale (SGS) models (Jiménez & Moser, AIAA J., vol. 38 (4), 2000, pp. 605-612). Second, we discuss the requirements for the slip condition to be used in conjunction with wall models and derive the equation that connects the slip boundary condition with the stress at the wall. Finally, a dynamic procedure for the slip coefficients is formulated, providing a dynamic slip wall model free of a priori specified coefficients. The performance of the proposed dynamic wall model is tested in a series of LES of turbulent channel flow at varying Reynolds numbers, non-equilibrium three-dimensional transient channel flow and a zero-pressure-gradient flat-plate turbulent boundary layer. The results show that the dynamic wall model is able to accurately predict one-point turbulence statistics for various flow configurations, Reynolds numbers and grid resolutions.

The Role of Forcing and Eddy Viscosity Variation on the Log-Layer Mismatch Observed in Wall-Modeled Large-Eddy Simulations

We investigate the role of eddy viscosity variation and the effect of zonal enforcement of the mass flow rate on the log-layer mismatch problem observed in turbulent channel flows. An analysis of the mean momentum balance shows that it lacks a degree-of-freedom (DOF) when eddy viscosity is large, and the mean velocity conforms to an incorrect profile. Zonal enforcement of the target flow rate introduces an additional degree-of-freedom to the mean momentum balance, similar to an external stochastic forcing term, leading to a significant reduction in the log-layer mismatch. We simulate turbulent channel flows at friction Reynolds numbers of 2000 and 5200 on coarse meshes that do not resolve the viscous sublayer. The second-order turbulence statistics agree well with the direct numerical simulation benchmark data when results are normalized by the velocity scale extracted from the filtered velocity field. Zonal enforcement of the flow rate also led to significant improvements in skin friction coefficients.

LES-based simulation of the time-resolved flow for rotor-stator interactions in axial fan stages

PurposeThe purpose of this paper is the development of a CFD methodology based on LES computations to analyze the rotor–stator interaction in an axial fan stage.Design/methodology/approachA wall-modeled large eddy simulation (WMLES) has been performed for a spanwise 3D extrusion of the central section of the fan stage. Computations were performed for three different operating conditions, from nominal (Q_N) to off-design (85 per cent Q_N and 70 per cent Q_N) working points. Circumferential periodic conditions were introduced to reduce the extent of the computational domain. The post-processing procedure enabled the segregation of unsteady deterministic features and turbulent scales. The simulations were experimentally validated using wake profiles and turbulent scales obtained from hot-wire measurements.FindingsThe transport of rotor wakes and both wake–vane and wake–wake interactions in the stator flow field have been analyzed. The description of flow separation, particularly at off-design conditions, is fully benefited from the LES performance. Rotor wakes impinging on the stator vanes generate a coherent large-scale vortex shedding at reduced frequencies. Large pressure fluctuations in the stagnation region on the leading edge of the vanes have been found.Research limitations/implicationsLES simulations have shown to be appropriate for the assessment of the design of an axial fan, especially for specific operating conditions for which a URANS model presents a lower performance for turbulence description.Originality/valueThis paper describes the development of an LES-based simulation to understand the flow mechanisms related to the rotor–stator interaction in axial fan stages.

Numerical assessment of wall modelling approaches in scale-resolving in-cylinder simulations

Wall modelling in internal combustion engines (ICEs) is a challenging task due to highly specific boundary layers and a dynamically changing flow environment. Recent experimental (Jainski et al., 2013, Renaud et al., 2018) and direct numerical simulation (DNS, Schmitt et al., 2015a) studies demonstrate that scaled near-wall velocity and temperature profiles in ICEs deviate considerably from the law of the wall. Utilising the DNS data, the present paper focusses on benchmarking a scale-resolving approach with a 1-D non-equilibrium wall model (HLR-WT, Keskinen et al., 2017) in ICE-like flows. Specific emphasis is put on the compression stroke using different grids and two additional wall-modelled large eddy simulation (WMLES) reference approaches. The standard wall law based WMLES-1 produces highly grid-dependent underprediction of wall fluxes, to which WMLES-2 (Plensgaard and Rutland, 2013) and HLR-WT, employing engine-targeted wall treatments, yield considerable improvement. Differences between the improved methods are noted in detailed metrics. Throughout the compression stroke, HLR-WT provides a good match to the DNS in scaled mean boundary layer profiles for both velocity and temperature. With relevance to local heat flux distribution, the characteristic impingement-ejection process observed in the DNS is qualitatively replicated with WMLES-2 and HLR-WT. The non-equilibrium formulation of the latter allows for slight improvements in terms of local heat transfer fluctuation predictions. In contrast, coarse near-wall grids appear to be detrimental for such predictions with all approaches. The study provides evidence on the potential of the HLR-WT and WMLES-2 approaches in ICE near-wall flow prediction, advocating further investigations in more realistic engine configurations.

High Reynolds Number Airfoil: From Wall-Resolved to Wall-Modeled LES

Wall-Modeled Large-Eddy Simulation (WMLES) alleviates the near-wall grid requirement by employing a wall-model to reconstruct the wall shear-stress. In this way, WMLES simultaneously reduces the computational cost associated with Wall-Resolved LES (WRLES) and opens the door towards higher Reynolds numbers (Piomelli, Wall-modeled large-eddy simulations: present status and prospects. Springer, Netherlands, 2010, [1], Larsson et al, Mech Eng Rev, 3, 2016, [2].