Validation Of Large Eddy Simulation Research Articles

Application of an Automatic Mesh Convergence Procedure for the Large Eddy Simulation of a Multipoint Injection System

AbstractLean combustion is an attractive alternative to limit pollutants levels in order to meet the imposed limitations for the next generation of civil aero-engines. However, its implementation involves important technological questions related to the augmentation of the air dedicated to the combustion process. An effort on the injection system design is required and large eddy simulation (LES) can be a useful tool in order to explore the design of novel concepts. At the state of the art, the validation of LES in high-pressure reactive conditions and in presence of the liquid phase is still limited. This shrinks the understanding and optimization of lean devices. The industrial project PERCEVAL, between Safran Tech and the CORIA laboratory, aims at extending the actual knowhow on lean combustion. Novel optical experimental techniques have been developed at CORIA to gain detailed information on industrial injection systems at high-pressure conditions. Within PERCEVAL, Safran Tech is in charge of the assessment of LES by using the experimental dataset collected at CORIA. In this framework, a novel automatic mesh convergence (AMC) procedure, based on adaptive mesh refinement, has been developed in the YALES2 platform to speed-up the calculation process. In the present paper, the AMC framework is described and then applied on the lean injection system designed at Safran Tech and tested during PERCEVAL. An analysis is carried out to evaluate the interest and gains offered by the AMC framework.

Verification and validation of large eddy simulation of cavitating flow in Venturi

Verification and validation of large eddy simulation of cavitating flow in Venturi

Numerical study of multi-component flow and mixing in a scaled fission product venting system

Numerical simulation of isothermal multi-component gas flow was conducted to study fluid flow and mixing in a scaled low-pressure, low-temperature test facility for the fission product venting system (FPVS) of a gas cooled fast reactor (GFR). The geometry of FPVS test facility was an open loop including gradual expansion coupling and two 90°pipe elbows.First, the validation of large eddy simulation (LES) wall-adapting local eddy-viscosity (WALE) turbulent model was performed for transitional flow in a circular pipe with gradual expansion. The reactingFOAM solver in OpenFOAM v8 was employed. By introducing appropriate turbulence disturbance at the flow inlet, such as turbulence intensity, the numerical results showed good agreement in (1) the velocity profile downstream of the pipe expansion measured by particle image velocimetry (PIV) in this study, and (2) the reattachment length reported in the literature.Using this validated model, the numerical simulation of flow and component mixing was performed for the FPVS with an inlet flowrate corresponding to a Reynolds number of 2,400 to investigate the flow behavior and component mixing. Standard deviation of the mass fraction of component, referred to as absolute mixing index (AMI), was calculated to quantify the mixing of components, showing that the component mixing length determined by AMI is related to the reattachment length downstream of the expansion. Finally, this study was able to identify the best location in the FPVS test facility to measure the velocity and concentration profies where multicomponent flow is well mixed.

Verification and Validation of Large Eddy Simulation for Tip Clearance Vortex Cavitating Flow in a Waterjet Pump

In this paper, large eddy simulation (LES) was adopted to simulate the cavitating flow in a waterjet pump with emphasis on the tip clearance flow. The numerical results agree well with the experimental observations, which indicates that the LES method can make good predictions of the unsteady cavitating flows around a rotor blade. The LES verification and validation (LES V&V) analysis was used to reveal the influence of cavitation on the flow structures. It can be found that the LES errors in cavitating region are larger than those in the non-cavitating area, which is mainly caused by more complicated cavitating and tip clearance flow structures. Further analysis of the interaction between the cavitating and vortex flow by the relative vorticity transport equation shows that the stretching, dilatation and baroclinic torque terms have major effects on the generation and transport of vortex structure. Meanwhile the Coriolis force term and viscosity term also exacerbate the vorticity transport in the cavitating region. In addition, the flow loss characteristics of this pump are also revealed by the entropy production theory. It is indicated that the tip clearance flow and trailing edge wake flow cause the viscous dissipation and turbulent dissipation, and the cavitation can further enhance the instability of the flow field in the tip clearance.

Advanced benchmark of the flow through a mixing vane grid – Large eddy simulation validation

Identifying what causes fuel assembly vibrations downstream of Mixing Vane Grids (MVG) in Pressurized Water reactor (PWR) is of paramount importance for nuclear community to understand grid-to-rod fretting wear. Experiments, called CALIFS, were carried out by the Atomic Energy Commission (CEA) on a 5 × 5 MVG at a hydraulic Reynolds number of 66,000, in order to measure the flow velocity and the pressure along the central rod. In parallel, a benchmark for Large Eddy Simulation (LES) was setup to compare the predictions of three different CFD codes: Star-CCM+, Code_Saturne and TrioCFD to experimental measurements. The computational domain is representative of a span of CALIFS mockup, composed of a 5 × 5 rod bundle with a MVG. The three computations overall give very satisfactory results, independently from the mesh created and the modelling options selected. It seems to suggest that whatever the software used, this kind of calculations has reached a significant level of robustness and accuracy. Nonetheless, some discrepancies remain concerning the predictions of pressure standard deviation decay far downstream of the mixing vane grid.

Validation of large eddy simulation of flow behind a circular cylinder

The flow in the wake behind a circular cylinder in a cross-flow at Reynolds number of 4815 was studied both experimentally and via mathematical modeling. The mathematical model was performed as a Large Eddy Simulation (LES), while the experiments were carried out using the time-resolved variant of the Particle Image Velocimetry (PIV) method. Both the simulation and experiment took into account the dynamical aspects of the studied phenomenon, which enabled a detailed validation of the mathematical model. The overall statistical properties of the simulated flow were validated via comparing the time-averaged measured and computed velocity and vorticity fields. To validate the dynamical behavior, the velocity spectra were examined first. Next, the Proper Orthogonal Decomposition (POD) of the spatio-temporal velocity data was performed on both the experimental and numerical data and a comparison of the obtained energetic modes was carried out. All the performed validations have shown a satisfactory agreement between the simulation and the experiment.

Wind Loading on Scaled Down Fractal Tree Models of Major Urban Tree Species in Singapore

Estimation of the aerodynamic load on trees is essential for urban tree management to mitigate the risk of tree failure. To assess that in a cost-effective way, scaled down tree models and numerical simulations were utilized. Scaled down tree models reduce the cost of experimental studies and allow the studies to be conducted in a controlled environment, namely in a wind or water tunnel, but the major challenge is to construct a tree model that resembles the real tree. We constructed 3D-printed scaled down fractal tree models of major urban tree species in Singapore using procedural modelling, based on species-specific growth processes and field statistical data gathered through laser scanning of real trees. The tree crowns were modelled to match the optical porosity of real trees. We developed a methodology to model the tree crowns using porous volumes filled with randomized tetrahedral elements. The wind loads acting on the tree models were then measured in the wind tunnel and the velocity profiles from selected models were captured using particle image velocimetry (PIV). The data was then used for the validation of Large Eddy Simulations (LES), in which the trees were modelled via a discretized momentum sink with 10–20 elements in width, height, and depth, respectively. It is observed that the velocity profiles and drag of the simulations and the wind tunnel tests are in reasonable agreement. We hence established a clear relationship between the measured bulk drag on the tree models in the wind tunnel, and the local drag coefficients of the discretized elements in the simulations. Analysis on the bulk drag coefficient also shows that the effect of complex crown shape could be more dominant compared to the frontal optical porosity.

Verification and validation of large eddy simulations of turbulent cavitating flow around two marine propellers with emphasis on the skew angle effects

Large eddy simulation (LES) was used to simulate turbulent cavitating flow around a conventional marine propeller (CP) and a highly skewed marine propeller (HSP) with emphasis on the skew angles effects. The LES verification and validation (V&V) analysis was carried out with cavitation influence on the flow structures. The current numerical results demonstrate that LES can give excellent predictions of the transient complex cavitating flows around a CP and a HSP with the numerical results agreeing well with experimental data. This study applies the LES V&V to the cavitating flow around two propellers with a simplified three-equation method. The results show that the LES errors for HSP are smaller than for CP, which is mainly resulted by more skewed blade of HSP than CP. In addition, the cavitation-vortex interactions around the propellers were studied using the relative vorticity transport equation. The results indicate that both the baroclinic torque term and the Coriolis force term have important influences on the vorticity generation and transport in the cavity closure region. Further analyses indicate that most of the important flow structures including the tip vortex, leading edge vortex, trailing vortex and internal jet are reproduced by the current LES simulations. Due to the different geometry features (less skewed blade of CP than HSP), significantly more intense and violent vortical structures and cavitation phenomena are observed on the CP than on the HSP.

Influence of obstacles on the wall heat transfer for 2D and 3D helically ribbed pipes

Obstacles placed on the inner walls of pipes lead to flow separation and subsequent reattachment with an overall beneficial effect on the local heat transfer. In this work, the effects of both a continuous and a discontinuous helicoidal turbulator on the local heat transfer are investigated and compared for Reynolds numbers in the range from 20,000 to 80,000. Both turbulators are cylindrical helixes with constant angle and fixed direction. The continuous geometry has a pitch-to-diameter-ratio (p/D) equal to 11, an inclination angle of 80° and a uniform obstacle height-to-diameter-ratio (e/D) equal to 3.6%. The discontinuous geometry is equivalent to the continuous one except that it has a varying obstacle height throughout the helix. Steady-state heat transfer measurements are carried out by means of Liquid Crystals Thermography (LCT). A recently developed calibration method for the narrow-band Thermochromic Liquid Crystals (TLCs) is successfully modified to lower the angular effects on the measurement uncertainty. The local wall heat transfer rates and the temperature distributions are strongly influenced by the flow reattachment and thus by the geometry of the investigated turbulators. Compared to a smooth pipe at the same Reynolds number, the continuous and the discontinuous turbulator increase the average Nusselt number by factors of approximately 1.8 and 1.5 respectively. These heat transfer measurements with respective uncertainties are meant to be a valuable dataset for the validation of Large Eddy Simulations (LES) of turbulent flows in pipes equipped with turbulators.

Verification and validation of large eddy simulation with Nek5000 for cold leg mixing benchmark

Large eddy simulation (LES) of an experimental benchmark investigating buoyant mixing among two fluids is developed for verification and validation. Flow physics involve a completely closed system where after a valve is opened, two initially stationary fluids with different densities are allowed to mix. The Boussinesq approximation and low-Mach equations are coded into spectral element code Nek5000 to predict the buoyancy forces, and miscibility is accounted for using the advection-diffusion equation for concentration. Due to computational costs of higher order solutions, current analysis consists of initial flow development after the valve is opened. Minor differences between the models and Schmidt number are found within the turbulent spectrum and time integrated quantities. Moreover, solutions for higher polynomial orders are compared to quantify error due to spatial discretization. Error estimates using a posteriori spectral derivation of truncation and quadrature error are also calculated. Validation with particle image velocimetry (PIV) reveals good prediction of both average and fluctuating components of the primary flow within the cold leg but less accurate results in the downcomer. It is concluded that instabilities due to resolving more of the interface using the high molecular Schmidt number is essential for accurately predicting this type of flow.

Verification and validation of LES of a triple parallel jet flow in the context of a thermal striping investigation

Large Eddy Simulation (LES) consists in explicitly resolving the largest turbulent scales while the small-scale motions are taken into account by means of a subgrid-scale (SGS) model. Although it remains computationally expensive, LES seems to constitute an increasingly employed tool for engineering applications in fluid mechanics. In a nuclear context e.g., LES is a relevant approach for the numerical study and prediction of the thermal striping, which can occur during the functioning of sodium-cooled nuclear reactors and may cause some damage to the structures. Numerous experiments where three differentially heated jets mix inside a cavity were designed and conducted by the Japan Atomic Energy Agency (JAEA) to study these phenomena. In order to evaluate the quality of LES using the CEA in-house TrioCFD code, a Verification and Validation study is proposed. Several LES of triple parallel jets are conducted with various grid sizes and SGS models. Following the idea of Celik et al. (2009), the effective SGS kinetic energy is written as a polynomial function of the grid size Δ:keff,sgs=amodΔpmod+anumΔpnum, where the two terms in the right-hand side are respectively the contributions of the numerical scheme and of the subgrid-scale model. In the Verification part, the orders of convergence of these two contributions are assessed for the Smagorinsky and WALE models. In the Validation part, an evaluation of the results is proposed by comparing computed velocity profiles to experimental measurements. The effects of grid resolution, SGS model and boundary conditions are discussed.

Verification and validation of Large Eddy Simulation of attached cavitating flow around a Clark-Y hydrofoil

This study uses implicitly filtered Large Eddy Simulation with a homogenous cavitation model to investigate the transient turbulent cavitating flow around a Clark-Y hydrofoil with emphasis on Verification and Validation (V&V). The numerical results indicate that the present simulation can predict the time evolution process of the periodic cavity shedding and agree reasonably with the available experimental data. This study presents the first practical application of LES V&V in a transient cavitating flow. The three-equation method is used to assess the LES error magnitudes in unsteady cavitating flow. The results show that a noticeable difference can be observed between the modeling error and numerical error, and the former has a larger magnitude than the latter in cavitating flow. It is demonstrated that the unsteady cavitation has a big impact on the LES errors. Additionally, compared with non-cavitating flow, the unsteady cavitation increases the values and fluctuation amplitudes of numerical, modeling, and total errors. Grid requirement for modeling the cavitating flow has been discussed from the viewpoint of LES V&V. The numerical results also reveal that the periodic cavity shedding causes the complex and turbulent flow feature by vortex and spectrum analyses, and this has a great influence on the simulation accuracy.

Wind load and wind-induced effect of the large wind turbine tower-blade system considering blade yaw and interference

The yaw and interference effects of blades affect aerodynamic performance of large wind turbine system significantly, thus influencing wind-induced response and stability performance of the tower-blade system. In this study, the 5MW wind turbine which was developed by Nanjing University of Aeronautics and Astronautics (NUAA) was chosen as the research object. Large eddy simulation on flow field and aerodynamics of its wind turbine system with different yaw angles (0, 5, 10,20, 30 and 45) under the most unfavorable blade position was carried out. Results were compared with codes and measurement results at home and abroad, which verified validity of large eddy simulation. On this basis, effects of yaw angle on average wind pressure, fluctuating wind pressure, lift coefficient, resistance coefficient, streaming and wake characteristics on different interference zone of tower of wind turbine were analyzed. Next, the blade-cabin-tower-foundation integrated coupling model of the large wind turbine was constructed based on finite element method. Dynamic characteristics, wind-induced response and stability performance of the wind turbine structural system under different yaw angle were analyzed systematically. Research results demonstrate that with the increase of yaw angle, the maximum negative pressure and extreme negative pressure of the significant interference zone of the tower present a V-shaped variation trend, whereas the layer resistance coefficient increases gradually. By contrast, the maximum negative pressure, extreme negative pressure and layer resistance coefficient of the non-interference zone remain basically same. Effects of streaming and wake weaken gradually. When the yaw angle increases to 45, aerodynamic force of the tower is close with that when there\' s no blade yaw and interference. As the height of significant interference zone increases, layer resistance coefficient decreases firstly and then increases under different yaw angles. Maximum means and mean square error (MSE) of radial displacement under different yaw angles all occur at circumferential 0 and 180 of the tower. The maximum bending moment at tower bottom is at circumferential 20. When the yaw angle is 0, the maximum downwind displacement responses of different blades are higher than 2.7 m. With the increase of yaw angle, MSEs of radial displacement at tower top, downwind displacement of blades, internal force at blade roots all decrease gradually, while the critical wind speed decreases firstly and then increases and finally decreases. The comprehensive analysis shows that the worst aerodynamic performance and wind-induced response of the wind turbine system are achieved when the yaw angle is 0, whereas the worst stability performance and ultimate bearing capacity are achieved when the yaw angle is 45.

Application of scale-resolving simulation to a hydraulic coupling, a hydraulic retarder, and a hydraulic torque converter

The paper describes the qualification and validation of large eddy simulation (LES) and hybrid Reynolds-averaged Navier–Stokes (RANS)/LES, the so-called scale-resolving simulation (SRS) approaches, which are currently employed in transient simulations of internal flow for fluid machineries. Firstly, the application of various turbulence models in ANSYS FLUENT is briefly introduced to acquire the external performance of three hydrokinetic devices and to compare it with experimental data. It was found that a remarkable improvement in external performance was achieved. The best results could be as low as 4% for the absolute error in hydraulic coupling, 2%–5% for the error for the hydraulic retarder, and 2%–4% for the hydraulic torque converter. Basically, all models had better error levels than that of around 10%–15% obtained by RANS. Then four typical SRS simulations were applied to conduct numerical simulations of the internal flow fields for hydraulic coupling, the hydraulic retarder, and the hydraulic torque converter. The results provided two indisputable facts, firstly, that SRS models are more accurate in certain flow situations than RANS models and, secondly, that SRS models can give additional information compared with RANS simulations. Finally, the BSL SBES DSL model, a dynamic hybrid RANS/LES (DHRL) turbulence model, was applied to simulate and analyze the flow mechanism of the hydraulic coupling to deepen our understanding of it. The detailed flow structure in hydraulic coupling was determined and was used to understand the flow mechanism.

On Street-Canyon Flow Dynamics: Advanced Validation of LES by Time-Resolved PIV

The advanced statistical techniques for qualitative and quantitative validation of Large Eddy Simulation (LES) of turbulent flow within and above a two-dimensional street canyon are presented. Time-resolved data from 3D LES are compared with those obtained from time-resolved 2D Particle Image Velocimetry (PIV) measurements. We have extended a standard validation approach based solely on time-mean statistics by a novel approach based on analyses of the intermittent flow dynamics. While the standard Hit rate validation metric indicates not so good agreement between compared values of both the streamwise and vertical velocity within the canyon canopy, the Fourier, quadrant and Proper Orthogonal Decomposition (POD) analyses demonstrate very good LES prediction of highly energetic and characteristic features in the flow. Using the quadrant analysis, we demonstrated similarity between the model and the experiment with respect to the typical shape of intensive sweep and ejection events and their frequency of appearance. These findings indicate that although the mean values predicted by the LES do not meet the criteria of all the standard validation metrics, the dominant coherent structures are simulated well.

The influence of building geometry on street canyon air flow: Validation of large eddy simulations against wind tunnel experiments

This study investigates the effect of roof and façade geometry on the mean wind flow and turbulence in street canyons, as well as the ability of numerical simulation techniques in capturing the flow features. Numerical experiments, using the large eddy simulation FLUENT code, have been conducted under neutral stability conditions to test 5 building geometries: i) flat roof, ii) pitched roof, iii) round roof, iv) terraced building and v) building with balconies. Wind tunnel experiments were also conducted for the first three geometries. The simulation and experimental setups were closely matched and both featured configurations consisting of seven building arrays. The results from the physical and numerical experiments concur that (i) in-canyon vortex dynamics and over-canopy flow conditions, are strongly dependent on the geometric features of the buildings, and (ii) pitched and round roof geometries increase in-canyon mean and turbulent velocities, as well as the depth of the shear layer. The findings provide novel insight on the sensitivity of the flow and turbulence fields, as well as the simulation quality, to urban topography, inflow conditions, and the Reynolds number. They also underline the influence on the flow of small-scale features such as balconies, which are often ignored in prior literature.

Large-eddy simulations for hill terrains: validation with wind-tunnel and field measurements

Development and validation of large-eddy simulation (LES) framework to study atmospheric boundary layer flows over complex terrains is reported here. The LES model uses the fourth-order time-accurate Runge–Kutta scheme and a fractional step method. The inflow boundary conditions are generated by using the so-called recycling (or mapping) technique. Evaluation of potential in-land wind park locations is the main application of the validated model. In this work, LES are carried out for turbulent boundary-layer flows over both simple and complex hill geometries. The prediction of the flow separation and reattachment length for a steeper wind-tunnel hill was closer to the measurements than previous numerical studies reported by other authors for the same hill geometry. For the complex hill case, the LES model showed good agreement with full-scale measurements.

A survey of modelling methods for high-fidelity wind farm simulations using large eddy simulation.

Large eddy simulations (LES) of wind farms have the capability to provide valuable and detailed information about the dynamics of wind turbine wakes. For this reason, their use within the wind energy research community is on the rise, spurring the development of new models and methods. This review surveys the most common schemes available to model the rotor, atmospheric conditions and terrain effects within current state-of-the-art LES codes, of which an overview is provided. A summary of the experimental research data available for validation of LES codes within the context of single and multiple wake situations is also supplied. Some typical results for wind turbine and wind farm flows are presented to illustrate best practices for carrying out high-fidelity LES of wind farms under various atmospheric and terrain conditions.This article is part of the themed issue 'Wind energy in complex terrains'.

LES validation of urban flow, part I: flow statistics and frequency distributions

Essential prerequisites for a thorough model evaluation are the availability of problem-specific, quality-controlled reference data and the use of model-specific comparison methods. The work presented here is motivated by the striking lack of proportion between the increasing use of large-eddy simulation (LES) as a standard technique in micro-meteorology and wind engineering and the level of scrutiny that is commonly applied to assess the quality of results obtained. We propose and apply an in-depth, multi-level validation concept that is specifically targeted at the time-dependency of mechanically induced shear-layer turbulence. Near-surface isothermal turbulent flow in a densely built-up city serves as the test scenario for the approach. High-resolution LES data are evaluated based on a comprehensive database of boundary-layer wind-tunnel measurements. From an exploratory data analysis of mean flow and turbulence statistics, a high level of agreement between simulation and experiment is apparent. Inspecting frequency distributions of the underlying instantaneous data proves to be necessary for a more rigorous assessment of the overall prediction quality. From velocity histograms local accuracy limitations due to a comparatively coarse building representation as well as particular strengths of the model to capture complex urban flow features with sufficient accuracy are readily determined. However, the analysis shows that further crucial information about the physical validity of the LES needs to be obtained through the comparison of eddy statistics, which is focused on in part II. Compared with methods that rely on single figures of merit, the multi-level validation strategy presented here supports conclusions about the simulation quality and the model’s fitness for its intended range of application through a deeper understanding of the unsteady structure of the flow.

LES validation of urban flow, part II: eddy statistics and flow structures

Time-dependent three-dimensional numerical simulations such as large-eddy simulation (LES) play an important role in fundamental research and practical applications in meteorology and wind engineering. Whether these simulations provide a sufficiently accurate picture of the time-dependent structure of the flow, however, is often not determined in enough detail. We propose an application-specific validation procedure for LES that focuses on the time dependent nature of mechanically induced shear-layer turbulence to derive information about strengths and limitations of the model. The validation procedure is tested for LES of turbulent flow in a complex city, for which reference data from wind-tunnel experiments are available. An initial comparison of mean flow statistics and frequency distributions was presented in part I. Part II focuses on comparing eddy statistics and flow structures. Analyses of integral time scales and auto-spectral energy densities show that the tested LES reproduces the temporal characteristics of energy-dominant and flux-carrying eddies accurately. Quadrant analysis of the vertical turbulent momentum flux reveals strong similarities between instantaneous ejection-sweep patterns in the LES and the laboratory flow, also showing comparable occurrence statistics of rare but strong flux events. A further comparison of wavelet-coefficient frequency distributions and associated high-order statistics reveals a strong agreement of location-dependent intermittency patterns induced by resolved eddies in the energy-production range. The validation concept enables wide-ranging conclusions to be drawn about the skill of turbulence-resolving simulations than the traditional approach of comparing only mean flow and turbulence statistics. Based on the accuracy levels determined, it can be stated that the tested LES is sufficiently accurate for its purpose of generating realistic urban wind fields that can be used to drive simpler dispersion models.