LES Computations Research Articles

Melt azimuthal rotation in direct current electric arc furnace without external axial magnetic field

This paper considers the azimuthal rotation of melt about the vertical axis of a DC EAF, which is ensured by an inclined installation of the power supply electrodes, which is a patented solution that is the basis for this article. Near inclined electrodes, the Lorentz force has a pronounced azimuthal component, which is the driver of the melt rotation without an external axial magnetic field – thus, additional energy consumption to create an external magnetic field is not necessary. The main design and technological solutions, formulated in the patent and presented by the authors, were obtained using observations and numerical LES study for a laboratory-scale experimental setup (capacity 4.8 kg of GaInSn), as well as using LES computations for an industrial-scale DC EAF (capacity 3.6 t of molten steel). Basic technological solutions studied: the flow of the melt may be controlled by varying the number of vertical and inclined electrodes of DC EAF, choosing the angle of inclination of the electrodes as well as choosing the sequence of turning on and turning off power supply through the electrodes.

Highly resolved LES and URANS computations of a differentially heated square cavity

This investigation focusses on the use of two wall-resolved LES approaches for the simulation of the buoyancy driven flow in a differentially heated square cavity, aiming to advance understanding of the flow phenomena involved. The study also assesses the predictive accuracy and cost-effectiveness of LES for natural convection flows. This assessment is pursued through comparisons of the resulting predictions, and also of their computational requirements, with those for DNS, produced in an earlier study, and those of a number of URANS models produced during the current investigation. URANS models tested here include both high- and low-Reynolds-number schemes. In the case of the high-Reynolds-number models a recently developed numerical variant of the Analytical Wall Function (AWF) has been employed to account for the effects of near-wall turbulence. Both LES approaches result in predictions that are close to those of the DNS but have considerably more modest resource requirements. The URANS approaches, which have the lowest resource requirements, for key parameters such as the local Nusselt number are not as accurate as the LES approaches and their performance is highly dependent on the turbulence model employed. Surprisingly, the simplest of the low-Re models employed, the Launder–Sharma two-equation model, is reasonably close to the DNS data. The approach that requires the least computational resource, the high-Re model with the recent version of the AWF, also offers a promising URANS route. Simulations were also conducted to investigate the flow behaviour at a Rayleigh number that is an order of magnitude higher than that of the available DNS. At such high Rayleigh number, at which the phenomena are of close relevance to cooling loops of nuclear reactors, comparisons are performed between the LES and the high-Re AWF combination. The findings show that as the Rayleigh number increases, the turbulence levels become stronger, but these are confined to more refined structures within the thinner near-wall boundary layers.

RANS and LES Computations of Natural Convection in a Square Cavity

RANS and LES Computations of Natural Convection in a Square Cavity

A foundational step towards understanding and improving the transition between URANS and LES in hybrid URANS-LES methodology

Appropriate transition between URANS and LES plays an integral role in the performance of hybrid URANS-LES techniques. However, dynamics of the transition region (also known as the Gray Area) have not yet been well understood, despite several efforts to mitigate negative impacts of inappropriate modeling of this region such as deviation from law-of-the-wall (also referred to as log-layer mismatch (LLM)) in turbulent attached boundary layers. The current paper centers on Scale Adaptive Simulation (SAS) hybrid URANS-LES methods and aims to address some shortcomings of the SAS methodology while providing new insights into the dynamics of the Gray Area. More specifically, necessary requirements, including the length scale, grid design and underlying RANS model to appropriately model essential dynamics of the Gray Area and, thus, mitigating the log-layer mismatch, are presented, which could be used for broader framework of hybrid URANS/LES and probably wall-modeled LES approaches.SAS methods traditionally have difficulty transitioning from URANS to LES without a sufficiently unstable base flow. Here we introduce an improved triggering mechanism based on departure-from-equilibrium dynamics formulated for the SAS method, primarily to enable the model to transition from URANS to scale resolving mode in attached/mildly separated flows, a well-known deficiency of the classical SAS formulations. The improved triggering mechanism considers deviations from equilibrium through a vortex stretching term in the dynamical equation for turbulent dissipation rate. The present study demonstrates that the length scale obtained for the gray area using straightforward blending of URANS and LES regions results in inaccurate transition dynamics. Therefore, we introduce a transport equation for the length scale of energy-containing eddies that allows an appropriate transition of length scale from URANS to LES mode. The modified framework uses the k−ɛ−ζ−f model as the underlying RANS model and is shown to trigger a transition from URANS to LES in stable attached stationary turbulent channel flow and mitigate the log-layer mismatch problem to a great extent. When applied to flows that feature boundary layer separation (flow over a periodic hill and flow over the wall-mounted hump), the current hybrid framework delivers results in agreement with existing experimental data and comparable to high fidelity LES calculations.

LES and unsteady RANS computations of natural convection cooling loops

Natural Circulation Loops (NCLs), where fluid is driven through a closed circuit solely by thermal imbalance, offer potential for use in passive cooling systems within nuclear power plants. However, the combination of uniform heat flux at the hot end and uniform temperature on the cold side promotes thermal imbalances which can cause large-scale flow instabilities, including flow reversal, that can severely challenge numerical methods. This study employs both unsteady RANS models and Large Eddy Simulation for the conjugate heat transfer analysis of an experimental NCL across a range of different heating powers, with a focus on comparing several widely used URANS models (Launder-Sharma k−ε, k−ω SST and Elliptic-Blending RSM) against the higher-fidelity LES. The results indicate that the more advanced EB-RSM provides the best agreement with the LES, with the computationally less demanding Launder-Sharma (LS) k−ε model also performing well. The k−ω SST model performed less well, tending to overpredict turbulence levels within the loop, leading to significantly cooler temperatures, higher heat transfer coefficients and flatter velocity profiles. The subsequent application of the LS k−ε model to a wide range of heating powers demonstrated good agreement with the experimental data, correctly capturing the trends presented by both the experimental data and existing correlations. Overall, the results demonstrate the suitability of the unsteady RANS approach in modelling flow behaviour typical of NCLs and contrast the performance of several widely used modelling approaches, providing insight for those designing, optimizing or experimenting with NCLs.

Numerical analysis of combustion noise in an atmospheric swirl-stabilized LDI burner through modal decomposition techniques

Combustion noise in gas turbine engines has recently become a relevant source of noise in the aircraft due to the appearance of new burner architectures that are intrinsically more unstable, and the optimization of other conventional noise sources in this mean of transport (e.g., jet, fan, airframe). In this work, a simulation setup for reactive conditions was prepared in the CONVERGE finite-volume package using the detailed chemistry SAGE solver to model the combustion of a benchmark case, which was solved using a LES approach with three different cell base sizes: 8,10,12 mm. A confined liquid-fueled swirl-stabilized burner located at the CORIA Laboratory, France, was used to validate the numerical results with the experimental measurements obtained at this facility. OH-PLIF measurements and PDA results for both phases were used to guarantee the accuracy of the numerical OH contours and the velocity profiles of both phases. These experimental measurements were collected at CORIA. After ensuring the stabilization of the numerical flame, the reactive simulations were extended with some adjustments in the time step to capture the acoustic motion. Several techniques like Fast Fourier Transform (FFT), Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) were used to analyze these results and confirm the presence of a Precessing Vortex Core (PVC) and a Vortex Breakdown Bubble (VBB) during the coupling of pressure, axial velocity and fuel mass fraction in reactive conditions. Furthermore, the acoustic analysis performed with a Helmholtz solver proved that the second longitudinal mode of the chamber (310 Hz) was present in the pressure signal (300 Hz in the LES calculations) and resonated with the Vortex Breakdown Bubble (VBB). However, this dominant frequency did not appear in the frequency distribution of the OH mass fraction and no feedback interaction between the acoustic and the combustion happened. Thus, only combustion noise was obtained.

Aerodynamic Behavior of a Biomimetic Wing in Soaring Flight – A Numerical Study

Multi-element wing tips based on bird wings appear attractive in soaring flight, where a minimal sink velocity is the design goal. The present study aims to reproduce the soaring flight observed from white storks (ciconia ciconia) in a biomimetic computational model in order to visualize and investigate the flow around and through the wing tip cascade. RANS and hybrid RANS–LES computations have been performed allowing access to all features of the flow field. The resulting properties in soaring flight have been compared to measured data of free flying birds from the literature to qualify the results of the re-engineered wing. Further, the flow field has been analyzed in detail to understand the underlying flow physics and to point out relevant phenomena such as a system of vortices associated with the wing and tip design which contributes to the observed aerodynamic properties.

Topology Rule-Based Methodology for Flow Separation Analysis in Turbomachinery

Boundary-layer flow separation is a common flow feature in many engineering applications. The consequences of flow separation in turbomachinery can be disastrous in terms of performance, stability and noise. In this context, flow separation is particularly difficult to understand because of its three-dimensional and confined aspects. Analyzing the skin friction lines is one key point to understanding and controlling this phenomenon. In the case of separation, the flow at the wall agglutinates around a manifold while the fluid from the boundary layer is ejected toward the flow away from the wall. The analysis of a three-dimensional separation zone based on topology is well addressed for a simple geometry. This paper aims at providing simple rules and methods, with a clear vocabulary based on mathematical background, to conduct a similar analysis with complex turbomachinery geometry (to understand a surface with a high genus). Such an analysis relies on physical principles that help in understanding the mechanisms of flow separation on complex geometries. This paper includes numerous typical turbomachinery surfaces: the stator row, vaneless diffuser, vaned diffuser, axial rotor and shrouded and unshrouded centrifugal impeller. Thanks to surface homeomorphisms, the generic examples presented can easily be converted into realistic shapes. Furthermore, classical turbomachinery problems are also addressed, such as periodicity or rotor clearance. In the last section, the proposed methodology is conducted on a radial diffuser of an industrial compressor. The flow at the wall is extracted from LES computations. This study presents the different closed separation zones in a high-efficiency operating condition.

LES and RANS calculations of particle dispersion behind a wall-mounted cubic obstacle

In the present paper, we evaluate the performances of three stochastic models for particle dispersion in the case of a three-dimensional turbulent flow. We consider the flow in a channel with a cubic wall-mounted obstacle, and perform large-eddy simulations (LESs) including passive particles injected behind the obstacle, for cases of low and strong inertial effects. We also perform Reynolds-averaged simulations of the same case, using standard turbulence models, and employ the two discrete stochastic models for particle dispersion implemented in the open-source code OpenFOAM and the continuous Lagrangian stochastic model proposed by Minier et al. (2004). The Lagrangian model is consistent with a Probability Density Function (PDF) model of the exact particle equations, and is based on the modelling of the fluid velocity seen by particles. This approach allows a consistent formulation which eliminates the spurious drifts flawing discrete models and to have the drag force in a closed form. The LES results are used as reference data both for the fluid RANS simulations and particle simulations with dispersion models. The present test case allows to evaluate the performance of dispersion models in highly non-homogeneous flow, and it used in this context for the first time. The continuous stochastic model generally shows a better agreement with the LES than the discrete stochastic models, in particular in the case of particles with higher inertia.

Assessment of low- and high-fidelity turbulence models for heat transfer predictions in low-prandlt number flows

Linear eddy viscosity-based Reynolds-averaged Navier-Stokes (RANS), hybrid RANS/Large Eddy Simulation (LES), and LES models are applied for simulations of plane channel flow at Reynolds number Reτ = 150 and 640. Results are obtained for fluids with Prandtl number (Pr) of 0.025 and 0.71, for cases without buoyancy (Gr = 0) and for a vertical channel flow with buoyancy (Ri = 0.1–0.13). The objective is to evaluate the predictive capability of each class of model for low-Pr flow typical of liquid metal reactor cooling systems. For this purpose, RANS, partially-averaged Navier-Stokes (PANS) and LES computations are performed on grids which contain 0.67%, 2%, and 5% of the grid points used in the Direct Numerical Simulation (DNS), respectively, and the predictions of the mean and turbulent flow and thermal quantities are validated against DNS results. RANS results obtained using Kays variable turbulent Prandtl number (PrT) formulation were more accurate than those using constant PrT for low-Pr forced convective flows, but did not play a significant role for mixed convective flows. The variable PrT formulation likewise did not have significant effect for hybrid RANS/LES or LES computations, wherein > 90% turbulence was resolved. The RANS model performed reasonably well for flows without buoyancy, but for mixed convective flows, significantly under predicted the sharp pressure gradient around the mid-channel, and the limitations were identified due to over prediction of heat transfer by wall-normal fluctuations on the aiding side. Both PANS and dynamic Smagorinsky (DSM) showed similar predictions, and under predicted the mid-channel temperature gradient for the low-Pr vertical channel case and over predicted flow turbulence on the opposing flow side and thermal fluctuations on the aiding flow side. The Wall-Adapting Local Eddy-viscosity (WALE) model performed best among the LES subgrid stress models. The improved predictions by WALE over PANS and DSM were identified because of higher modeled dissipation predictions away from the wall. Future work will focus on investigation of non-linear RANS models and extend the validation for higher Re flows.

An efficient numerical approach for simulating airflows around an isolated building

The efficiency and precision of numerical simulation are very important to study the airflow around buildings. A new simulating approach for predicting airflow around a building efficiently is proposed. The modeled turbulence kinetic energy is converted into resolved energy using the vortex method at the interface away from the inlet. This treatment helps to converge. A novel hybrid turbulence model, namely production-limited eddy simulation (PLES), works in the whole computational region. Then, the PLES model needs not treat the LES-RANS (large eddy-simulation-Reynolds-averaged Navier-Stokes) interface, making the computation running more easily. For testing the accuracy and efficiency of the PLES model, the WALE (wall-adapting local eddy-viscosity) model, an LES model, is applied to simulate the airflow around an isolated building on the same grid. And the effects of the underlying RANS models on the performance of the PLES models are investigated. The underlying RANS models are the BSL (baseline) k-ω and the SST (shear stress transport) k-ω models. The BSL PLES model obtains good predictions of the time-averaged streamwise velocity, but over-predicts the resolved turbulence kinetic energy (TKE) in comparison with the WALE model. The performance of the SST PLES model in predicting the time-averaged streamwise velocity is comparable to the WALE and the BSL PLES models. Furthermore, the SST PLES model gives better resolved TKE comparing with two other models. Meanwhile, the computing time needed in the new approach is 35% less than that needed in the LES computation.

Characterization of pressure fluctuations within a controlled-diffusion blade boundary layer using the equilibrium wall-modelled LES

In this study, the generation of airfoil trailing edge broadband noise that arises from the interaction of turbulent boundary layer with the airfoil trailing edge is investigated. The primary objectives of this work are: (i) to apply a wall-modelled large-eddy simulation (WMLES) approach to predict the flow of air passing a controlled-diffusion blade, and (ii) to study the blade broadband noise that is generated from the interaction of a turbulent boundary layer with a lifting surface trailing edge. This study is carried out for two values of the Mach number, {{rm Ma}}_{infty } = 0.3 and 0.5, two values of the chord Reynolds number, {{rm Re}}=8.30 times 10^5 and 2.29 times 10^6, and two angles of attack, AoA =4^circ and 5^circ. To examine the influence of the grid resolution on aerodynamic and aeroacoustic quantities, we compare our results with experimental data available in the literature. We also compare our results with two in-house numerical solutions generated from two wall-resolved LES (WRLES) calculations, one of which has a DNS-like resolution. We show that WMLES accurately predicts the mean pressure coefficient distribution, velocity statistics (including the mean velocity), and the traces of Reynolds tensor components. Furthermore, we observe that the instantaneous flow structures computed by the WMLES resemble those found in the reference WMLES database, except near the leading edge region. Some of the differences observed in these structures are associated with tripping and the transition to a turbulence mechanism near the leading edge, which are significantly affected by the grid resolution. The aeroacoustic noise calculations indicate that the power spectral density profiles obtained using the WMLES compare well with the experimental data.

Dynamical p−adaptivity for LES of compressible flows in a high order DG framework

We investigate the possibility of reducing the computational burden of LES configurations by employing locally and dynamically adaptive polynomial degrees in the framework of a high order DG method. A degree adaptation technique especially featured to be effective for LES applications, that was previously developed by the authors and tested in the statically adaptive case, is applied here in a dynamically adaptive fashion. In this first study, no dynamic load balancing was carried out. Two significant benchmarks are considered, comparing the results of adaptive and non-adaptive simulations. The proposed dynamically adaptive approach allows for a significant reduction of the computational cost of representative LES computation, while allowing to maintain the level of accuracy guaranteed by LES carried out with constant, maximum polynomial degree values.

Demonstration of the STRUCT turbulence model for mesh consistent resolution of unsteady thermal mixing in a T-junction

A STRUCTure-based (STRUCT) second generation (2G) URANS turbulence model is presented in this paper and assessed on the simulation of unsteady turbulent flow, with application to thermal striping evaluation in T-junctions. The closure methodology leverages the robustness of the Unsteady Reynolds-Averaged Navier-Stokes (URANS) formulation and enhances it with the addition of physics-based local resolution of flow structures. Numerical predictions of the velocity and temperature fields for the STRUCT model compare favorably with experimental data, where the main inlet Reynolds number ranges from 8790 to 16,300. Spectral analyses of the temperature signals are also performed in order to investigate the mechanism of high-cycle thermal fatigue in the flow mixing region. The computed spectra from the STRUCT solution agree with the reference LES calculations and experimental data, accurately reproducing both shape and magnitude of the power spectra density. Overall, the STRUCT model demonstrates significant potential to accurately and efficiently model the thermal striping phenomenon. Its application produces LES quality results, while the computational cost is comparable to the URANS solution.

What can we learn from information‐entropy about turbulence and Large‐Eddy‐Simulation?

AbstractThe Shannon entropy is a rigorous measure of information that can be used to evaluate the properties of information in turbulent flows. In this study, the behavior of the Shannon entropy is investigated for LES of an incompressible turbulent channel flow based on the classical DNS of Moser et al.. The Shannon entropy is calculated from the velocity fluctuation fields and is compared to the turbulent kinetic energy. The quality and resolution of the LES calculations on different grids has been evaluated against DNS results. The Shannon entropy is calculated for velocities and the turbulent viscosity to observe how it behaves and how it is related to the simulation quality and grid resolution.

Synthetic Freestream Disturbance for the Numerical Reproduction of Experimental Zero-Pressure-Gradient Bypass Transition Test Cases

The present work introduces a means of forcing bypass transition within a zero-pressure-gradient smooth flat-plate boundary layer (ZPGSFPBL), suitable for DNS or LES computations for reproducing experimental datasets. In this type of bypass transition, one of the means of forcing transition within the boundary layer is via the introduction of a specific disturbance along the inflow boundary. Following this principle, the current method, introduces synthetic turbulence, generated by the method of Klein et al. (J. Comput. Phys. 186(2), 652–665, 2003), at the inflow, a certain height above the boundary layer, thereby confining it within the freestream. The principle parameter which dictates the transition behaviour is the height above the boundary layer at which the freestream turbulence is injected. By adjusting this parameter, as well as the integral length-scale and the intensity (either estimated or known a priori from experiments), ILES computations providing good agreement with experimental data sets can be achieved upon a variety of grids. This procedure has been validated upon the ERCOFTAC T3A experimental test case (freestream turbulent intensity of 3%), where good matching is achieved on streamwise quantities like skin-friction coefficient, shape factor, boundary layer thickness and fluctuating velocity growth rates as well as for profiles of mean velocity and fluctuating velocities in the wall-normal direction.

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.

An a priori analysis of a DNS database of turbulent lean premixed methane flames for LES with finite-rate chemistry

Abstract An a priori analysis of a DNS database of turbulent lean premixed methane flames is presented considering the relative effects of turbulence and LES filtering, along with a potential modelling approach for LES with finite-rate chemistry. The leading-order effect was found to be due to the filter operation; flame response to turbulence was a secondary effect, and manifested primarily as an increase in standard deviations about conditional means. It was found that the radicals O, H and OH were less impacted by the filter than other high-temperature radicals, which were significantly reduced in magnitude by the filter. By considering reaction path diagrams, key reactions have been identified that are responsible for disparities between the desired filtered reaction rates and the reaction rates evaluated using quantities available in LES calculations (i.e. the filtered species and temperature). More specifically, the hydrogen abstraction reactions that take CH 4 to CH 3 (by O, H and OH) were found to have particularly enhanced reaction rates, and dominate the reaction path. Under the conditions presented, reaction paths were found to be independent of turbulence intensity. In general, the filtered reaction rates from the DNS were found to align more closely with the filtered laminar profile than the reaction rate of filtered species and temperature, (but disparities were found to decrease with increasing Karlovitz number). A simple model for scaling reaction rates is considered based on filtered laminar flame profiles, and the resulting reaction paths demonstrate proof-of-concept of a simple approach for formulating a reaction rate model for LES with finite-rate chemistry.

Challenges in using scanning lidars to estimate wind resources in complex terrain

Pairs of synchronously scanning Doppler lidars measure the average wind speed of flows crossing the parallel ridges at Perdigão, Portugal, with the ultimate purpose of wind resource estimation. The availability of the data from the lidars when they are running is quite low (50–70%). Furthermore, the instruments did only run less than half the time of the experimental period. These figures have to be improved in order for scanning lidars to be a viable option for wind resource estimation. The variations along the ridges are compared to neutral LES calculations making a good match at the upstream ridge but a significantly worse prediction at the downstream ridge. One reason could be an insufficient representation of the terrain. Another unknown is the influence of the atmospheric stability on the flow which is clearly seen by the scanning lidars.

Measurements and LES computations of a turbulent particle-laden flow inside a cubical differentially heated cavity

The estimation of particle removal rates in closed atmospheric environments such as rooms or airplanes is of great interest because it is directly relevant to air quality and disease transmission. In this work we investigate experimentally and numerically the depletion of micron-sized particles inside a turbulent, naturally driven enclosure flow. The enclosure is a cubical cavity with side 0.7 m in which two opposite vertical walls are held at different temperatures while other walls are adiabatic. The transport of monodisperse SiO2 particles with diameter 0.5, 1.0, and 2.5 μm was studied at Rayleigh numbers varying in the range of 3 × 108 to 109, corresponding to temperature differentials ΔT from 10 to 40 K. The decay time constant of the airborne particle concentration was calculated from discrete measurements performed with an Electrical Low-Pressure Impactor. It is found that the temperature difference greatly influences the deposition of particles with sizes equal to or less than 1 μm in diameter. For this particle range, thermophoresis becomes important and hence the larger the ΔT, the larger the deposition, a significant part of which taking place on the cold wall by the thermophoretic effect. As a result, the decay time constant of submicron particles is up to 5 times smaller than predicted by the stirred settling model, depending on the imposed ΔT. For the largest particles with diameter 2.5 μm, the influence of thermophoresis and hence ΔT is negligible, and particle removal rates are in line with the stirred settling model predictions, with mostly gravity dominated deposition on the bottom wall. A Large Eddy Simulation of the tests was performed using an Euler-Lagrange approach, and predictions of the decay time constant are within less than 20% of the measured values.