Large Eddy Simulation Computations Research Articles

A mechanism for unsteady separation in over-expanded nozzle flow

Shock wave induced separation in an over-expanded planar nozzle is studied through numerical simulation. These Large-Eddy Simulations (LES) model previous experiments which have shown unsteady motion of the shock wave in flows with similar geometries but offered little insight into the underlying mechanism. Unsteady separation in nozzle flow leads to “side loads” in the rocket engine which can adversely affect the stability of the rocket. A mechanism for the low-frequency shock motion is identified and explained using the LES data. This mechanism is analyzed for a series of over-expanded planar nozzles of various area ratios and nozzle pressure ratios. The effect of grid resolution and Reynolds number on the instability is discussed. A simple reduced order model for the unsteady shock behavior is used to further validate the proposed mechanism. This model is derived from first principles and uses data from the LES calculations to capture the effects of the turbulent boundary layer and shear layer.

A skin friction model for axisymmetric turbulent boundary layers along long thin circular cylinders

Only a few engineering design models are presently available that adequately depict the axisymmetric skin friction (Cf) maturity along long thin turbulent cylinders. This deficit rests essentially on the experimental and numerical difficulties of measuring (or computing) the spatial evolution of the thin cylinder turbulence. Consequently, the present axisymmetric Cf models have questionable accuracy. Herein, we attempt to formulate a more robust Cf model that owns acceptable error. The formulation is founded on triple integration of the governing equation system that represents a thin cylinder turbulent boundary layer (TBL) at statistical steady-state in appropriate dimensionless units. The final model requires only the radius-based Reynolds number (Rea) and transverse curvature (δ/a) as input parameters. We tuned the accompanying coefficients empirically via an expanded statistical database (over 60 data points) that house new Cf values from large-eddy simulations (LES). The LES computations employed a turbulence inflow generation procedure that permits spatial resolution of the TBL at low-high Reynolds numbers and transverse curvatures. Compared to the new skin friction database, the Cf model revealed averaged predictive errors under 5% with a 3.5% standard deviation. Apart from owning higher values than the flat plate TBL, the most distinguishing characteristic of the axisymmetric skin friction is its rising levels when the boundary layer thickness exceeds the cylinder radius. All Cf levels diminish with increasing Reynolds number. These unique features differentiate the axisymmetric TBL along thin cylinders as a separate canonical flow when compared to the turbulent wall shear-layers of channels, pipes, and planar-type geometries.

Heat transfer and vortical structures around a rotating cylinder with a spanwise disk and low-velocity crossflow

In this paper, we study the flow field characteristics and heat transfer of a rotating cylinder with a spanwise disk attached and subjected to low-velocity crossflow with a disk-cylinder diameter ratio, Dd/Dc=3.06. Reynolds Averaged Navier–Stokes (RANS) simulations using the k-∊ realizable turbulence model with enhanced wall treatment were performed for various crossflow and rotational velocities. We were particularly interested in the case of a rotating cylinder, with a spanwise disk attached, in an air stream with a crossflow Reynolds number, ReU=23,560, and rotational Reynolds numbers, ReΩ, ranging from 5500 to 109,800. Large-eddy simulations (LES) using the dynamic kinetic energy subgrid-scale model were performed for ReΩ=54,900 and 109,800. Heat transfer results from the RANS simulations were compared to results from previous studies. Flow statistics computed from the LES calculations were compared to results measured by particle image velocimetry experiments. Unsteady flow patterns and thermal behaviours are discussed with reference to the instantaneous LES results.

Large-eddy simulations of velocity and temperature fluctuations in hot and cold fluids mixing in a tee junction with an upstream straight or elbow main pipe

Thermal striping resulting in thermal fatigue is an important safety issue for nuclear power plants. In this work, temperature and velocity fluctuations in hot and cold fluids mixing in a tee junction with the main pipe connected either to an upstream straight or elbow pipe have been numerically predicted using large-eddy simulations (LES) on the FLUENT platform with the assumption of fully-developed velocity at both main and branch pipe inlets. The numerical results for the case with an upstream straight pipe were found to be in reasonable agreement with the available experimental data. The reason for the small discrepancy between the numerical results and experimental data can be attributed to the turbulence velocity being 10% of the fully-developed velocity at the main and branch pipe inlets in the LES calculations, while in the experiments the turbulence velocity was about 10% of the average velocity upstream of the tee junction. The simulated normalized mean and root-mean square (RMS) temperatures and the velocities at both straight and elbow tees were then compared, as well as the power spectrum densities (PSD) of the temperature fluctuations. The elbow pipe upstream of the main pipe has a significant influence on the mixing, resulting in increased temperature and velocity fluctuations. The flow pattern of the elbow tee deviates from the wall jet due to the secondary flow in the upstream elbow pipe.

Effects of residual burnt gas heterogeneity on early flame propagation and on cyclic variability in spark-ignited engines

This paper investigates the effects of Residual Burnt Gas (RBG) heterogeneity on premixed early flame propagation. The aim is to quantify the contribution of RBG spatial fluctuations on Cycle-to-Cycle Variations (CCVs) observed in Spark-Ignited (SI) engines under stoichiometric operation. We performed Direct Numerical Simulations (DNSs) of turbulent flame propagation with a semi-detailed kinetic mechanism for iso-octane (29 species and 48 reactions). To obtain relevant results for SI engines, aerothermochemical conditions (pressure, temperature, turbulence characteristics) representative of low-load engine operation were extracted from experimental data and engine Large Eddy Simulation computations. The main findings are as follows: (1) RBG heterogeneity combines two opposing effects. Hot temperature pockets tend to locally increase the laminar flame speed, while diluted mixtures slow down flame propagation. (2) RBG heterogeneities locally enhance flame propagation because of the dominant effect of temperature over dilution. (3) RBG heterogeneities are able to create additional flame wrinkling even without the introduction of sensible enthalpy fluctuations. (4) DNS results contain valuable information to further our understanding of previously misinterpreted experimental observations in engines [Sztenderowicz and Heywood, SAE paper 902142, 1990]. The influence of RBG heterogeneities on the flame development process is minor compared to their contribution on heat release fluctuations and therefore on engine CCVs as a result of the non-linear chemical response to local mixture conditions.

Large eddy simulation of unsteady turbulent flow in a semi-industrial size spray dryer

This paper is concerned with the numerical simulation of gas flow inside spray dryers, which is known to be highly transient. Experimental evidence presented in the literature to date is acquired either at a scale much smaller than industrial or at a limited number of locations in the system (e.g. the central axis). Alternatively, computational fluid dynamics (CFD) simulation, in particular methods based on the Reynolds-averaged Navier–Stokes (RANS) equations, may provide the information of the complete flow field at industrial scales. However, RANS methods to some extent are hampered by the limitations of turbulence modelling. In this study, it has been investigated whether the current limitations of experiments and RANS CFD simulations can be omitted by the use of large eddy simulation (LES). The spray dryer studied was of semi-industrial size with a volume of 60 m3. Via grid size and time step refinement studies, the minimum numerical requirements for obtaining converged time averaged velocity profiles could be identified. Spectral analysis and estimates of the Kolmogorov scales demonstrated that LES resolves scales well into the inertial subrange of turbulence scales. The LES calculation shows a complex precession of the jet. The jet moves around in the drying chamber seemingly with no reoccurring modes. Even with very long sampling times, no typical frequencies could be found. This work demonstrates the feasibility of LES for a spray dryer of semi-industrial scale and the ability of LES to predict large-scale motions. Future work will include the further validation of the LES via comparison with laser Doppler anemometry measurements and, subsequently, the comparison of RANS, unsteady RANS and LES simulations.

Comparisons of LES and RANS Computations with PIV Experiments on a Cylindrical Cavity Flow

A comparison study on the numerical computations by large eddy simulation (LES) and Reynolds-averaged Navier-Stokes (RANS) methods with experiment on a cylindrical cavity flow was conducted in this paper. Numerical simulations and particle image velocimetry (PIV) measurement were performed for two Reynolds numbers of the flow at a constant aspect ratio of H/R = 2.4 ( R is the radius of the cylindrical cavity, and H is liquid level). The three components of velocity were extracted from 100 sequential PIV measured velocity frames with averaging, in order to illustrate the axial jet flow evolution and circulation distribution in the radial direction. The results show that LES can reproduce well the fine structure inside the swirling motions in both the meridional and the horizontal planes, as well as the distributions of velocity components and the circulation, in good agreement with experimental results, while the RANS method only provided a rough trend of inside vortex structure. Based on the analysis of velocity profiles at various locations, it indicates that LES is more suitable for predicting the complex flow characteristics inside complicated three-dimensional geometries.

Experimental and Numerical Investigation of a Wing-Body Junction Flow

This article presents a study of a simplified wing-body junction. The main purpose of this work is to provide insights on the abilities of some industrial and research numerical approaches to accurately reproduce the flow for that type of configuration. To this end, a test case based on a NACA012 wing is designed at Rec 2:8 10. An experimental database was created and used as validation data for the evaluation of Reynolds-averaged Navier– Stokes and large-eddy simulation computations. The sensitivities of the configuration to flow conditions such as the incoming boundary-layer thickness and the angle of attack are explored. The study points out the limitations of the Reynolds-averaged Navier–Stokes models investigated, which are highly sensitive to the grid density. Conversely, the large-eddy simulation is shown to be able to reproduce the experimentally observed flow features within the junction.

Computational modelling of inflow turbulence effects on transitional flow in a highly transonic linear turbine

PurposeThe purpose of this paper is to numerically study inflow turbulence effects on the transitional flow in a high pressure linear transonic turbine at the design incidence.Design/methodology/approachThe three‐dimensional (3‐D) compressible turbulent flow in a turbine inlet guide vane is simulated using a finite volume based fluid solver coupled with dynamic large eddy simulation (LES) computations to investigate the effects of varying inflow turbulence length scale and the turbulence intensity on the aero‐thermal flow characteristics and the laminar‐turbulent transition phenomena. The computational analyses are extended to very high exit Reynolds number flow conditions to further study the effect of high exit Reynolds numbers on the transitional behavior of the present flow around the inlet guide vane cascades of the turbine. The calculations are performed with varying degree of inflow turbulence intensity values ranging from 0.8 to 6 percent and the inflow turbulence length scales ranging from one to five percent of pitch for different exit isentropic Mach and Reynolds numbers.FindingsThe numerical predictions in comparison with the experimental data demonstrate that the level of inflow turbulence closure provided by the present LES computations offers a reliable framework to predict complex turbulent flow and transition phenomena in high free‐stream turbulence environments of high pressure linear turbines.Originality/valueThis is the first instance in which both artificially modified random flow generation method in association with the dynamic procedure of LES application is employed to represent the realistic inflow turbulence conditions in the high pressure turbine and to resolve the transitional flow in a dynamic approach.

Spatial Resolution of the Axisymmetric Turbulent Statistics Along Thin Circular Cylinders at High Transverse Curvatures and Low-Re

After three decades of accumulated experimental and numerical results, a comprehensive understanding of the spatial evolution of axisymmetric turbulent boundary layers (ATBL) along long thin cylinders still eludes both scientists and engineers. While experimentalists dealt with axial alignment complexities, computationalists lacked proper inflow boundary conditions. Herein, we correct this latter deficiency and initiate an investigation of the thin cylinder turbulence under low Reynolds numbers and high transverse curvatures (boundary layer thicknesses to radius). Using the large-eddy simulation (LES) methodology, we are particularly interested in the radial propagation of the transverse curvature on the ATBL statistics. A ten-simulation matrix was constructed to examine these effects with validation against the experimental evidence. These tests investigated the ATBL maturity up to transverse curvatures approaching 2 orders of magnitude. A recently developed turbulent inflow procedure for the thin cylinder was implemented that couples a dynamic form of Spalding’s expression for rescaling the mean streamwise velocity with recycling of all superimposed turbulent fluctuations. The technique specifically circumvents intensive computations from the cylinder leading edge, and the rescaling-recycling combination minimizes the inflow turbulent regeneration length under very high transverse curvatures. After the initial transition phase in each LES computation, the respective numerical uncertainty was quantified to ensure sufficient spatial resolution within the discretized domain for resolving the energy-bearing scales of the turbulent motion. For the present low-Re conditions, the strength of the log layer steadily diminishes under continuous rise in the transverse curvature whereas the scaled fluctuating intensities elevate (except for the dominate shear stresses) with no sign towards full maturity. Each simulation reveals a boundary layer thickness that grows downstream by a factor of 7 relative to the momentum thickness with a linear influence of the transverse curvature on the wall-shear stress coefficient.

A computational study of the HyShot II combustor performance

Experimental and flight data for hypersonic air-breathing engines are both difficult and extremely expensive to obtain, motivating the use of computational models to enhance the understanding of the complex physics involved. Here, a comprehensive numerical study has been carried out for the HyShot II scramjet combustor. This study makes use of Reynolds Average Navier Stokes (RANS) based models and Large Eddy Simulation (LES) based models with semi-detailed reaction kinetics. In this investigation we focus on the underlying flow-mixing-combustion physics at different operating conditions tested in the High Enthalpy Shock Tunnel Göttingen (HEG). To account for the complex flow in the HEG facility a zonal approach is employed in which RANS is used to simulate the flow in the HEG nozzle and test-section, providing the necessary inflow boundary conditions for the combustor RANS and LES, being the focus of this analysis. Specifically, we here combine results from RANS and LES computations with data from the HEG experiments and the target HyShot II flight-tests at two different flight-altitudes (28 and 33km). The LES model is observed to capture the experimental wall-pressure and heat-flux data very well for both the 33 and 28km altitude cases, whereas the RANS model is only able to predict the wall-pressure and heat flux data for the 28km altitude case. Based on the LES results, the flow at both altitudes is found to be unsteady, but with unsteady transitional flow features dominating the 33km case. Moreover, these results show that the equivalence ratio is of key importance to the resulting flow, mixing and combustion physics, with richer mixtures being prone to transitional flow features. The LES results are also used to describe the flow physics in detail for both altitudes, and the key processes responsible for the transition between the two combustion modes observed.

The Role of the Combustion Submodel for Large Eddy Simulation of Transient Premixed Flame-Vortex Interactions in Gas Explosions

In this paper, the sensitivity of large eddy simulation (LES) to the presence of the combustion submodel was investigated for transient interactions between premixed flame fronts and toroidal vortex structures generated at the wake of a circular orifice. To this end, LES computations were run, with and without the combustion submodel, for two orifice diameters: 40 mm and 20 mm. Nonuniform unstructured grids with a cell characteristic length varying in the range of 0.5-1 mm were used. In going from the 40-mm orifice to the 20-mm orifice, both the size and velocity of the vortex increase, leading to a different regime of interaction with the flame: the vortex only wrinkles the flame front in the 40-mm case (wrinkled regime) and also disrupts the continuity of the front, giving rise to the formation of separate reaction zones (i.e., flame pockets that leave the main front), in the 20-mm case (breakthrough regime). It has been found that the impact of the combustion submodel on LES predictions is strongly dependent on the regime of interaction. Results for the 40-mm orifice are substantially the same, regardless of the presence of the combustion submodel. Conversely, at the wake of the 20-mm orifice, the intensity of the flame-vortex interaction is such that the combustion submodel is strictly needed to reproduce both the qualitative (evolution of the pockets formed and their interaction with the main front) and quantitative (flame speed) characteristics of the flame propagation correctly.

Study on micro-atmospheric environment by coupling large eddy simulation with mesoscale model

This paper investigates wind field and traffic pollutant dispersion at street level in a local urban area. A coupling method is employed that the time-varying boundary values of velocity and temperature at the local urban area are provided by the Weather Research and Forecasting (WRF) mesoscale model which computes the atmospheric flow over the entire city while the atmospheric environmental flows are computed by a large eddy simulation (LES) code developed by the authors in the local urban area. In order to reduce the LES computation cost, a combined model for buildings in the urban area is proposed that in a central area, where the wind and pollution is most concerned, the buildings are properly resolved in fine grids and non-slip flow condition is prescribed at rigid boundaries whereas in the surrounding region the buildings are treated as drag element and coarse grids are used in computation. The wind field, temperature and carbon monoxide concentration fields are computed from 9 a.m. of October 26, 2009 to 8 a.m. of the next day in a built-up district of Beijing and the results show good agreement with observations. The advantage of proposed approach is that the multi-scale effect of atmospheric motion is considered and temporal evolution of flow structure in local urban area can be simulated by large eddy simulation at moderate computation cost.

The influence of large-scale structures on entrainment in a decelerating transient turbulent jet revealed by large eddy simulation

To provide a better understanding of the fluid mechanical mechanisms governing entrainment in decelerating jets, we performed a large eddy simulation (LES) of a transient air jet. The ensemble-averaged LES calculations agree well with the available measurements of centerline velocity, and they reveal a region of increased entrainment that grows as it propagates downstream during deceleration. Within the temporal and spatial domains of the simulation, entrainment during deceleration temporarily increases by roughly a factor of two over that of the quasi-steady jet, and thereafter decays to a level lower than the quasi-steady jet. The LES results also provide large-structure flow details that lend insight into the effects of deceleration on entrainment. The simulations show greater growth and separation of large vortical structures during deceleration. Ambient fluid is engulfed into the gaps between the large-scale structures, causing large-scale indentations in the scalar jet boundary. The changes in the growth and separation of large structures during deceleration are attributed to changes in the production and convection of vorticity. Both the absolute and normalized scalar dissipation rates decrease during deceleration, implying that changes in small-scale mixing during deceleration do not play an important role in the increased entrainment. Hence, the simulations predict that entrainment in combustion devices may be controlled by manipulating the fuel-jet boundary conditions, which affect structures at large scales much more than at small scales.

Effect of Electromagnetic Ruler Braking (EMBr) on Transient Turbulent Flow in Continuous Slab Casting using Large Eddy Simulations

Static electromagnetic braking (EMBr) fields affect greatly the turbulent flow pattern in steel continuous casting, which leads to potential benefits such as decreasing flow instability, surface defects, and inclusion entrapment if applied correctly. To gain a fundamental understanding of how EMBr affects transient turbulent flow, the current work applies large eddy simulations (LES) to investigate the effect of three EMBr ruler brake configurations on transient turbulent flow through the bifurcated nozzle and mold of a liquid-metal GaInSn model of a typical steel slab-casting process, but with deep nozzle submergence and insulated walls with no solidifying shell. The LES calculations are performed using an in-house graphic-processing-unit-based computational-fluid-dynamics code (LES-CU-FLOW) on a mesh of ~7 million brick cells. The LES model is validated first via ultrasonic velocimetry measurements in this system. It is then applied to quantify the mean and instantaneous flow structures, Reynolds stresses, turbulent kinetic energy and its budgets, and proper orthogonal modes of four cases. Positioning the strongest part of the ruler magnetic field over the nozzle bottom suppresses turbulence in this region, thus reducing nozzle well swirl and its alternation. This process leads to strong and focused jets entering the mold cavity making large-scale and low-frequency (<0.02 Hz) flow variations in the mold with detrimental surface velocity variations. Lowering the ruler below nozzle deflects the jets upward, leading to faster surface velocities than the other cases. The double-ruler and no-EMBr cases have the most stable flow. The magnetic field generates large-scale vortical structures tending toward two-dimensional (2-D) turbulence. To avoid detrimental large-scale, low-frequency flow variations, it is recommended to avoid strong magnetic fields across the nozzle well and port regions.

Large eddy simulation of a pulsed jet in cross-flow

AbstractThis study quantifies the mixing that results from a pulsed jet in cross-flow in the near jet region. By large eddy simulation computations, it also helps to understand the physical phenomena involved in the formation of the pulsed jet in cross-flow. The boundary conditions of the jet inlet are implemented via a Navier–Stokes characteristic boundary condition coupled with a Fourier series development. The signals used to pulse the jet inlet are a square or a sine wave. A new way of characterizing the mixing is introduced with the goal of easily interpreting and quantifying the complicated mixing process involved in a pulsed jet in cross-flow flow fields. Different flow configurations, pulsed and non-pulsed, are computed and compared, keeping the root mean square value of the signal constant. This comparison not only allows the characterization of the mixing but also illustrates some of the properties of the mixing characterization.

Sensitivity to the Presence of the Combustion Submodel for Large Eddy Simulation of Transient Premixed Flame–Vortex Interactions

In this paper, the sensitivity of large eddy simulation (LES) to the presence of the combustion submodel was investigated for transient interactions between premixed flame fronts and toroidal vortex structures generated at the wake of a circular orifice. To this end, LES computations were run, with and without the combustion submodel, for two orifice diameters: 40 mm and 20 mm. Nonuniform unstructured grids with a cell characteristic length varying in the range of 0.5–1 mm were used. In going from the 40-mm orifice to the 20-mm orifice, both the size and velocity of the vortex increase, leading to a different regime of interaction with the flame: the vortex only wrinkles the flame front in the 40-mm case (wrinkled regime) and also disrupts the continuity of the front, giving rise to the formation of separate reaction zones (i.e., flame pockets that leave the main front), in the 20-mm case (breakthrough regime). It has been found that the impact of the combustion submodel on LES predictions is strongly dep...

処理水放流口近傍初期混合過程のLESシミュレーション

Large Eddy Simulation (LES) method for calculating the mixing process in the vicinity of a freshwater outfall in a coastal region has been developed. The LES calculation solves the three-dimensional non-hydrostatic equations of motion and is capable of directly representing the large-scale three dimensional fluid motion that is responsible for the mixing of the effluent and the ambient fluid. The present results indicate that the initial mixing of buoyant effluent can be reproduced fairly well for different discharge conditions, but the spatial extent and the time span of the simulation are limited to a few hundred meters in the horizontal direction and tens of minutes, respectively.

Reynolds 応力のコレスキー分解に基づく人工的なLES流入変動風生成手法の検討

For merging of mesoscale/regional meteorological models and engineering large-eddy simulation (LES) models seamlessly, to generate appropriate velocity fluctuations at the inflow boundary (inflow turbulence) for engineering LES models is a very essential issue. In this study, we introduced three artificial generation methods of inflow turbulence based on the Cholesky decomposition of Reynolds stresses in the LES analysis of a plane boundary layer flow, and assessed the performance of those methods by comparing their results with those froma LES computation with periodic inlet-outlet conditions. In particular, we investigated the effects of space and/or time correlations of the generated inflow turbulence on the downstream turbulence characteristics.

Numerical study of the thermal development in a rotating cooling passage

In the present study, the fully developed turbulent secondary flows inside rotating square-upstream section and rectangular downstream section oriented at several angles with axis rotation are numerically simulated using the low Reynolds Non-linear \( k - \omega \), \( k - \varepsilon \) Reynolds-Stress Model (RSM), and Large Eddy Simulation (LES) models. The two flat walls are heated (leading and trailing sides), while the outer wall and the splitter plate are thermally insulated. The simulation has been done at Re = 36,000 with different rotation numbers from Ro = −0.4 to Ro = 0.4, where negative Ro values refer to counter-clock wise and positive Ro values refer to clock wise sense of rotation. This enables to investigate the effects on the thermal development of the rotation numbers. The effects of minor modifications, in cross section area at the bend exit, on thermal development, under rotating conditions, can also be explored. The interactions between secondary flows and separation lead to very complex flow patterns. To accurately simulate these flows and heat transfer, both refined turbulence models and higher-order numerical schemes are indispensable for turbine designers to improve the cooling performance. Previous studies have shown that the flow and heat transfer features through curved bends, even with a moderate curvature, cannot be accurately simulated. It is the conventional belief and practice that the usage of a proper turbulence model and a reliable numerical method for achieving accurate computations. It is shown that the present RSM model produces satisfactory predictions of the flow development inside the sharp U-bend comparing with the experiments (Iacovides et al. 2006). In the present study, three turbulence models are used to predict Nusselt number distribution. The results were further compared with the LES computations and discussed the difference between the turbulence models and the LES as well.