Large Eddy Simulation Turbulence Model Research Articles

END OF INJECTION PROCESS IN A SINGLE-HOLE DIESEL INJECTOR

The end of injection processes in a single-hole high-pressure diesel injector is investigated experimentally and numerically. Experimental measurements are performed using a laser-based backlit imaging technique. Numerical investigation of in- and near-nozzle fluid dynamics is conducted in a Eulerian framework using a volume-of-fluid interface capturing technique integrated with large eddy simulation turbulence modeling. An incompressible noncavitating and a compressible cavitating model are employed to gain a clearer understanding of the nozzle air ingestion mechanism at the end of injection. In the compressible model, a basic cavitation model is allowing liquid fuel to flash to gas at the fuel vapor pressure. The results show that upon needle valve closure, the high-energy core fluid maintains outward flow but the peripheral flow has reversed, as required by continuity, thus ingesting chamber air into the nozzle. The remaining unstable flow forms an asymmetry enhancing the air ingestion. Numerical results of the incompressible noncavitating model show a single bubble of chamber gas remains embedded within the liquid in the nozzle hole only after velocities have largely dissipated. The results of the compressible cavitating model demonstrate how chamber gas is entrained into the sac volume through the air passage previously generated by hydraulic flip. These results provide an explanation of a mechanism for air ingestion at the end of injection recently described using X-ray imaging. This mechanism also provides a possible explanation for the presence of air within the emerging jet of subsequent injections.

Experimental study and large Eddy simulation of a coaxial jet with perforated obstacles to control thermal mixing characteristics

ABSTRACTThermal mixing phenomena of a coaxial jet with perforated obstacles are analyzed both experimentally and numerically. Perforated obstacles are used in front of a coaxial jet that has different temperatures of the fluid to control the thermal mixing behavior. An experimental set-up was constructed to perform several cases and all cases are simulated by using large eddy simulation (LES) turbulence model. Results of the study presented that inserting perforated obstacle affects the mixing performance positively and the best mixing performance is obtained in the case that the obstacle has the highest permeability. Experimental and computational results were compared and a good agreement was obtained.

Large eddy simulation of a homogeneously charged turbulent jet ignition system

Transient jet ignition of a homogeneous methane air mixture in a turbulent jet ignition system is studied computationally using a large eddy simulation turbulence model. The jet discharges from a prechamber into a main combustion chamber via one or more orifice(s) and provides a distributed ignition source in turbulent jet ignition. The effect of orifice size and stoichiometry is studied numerically using the Converge computational fluid dynamics code. A reduced kinetic mechanism is used for combustion along with a Smagorinsky sub-model for turbulence modeling. The computed pressure traces are compared with experimental measurements through rapid compression machine tests. Computational fluid dynamics results are in acceptable agreement with the experimental data during compression and the early stage of combustion; however, an over-prediction of peak pressure was reported. Peak pressure error is in the range of 0.1%–4% for Reynolds-averaged Navier–Stokes simulation estimation compared to the experimental measurements. This error is a function of mixture stoichiometry and unburned gas temperature. The error calculation showed that with the large eddy simulation model, 1% and 12% improvements in peak pressure and burn rate estimations, respectively, were achieved compared to Reynolds-averaged Navier–Stokes results. The reduced large eddy simulation error relative to the Reynolds-averaged Navier–Stokes simulations were considered to be in the acceptable range; however, further improvements could be achieved through validation and testing of additional turbulence models. In addition, computational fluid dynamics temperature contours for various nozzle orifices and air–fuel ratios are compared to achieve deeper insight into the turbulent jet ignition combustion process in the rapid compression machine combustion cylinder. The numerical iso-surface temperature contours were obtained which enabled three-dimensional views of the flame propagation, the jet discharge, ignition and extinction events. The heat release process and regeneration of mid-range temperature iso-surfaces (1200 K) were not visible through the experimental images.

Computational study for noise reduction and characteristic of unsteady flow field/flow-induced noise generated in a small radial fan

This study is an attempt to characterize unsteady flow field and predict flow noise produced by a radial fan installed in a system that has a tiny gap between the fan casing and upper wall (similar conditions found in actual products). The large-eddy simulation turbulence model was used to simulate unsteady flow conditions, and an impeller was rotated 13 times in total to sufficiently develop unsteady flow and obtain data for Computational aeroacoustics (CAA). Unsteady flow field due to the radial fan structure and narrow flow channel in a system was verified, and a location wherein flow properties rapidly changed over time due to the complicated flow field was identified. The noise spectrum obtained through CAA was compared with that measured through experiment. Both tonal noise components consisting of blade passing frequency with its harmonic frequencies and broadband noise were matched. The major source of noise of the radial fan was on the inner surface of the casing. However, the sources of noise in the upper and lower sides of the casing were not equivalent to each other due to the interaction between the flow coming through an inlet located in the upper part of the casing and the flow discharged through an outlet installed at its side, as well as the interaction between impeller rotation and cut-off. Sources of noise located at the upper casing are caused by the flow around impeller tips, whereas sources on the lower casing are related to the interaction of the flow between the impeller and casing. A low noise model with a modified impeller tip was proposed, and its noise reducing effects were evaluated. The modified model reduced overall sound pressure level by 0.8 dB compared with the base model.

Large eddy simulation of turbulent flow using the parallel computational fluid dynamics code GASFLOW-MPI

Abstract GASFLOW-MPI is a widely used scalable computational fluid dynamics numerical tool to simulate the fluid turbulence behavior, combustion dynamics, and other related thermal–hydraulic phenomena in nuclear power plant containment. An efficient scalable linear solver for the large-scale pressure equation is one of the key issues to ensure the computational efficiency of GASFLOW-MPI. Several advanced Krylov subspace methods and scalable preconditioning methods are compared and analyzed to improve the computational performance. With the help of the powerful computational capability, the large eddy simulation turbulent model is used to resolve more detailed turbulent behaviors. A backward-facing step flow is performed to study the free shear layer, the recirculation region, and the boundary layer, which is widespread in many scientific and engineering applications. Numerical results are compared with the experimental data in the literature and the direct numerical simulation results by GASFLOW-MPI. Both time-averaged velocity profile and turbulent intensity are well consistent with the experimental data and direct numerical simulation result. Furthermore, the frequency spectrum is presented and a –5/3 energy decay is observed for a wide range of frequencies, satisfying the turbulent energy spectrum theory. Parallel scaling tests are also implemented on the KIT/IKET cluster and a linear scaling is realized for GASFLOW-MPI.

Flow-induced vibration analysis of a helical coil steam generator experiment using large eddy simulation

This paper describes a numerical study of flow-induced vibration in a helical coil steam generator experiment conducted at Argonne National Laboratory in the 1980s. In the experiment, a half-scale sector model of a steam generator helical coil tube bank was subjected to still and flowing air and water, and the vibrational characteristics were recorded. The research detailed in this document utilizes the multi-physics simulation toolkit SHARP developed at Argonne National Laboratory, in cooperation with Lawrence Livermore National Laboratory, to simulate the experiment. SHARP uses the spectral element code Nek5000 for fluid dynamics analysis and the finite element code DIABLO for structural analysis. The flow around the coil tubes is modeled in Nek5000 by using a large eddy simulation turbulence model. Transient pressure data on the tube surfaces is sampled and transferred to DIABLO for the structural simulation. The structural response is simulated in DIABLO via an implicit time-marching algorithm and a combination of continuum elements and structural shells. Tube vibration data (acceleration and frequency) are sampled and compared with the experimental data. Currently, only one-way coupling is used, which means that pressure loads from the fluid simulation are transferred to the structural simulation but the resulting structural displacements are not fed back to the fluid simulation.

A numerical study of intake valve jet flapping in a gasoline direct injection engine

This paper presents findings from a numerical study of intake valve jet flapping within a gasoline direct injection (GDI) engine, using a large eddy simulation (LES) turbulence modelling approach. The experimental test case and computational setup, including choice of sub-grid scale (SGS) turbulence model, are presented and discussed. An example cycle where intake valve jet flapping is seen to be prominent is discussed in detail. Intake valve jet flapping was found to be initiated as a consequence of turbulent fluctuations in the intake valve curtains. Cycle-by-cycle variations in valve curtain mass flux and the subsequent jet flapping events are investigated and significant cyclic variability is found. It was observed that when an ensemble-averaging procedure, typically used in LES simulations and experimental PIV data post-processing, is applied, due to the cyclic variability of the variations in valve curtain mass flux, most of the information related to this flow phenomenon is lost.

Investigation of the Gas‐Jet Wiping Process − Two‐Phase Large Eddy Simulations Elucidate Impingement Dynamics and Wave Formation on Zinc Coatings

In addition to corrosion resistance and processing properties, high coating uniformity is a key quality criterion for galvanized steel sheets. Hydrodynamic gas jet wiping has proved to be an efficient method to control the coating thickness. However, the occurrence of non‐uniformities is attributed to the unsteadiness of the impinging jet. For the first time, vertical surface non‐uniformities resulting from the interaction of the impinging jet with the liquid coating are numerically predicted under industrial boundary conditions using the ANSYS Fluent®. The turbulent flow field of the compressible wiping gas is accomplished by the LES (Large‐Eddy‐Simulation) turbulence model, whereas the interphase between the wiping gas and the liquid coating is modeled by the VOF (Volume‐of‐Fluid) method. It is found that the liquid coating reacts relatively slowly to the high frequent flapping gas jet. Only, when the jet is deflected for a comparatively long period, significant waves are able to develop. The waviness predicted by the simulation model is in good agreement with experimental results. Thus, the model enables a careful study of process settings on the vertical coating uniformity characteristics. For the studied case, an increase of nozzle inclination is found to enhance the performance in terms of coating uniformity significantly.

Large Eddy Simulation of Cylindrical Jet Breakup and Correlation of Simulation Results With Experimental Data

Modern engines with increasing power densities have put additional demands on pistons to perform in incrementally challenging thermal environments. Piston cooling is therefore of paramount importance for engine component manufacturers. The objective of this computational fluid dynamics (CFD) study is to identify the effect of a given piston cooling nozzle (PCN) geometry on the cooling oil jet spreading phenomenon. The scope of this study is to develop a numerical setup using the open-source CFD toolkit OpenFoam® for measuring the magnitude of oil jet spreading and comparing it to experimental results. Large eddy simulation (LES) turbulence modeling is used to capture the flow physics that affects the inherently unsteady jet breakup phenomenon. The oil jet spreading width is the primary metric used for comparing the numerical and experimental results. The results of simulation are validated for the correct applicability of LES by evaluating the fraction of resolved turbulent kinetic energy (TKE) at various probe locations and also by performing turbulent kinetic energy spectral analysis. CFD results appear promising since they correspond to the experimental data within a tolerance (of ±10%) deemed satisfactory for the purpose of this study. Further generalization of the setup is underway toward developing a tool that predicts the aforementioned metric—thereby evaluating the effect of PCN geometry on oil jet spreading and hence on the oil catching efficiency (CE) of the piston cooling gallery. This tool would act as an intermediate step in boundary condition formulation for the simulation determining the filling ratio (FR) and subsequently the heat transfer coefficients (HTCs) in the piston cooling gallery.

Comparison of flow characteristics around an equilateral triangular cylinder via PIV and Large Eddy Simulation methods

The flow structures around an equilateral triangular cylinder, which is commonly used as a vortex shedder in the vortex flowmeter, were investigated experimentally and numerically. Flow characteristics such as vorticity contours, patterns of sectional streamlines, velocity vectors, velocity fields, Reynolds stress correlations, Strouhal numbers and drag coefficients were examined using the Particle Image Velocimetry (PIV) technique and the Large Eddy Simulation (LES) turbulence model. Experimental studies were performed in an open water channel for Re=2.9×103, Re=5.8×103 and Re=1.16×104 based on the equilateral triangle edge. A sharp-tip corner of the cylinder with a triangle cross-section was exposed to the upstream side while the other two sharp-tip corners were placed on the downstream side. Numerical studies were also completed at Reynolds numbers in the range of 2.9×103≤Re≤1.16×105 to obtain the changes in the Strouhal numbers and drag coefficients. When the results of PIV and LES are considered in the same interval of Reynolds numbers, the maximum and minimum values of each flow pattern were nearly the same. The time-averaged patterns had considerable symmetry with respect to the axis line passing through the sharp-tip corner of the cross-section of the triangular cylinder. The Strouhal number was independent of the Reynolds number and was found to be approximately 0.22. The drag coefficient decreased with increasing Reynolds numbers while increasing the Power Spectral Density (PSD) and the vortex shedding frequency. For the same Reynolds numbers, the experimental and numerical results were in good agreement. Therefore, the LES turbulence model is recommended for applications of flow around this type of bluff body that is generally used in the design of vortex flowmeters to generate vortex shedding.

Research on CO dispersion of a vehicular exhaust plume using Lattice Boltzmann Method and Large Eddy Simulation

The exhaust gas dispersion from the vehicles has a direct impact on human health, particularly on vehicle drivers, passengers, and people nearby. Because of the numerical implementation simplicity and well captured solution for fluid dynamics around multiple bodies, Lattice Boltzmann Method (LBM) has been adopted in lots of domains during the last two decades. This paper combines LBM together with Large Eddy Simulation (LES) turbulence model to study exhaust gas emission and dispersion characteristics. Carbon monoxide (CO) relative concentration and velocity distribution from a vehicular exhaust plume in the near-wake region is investigated for low and high idling driving conditions under different ambient wind speeds. Validation has been made to test the performance of gas simulation of the LBM-LES method in XFlow, after which 3D simulation of exhaust gas dispersion is considered, and the time-averaged CO relative concentration and velocity are analyzed. Results indicate that flow wake by ambient wind can affect the dispersion of exhaust gas. The exhaust gas moves downstream and spread to a wider range under higher ambient wind speed. While, the flow field becomes complicated due to the interflow effect when the wind speed and exhaust gas velocity are close, which enhances the dispersion characteristics of exhaust gas to a certain extent.

Using computational fluid dynamic and artificial neural networks to predict the performance and cavitation volume of a propeller under different geometrical and physical characteristics

ABSTRACTIn the current study, simulations of hydrodynamic characteristics of a propeller under different geometrical and physical characteristics are conducted by the computational fluid dynamic (CFD). Then, by designing appropriate artificial neural networks (ANNs), the hydrodynamic performance and cavitation volume of propellers are predicted under intended conditions. For this accomplishment, finite volume-based Navier–Stokes equations associated with incompressible large eddy simulation turbulence model are used. In order to verify the computational procedure, mesh sensitivity analysis and validation study are conducted and appropriate accuracy is observed. In the CFD simulations, propeller thrust, torque and cavitation volume are computed under different pitch ratio (P/D), rake angle (RA) and skew angle (SA), advance velocity ratio (J) and cavitation number (σ). By the CFD results, a significant increase in propeller thrust and torque is observable by enhancement of P/D and positive value of RA. Moreover, maximum mean square errors of ANNs output in the prediction of propeller thrust, torque and cavitation volume achieved are 0.000111, 7.4206E−5 and 0.000667, respectively. Also, related to ANNs’ weights and bias, four set of equations are proposed to predict the performance and cavitation volume of propellers.

Effect of bed vicinity on vortex shedding and force coefficients of fluid flow on an offshore pipeline

The effect of rigid bed proximity on flow parameters and hydrodynamic loads in offshore pipelines exposed to turbulent flow is investigated numerically. The Galerkin finite volume method is employed to solve the unsteady incompressible 2D Navier–Stokes equations. The large eddy simulation turbulence model is solved using the artificial compressibility method and dual time-stepping approach. The proposed algorithm is developed for a wide range of turbulent flows with Reynolds numbers of 9500 to 1.5×104. Evaluation of the developed numerical model shows that the proposed technique is capable of properly predicting hydrodynamic forces and simulating the flow pattern. The obtained results show that the lift and drag coefficients are strongly affected by the gap ratio. The mean drag coefficient slightly increases as the gap ratio increases, although the mean lift coefficient rapidly decreases. The vortex shedding suppression happen at the gap ratio of less than 0.2.

すれ違う 2 つの物体の流体干渉に関する研究

This paper describes the method and results of an experiment on the aerodynamic effects between cars during passing maneuver. Two simplified shape bodies were placed in a towing water tank and the aerodynamic forces were measured and analyzed. A computational fluid dynamics (CFD) simulation using Large Eddy Simulation (LES) turbulence model was performed as to simulate the transient aerodynamic phenomena. The results obtained show that the spacing between cars has a significant effect on the handling stability.

Numerical simulation of industrial hydrocyclones performance: Role of turbulence modelling

Abstract Flow in industrial hydrocyclones is always turbulent, selection of suitable turbulence model is key for accurate predictions. This paper aims to find the appropriate turbulence model for the hydrodynamic predictions in industrial hydrocyclones. Two-phase and multiphase simulations are conducted in various size industrial hydrocyclones using volume of fluid and modified mixture models coupled with Reynolds Stress Model (RSM), Detached Eddy Simulation and Large Eddy Simulation (LES) turbulence models. Assessment of turbulence model effect on two phase flow field is made with respect to air-core, flow split, mean and turbulent velocities. The simulated flow field in 75 and 250 mm hydrocyclones is validated against literature based Laser Doppler Velocimetry, in-house high speed video and Electrical Resistance Tomography measurements. Mean density segregation contours and radial density profiles variation at different axial positions in a 350 mm dense medium cyclone is compared against Gamma Ray Tomography data. Turbulent intensity profiles are compared to display the turbulence levels. Further turbulence effect on the particles in various size hydrocyclones is analyzed by dispersion index formulation. Multiphase flow predictions in 75 and 250 mm hydrocyclones with RSM and LES models are compared against classical experimental classification performance in terms of cut size and sharpness of separation.

Experimental and computational analysis of thermal mixing characteristics of a coaxial jet

In this study, experimental and numerical analyses have been performed to investigate the mixing behavior of hot and cold fluids for a coaxial jet. Thermal mixing phenomenon is an important issue for many industrial applications, since thermal stress occurs when fluids do not mix completely. Several test cases were considered to investigate the mixing quality and frequency of temperature fluctuations. Computational study was performed using Large Eddy Simulation (LES) turbulence model, since this model is proven in this type calculations. A commercial CFD code, ANSYS-Fluent is used for numerical calculations. The study is performed for governing parameters as ratio of mass flow rate of hot (annular) jet to cold (central) jet and temperature difference between hot and cold jet. It is found that there is a good agreement between experimental and numerical studies with respect to spectral analyses. The dominant frequency of temperature fluctuation is found as 5Hz as compatible with literature. Thermal mixing efficiency increases with increasing temperature difference between hot and cold jet. Also, thermal mixing performance getting better with enhancing flow rate of hot jet in the first half of the channel and the best mixing observed at ṁh/ṁc=2 along second half of the channel.

A 5-D Implementation of FGM for the Large Eddy Simulation of a Stratified Swirled Flame with Heat Loss in a Gas Turbine Combustor

Numerical simulations are foreseen to provide a tremendous increase in gas-turbine burners efficiency in the near future. Modern developments in numerical schemes, turbulence models and the consistent increase of computing power allow Large Eddy Simulation (LES) to be applied to real cold flow industrial applications. However, the detailed simulation of the gas-turbine combustion process remains still prohibited because of its enormous computational cost. Several numerical models have been developed in order to reduce the costs of flame simulations for engineering applications. In this paper, the Flamelet-Generated Manifold (FGM) chemistry reduction technique is implemented and progressively extended for the inclusion of all the combustion features that are typically observed in stationary gas-turbine combustion. These consist of stratification effects, heat loss and turbulence. Three control variables are included for the chemistry representation: the reaction evolution is described by the reaction progress variable, the heat loss is described by the enthalpy and the stratification effect is expressed by the mixture fraction. The interaction between chemistry and turbulence is considered through a presumed beta-shaped probability density function (PDF) approach, which is considered for progress variable and mixture fraction, finally attaining a 5-D manifold. The application of FGM in combination with heat loss, fuel stratification and turbulence has never been studied in literature. To this aim, a highly turbulent and swirling flame in a gas turbine combustor is computed by means of the present 5-D FGM implementation coupled to an LES turbulence model, and the results are compared with experimental data. In general, the model gives a rather good agreement with experimental data. It is shown that the inclusion of heat loss strongly enhances the temperature predictions in the whole burner and leads to greatly improved NO predictions. The use of FGM as a combustion model shows that combustion features at gas turbine conditions can be satisfactorily reproduced with a reasonable computational effort. The implemented combustion model retains most of the physical accuracy of a detailed simulation while drastically reducing its computational time, paving the way for new developments of alternative fuel usage in a cleaner and more efficient combustion.

Analysis of diesel spray dynamics using a compressible Eulerian/VOF/LES model and microscopic shadowgraphy

This paper presents numerical and experimental analysis of diesel engine spray dynamics in the region very close to the nozzle exit. Diesel fuel is injected through a single solid cone injector with sharp-edged nozzle inlet. Numerical investigations are conducted in an Eulerian framework by applying a Volume of Fluid interface capturing technique integrated with Large-Eddy Simulation turbulence modelling. Cavitation is modelled, by allowing liquid fuel to flash to gas at the fuel vapor pressure. The computational domain and settings mimic the experimental injector internal geometry and experimental operating conditions. In-nozzle disturbances are qualitatively well modelled by implementing the no-slip condition at the injector walls as well as cavitation and compressibility effects for each phase. A mesh dependency study is conducted with four different grid resolutions. Data are presented around the start of penetration (SOP) and up to the time when shock waves at the gas-liquid interface are well developed, the quasi-steady stage of injection. At SOP, an umbrella-shaped leading edge is captured in both the numerical and experimental studies however only the experimental images demonstrated a semi-transparent cloud of air-fuel mixture at the leading edge. A previously undescribed toroidal starting vortex near the nozzle exit is captured experimentally and numerically. Development of cavitation, down to the end of nozzle hole leads to the detachment of liquid from the nozzle hole walls and subsequently the diminution of boundary layer effects and thus reduced in-nozzle turbulence, and increased liquid jet velocity.

Unstable flow structure around partially buried objects on a simulated river bed

The unsteady flow characteristics around two partially buried objects, a short cylinder and a truncated cone, were examined with a three-dimensional, non-hydrostatic hydrodynamic model under similar steady unidirectional currents with flow Reynolds numbers, Re, of 86,061 and 76,209, respectively. Model simulations were conducted with the two objects partially buried in a simulated rippled river bed. A Reynolds-averaged Navier–Stokes (RANS) equation model coupled with a κ-ε turbulence closure was used to validate the experimental velocity measurements. A large eddy simulation (LES) turbulence model was subsequently used to characterize the unsteady flow structure around the objects. The LES closure allowed for the characterization of highly unsteady coherent turbulent structures such as the horse-shoe vortex, the arch-shaped vortex, as well as vortex shedding in the wake of the object.

Experimental study and Large Eddy Simulation of thermal mixing phenomena of a parallel jet with perforated obstacles

Thermal mixing and flow field due to two parallel jets which have different temperatures are investigated both experimentally and numerically. The perforated passive obstacles are used with different geometrical specifications. They are located in front of the jets to control flow field and mixing behavior of the jets. An experimental setup designed and manufactured and Large Eddy Simulation turbulence model with the WALE subgrid-scale stress model were used to simulate same experimental conditions. Three perforated obstacles (POs) with different porosity values were used in the study and these obstacles inserted into a rectangular cross-section confined channel. Results demonstrated that increasing the values of temperature differences enhance thermal mixing performance along the channel. The best mixing quality was captured at the same flow rates of the jets. The perforated obstacle has significant positive effect on the mixing performance and this effect increase with higher porosity values. Dominant frequency of mixing region was found as 5 Hz in all cases. The temperature profiles showed that as porosity decreases thermal oscillations are starting to reach the wall. Both 2D and 3D velocity profiles demonstrated that using of POs reduces the domination of turbulence area in the channel.