Large Eddy Simulation Turbulence Model Research Articles

A MODIFIED WAVEMAKER BOUNDARY CONDITION FOR A NUMERICAL WAVE TANK BASED ON THE WCSPH METHOD

In this paper a space-averaged Navier–Stokes approach was deployed to Modified Wavemaker Boundary condition for a numerical wave tank. The developed model is based on the smoothed particle hydrodynamic (SPH) method which is a pure Lagrangian approach and can handle large deformations of the free surface with high accuracy. In this study, the large eddy simulation (LES) turbulent model was coupled with the weakly compressible version of the smoothed particle hydrodynamics (WCSPH) method to Modified Wavemaker Boundary condition for a numerical wave tank. An absorbing wavemaker boundary condition was developed to absorb the second reflecting waves from the wavemaker. The capacity of absorbing secondary reflecting waves and incoming waves in absorbing wavemaker was validated through comparisons of the numerical results with general wavemaker.

Boundary layer effects on the critical nozzle of hydrogen sonic jet

When hydrogen flows through a small finite length constant exit area nozzle the viscous effects create a fluid throat which acts as a converging-diverging nozzle and lead to Mach number greater than one at the exit if the jet is under-expanded. This phenomenon influences the mass flow rate and the dispersion cloud size. In this study, the boundary layer effect on the unsteady hydrogen sonic jet flow through a 1 mm diameter pipe from a high pressure reservoir (up to 70 MPa) is studied using computational fluid dynamics with a large eddy simulation turbulence model. This viscous flow simulation is compared with a non-viscous simulation to demonstrate that the velocity is supersonic at the exit of a small exit nozzle and that the mass flow is reduced.

A Computational Fluid Dynamics Model for Investigating Air-Pumping Mechanisms in Air-Borne Tire Noise

ABSTRACT The reduction in power train noise over the past decade has led to an increased focus in reducing tire/road noise, largely due to the environmental concerns related to road traffic noise in industrial countries. Computational fluid dynamic (CFD) simulations conducted using ANSYS FLUENT are presented here with the objective of understanding air-pumping noise-generation mechanisms due to tire/road interaction. The CFD model employs a large eddy simulation turbulence modeling approach, in which the filtered compressible Navier-Stokes equations are solved to obtain temporally and spatially accurate near-field pressure fluctuations for a two-dimensional (2D) tire geometry with (1) one groove and (2) two grooves. In addition, the Ffowcs-Williams and Hawkings (FW-H) acoustic model is used to predict far-field acoustics. The deformation of the grooves, as the tire rotates, is represented by prescribed sidewall movements. Consequently, the solution to the numerical problem is obtained through a single process, thereby enabling the prediction of small-scale air pumping, horn effect, and far-field acoustics in a single simulation. The acoustic characteristics associated with air pumping are studied through spectral analysis tools, and comparisons show that the additional groove on the horn geometry alters the spectral characteristics of air pumping. Validation of the model is conducted through qualitative and quantitative comparisons with previous studies. These simulations are intended to provide a deeper understanding about the small-scale noise generation as well as the near-field and far-field acoustics, thereby paving the way for the automotive manufacturer to compare a variety of air-related tire noise characteristics without spending time and money for vehicle pass-by tests.

Assessment of Different Turbulence Models for the Motion of Non-metallic Inclusion in Induction Crucible Furnace

Turbulent fluid flow due to the electromagnetic forces in induction crucible furnace (ICF) is modeled using k-ϵ, k-ω SST and Large Eddy Simulation (LES) turbulence models. Fluid flow patterns calculated by different turbulence models and their effects on the motion of non-metallic inclusions (NMI) in the bulk melt have been investigated. Results show that the conventional k-ϵ model cannot solve the transient flow in ICF properly. With k-ω model transient flow and oscillation behavior of the flow pattern can be solved, and the motion of NMI can be tracked fairly well. LES model delivers the best modeling result on both details of the transient flow pattern and motion trajectories of NMI without the limitation of NMI size. The drawback of LES model is the long calculation time. Therefore, for general purpose to estimate the dynamic behavior of NMI in ICF both k-ω SST and LES are recommended. For the precise calculation of the motion of NMI smaller than 10 μm only LES model is appropriate.

Analysis of the Effect of the Swirl Flow Intensity on Combustion Characteristics in Liquid Fuel Powered Confined Swirling Flames

This article examines the implementation of CFD technology in the design of the industrial liquid fuel powered swirl flame burner. The coupling between the flow field and the combustion model is based on the eddy dissipation model. The choice of the LES (Large Eddy Simulation) turbulence model over standard RANS (Reynolds Averaged Navier-Stokes) offers a possibility to improve the quality of the combustion-flow field interaction. The Wall Adapting Local Eddy-Viscosity (WALE) sub-grid model was used. The reaction chemistry is a simple infinitely fast one step global irreversible reaction. The computational model was setup with the Ansys-CFX software. Through the detailed measurements of industrial size burner, it was possible to determine the natural operational state of the burner according to the type of fuel used. For the inlet conditions, axial and radial velocity components were calculated from known physical characteristics of both the fuel and air input, with the initial tangential velocity of the fuel assumed as 18% of the initial axial fuel velocity. Different swirl number (S) values were studied. Addition of a surplus (in comparison to conventional flame stabilization) of tangential air velocity component (W), the rotational component increases itself with a considerably high magnitude, contributing to the overall flame stabilization. The level of S especially influences the turbulent energy, its dissipation rate and turbulent (Reynolds) stresses. In the case of high swirl number values (S > 0,65) it is possible to divide the flow field in three principle areas: mixing area (fuel-air), where exothermal reactions are taking place, central recirculation area and outer recirculation area, which primarily contains the flow of burnt flue gases. The described model was used to determine the flow and chemical behavior, whereas the liquid atomization was accounted for by LISA (Linear Instability Sheet Atomization) model incorporating also the cavitation within injection boundary condition. The boundary conditions were determined based on the data from the experimental hot water system. Depending on system requirements, especially with continuous physical processes as well as the results of experimental measurements, the paper reports on determination of the mixing field and its intensity in the turbulent flow, the description of heat release and interaction of turbulent flow field and chemical kinetics in the case of confined swirling flames.

Turbulence-induced noise of a submerged cylinder using a permeable FW–H method

Among underwater noise sources around submerged bodies, turbulence-induced noise has not been well investigated because of the difficulty of predicting it. In computational aeroacoustics, a number of studies has been conducted using the Ffowcs Williams–Hawkings (FW–H) acoustic analogy without consideration of quadrupole source term due to the unacceptable calculation cost. In this paper, turbulence-induced noise is predicted, including that due to quadrupole sources, using a large eddy simulation (LES) turbulence model and a developed formulation of permeable FW–H method with an open source computational fluid dynamics (CFD) tool-kit. Noise around a circular cylinder is examined and the results of using the acoustic analogy method with and without quadrupole noise are compared, i.e. the FW–H method without quadrupole noise versus the permeable FW–H method that includes quadrupole sources. The usability of the permeable FW–H method for the prediction of turbulence-noise around submerged bodies is shown.

The numerical simulation of draft tube cavitation in Francis turbine at off-design conditions

Purpose – The vortex ropes in draft tube of Francis turbine always cause fluctuation and vibration, which consequently threaten the safety and stability of hydro turbines. The purpose of this paper is to use a cavitation flow computational method to simulate spiral vortex ropes under part load conditions and columnar vortex ropes under high-load conditions in draft tube. The unsteady cavitating flow characteristics in draft tube and its interaction with runner cavitation were analyzed. Design/methodology/approach – The calculation method was verified by cavitation simulation around a 3D hydrofoil. The results show that the Large Eddy Simulation (LES) turbulence model with the Zwart-Gerber-Blemari cavitation model have comparative advantage in cavitation simulations whether from capture of cavity shape or prediction of pressure changes. So it was chosen to simulate the two-phase cavitation flow in Francis turbine. The boundary conditions for inlet and outlet were set to inlet total pressure and outlet static pressure. The finite volume method with the central difference was adopted to discretize the equations. Findings – The calculated Thoma number agreed well with the experimental data. The vortex rope diameter and length increased with the cavitation development for both of the two types of vortex ropes conditions. The maximum peak-to-peak values of pressure pulsations located in the draft tube elbow part under all of the Thoma numbers conditions. Under spiral vortex rope conditions, the pressure pulsation in the same section of draft tube cone show obviously phase shift. The vortex rope affects the development of runner cavitation, which induces the symmetric and axisymmetric cavitation region in the suction side of blades for spiral and columnar vortex rope condition, respectively. Research limitations/implications – The mesh independence had been checked only in non-cavitation flow; in addition, the mesh density did not well satisfy the requirements of LES due to the limitations of computing power. The higher mesh density on a simplified model with one blade flow path and the entire draft tube may be helpful for obtaining more precise results. Originality/value – The spiral and columnar vortex ropes in a Francis turbine were compared and analyzed. The annular hydraulic jump appeared in the columnar vortex rope conditions has little effects on the pressure pulsations. The uneven flow field caused by spiral vortex led to the asymmetric cavitation development.

Large eddy simulation of a pitched blade impeller mixed vessel – Comparison with LDA measurements

This study deals with a large eddy simulation (LES) of the single-phase turbulent flow in a fully baffled mixed vessel stirred by a pitched six-blade impeller. The ANSYS FLUENT 15.7 commercial CFD programme was employed. The LES turbulent model is used with the dynamic Smagorinsky–Lilly model. The Werner–Wengle wall function is used to cope with the requirements of near the wall cell size of the LES model. The sliding mesh (SM) model was used for simulating the impeller movement. The results of the simulations are compared with experimental data obtained by previous LDA measurements of the radial profiles of the axial component of the ensemble averaged mean velocity in the impeller discharge flow. The volume flow rates and the flow rate criteria were calculated from these velocity profiles, and were compared with the flow rates determined from the horizontal vessel cross section from simulation.

Numerical research of diesel spray and atomization coupled cavitation by Large Eddy Simulation (LES) under high injection pressure

A special spray model is applied to study the spray behavior with high injection pressure and micro-hole nozzle. To reveal the cavitation in diesel nozzle and its influence on spray and atomization, the Large Eddy Simulation (LES) turbulence model is adopted to detect the cavitation, and then the special spray model coupling the cavitation is build. From research results, three important conclusions can be drawn. Firstly, the cavitation flow can raise the effective velocity at the nozzle exit and such effect become even more obvious with higher injection pressure, e.g.180MPa. Secondly, the applied spray model is in good agreement with the spray characteristics and images obtained from the EFS8400 spray test platform. Thirdly, the cavitation with high injection pressure and micro-hole nozzle can increase the spray cone angle and reduce the spray penetration; the cavitation intensity has a great impact on the spray velocity field and vorticity intensity, especially at the initial spray field under the condition of high injection pressure.

Scale Effect of Premixed Methane-Air Combustion in Confined Space Using LES Model

Gas explosion is the most hazardous incident occurring in underground airways. Computational Fluid Dynamics (CFD) techniques are sophisticated in simulating explosions in confined spaces; specifically, when testing large-scale gaseous explosions, such as methane explosions in underground mines. The dimensions of a confined space where explosions could occur vary significantly. Thus, the scale effect on explosion parameters is worth investigating. In this paper, the impact of scaling on explosion overpressures is investigated by employing two scaling factors: The Gas-fill Length Scaling Factor (FLSF) and the Hydraulic Diameter Scaling Factor (HDSF). The combinations of eight FLSFs and five HDSFs will cover a wide range of space dimensions where flammable gas could accumulate. Experiments were also conducted to evaluate the selected numerical models. The Large Eddy Simulation turbulence model was selected because it shows accuracy compared to the widely used Reynolds’ averaged models for the scenarios investigated in the experiments. Three major conclusions can be drawn: (1) The overpressure increases with both FLSF and HDSF within the deflagration regime; (2) In an explosion duct with a length to diameter ratio greater than 54, detonation is more likely to be triggered for a stoichiometric methane/air mixture; (3) Overpressure increases as an increment hydraulic diameter of a geometry within deflagration regime. A relative error of 7% is found when predicting blast peak overpressure for the base case compared to the experiment; a good agreement for the wave arrival time is also achieved.

Three-dimensional fluid–structure interaction simulation with a hybrid RANS–LES turbulence model for applications in transonic flow domain

Current industrial practice for the fluid–structure interaction (FSI) analyses and prediction of aeroelastic phenomena, such as flutter, is heavily based on linear methods. These methods involve many of design limitations and envelope restrictions for aircraft. In this paper novel hybrid Reynolds-Averaged Navier–Stokes – Large Eddy Simulation (RANS–LES) turbulence model, i.e. k–Omega Shear Stress Transport Scale-Adaptive Improved Delayed Detached Eddy Simulation (k–Omega SST SA IDDES) is tested and implemented in the FSI procedure and is applied in transonic flow. This model is also compared with the lower fidelity RANS models, i.e. k–ωSST and Spalart–Allmaras. More precisely, a strongly coupled three-dimensional (3D) FSI solver is combined with the turbulence model and large deformation updated Lagrangian finite volume structural solver in order to resolve standard computational fluid dynamics (CFD) and aeroelastic benchmark cases of transonic flow. The turbulence model combines the advanced capabilities of the existing SST, SAS and IDDES turbulence models. Unsteadiness detection deficiency of SAS is automatically supplemented by the IDDES term included in kinetic energy equation. The numerical results of Onera M6 and AGARD 445.6 validation cases are presented and compared with the existing experimental results. Discretization of the governing equations is performed by cell-centered finite volume method (FVM) on unstructured meshes. Further application of the FSI procedure for the FSI analyzes of the whole aircraft structures is one of the aims. The emphasis is made on turbulence modeling which appears to have a major impact to the prediction of FSI behavior in transonic flow domain. In this work the aeroelasticity is treated as one of the many FSI branches. Described FSI solver is custom written and implemented in OpenFOAM.

CFD based approach for modeling steam–water direct contact condensation in subcooled water flow in a tee junction

The phenomenon of direct contact condensation (DCC) is encountered in several components of a nuclear power plant in case of some transient and accident conditions. Studies on steam jet discharged directly into a stagnant water pool have been studied extensively; however, it is sparse for direct contact condensation of steam in subcooled water in pipes. The present paper aims to investigate the low mass flux saturated steam discharged directly into subcooled water flow in a tee junction. The Volume of Fluid (VOF) two-phase flow model and Large Eddy Simulation (LES) turbulent flow model of FLUENT have been used. Also, a thermal equilibrium model considering heat and mass transfer was introduced to model steam condensation using user-defined function (UDF). The simulation results show that, for steam mass flux of 10 kg/m2 s, large chugging, small chugging and bubbling are obtained at water temperature of 303.15 K, 343.15 K and 363.16 K, respectively. In addition, it was found that, in chugging, there exist temperature and pressure oscillations at the location where the vertical branch pipe and the horizontal main pipe intersect. The phenomena of temperature and pressure oscillations at this location are called thermal cycling and water hammer, respectively. Moreover, the pressure oscillation frequency increases and the spike pressure decreases with increasing of subcooled water temperature in chugging. The Computational Fluid Dynamics (CFD) simulation results were qualitatively validated against the test results. It would be possible to improve the accuracy of the test results by employing multiple cameras.

CFD simulations of the flow around a cyclist subjected to crosswinds

For the first time, an extensive numerical study of the effect of crosswinds on the flow around a cyclist on a bicycle with stationary wheels has been undertaken for crosswind (yaw) angles ranging from 0° to 90°. The flow field and the aerodynamic forces have been obtained using three numerical techniques: Reynolds Averaged Navier Stokes (RANS), Detached Eddy Simulation (DES) and Large Eddy Simulation (LES). RANS simulations have been undertaken for all the range of yaw angles to provide a general insight of the flow around a cyclist, whilst DES and LES have been undertaken at 15° yaw angle in order to investigate the time-varying flow physics in detail. The aerodynamic forces have been compared with a series of wind tunnel experiments. The RANS results showed the development of large flow separation around the bicycle with increasing yaw angles. The instantaneous flow structures and the auto-spectral densities of the time histories of the force coefficients are identified and revealed that the DES and LES turbulence models are able to predict the dominant frequencies found in the physical experiments. This work provides an improved understanding of the flow characteristics around a cyclist in crosswinds that will hopefully help to improve the safety of cyclists.

Numerical investigation of electro hydrodynamics (EHD) enhanced water evaporation using Large Eddy Simulation turbulent model

In this paper, the electric field effect on enhancement of the water evaporation rate in a channel is numerically investigated. The coupled equations of electrical field, flow field, temperature field and species concentration fields are discretized using Finite Volume Method (FVM) and the implicit/hybrid difference form. This equations are solved via SIMPLE algorithm and the Kaptsov hypothesis. The turbulent flow is modelled by Large Eddy Simulation (LES). The numerical results show that the water evaporation rate is increased with the presence of electric field, but the effect of electric field is diminished at high Reynolds number.

Implementation and Validation of the Jet Flame Model Using C++ Open Source Codes Computational Fluid Dynamics Software

The open source code software, OpenFOAM, based on the computational fluid dynamics (CFD) was implemented to simulate the methane jet flame. The large eddy simulation (LES) of turbulence model was written using C++ language. The mixture fraction approach and infinitely-fast chemistry assumption was combined with this LES turbulence model. The results of jet flame simulation were validated with Yi Zeng etal., (2011) experiments based on flame lengths under flow rate and pressure conditions of 5.95-23.81 mg/s and 50-100 kPa, respectively. It was found that an average error of flame lengths obtained from the developed CFD model was 8.57% when referred the 1% O2 remaining was a flame shape.

Numerical Simulation of Combustion Enhancement Through a Repetitive Pulsed Plasma Actuator

This work presents a numerical study of combustion enhancement using a repetitive pulsed plasma actuator. A large-eddy simulation turbulence model is considered to investigate the nonpremixed turbulent combustion of methane-air flow. The baseline case shows agreement with published numerical data. Based on recent studies, it is assumed that the repetitive pulsed actuators are able to create acoustic perturbations for flow control. Numerical results show that induced acoustic perturbations may enhance chemical reactions and result in modification of the flame shape. These numerical results may be useful for the design and improvement of a coaxial jet combustor.

Lattice Boltzmann Method Contribution to the Second High-Lift Prediction Workshop

This paper presents the results of the computational-fluid-dynamics code XFlow for the test case proposed in the 2nd High-Lift Prediction Workshop. The target of this international workshop is to assess the accuracy of numerical predictions provided by a wide selection of computational-fluid-dynamics codes and applied to the aeronautical industry. It deals with a full aircraft in high-lift configuration with unstowed flap and slat, including slat tracks, flap track fairings, and slat pressure tube bundles. XFlow is a particle-based kinetic solver, based on the lattice Boltzmann method. It is well suited for such low-speed flows because the standard lattice Boltzmann method is limited to low Mach number. It employs large-eddy simulation turbulence modeling with the wall-adapting local eddy model, coupled with a generalized law of the wall to model the boundary-layer prediction. Traditional computational-fluid-dynamics software requires a time-consuming meshing process that is prone to errors, which significantly affects the mesh quality and thus the outcome of a simulation. Furthermore, the complex geometries such as elements in small gaps add an extra complication to generate the mesh. In contrast, the method employed by XFlow avoids the traditional meshing process using an octree structure, with spatial refinement related by a factor of 2 based on the input geometry. The discretization stage is strongly accelerated, thus reducing engineering costs and complex geometry computations are affordable in a straightforward way. The research presented in this paper starts with a grid validation study (case 1) with the DLR F11 geometry in configuration 2. Because the lattice Boltzmann method approach used by XFlow simplifies the integration of geometrical details, the full polar characteristic is directly investigated with configuration 5 (case 3) that includes the actual model configuration on the aerodynamic performances. The configuration 5 geometry includes slat tracks and flap track fairings, as well as slat pressure tube bundles. This is the highest-fidelity geometry in comparison with wind-tunnel testing. This study is conducted for both low- and high-Reynolds-number conditions, 1.35 and 15.1 million, respectively. In addition, the study focuses on the poststall region and the influence of geometrical details on turbulence and flow patterns. The interest for the poststall region is justified by the lack of consistency in the stall prediction found during the 2nd AIAA High-Lift Prediction Workshop by all the participants.

Hybrid Large-Eddy Simulation Optimization of a Fundamental Turbine Blade Turbulated Cooling Passage

An efficient methodology for shape optimization based on computational fluid dynamics is developed and applied to a fundamental turbulated square duct. The present study uses a hybrid large-eddy simulation turbulence modeling approach for all computational fluid dynamics evaluation, as it is shown to give much more reliable results over a range of geometries. Four geometric parameters defining the cross-sectional profile and angle of the turbulators were optimized for two competing performance metrics: heat transfer and pressure drop. An initial database of computational fluid dynamics runs, sampled from the design space using a space-filling design of experiments, is used as a starting point. A metamodel is then fitted through the training data in the database. The metamodel provides the means for fast approximation of the objective functions at new design points, allowing the use of a genetic algorithm for this multiobjective optimization. The resulting Pareto front is verified with a second round of computational fluid dynamics evaluations at the end. The optimization demonstrates the potential for improvement in thermal performance when using turbulators with an upstream ramp. Various metamodels are also explored for their ability to deal with design space nonlinearity and noisy data.

Numerical simulation of vapor bubble condensation in turbulent subcooled flow boiling

Subcooled flow boiling is a significant heat transfer regime in pressurized water reactors (PWRs) and also occurs in boiling water reactors (BWRs) prior to the onset of saturated nucleate boiling. In subcooled boiling, individual bubbles experience a life-cycle of nucleation from the wall, (possible) departure from the wall, and transport along the channel during condensation/collapse. The present paper focuses on the last stage of this life cycle. CFD analysis of a single bubble behavior in upward subcooled flow boiling was performed using volume-of-fluid (VOF) interface tracking method and large eddy simulation (LES) turbulence model. High pressure steam-water conditions at high flow velocity were simulated due to their relevance to the nuclear application. The numerical solutions were compared with terminal velocity correlations. Bubble behavior was investigated at different system pressure, subcooling temperature, bubble diameter, pipe diameter, and bulk fluid velocity, corresponding to bulk Reynolds number range of 0–410,000. The effects of these parametric variations on bubble rise velocity, deformation rate, trajectory, and distortion are presented. Results show that bubble rise velocity increases with bulk velocity, and bubble size, but decreases with increasing pipe diameter. A reverse bubble velocity trend with varying system pressure is observed with and without bulk velocity. Bubble condensation rate decreased with increasing system pressure, bubble size and pipe diameter, but increased with increasing subcooling temperature and bulk fluid velocity.

A new class of finite element variational multiscale turbulence models for incompressible magnetohydrodynamics

New large eddy simulation (LES) turbulence models for incompressible magnetohydrodynamics (MHD) derived from the variational multiscale (VMS) formulation for finite element simulations are introduced. The new models include the variational multiscale formulation, a residual-based eddy viscosity model, and a mixed model that combines both of these component models. Each model contains terms that are proportional to the residual of the incompressible MHD equations and is therefore numerically consistent. Moreover, each model is also dynamic, in that its effect vanishes when this residual is small. The new models are tested on the decaying MHD Taylor Green vortex at low and high Reynolds numbers. The evaluation of the models is based on comparisons with available data from direct numerical simulations (DNS) of the time evolution of energies as well as energy spectra at various discrete times. A numerical study, on a sequence of meshes, is presented that demonstrates that the large eddy simulation approaches the DNS solution for these quantities with spatial mesh refinement.