Highly-resolved LES and PIV Analysis of Isothermal Turbulent Opposed Jets for Combustion Applications

Turbulent opposed jet (TOJ) burners are an interesting test case for fundamental combustion research and a good benchmark for the available modelling approaches. However, these opposed jet flames strongly depend on the turbulence generation inside the nozzle, which is usually achieved through a perforated plate upstream of the nozzle exit. The present work investigates the flow from these perforated plates and the subsequent turbulence generation in great detail. We present results from highly-resolved large eddy simulations (LES) of the in-nozzle flow in turbulent opposed jets alongside state-of-the-art particle image velocimetry (PIV) at standard and high repetition rates taken inside a glass nozzle. The in-nozzle PIV data provides the LES inflow conditions with unprecedented detail, which are used to follow the initial jet development behaviour known from PIV, before jet coalescence, turbulence production and decay further downstream in the nozzles are successfully predicted. In regions where the PIV experiment suffers from inherent limitations like reflections and the velocity bias, the LES data is available to still obtain a detailed picture of the flow. The sensitivity of the simulations to various physical and numerical parameters is discussed in detail. Results from LES and PIV are compared qualitatively and quantitatively in terms of first and second moments of velocity, temporal autocorrelations, and energy density spectra. Significant deviations are found in the frequency (20%) and strength of vortex shedding from the inlet plane only, whereas the qualitative and quantitative agreement between simulation and experiment is otherwise excellent throughout, implying that a successful large eddy simulation of a turbulent opposed jet can be performed in a domain that includes the perforated plates.     

On The Validation of LES Applied to Internal Combustion Engine Flows: Part 1: Comprehensive Experimental Database

Improved understanding of in-cylinder flows requires knowledge from well-resolved experimental velocimetry measurements and flow simulation modeling. Engine simulations using large eddy simulations (LES) are making large progress and the need for well documented velocimetry measurements for model validation is high. This work presents velocimetry measurements from PIV, high-speed PIV, stereoscopic PIV, and tomographic PIV to extensively describe the in-cylinder flow field in a motored optical engine operating at 800 RPM. These measurements also establish a comprehensive database designed for LES model development and validation. Details of the engine, engine accessory components, and well-controlled boundary conditions and engine operation are presented. The first two statistical moments of the flow field are computed and show excellent agreement among the PIV database. Analysis of statistical moments based on limited sample size is presented and is important for modeling validation purposes. High-speed PIV resolved the instantaneous flow field throughout entire engine cycles (i.e. 719 consecutive crank-angles), while tomographic PIV images are further used to investigate the 3D flow field and identify regions of strong vortical structures identified by the Q-criterion. Principle velocity gradient components are computed and emphasize the need to resolve similar spatial scales between experimental and modeling efforts for suitable model validation.     

Large-eddy Simulation of Film Cooling Flows with Variable Density Jets

The present paper investigates the impact of the velocity and density ratio on the turbulent mixing process in gas turbine blade film cooling. A cooling fluid is injected from an inclined pipe at α=30° into a turbulent boundary layer profile at a freestream Reynolds number of Re_∞ = 400,000. This jet-in-a-crossflow (JICF) problem is investigated using large-eddy simulations (LES). The governing equations comprise the Navier–Stokes equations plus additional transport equations for several species to simulate a non-reacting gas mixture. A variation of the density ratio is simulated by the heat-mass transfer analogy, i.e., gases of different density are effused into an air crossflow at a constant temperature. An efficient large-eddy simulation method for low subsonic flows based on an implicit dual time-stepping scheme combined with low Mach number preconditioning is applied. The numerical results and experimental velocity data measured using two-component particle-image velocimetry (PIV) are in excellent agreement. The results show the dynamics of the flow field in the vicinity of the jet hole, i.e., the recirculation region and the inclination of the shear layers, to be mainly determined by the velocity ratio. However, evaluating the cooling efficiency downstream of the jet hole the mass flux ratio proves to be the dominant similarity parameter, i.e., the density ratio between the fluids and the velocity ratio have to be considered.     

On the influences of key modelling constants of large eddy simulations for large-scale compartment fires predictions

An in-house large eddy simulation (LES) based fire field model has been developed for large-scale compartment fire simulations. The model incorporates four major components, including subgrid-scale turbulence, combustion, soot and radiation models which are fully coupled. It is designed to simulate the temporal and fluid dynamical effects of turbulent reaction flow for non-premixed diffusion flame. Parametric studies were performed based on a large-scale fire experiment carried out in a 39-m long test hall facility. Several turbulent Prandtl and Schmidt numbers ranging from 0.2 to 0.5, and Smagorinsky constants ranging from 0.18 to 0.23 were investigated. It was found that the temperature and flow field predictions were most accurate with turbulent Prandtl and Schmidt numbers of 0.3, respectively, and a Smagorinsky constant of 0.2 applied. In addition, by utilising a set of numerically verified key modelling parameters, the smoke filling process was successfully captured by the present LES model.     

Numerical investigation of helical coil tube bundle in turbulent cross flow using large eddy simulation

A realistic five-layered helical coil tube bundle in turbulent cross flow is numerically investigated using large eddy simulation (LES). The geometry of the helical coil tube bundle, which is a 1/45 sector of a helical coil steam generator design for an advanced small modular reactor, has three counterclockwise helical layers and two clockwise helical layers stacked in alternating fashion in the radial direction. A Reynolds number of 21,800 is considered based on the mean gap velocity, diameter of the coil tube, and kinematic viscosity of the working fluid. The WALE sub-grid scale model is employed for LES to resolve scale smaller than the grid size. Additionally, LES for an ideal five-layered coil bundle model without a helical geometry is utilized to generate comparison data. Instantaneous flow fields are explored by analyzing velocity magnitude, vorticity, static pressure, and vortical structures. The detachments of vortical structures from the coil tube surface are observed by visualizing vortical structures and wall shear stress distributions simultaneously. Various turbulent statistics at multiple monitoring locations are presented with an emphasis on feature difference between realistic and ideal coil bundle models. The results of spectral analyses, including the power spectral density and continuous wavelet transform analyses, are addressed from the perspective of flow-induced vibration. The key feature of this paper is the discussion of three-dimensional effects based on the helical geometry of a coil bundle.     

Simulation of Transverse Combustion Instability in a Multi-Injector Combustor using the Time-Domain Impedance Boundary Conditions

The time-domain impedance boundary condition (TDIBC) is used as a reduced-order model (ROM) in large-eddy simulation (LES) to study self-sustained transverse oscillations in an experimentally studied high-pressure, shear coaxial multi-injector combustor. This work is an extension of the recent study using ROM-LES to simulate a single-element combustor that exhibited longitudinal instability. Here, we focus on transverse instability in a seven-injector combustor. The fuel and oxidizer inlets are truncated and the conventional inflow boundary conditions at the original inlet are replaced by an impedance describing function (IDF) in the form of a reflection coefficient that couples with LES through characteristic based boundary conditions at the truncated inlet. The impedance model is also generalized to include the effects of entropy fluctuations at the inflow. The hybrid ROM-LES simulations are compared with LES simulations with the full combustor geometry. Results show very good agreement and confirm that the use of TDIBC within LES is a viable tool to account for complex acoustic/boundary interaction in a physical way without explicitly solving the full geometry at LES level. Some simplifications and approximations have to be invoked and these constraints are also discussed.     

Large Eddy Simulation of an Internal Combustion Engine Using an Efficient Immersed Boundary Technique

This paper presents highly resolved large eddy simulations (LES) of an internal combustion engine (ICE) using an immersed boundary method (IBM), which can describe moving and stationary boundaries in a simple and efficient manner. In this novel approach, the motion of the valves and the piston is modeled by Lagrangian particles, whilst the stationary parts of the engine are described by a computationally efficient IBM. The proposed mesh-free technique of boundary representation is simple for parallelization and suitable for high performance computing (HPC). To demonstrate the method, LES results are presented for the flow and the combustion in an internal combustion engine. The Favre-filtered Navier-Stokes equations are solved for a compressible flow employing a finite volume method on Cartesian grids. Non-reflecting boundary conditions are applied at the intake and the exhaust ports. Combustion is described using a flame surface density (FSD) model with an algebraic reaction rate closure. A simplified engine with a fixed axisymmetric valve (see Appendix A) is employed to show the correctness of the method while avoiding the uncertainties which may be induced by the complex engine geometry. Three test-cases using a real engine geometry are investigated on different grids to evaluate the impact of the cell size and the filter width. The simulation results are compared against the experimental data. A good overall agreement was found between the measurements and the simulation data. The presented method has particular advantages in the efficient generation of the grid, high resolution and low numerical dissipation throughout the domain and an excellent suitability for massively parallel simulations.     

Reynolds-Averaged,Scale-Adaptive and Large-Eddy Simulations of Premixed Bluff-Body Combustion Using the Eddy Dissipation Concept

A lean premixed propane/air bluff-body stabilized flame (Volvo test rig) is calculated using the Scale-Adaptive Simulation turbulence model (SAS) and Large-Eddy simulations (LES) as well as the conventional Reynolds-averaged approach (RAS). RAS and SAS are closed by the standard k-?? and the k-ω Shear Stress Transport (SST) turbulence models, respectively. The conventional Smagorinsky and the k-equation sub-grid scales models are used for the LES closure. Effects of the sub-grid scalar flux modeling using the classical gradient hypothesis and Clark’s tensor diffusivity closures both for the inert and reactive LES flows are discussed. The Eddy Dissipation Concept (EDC) is used for the turbulence-chemistry interaction. It assumes that molecular mixing and the subsequent combustion occur in the ’fine structures’ (smaller dissipative eddies, which are close to the Kolmogorov scales). Assuming the full turbulence energy cascade, the characteristic length and velocity scales of the ’fine structures’ are evaluated using different turbulence models (RAS, SAS and LES). The finite-rate chemical kinetics is taken into account by treating the ’fine structures’ as constant pressure and adiabatic homogeneous reactors, calculated as a system of ordinary-differential equations (ODEs) described by a Perfectly Stirred Reactor (PSR) concept. Several further enhancements to model the PSRs are proposed, including a new Livermore Solver (LSODA) for integrating stiff ODEs and a new correction to calculate the PSR time scales. All models have been implemented as a stand-alone application \(\text {edcPisoFoam}\) based on the OpenFOAM technology. Additionally, several RAS calculations were performed using the Turbulence Flame Speed Closure model in Ansys Fluent to assess effects of the heat losses by modeling the conjugate heat transfer between the bluff-body and the reactive flow. Effects of the turbulence Schmidt number on RAS results are discussed as well. Numerical results are compared with available experimental data. Reasonable consistency between experimental data and numerical results provided by RAS, SAS and LES is observed. In general, there is satisfactory agreement between present LES-EDC simulations, numerical results by other authors and measurements without any major modification to the EDC closure constants, which gives a quite reasonable indication on the adequacy and accuracy of the method and its further application for turbulent premixed combustion simulations.     

Numerical and experimental study of shock waves emanating from an open-ended rectangular tube

We examine the dynamics of a high-speed shock-induced flow near the open end of a shock tube using the particle image velocimetry (PIV) and the background oriented schlieren (BOS) methods along with two- and three-dimensional numerical simulations. In experiments, planar shock waves (\(M=1.3\)–1.6) are discharged from a rectangular (\(24\,\hbox {mm} \times 48\,\hbox {mm}\)) low-pressure section of a shock tube open to the atmosphere. Due to the rectangular exit geometry, the resulting flow is highly three-dimensional and, thus, more complicated, compared to well-studied circular/axisymmetric geometries. The study focuses on the spatio-temporal flow structure up to 1 ms after the shock wave diffraction. PIV and BOS visualization techniques share the same post-processing principle, and the iterative multi-step cross-correlation algorithm applied in the PIV software is adapted here for the calculation of background pattern displacement on the BOS images. Particular attention is given to the resolution of flow regions where sharp gradients are present, such as a diffracted shock front or embedded shocks. Computational fluid dynamic simulations of the problem are also conducted to validate the experimental results and methods and to gain more insight into the three-dimensional flow dynamics. PIV and BOS images are found to be consistent with the corresponding numerical flow visualizations.     

Lagrangian coherent structures in the human carotid artery bifurcation

The carotid artery bifurcation is known as a site of atheromatous plaque formation which is closely related to hemodynamics. To investigate the fluid mechanics inside the bifurcation, a transparent model of the carotid geometry was built to estimate the feasibility of using stereoscopic particle image velocimetry (PIV) in a complex three-dimensional geometry. As a first approach, steady inflow conditions are considered. Velocity data are acquired in cross-sectional planes and combined to yield the full three-dimensional velocity vector field in the region of the bifurcation. The finite-time Lyapunov exponent (FTLE) is used as a criterion to reveal the complex flow structure and is found to be particularly efficient in discriminating between reverse flow and recirculation regions. The Lagrangian criterion is also computed with time-resolved, two-component PIV measurements obtained by increasing the Reynolds number up to the onset of unsteadiness. The FTLE field produces in this case a detailed visualization of the instability development.     

Large-eddy simulation of turbulent channel flow at transcritical states

We present well-resolved large-eddy simulations (LES) of a channel flow solving the fully compressible Navier–Stokes equations in conservative form. An adaptive look-up table method is used for thermodynamic and transport properties. A physically consistent subgrid-scale turbulence model is incorporated, that is based on the Adaptive Local Deconvolution Method (ALDM) for implicit LES. The wall temperatures are set to enclose the pseudo-boiling temperature at a supercritical pressure, leading to strong property variations within the channel geometry. The hot wall at the top and the cold wall at the bottom produce asymmetric mean velocity and temperature profiles which result in different momentum and thermal boundary layer thicknesses. Different turbulent Prandtl number formulations and their components are discussed in context of strong property variations.     

The development of high-speed particle image velocimetry (20 kHz) for large eddy simulation code refinement in bluff body flows

Flow interaction with a bluff body generates a highly complex flow field and has been the subject of much experimental and theoretical analysis. It has been shown that large eddy simulation (LES) modelling provides a more realistic analysis of the flow for such situations where the large scales of turbulence must be resolved. The inherent small-scale spatial velocity averaging in particle image velocimetry (PIV) is commensurate with the sub-grid scale modelling of LES and, therefore, offers potential as a code refinement technique. To demonstrate this potential, however, PIV must be performed with a temporal resolution of typically kHz and a spatial resolution of sub-mm² to be relevant for the vast majority of flows of engineering interest. This paper reports the development of a high-speed PIV system capable of operating at 20 kHz with a spatial resolution of 0.9 mm². This is the combined highest speed, highest resolution PIV data reported to date. The experiment chosen to demonstrate the system is the study of the steady flow interaction with circular and square cross-section obstacles. A Reynolds number of 3,900 is chosen for the cylinder flow to extend the database used by Breuer M. (1998 Int J Heat Fluid 19:512–521) in his extensive LES modelling of this flow. Data presented include a sequence of two-dimensional velocity and vorticity fields, including flow streamlines. Importantly, the random error, inherent in a PIV measurement, is discussed and a formula presented which allows the error to be estimated and regions of the flow identified where LES comparisons would be uncertain.     

Three-component velocity field measurement in confined liquid flows with high-speed digital image plane holography

In the last years, several techniques have been developed for the measurement of the three velocity components in a fluid plane or volume. Techniques as stereoscopic particle image velocimetry (SPIV) or tomographic PIV need a complex set-up and present serious restrictions when applied to confined liquid flows. Other like digital holographic PIV has some limitations in the particle concentration that can be measured. In this work, high-speed digital image plane holography has been applied for the measurement of the three velocity components in a complex geometry brain aneurysm model, using a two-cavity high-speed laser, one double frame camera and normal visualization, like in regular PIV. A portable and compact system has been built for adapting the high-speed laser short coherence length to the measurement of larger areas.     

Nanoparticle coagulation in a planar jet via moment method

Large eddy simulations of nanoparticle coagulation in an incompressible pla- nar jet were performed.The particle is described using a moment method to approximate the particle general dynamics equations.The time-averaged results based on 3000 time steps for every case were obtained to explore the influence of the Schmidt number and the Damkohler number on the nanoparticle dynamics.The results show that the changes of Schmidt number have the influence on the number concentration of nanoparticles only when the particle diameter is less than 1 nm for the fixed gas parameters.The number concentration of particles for small particles decreases more rapidly along the flow di- rection,and the nanoparticles with larger Schmidt number have a narrower distribution along the transverse direction.The smaller nanoparticles coagulate and disperse easily, grow rapidly hence show a stronger polydispersity.The smaller coagulation time scale can enhance the particle collision and coagulation.Frequented collision and coagulation bring a great increase in particle size.The larger the Damkohler number is,the higher the particle polydispersity is.     

Large Eddy Simulation of a Motored Single-Cylinder Piston Engine: Numerical Strategies and Validation

This paper describes a compressible Large Eddy Simulation (LES) used to investigate cyclic variations for nonreacting flow in an optical single cylinder engine setup. The simulated operating point is part of a large experimental database designed to validate LES for cycle-to-cycle prediction, and constitutes a first step towards the realization of fired operating points. The computational domain covers almost the whole experimental setup (intake and exhaust plenums, intake and exhaust ducts, cylinder) to account for acoustic phenomena. The assessment of the computation is performed in two regions of the domain: the intake and exhaust duct predictions are compared to the results of a Helmholtz solver and the experiment (pressure transducers and Particle Image Velocimetry (PIV)) while the in-cylinder dynamics are compared to PIV measurements. The ability of the developed methodology to capture the correct level of cycle-to-cycle variations is demonstrated considering in-cylinder pressure and velocity fields predictions. Cycle-to-cycle variations in velocity are highlighted and localized using a proper orthogonal decomposition analysis.     

Impingement of buoyancy-driven flows at a stratified interface

Laser-induced fluorescence (LIF) and particle-image velocimetry (PIV) are used to study both thermals and plumes impinging on a stratified interface. Data are obtained for a central slice of the flow near the stratified interface. Both the thermal and plume are generated by releasing fresh water at the bottom of a tank filled with two layers of salt water of different densities. Thermals and plumes are studied at Reynolds numbers ranging from 3,000 to 8,000, above the value for the mixing transition, a Schmidt number of about 600, and Richardson numbers from 1 to 22. The Richardson and Reynolds numbers are based on the thermal or plume characteristics (size and vertical velocity) before impingement and the initial density difference across the interface. Laser-induced fluorescence (LIF) is used to determine the maximum penetration height, rebound distance and lateral spreading velocity. The vorticity results obtained from the PIV data reveal the vortical structure near impingement. When the thermal impinges upon the stratified interface, a baroclinic eddy generated at the interface appears to merge with eddies comprising the thermal itself to form a vortex ring. This ring remains near the interface, moving mainly along the lateral or horizontal direction away from the region of impingement. These results suggest that lateral transport is significant for thermals impinging on stratified interfaces, and that ignoring such transport may greatly underestimate overall transport and mixing in such flows.     

Large Eddy Simulations of Swirling Non-premixed Flames With Flamelet Models: A Comparison of Numerical Methods

This work investigates the application of large eddy simulation (LES) to selected cases of the turbulent non-premixed Sydney swirl flames. Two research groups (Loughborough University, LU and Imperial College, IC) have simulated these cases for different parameter sets, using two different and independent LES methods. The simulations of the non-reactive turbulent flow predicted the experimental results with good agreement and both simulations captured the recirculation structures and the vortex breakdown without major difficulties. For the reactive cases, the LES predictions were less satisfactory, and using two independent simulations has helped to understand the shortcomings of each. Furthermore one of the flames (SMH2) was found to be exceptionally hard to predict, which was supported by the lower amount of turbulent kinetic energy that was resolved in this case. However, the LES has identified modes of flame instability that were similar to those observed in some of the experiments.     

Large Eddy and Reynolds-Averaged Navier - Stokes Simulations of Turbulent Flow Over an Airfoil

A Large Eddy Simulation (LES) of turbulent flow over an airfoil near stall is performed. Results of the LES are compared with those of Reynolds-Averaged Navier-Stokes (RANS) simulations using two well-known turbulence models, namely the Baldwin-Lomax model and the Spalart-Allmaras model. The subgrid scale model used for the LES is the filtered structure function model. All simulations are performed using the same structured multi-block code. In order to reduce the CPU time, an implicit time stepping method is used for the LES. The purpose of this study is to show the possibilities and limitations of LES of complex flows associated with aeronautical applications using state of the art simulation techniques. Typical flow features are captured by the LES such as the adverse-pressure gradient and flow retardation. Visualization of instantaneous flow fields shows the typical streaky structures in the near-wall region.     

Homogenization Based LES for Turbulent Combustion

In this work we propose a novel methodology for performing Large Eddy Simulations (LES) of premixed, non-premixed and partially premixed laminar and turbulent flames. The motivation behind this study is the need for more accurate and flexible LES computations of increasingly complex engineering applications, for which current LES models are limited. The main drawback of present LES methods for reactive flows is that most of the chemical activity, and thus also most of the exothermicity, occurs on the subgrid scales, and is hence subject to modeling using only information about the resolved scale flow. Reasonable results have been achieved in several studies with present LES models but improved accuracy, flexibility and reliability is needed. Here, we use a homogenization-based approach based on a multi-scale expansion technique to convert the reactive Navier–Stokes equations, with finite rate chemistry, into a cascade of equations for different scales. The equations of motion for the large-scale dependent variable dynamics are explicitly simulated, whereas the equations of motion for the small-scale dependent variable dynamics are simplified by reducing the spatial dimensions from three to one, thus permitting affordable simulations in a grid within the grid approach. Presently, the methodology is limited to low Ma number variable density flows, but can be extended to high Ma number reactive flows. This method has some similarities with the LES–LEM and the TLS models of Menon et al., but differs in some important aspects. The model developed is here applied to a bluff-body stabilized flame and comparisons with both experimental data and conventional flamelet and finite rate chemistry LES models are made. The results show that the performance of this model is as good or better than any of the other models, and to a reasonable computational cost.