Large Eddy Simulation Tools Research Articles

Energy optimal 3D flight path planning for unmanned aerial vehicle in urban environments

This paper presents a general approach to compute energy optimal flight paths for unmanned aerial vehicle (UAV) in urban environments. To minimize the energy required, the flight path is optimized by exploiting local wind phenomena, i.e., upwind and tailwind areas from the airflow around buildings. A realistic wind field of a model urban environment typical for continental Europe is generated using PALM, a Large Eddy Simulation tool. The calculated wind field feeds into the flight path planning algorithm to minimize the energy required. A specifically tailored A-Star-Algorithm is used to optimize flight trajectories. The approach is demonstrated on a delivery UAV benchmark scenario. Energy optimal flight paths are compared to shortest way trajectories for 12 different scenarios. It is shown that energy can be saved significantly while flying in a city using knowledge of the current wind field.

Energetics of buoyancy-generated turbulent flows with active scalar: pure buoyant plume

A framework for the budget of Reynolds stress, temperature and density variations, turbulent mass flux and turbulent heat flux is presented for buoyancy-generated turbulent flows with a focus on turbulent plumes. The dynamical interactions between the turbulence generated from the velocity and thermodynamic fluctuations in thermal and heated gas plumes is studied. A large-eddy simulation tool has been used to simulate heavier and lighter than air plumes released from a circular heated source for high Reynolds number (Re). The budget equations are developed using a Favre-averaging approach. Both heated air plumes and heated gas plumes, namely, heated ${\rm SO}_2$ and heated ${\rm CH}_4$ plumes, are included in the study. The study focuses on addressing key questions – does the turbulence kinetic energy (TKE) spectrum follow the classical inertial $-5/3$ spectrum or is there a $-3$ buoyancy regime with a different scaling law? Fundamental pathways for generation of turbulence through velocity fluctuations, and temperature and density fluctuations are discussed. Analysis of the TKE and scalar variance budget equations has demonstrated that multiple mechanisms dominate the transport of Reynolds stresses, and in particular that correlations between the density and the velocity dominates the turbulence production mechanisms. Entrainment is connected to the turbulent transport and pressure-dilation processes.

Validation of a coupled atmospheric–aeroelastic model system for wind turbine power and load calculations

Abstract. The optimisation of the power output of wind turbines requires the consideration of various aspects including turbine design, wind farm layout and more. An improved understanding of the interaction of wind turbines with the atmospheric boundary layer is an essential prerequisite for such optimisations. With numerical simulations, a variety of different situations and turbine designs can be compared and evaluated. For such a detailed analysis, the output of an extensive number of turbine and flow parameters is of great importance. In this paper a coupling of the aeroelastic code FAST (fatigue, aerodynamics, structures, and turbulence) and the large-eddy simulation tool PALM (parallelised large-eddy simulation model) is presented. The advantage of the coupling of these models is that it enables the analysis of the turbine behaviour, among others turbine power, blade and tower loads, under different atmospheric conditions. The proposed coupling is tested with the generic National Renewable Energy Laboratory (NREL) 5 MW turbine and the operational eno114 3.5 MW turbine. Simulating the NREL 5 MW turbine allows for a first evaluation of our PALM–FAST coupling approach based on characteristics of the NREL turbine reported in the literature. The basic test of the coupling with the NREL 5 MW turbine shows that the power curve obtained is very close to the one when using FAST alone. Furthermore, a validation with free-field measurement data for the eno114 3.5 MW turbine for a site in northern Germany is performed. The results show a good agreement with the free-field measurement data. Additionally, our coupling offers an enormous reduction of the computing time in comparison to an actuator line model, in one of our cases by 89 %, and at the same time an extensive output of turbine data.

Implicit large Eddy simulation of the NASA CRM high-lift configuration near stall

Recent progress in adaptive high-order numerical methods, high-order mesh generation, and scalable implicit time integration approaches has made large eddy simulation (LES) more affordable than ever before. In the present study, we demonstrate the progress by performing implicit LES of a benchmark problem from the 3rd AIAA High-Lift Prediction Workshop: NASA's Common Research Model (CRM) high-lift configuration. The LES tool, hpMusic, is based on the flux reconstruction method capable of handling high-order mixed unstructured meshes. P-refinement studies are used to assess the mesh-dependence of the numerical solutions. Comparisons are also made with available experimental data, and a very good agreement was obtained in lift and drag coefficients between the present computation and wall-corrected experimental data.

Numerical Investigation of Flow through a Valve during Charge Intake in a DISI -Engine Using Large Eddy Simulation

Apart from electric vehicles, most internal combustion (IC) engines are powered while burning petroleum-based fossil or alternative fuels after mixing with inducted air. Thereby the operations of mixing and combustion evolve in a turbulent flow environment created during the intake phase and then intensified by the piston motion and influenced by the shape of combustion chamber. In particular, the swirl and turbulence levels existing immediately before and during combustion affect the evolution of these processes and determine engine performance, noise and pollutant emissions. Both the turbulence characteristics and the bulk flow pattern in the cylinder are strongly affected by the inlet port and valve design. In the present paper, large eddy simulation (LES) is appraised and applied to studying the turbulent fluid flow around the intake valve of a single cylinder IC-engine as represented by the so called magnetic resonance velocimetry (MRV) flow bench configuration with a relatively large Reynolds number of 45,000. To avoid an intense mesh refinement near the wall, various subgrid scale models for LES; namely the Smagorinsky, wall adapting local eddy (WALE) model, SIGMA, and dynamic one equation models, are employed in combination with an appropriate wall function. For comparison purposes, the standard RANS (Reynolds-averaged Navier–Stokes) k- ε model is also used. In terms of a global mean error index for the velocity results obtained from all the models, at first it turns out that all the subgrid models show similar predictive capability except the Smagorinsky model, while the standard k- ε model experiences a higher normalized mean absolute error (nMAE) of velocity once compared with MRV data. Secondly, based on the cost-accuracy criteria, the WALE model is used with a fine mesh of ≈39 millions control volumes, the averaged velocity results showed excellent agreement between LES and MRV measurements, revealing the high prediction capability of the suggested LES tool for valve flows. Thirdly, the turbulent flow across the valve curtain clearly featured a back flow resulting in a high speed intake jet in the middle. Comprehensive LES data are generated to carry out statistical analysis in terms of (1) evolution of the turbulent morphology across the valve passage relying on the flow anisotropy map, (2) integral turbulent scales along the intake-charge stream, (3) turbulent flow properties such as turbulent kinetic energy, turbulent velocity and its intensity within the most critical zone in intake-port and along the port length, it further transpires that the most turbulence are generated across the valve passage and these are responsible for the in-cylinder turbulence.

University of Warsaw Lagrangian Cloud Model (UWLCM) 1.0: a modern large-eddy simulation tool for warm cloud modeling with Lagrangian microphysics

Abstract. A new anelastic large-eddy simulation (LES) model with an Eulerian dynamical core and Lagrangian particle-based microphysics is presented. The dynamical core uses the multidimensional positive-definite advection transport algorithm (MPDATA) advection scheme and the generalized conjugate residual pressure solver, whereas the microphysics scheme is based on the super-droplet method. Algorithms for coupling of Lagrangian microphysics with Eulerian dynamics are presented, including spatial and temporal discretizations and a condensation substepping algorithm. The model is free of numerical diffusion in the droplet size spectrum. Activation of droplets is modeled explicitly, making the model less sensitive to local supersaturation maxima than models in which activation is parameterized. Simulations of a drizzling marine stratocumulus give results in agreement with other LES models. It is shown that in the super-droplet method a relatively low number of computational particles is sufficient to obtain correct averaged properties of a cloud, but condensation and collision–coalescence have to be modeled with a time step of the order of 0.1 s. Such short time steps are achieved by substepping, as the model time step is typically around 1 s. Simulations with and without an explicit subgrid-scale turbulence model are compared. Effects of modeling subgrid-scale motion of super-droplets are investigated. The model achieves high computational performance by using graphics processing unit (GPU) accelerators.

Load Estimation of Offshore Wind Turbines

The influence of 3 MW Hywind-II wind turbine wakes from an upstream offshore floating wind turbine on a downstream turbine with a separation distance of seven rotor diameters was studied for a site in the Gulf of Maine. The turbines and the platforms were subjected to atmospheric boundary layer flows. Various sensitivity studies on fatigue loads with respect to the positions of the downstream turbine were performed and validated with a large-eddy simulation tool. In particular, the effect of various lateral positions of the downstream turbine relative to the upstream turbine were considered using time-series turbine wake data generated from the large-eddy simulation tool which served as an input to an aero-elastic wind turbine model to assess the loads. The load response from the rotor, tower, and the floating platform for the downstream turbine were sensitive to the lateral offset positions where turbines that were partially exposed to upstream turbine wakes yielded significant increases in the cyclic load range. For the given set of lateral positions for the downstream turbine, the largest damage equivalent load occurred when the turbine was one rotor diameter to the left of the centerline, when looking upstream, which is the position of the turbine fully exposed to upstream turbine wake. On the other hand, the fatigue load on the downstream turbine placed on the right side of the position fully exposed to the upstream turbine wake, yielded lower stress due to the non-symmetric shape of the turbine wake. The configuration associated with the largest damage equivalent loads was further investigated in a large-eddy simulation, modeling both the upstream and downstream turbines. It was found that the energy spectra at the blade rotational frequency were a magnitude order higher for the downstream turbine, especially for surge, heave, pitch, and yaw motion of the platform. The increase of the damage equivalent load for the flapwise blade root moment was 45% compared to the upstream turbine, which can potentially reduce the turbine service life time.

A minimally-dissipative low-Mach number solver for complex reacting flows in OpenFOAM

Large eddy simulation (LES) has become the de-facto computational tool for modeling complex reacting flows, especially in gas turbine applications. However, readily usable general-purpose LES codes for complex geometries are typically academic or proprietary/commercial in nature. The objective of this work is to develop and disseminate an open source LES tool for low-Mach number turbulent combustion using the OpenFOAM framework. In particular, a collocated-mesh approach suited for unstructured grid formulation is provided. Unlike other fluid dynamics models, LES accuracy is intricately linked to so-called primary and secondary conservation properties of the numerical discretization schemes. This implies that although the solver only evolves equations for mass, momentum, and energy, the implied discrete equation for kinetic energy (square of velocity) should be minimally-dissipative. Here, a specific spatial and temporal discretization is imposed such that this kinetic energy dissipation is minimized. The method is demonstrated using manufactured solutions approach on regular and skewed meshes for a canonical flow and a lab-scale turbulent flow problem.

Large-Eddy Simulation of Supersonic Hydrogen/Oxygen Combustion: Application to a Rocketlike Igniter

The three-dimensional simulation under consideration is related to the supersonic combustion of hydrogen with oxygen. The application under study corresponds to the early phase of ignition in a rocketlike engine. The configuration is a sonic jet of hot burned gases underexpanding into a combustion chamber filled with gaseous hydrogen. Two- and three-dimensional simulations are performed to assess the numerical strategy, define the mesh requirements, and to assist in the choice of the kinetic scheme for hydrogen/oxygen combustion. The large-eddy simulation tool is used with a reduced chemistry (five species and three global steps) on a mesh resolution fine enough to use the laminar model assumption (, 680 million points). The Pope criterion and a subgrid-scale Damköhler number are provided with various definitions for the Damköhler number to validate the retained procedure for three-dimensional simulations. The sequence of ignition is detailed, revealing the flame behavior and shock arrangements. A diffusion flame develops in the supersonic mixing layer formed by the igniter flow and hydrogen injections. The flame structure is recovered with a counterflow flame configuration. A comparison with a nonreacting case shows that the driving physical mechanism is mainly linked to the aerodynamics of the igniter jet.

Direct Numerical Simulation and Large-Eddy Simulation of Supersonic Channel Flow

Compressible isothermal-wall channel flows are studied with direct numerical simulation and large-eddy simulation tools. Computations are carried out using a high-order, low-dissipation, bandwidth-optimized weighted essentially nonoscillatory numerical scheme to describe the hyperbolic terms of the Navier–Stokes equations. Periodic supersonic channel flow direct numerical simulation (, , , and ) is used to validate the procedure and the numerical scheme; a new subgrid term contribution based on pressure drop is proposed for the driving term required in momentum and energy equations for large-eddy simulation. Coherent structures of the flowfield are analyzed with scatter plots, criterion, and vorticity fields. As expected, the strong Reynolds analogy is not valid for this nonadiabatic flow. Streaks and horseshoe-like structures are highlighted and detailed. The authors propose a scenario for the formation of horseshoe-like structures. With large-eddy simulation tools, a dynamic procedure to evaluate the turbulent Prandtl number is required because results are found more accurate and computations more stable. Wall temperature and pressure impact are also emphasized on the normalized van Driest velocity profile in the logarithmic region. The classical log law is recovered: with , and a constant depending on and . An analog law is also recovered for the normalized temperature with the maximum of the Prandtl number . An a priori study on mesh requirements determination for a large range of pressure levels is realized through highly near-wall resolved large-eddy simulation.

Large Eddy Simulation of Pulverized Coal Jet Flame Ignition Using the Direct Quadrature Method of Moments

The goal of this study is to show that large eddy simulation (LES) combined with the direct quadrature method of moments (DQMOM) can accurately predict coal flame ignition mechanisms and more generally be a valuable tool for predicting and understanding coal flame characteristics. A validation study of the ARCHES LES tool is performed. LES is applied to a pulverized coal jet flame ignited by a preheated gas flow. A DQMOM has been developed to track the particle phase in an Eulerian framework. The simulation results are compared qualitatively and quantitatively to experimental observations for different inlet stoichiometric ratios. LESs are able to capture the various combustion regimes observed during flame ignition and to accurately model the flame stand-off distance sensitivity to the stoichiometric ratio. The gas temperature and coal burnout predictions are also examined and show good agreement with experimental data. Overall, this study shows that high-fidelity LESs combined with DQMOM can yield a deeper understanding of complex coal flames and be a valuable tool for the design, retrofit, and scale-up of oxy-coal boilers.

Investigation of Two-Fluid Methods for Large Eddy Simulation of Spray Combustion in Gas Turbines

An extension of the large eddy simulation (LES) technique to two-phase reacting flows, required to capture and predict the behavior of industrial burners, is presented. While most efforts reported in the literature to construct LES solvers for two-phase flow focus on Euler–Lagrange formulation, the present work explores a different solution (‘two-fluid’ approach) where an Eulerian formulation is used for the liquid phase and coupled with the LES solver of the gas phase. The equations used for each phase and the coupling terms are presented before describing validation in two simple cases which gather some of the specificities of real combustion chamber: (1) a one-dimensional laminar JP10/air flame and (2) a non-reacting swirled flow where solid particles disperse (Sommerfeld and Qiu, Int. J. Multiphase Flow 19(6):1093–1127, 1993). After these validations, the LES tool is applied to a realistic aircraft combustion chamber to study both a steady flame regime and an ignition sequence by a spark. Results bring new insights into the physics of these complex flames and demonstrate the capabilities of two-fluid LES.

Foreword to the Special Issue on Computational Heat Transfer

Foreword to the Special Issue on Computational Heat Transfer

A NUMERICAL METHOD OF 3-D FLOW AROUND SUBMERGED SPUR-DIKES

A large-eddy simulation tool is adopted to simulate the three-dimensional flow around the submerged spur-dikes. A numerical method is put forward to compute the three-dimensional flow with free surface and complex boundaries. The numerical method is established on the base of finite-volume/finitedifference method with staggered grid. A method of the equivalent Cartesian grid in a fluid cell, whosevolume is the same with that of the irregular fluid cell, is used to deal with the irregular cells and the eddy viscosity coefficient at the boundary cells in the flow field with complex boundaries. The water surface and the velocity distribution around submerged spur-dikes of simulation are compared with those of experiment. It is shown that the results of simulation are in agreement with those of experiment and it is possible to apply the numerical model to simulate the complex flow with complex boundaries.