Large Eddy Simulation Of Flow Research Articles

Controversial turbulent Schmidt number value in particle-laden boundary layer flows

When solid particles, such as sediments, are transported by a turbulent boundary layer flow, the wall-normal turbulent diffusion of particle concentration has been observed to be much higher than the turbulent diffusion of momentum (the eddy viscosity). This is characterized by a turbulent Schmidt number lower than unity. By using the few available existing direct measurements of Reynolds-averaged wall-normal particle flux and our two-phase flow large eddy simulations we demonstrate that the actual turbulent Schmidt number is indeed higher than unity. The misinterpretation may be explained by settling retardation of particles in turbulent flows compared with quiescent water.

Assessment of Wind Field Generation Methods on Predicted Wind Turbine Power Production Using a Free Vortex Filament Wake Approach

Abstract The generated power and thrust of a wind turbine strongly depend on the flow field around the turbine. In the present study, three different inflow methods, i.e., a time series (TS) from large eddy simulation (LES) of atmospheric boundary layer flow field, a synthetic turbulent flow field using the Mann model (MM), and a steady-state mean wind profile with shear, are integrated with the free vortex filament wake method to investigate the effect of wind field generation methods on the wind turbine performance where the impact of the turbine and the trailing wake vortices on the turbulent flow fields is ignored. For this purpose, an in-house vortex lattice free wake (VLFW) code is developed and used to predict the aerodynamic loads on rotor blades. The NREL 5-MW reference wind turbine is used for the VLFW simulations. For a fair assessment of different inflow generation methods on power production of a wind turbine, it is not sufficient that the generated wind fields employed in the TS and MM methods have the same streamwise mean velocity and turbulence intensity at hub height. Instead, the generated inflows must have equivalent power-spectral densities especially at low frequencies since the rotor blades essentially respond to the large-scale fluctuations (macroscopic scales) rather than small-scale fluctuations (microscopic scales). A faster energy decay rate of LES inflow leads to a higher energy content in the TS method at low frequencies (associated with the macroscopic dynamics of the rotor blades). This extra kinetic energy results in a slightly higher mean power production while using the TS method although the inflow conditions at hub height/rotor plane are the same for both the TS and MM methods. Moreover, the impact of simulation time (the length of time integration) on the power production of a wind turbine (exposed to an unsteady inflow) must be taken into account. A short simulation time remarkably affects the mean wind speed over the rotor area for identical turbulent inflows. For Taylor’s hypothesis application using a single LES flow field, the results show a significant difference in the mean powers corresponding to the different realizations due to large turbulent fluctuations.

LES of Turbulent Flow through Square Duct with Rounded Corners

In the present study, large eddy simulation (LES) for turbulent flows through square ducts with four rounded corners is performed using immersed boundary (IB) method. The geometrical shape of the duct is determined by an equation |y|α+ |z|α= Rα with a deformation parameter α. The distribution and magnitude of the mean secondary flow with α≧4 is almost close to those of the square-duct case. The center position of secondary circulation flows with α≦4 approaches the square-side bisector in contrast to the square-duct case. The profiles of the Reynolds stress and behaviour of the instantaneous turbulent structures in case of α≧4 are similar with those in square duct case.

Fragmentation and atomization characteristics of near-orifice liquid jet under transverse gas film

In order to understand the fragmentation and atomization characteristics of the liquid jet in transverse gas film, a pintle injection element using air and water as simulants is designed. The two-phase flow large eddy simulation and backlight imaging are used to study the liquid-jet breakup process and spray-field dynamic characteristics in the nearorifice area of pinte injection element under the atmospheric environment. The primary fragmentation process of the liquid jet dominated by surface wave is obtained by large eddy simulation, which reveals the establishment process of the spray field in near-orifice area of the gas-liquid pintle injector. After the subsonic airflow leaves the slit, it expands and accelerates into supersonic state. Then the deceleration and pressurization phenomenon occurs once the supersonic airflow passes through the detached bow shock upstream of the liquid jet. The liquid jet bends downstream due to the difference in pressure between upstream and downstream, and the Rayleigh-Taylor (R-T) unstable surface wave appears on the jet windward surface. As the surface wave develops, the penetration of the wave trough by airflow causes the continuous liquid jet to fragment. Proper orthogonal decomposition (POD) method can effectively reconstruct spray snapshot. The POD mode shows that the low-frequency spray oscillation in near-orifice area is caused by the overall expansion/contraction process of the spray field, while the high-frequency one is due to the “impact wave” movement of the liquid block or liquid mist group on the windward side. The latter is produced by the R-T unstable surface wave before the jet breakup, and can be categorized as traveling wave structure. The dimensionless traveling wave wavelength has a power-law relationship with Weber number.

Numerical simulations and analysis of the turbulent flow field in a practical gas turbine engine combustor

The turbulent flow field in a practical gas turbine combustor is very complex because of the interactions between various flows resulting from components like multiple types of swirlers, dilution holes, and liner effusion cooling holes. Numerical simulations of flows in such complex combustor configurations are challenging. The challenges result from (a) the complexities of the interfaces between multiple three-dimensional shear layers, (b) the need for proper treatment of a large number of tiny effusion holes with multiple angles, and (c) the requirements for fast turnaround times in support of engineering design optimization. Both the Reynolds averaged Navier–Stokes simulation (RANS) and the large eddy simulation (LES) for the practical combustor geometry are considered. An autonomous meshing using the cut-cell Cartesian method and adaptive mesh refinement (AMR) is demonstrated for the first time to simulate the flow in a practical combustor geometry. The numerical studies include a set of computations of flows under a prescribed pressure drop across the passage of interest and another set of computations with all passages open with a specified total flow rate at the plenum inlet and the pressure at the exit. For both sets, the results of the RANS and the LES flow computations agree with each other and with the corresponding measurements. The results from the high-resolution LES simulations are utilized to gain fundamental insights into the complex turbulent flow field by examining the profiles of the velocity, the vorticity, and the turbulent kinetic energy. The dynamics of the turbulent structures are well captured in the results of the LES simulations.

Secondary vortex, laminar separation bubble and vortex shedding in flow past a low aspect ratio circular cylinder

Large eddy simulation of flow past a circular cylinder of low aspect ratio ( $AR=1$ and $3$ ), spanning subcritical, critical and supercritical regimes, is carried out for $2\times 10^3 \le Re \le 4\times 10^5$ . The end walls restrict three-dimensionality of the flow. The critical $Re$ for the onset of the critical regime is significantly lower for small aspect ratio cylinders. The evolution of secondary vortex (SV), laminar separation bubble (LSB) and the related transition of boundary layer with $Re$ is investigated. The plateau in the surface pressure due to LSB is modified by the presence of SV. Proper orthogonal decomposition of surface pressure reveals that although the vortex shedding mode is most dominant throughout the $Re$ regime studied, significant energy of the flow lies in a symmetric mode that corresponds to expansion–contraction of the vortex formation region and is responsible for bursts of weak vortex shedding. A triple decomposition of the time signals comprising of contributions from shear layer vortices, von Kármán vortex shedding and low frequency modulation due to the symmetric mode of flow is proposed. A moving average, with appropriate size of window, is utilized to estimate the component due to vortex shedding. It is used to assess the variation, with $Re$ , of strength of vortex shedding as well as its coherence along the span. Weakening of vortex shedding in the high subcritical and critical regime is followed by its rejuvenation in the supercritical regime. Its spanwise correlation is high in the subcritical regime, decreases in the critical regime and improves again in the supercritical regime.

Large-eddy simulation of flow around an isolated building: A step-by-step analysis of influencing factors on turbulent statistics

We designed and exclusively performed a wind tunnel experiment to obtain high-quality and high-resolution measurement results for three components of mean and fluctuating velocities around a 1: 1: 2 isolated building to establish a benchmark case for large-eddy simulation (LES) as a part of the activities of the Architectural Institute of Japan. Subsequently, we conducted LES to investigate the effect of computational grid arrangement on turbulent statistics by changing three grid arrangements. Finally, we performed a comparison by changing spatial discretization schemes, sub-grid scale (SGS) turbulence models, and convergence criteria. A grid in which the width of a building was discretized into 20 grids yielded sufficient resolution to reproduce the mean flow and second-order turbulent statistics. Under this appropriate grid arrangement, in the condition that the blending ratio of the first-order upwind interpolation scheme with the second-order linear interpolation scheme from 5 to 10%, the appropriate second-order statistics of wind velocity around the isolated building was obtained and numerical oscillation was effectively removed. Then, with this grid, any SGS model yielded appropriate results. In addition, the convergence criteria had little effects on the LES results if stable solution was obtained. Because the convergence criteria greatly affect the total calculation cost, weaker appropriate criteria should be set.

Large Eddy Simulation of the By-pass Transition Process under Different Inlet Turbulence Conditions

The transition process of the boundary layer developing over a flat plate with elevated inlet Free Stream Turbulence Intensity (FSTI) has been studied by means of Large Eddy Simulation (LES). To this purpose, four cases with different inflow disturbances have been tested varying the magnitude and the length scale of turbulence. LES has been performed by using the finite-volume ANSYS Fluent code. The computational domain, which was constituted by a rectangular domain with a zero thickness plate, was based on an ERCOFTAC test case in order to provide a validation with a well-known set of data by comparing the boundary layer integral parameters and mean and fluctuating streamwise velocity profiles.The four cases were discussed within the paper by looking at classical statistical properties as well as advanced post-processing tools. It was shown that the decrease in the free stream turbulence level postpones the transition location, whereas the variation of the integral length scale has a very low influence on the distribution of the time-mean flow properties. Proper Orthogonal Decomposition (POD) has been applied to the instantaneous LES flow fields in order to provide a statistical representation of the structures responsible for transition and their response to free-stream turbulence intensity and length scale. The presence of vortical filaments parallel to the wall, typically referred as boundary layer streaks, is clearly identified; their characteristic dimensions and how they change as a function of FSTI properties were analyzed within the paper.

A divergence-free inflow turbulence generator using spectral representation method for large-eddy simulation of ABL flows

The generation of inflow turbulence is the first necessary step to conduct a successful large-eddy simulation (LES) of atmospheric boundary layer (ABL) flows. It is required that the generated inflow turbulence is divergence-free and satisfies essential target statistical properties, including the two-point statistics of the flow described by the coherency function and the spatial correlation. In this paper, a computationally efficient synthetic inflow turbulence generation technique with explicitly defined two-point flow statistics is proposed based on the spectral representation method. The divergence-free condition is imposed on the generated turbulence through a posteriori procedure embedded in the Pressure-Implicit with Splitting of Operators (PISO) solver of OpenFOAM. The efficacy of the proposed method to represent one-point and two-point statistics is demonstrated by comparing the generated turbulence with wind field measurements taken at the Boundary Layer Wind Tunnel Laboratory of Western University. The proposed method is then applied to LES of ABL flows for three exposure conditions, and the wind profiles downstream of the inlet are examined. The mean velocity, turbulence intensity, integral length scale profiles, and the velocity spectra are in good agreement with the target experimental data, thus, indicating the potential of the method for computational wind engineering applications such as wind load evaluation studies. The implementation of the proposed method is available at https://github.com/GBitsuamlak/DFSR.

Subgrid-scale helicity equation model for large-eddy simulation of turbulent flows

A new one-equation eddy-viscosity model based on subgrid-scale (SGS) helicity is introduced in this paper for large-eddy simulation (LES) of turbulent flows. First, the governing equation of SGS helicity is deduced from the incompressible Navier–Stokes equations, and it reflects the transfer of the small-scale helicity that has been filtered out. We deduce a certain functional relation between the eddy viscosity and SGS helicity based on the kinetic energy and helicity spectra in the homogeneous and isotropic helical turbulence. For improving the accuracy, each unclosed term in the governing equation of SGS helicity is modeled independently, and the coefficients of these unclosed terms are constants or are determined dynamically. The new one-equation eddy-viscosity model is first tested and validated in the simulation of the homogeneous and isotropic helical turbulence. The a priori tests from the direct numerical simulation of forced homogeneous and isotropic turbulence show that the energy and helicity fluxes exhibit scale invariance in the inertial subrange. Additionally, the a posteriori tests demonstrate that the constant-coefficient and dynamic SGS helicity equation models can predict both the energy and helicity spectra more precisely than the common SGS models. For the LES of channel flow, the SGS helicity equation model can accurately predict the mean velocity, the turbulent stress, and the viscous shear stress and supply more abundant flow structures than the compared SGS model under the same grid resolution.

Source term estimation in complex urban environments based on Bayesian inference and unsteady adjoint equations simulated via large eddy simulation

Establishing an accurate source–receptor relationship is essential for identifying unknown sources of air pollution in complex urban environments. Existing source term estimation (STE) methods determine this relationship via steady simulations of adjoint equations with ensembled-averaged flow fields. However, the effect of turbulent diffusion on the dispersion of pollutants is simplified according to the gradient dispersion hypothesis, which may result in considerable STE errors, especially in the complex urban areas. Therefore, we developed a new STE method by embedding unsteady adjoint equations modeled via large eddy simulation (LES) into a Bayesian inference framework. The tremendous storage requirement of the unsteady simulation was mitigated using a wavelet-based compression method. The performance of the proposed method was then evaluated based on dispersion measurements collected in a wind tunnel experiment for a regular, block-arrayed building group model. The estimation results were compared with those derived from an existing method, in which the steady simulation of adjoint equations was employed based on the mean LES flow field. Our results suggest that the LES-based approach significantly improved the modeling accuracy of the adjoint equations and the resulting STEs. If the 50th percentile of the posterior probability was regarded as the point estimate, the absolute errors in source location and strength were reduced by 89% and 99%, respectively, when compared to the existing method.

Validation of large eddy simulation of flow behind a circular cylinder

The flow in the wake behind a circular cylinder in a cross-flow at Reynolds number of 4815 was studied both experimentally and via mathematical modeling. The mathematical model was performed as a Large Eddy Simulation (LES), while the experiments were carried out using the time-resolved variant of the Particle Image Velocimetry (PIV) method. Both the simulation and experiment took into account the dynamical aspects of the studied phenomenon, which enabled a detailed validation of the mathematical model. The overall statistical properties of the simulated flow were validated via comparing the time-averaged measured and computed velocity and vorticity fields. To validate the dynamical behavior, the velocity spectra were examined first. Next, the Proper Orthogonal Decomposition (POD) of the spatio-temporal velocity data was performed on both the experimental and numerical data and a comparison of the obtained energetic modes was carried out. All the performed validations have shown a satisfactory agreement between the simulation and the experiment.

Turbulent flow and heat flux analysis from validated large eddy simulations of flow past a heated cylinder in the near wake region

We conduct non-isothermal large eddy simulations (LESs) of flow past a heated cylinder (Re = 3900) to investigate flow physics throughout the wake region and develop a foundation upon which future heat flux wall models can be built (both for wall-modeled LES and other lower fidelity models) for mathematical closure of the energy equation. A rigorous validation of the mesh is made under isothermal conditions with results showing a closer match to experimental data than any other LES studies to date. The insights gained into the mesh design and approach are discussed. Simulation of non-isothermal flow is performed on the validated mesh for temperature differences between the cylinder surface and the freestream of 25 K and 300 K. The mesh design and realistic (temperature-dependent) thermodynamic property variations play key roles in predicting delayed separation, larger re-circulation zones, and enhanced turbulence intensity for the higher temperature difference case. The effect of both temperature differences on the flow is analyzed, and a new scaling of the flow domain is proposed to gain further insight into non-isothermal flow physics. Key scaling variables, friction temperature and friction velocity, are able to reduce nearly all of the temperature dependence of first and second order flow statistics, including turbulent heat fluxes. This leads to the finding that the turbulent heat flux in the wake region scales with the wall heat flux irrespective of the temperature difference in the flow.

Construction of urban turbulent flow database with wavelet-based compression: A study with large-eddy simulation of flow and dispersion in block-arrayed building group model

Urban Turbulent flow database, which includes accurately simulated velocity field with full spatial distribution and time-dependent dynamics, is beneficial for fast analysis of pollutant dispersion and comprehensive validation. However, the enormous data volume makes storage and sharing difficult. This research investigated the feasibility of constructing a small-sized database by compression. A wavelet-based compression method was selected because of its light calculation burden and independence of time series data. The database of the flow field in urban-like arrayed blocks was obtained by large-eddy simulation, and then the velocity and diffusion coefficient fields were compressed with different error controls to construct databases. The compression ability of the wavelet-based method was evaluated, and it was found that the maximum compression ratio is reached when most information in the wavelet coefficients is discarded. The effects of compression error on both the single snapshot and the time-series results were studied. The applicability of the compressed database was verified by the resimulation of passive scalar dispersion in the flow. According to results, the database with approximately 100-times compression is suitable for such applications as the visualization and dispersion simulation of passive scalars with sufficient accuracy.

Scale-adaptive turbulence modeling for LES over complex terrain

The large-eddy simulation (LES) is an efficient method for the study of atmospheric boundary layer (ABL) flow over complex terrain. However, the robustness of LES is influenced by dynamics methods for the subgrid-scale stress, which remains one of the challenging issues. Here, we present a scale-adaptive LES methodology to study ABL flow over complex terrain. We present a canopy stress method for LES over complex terrain, which provides no-slip and shear-stress boundary conditions using a cost-effective Cartesian mesh. We consider measurements from wind tunnel experiments for the investigation of a scale-adaptive subgrid-scale modeling framework for LES of wind flow over a hilly terrain. A flow simulation is then performed on six cases of ABL flow over an idealized complex terrain. The LES result is also compared against the measurement of wind over Askervein hill. The dynamic and non-dynamic versions of the turbulence kinetic energy-based subgrid model are also considered to illustrate the scale-adaptive nature of subgrid-scale turbulence. It was observed that ignoring scale-adaptivity of subgrid turbulence, LES can under-predict shear-stress, turbulence kinetic energy, and other statistical measures of atmospheric turbulence.

Flow and Noise Predictions of Coaxial Jets

Flow and noise solutions of the two Large-Eddy Simulation (LES) approaches are evaluated for the jet flow conditions corresponding to a benchmark coaxial jet case from the European Union Computation of Coaxial Jet Noise experiment. The jet is heated and issues for a short-cowl axisymmetric nozzle with a central body at a transonic speed. The first LES method is based on the Compact Accurately Boundary-Adjusting High-Resolution Technique (CABARET) scheme, for which implementation features include asynchronous time stepping at an optimal Courant–Friedrichs–Lewy number, a wall model, and a synthetic turbulence inflow boundary condition. The CABARET LES is implemented on Graphics Processing Units. The second LES approach is based on the hybrid Reynolds-average Navier–Stokes (RANS)/implicit LES method that uses a mixture of a high-order Roe and Monotonicity-Preserving reconstruction of the 9th order (MP9 scheme) and a wall distance model of the improved delayed detached-eddy simulation type. The RANS/implicit LES method is run on a Message Passing Interface (MPI) cluster. Two grid generation approaches are considered: the unstructured grid using OpenFOAM utility snappyHexMesh and the conventional structured multiblock body-fitted curvilinear grid. The LES flow solutions are compared with the experiment and also with solutions obtained from the standard axisymmetric RANS method using the turbulence model. For noise predictions, the LES solutions are coupled with the penetrable surface formation of the Ffowcs Williams–Hawkings method. The results of noise predictions are compared with the experiment, and the effect of different LES grids and acoustic integration surfaces is discussed.

Residence times of airborne pollutants in the urban environment

This study investigates the applicability and relevance of the residence time to the characterisation of airborne pollutant removal in the urban environment. Using large-eddy simulations of flow over an idealised building array and a realistic urban area, the statistical and spatial distributions of residence times are analysed for different source locations and wind conditions. For the idealised building array, the mean residence times in the avenue and intersection units depend on the mean flow, turbulent kinetic energy and wind direction. The probability density function (PDF) shows distinct early and late regimes when there are pollutant removal processes associated with different boundaries. For the realistic urban domain, the residence times are significantly shorter and stronger fluctuations are observed in the PDF, especially when the inflow winds are driven by the output from a high-resolution mesoscale model. The residence time is especially useful for applications to assessment and planning, when information about the timescale of pollutant removal is required and the flow is inhomogeneous and unsteady.

Large eddy simulation of flow and mixing characteristics in a T-junction under inflow pulsation

Large eddy simulation is performed to investigate the mixing characteristics between the hot and cold fluids in a T-junction with different amplitudes and periods of inlet velocity. The distributions of temperature field and velocity field as well as the properties of vortex structures are focused in the present study. In addition, the wavelet power spectrum density (WPSD) of both temperature and velocity fluctuations are investigated by wavelet transform. The results show that the instantaneous temperature and velocity in the mixing area vary periodically, while the variation period is the same as that of inlet velocity. A typical phenomenon is observed near the moment of lowest inlet velocity, the backflow occurs near the upstream of the opposite wall of the jet entrance. Moreover, the WPSD results show that both mixing velocity and temperature fluctuations have the identical prominent frequency which is equal to that of inlet velocity. The period of inlet velocity has a large effect on the WPSDs of velocity and temperature fluctuations in the low frequency region, while does the amplitude of inlet velocity in the high frequency region. In addition, the effect of either period or amplitude on velocity fluctuations is not synchronized with that of temperature fluctuations.

A new LES approach to trans-critical mixing and combustion processes in high-pressure liquid-injectant engines

The understanding of flame holding mechanism is very important for combustor design. To elucidate the flame holding mechanism at the injector exit in liquid rocket engines, it is indispensable to know the underlying physics of trans-critical mixing process, because the flame holding location is exactly where trans-critical mixing process, which is still veiled, takes place. In the present paper, a new Large Eddy Simulation (LES) method is developed by gaining deeper physical insights into possible sub-grid-scale (SGS) thermodynamic and fluid dynamic instabilities excitable at thin large-density-gradient-magnitude portions to describe trans-critical processes. The presence of thin large-density-gradient-magnitude portions in trans-critical mixing process is troublesome for numerical stability of supercritical fluid flow LES from various points of view. Our idea is to utilize an SGS model functioning at the crossover between the liquid-like and gas-like supercritical fluids in a hybrid scheme of Lagrangian particle tracking and two-phase flow LES. Reduced dynamic viscosity at high pressure may cause SGS thermodynamic and hydrodynamic instabilities in the trans-critical mixing flow field. They are (1) phase separation due to thermodynamic instability, (2) Landau's hydrodynamic instability occurring at the pseudo-boiling location within a heated injector, and (3) Rayleigh–Taylor instability at the pseudo-boiling location, which is induced by Kelvin–Helmholtz instability vortices. These are analyzed and organized into new SGS models characterizing the trans-critical jet mixing process.

Numerical experiments in separating and reattaching flows

We report high-order implicit large Eddy simulations of flows around flat plates with massive flow separation and reattachment. The aim is to provide evidence of the influence of relevant flow parameters such as the geometry of the leading-edge corner, the presence of a trailing-edge flow separation, and of a flow coupling between the two sides of the plate. The results reveal that flows with right-angled corners develop taller flow recirculations, which promote very-slow instability of the bubble itself. This large-scale unsteadiness is then found to be the basis of negative turbulence production mechanisms that in turn enhance the height of the bubble itself, thus closing a self-sustained cycle. The absence of these phenomena in flows with smooth leading-edge corners is also found to explain their high sensitivity to free-stream turbulence. The observed behaviors may have strong repercussions for theories and closures of separating and reattaching flows and should be carefully taken into account in control strategies used in the applications.