Large Eddy Simulation Of Flow Research Articles

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.

The Coriolis force and the direction of rotation of the blades significantly affect the wake of wind turbines

The aerodynamic interactions of wind turbines within a wind farm cause major energy losses. Yaw control is a promising active strategy to tackle this issue in real time during the operation of the plant. The wake of a wind turbine can be steered away from its downstream counterparts by imposing a yaw angle to its rotor. Studying the impact of intentional yaw misalignment on the performance of wind farms located in the northern hemisphere has revealed that, for columns aligned with the wind direction, positive yaw misalignment can lead to an overall increase in the annual energy production, while negative misalignment reduces it. Note that our emphasis is on the front-row turbine being the only yawed turbine of the column. Two unverified reasons have been proposed for the difference between the impact of positive and negative yaw misalignment: (i) the clockwise rotation of the turbine blades and (ii) the Coriolis effect. This paper investigates these two potential explanations by conducting six large-eddy simulations of flow through a wind farm of ten wind turbines located in the northern hemisphere. Results indicate that the Coriolis force and the direction of rotation of the blades both contribute to the inconsistency between the impact of positive and negative yaws on the net power production. The Coriolis force appears to be more influential. The difference between applying a positive and a negative yaw angle to the front-row turbine was found to be approximately 17%. This difference was reduced to almost 7% when the Coriolis force was relaxed, and to approximately 11% when the turbines were set to rotate counter clockwise. Note that a separate study is required to investigate this concept in the southern hemisphere.

A New Explicit Algebraic Wall Model for LES of Turbulent Flows Under Adverse Pressure Gradient

A new explicit algebraic wall law for the Large Eddy Simulation of flows with adverse pressure gradient is proposed. This new wall law, referred as adverse pressure gradient power law (APGPL), is developed starting from the power-law of Werner and Wengle (Turbulent Shear Flows, vol 8, Springer, New York, pp 155–168, 1993) in order to mimic an implicit non-equilibrium log-law based on Afzal’s law (Afzal, IUTAM Symposium on Asymptotic Methods for Turbulent Shear Flows at High Reynolds Numbers, Kluwer Academic Publishers, Bochum, pp 95–118, 1996). No iterative method is needed for the evaluation of the wall shear stress from the APGPL contrary to the majority of models available in the literature. The APGPL model relies on the definition of three modes: the equilibrium power-law is used in regions of no or favourable pressure gradient, the APGPL is used in regions of adverse pressure gradient, and no wall model is used in separated flow regions. This model is assessed via Large Eddy Simulations of flows involving adverse pressure gradient and boundary layer separation using the Lattice Boltzmann Method on uniform nested grids. The flow around a clean and iced NACA23012 airfoil at Reynolds number $$Re = 1.88 \times 10^6$$ and the flow over the LAGOON landing gear at $$Re = 1.59 \times 10^6$$ are considered. Results are found in good agreement with those obtained by the non-equilibrium log-law and experimental and numerical data available in the literature.

Large eddy simulation of flow over square cylinder arrays in an octagonal configuration at subcritical Reynolds numbers

AbstractIn this study, fluid flow over an array of eight, 0.029 m × 0.029 m, square cross‐section cylinders in an octagonal configuration is studied numerically. The mean force coefficients (drag and lift) and the vortex formation characteristics of the array are calculated numerically by utilizing a three‐dimensional large eddy simulation mathematical model for turbulence. The numerical simulation is performed with commercial software ANSYS Fluent 19R1. To investigate the parametric influences, three spacings between the cylinders (0.07, 0.14, and 0.2 m), two array attack angles (0° and 15°), and two Reynolds numbers (4060 and 45 800) are considered. The results comprise flow patterns and force coefficients' variations with Reynolds numbers. The lift force of the downstream cylinder reaches its maximum at α = 15°, and the drag force of the upstream cylinders finds its peak at α = 0°. It is observed through velocity and viscosity contour plots that vortex formation length near the cylinder increases at higher Reynolds number. Velocity vector plots are also presented to show fluid flow behavior near the cylinder. Furthermore, the predicted mean forces on the cylinders are slightly different for different Reynolds numbers, spacings, and angles of attack.

Combining analytical models and LES data to determine the transfer function from swirled premixed flames

A methodology is developed where the acoustic response of a swirl stabilized flame is obtained from a reduced set of simulations. Building upon previous analytical flame transfer functions, a parametrization of the flame response is first proposed, based on six independent physical parameters: a Strouhal number, the mean flame angle with respect to the main flow direction, the vortical structures convection speed, a swirl intensity parameter, a time delay between acoustic and vortical perturbations, as well as a phase shift between bulk and local velocity signals. It is then shown how these parameters can be deduced from steady and unsteady simulations. The methodology is applied to a laboratory scale premixed swirl stabilized flame exhibiting features representative of real aero-engines. In this matter, cold and reactive flow Large Eddy Simulations are first validated by comparing results with reference data from experiments. The high fidelity simulations are seen to be able to capture the flame structure and velocity profiles at different locations while forced flame dynamics for the frequency range of interest also match the experimental data. From the same analytical transfer function model, three methodologies of increasing complexity are presented for the determination of the model parameters, depending on the available data or computational resources. A first estimation of the flame acoustic response is obtained by evaluating parameters from a single stationary flame simulation in conjunction with analytical estimations for the acoustic-convective time delay. Flame dynamics and swirl related parameters can then be determined from a series of robust treatments on pulsed simulations data to improve the model accuracy. It is shown that good qualitative agreement for the flame transfer function can be obtained from a single non-forced simulation while quantitative agreement over the frequency range of interest can be obtained using additional reactive or non-reactive pulsed simulations at one single forcing frequency corresponding to a local gain minimum. The method also naturally handles different perturbation levels.

A-Priori Assessment of Interfacial Sub-grid Scale Closures in the Two-Phase Flow LES Context

Due to the continuous increase in available computing power, the Large Eddy Simulation (LES) of two-phase flows started to receive more attention in recent years. Well-established models from single-phase flows are often used to close the sub-grid scale convective momentum transport and recently some modifications have been suggested to account for the jump of density and viscosity at the interface of multi-phase flows. However, additional unclosed terms in multi-phase flows, which are absent in single-phase flows, often remain ignored. This paper focuses on the crucial gaps in literature, namely the modeling of volume fraction advection and surface tension effects on sub-grid level. An a-priori analysis has been conducted for this purpose, i.e. the Direct Numerical Simulation of an academic two-phase flow configuration (single wobbling bubble in a turbulent background flow) has been explicitly filtered (corresponding to implicit filtering in actual LES) for varying filter width and the corresponding sub-grid terms have been compared to potentially suitable model expressions. Besides other approaches, adequately formulated models based on the scale similarity principle emerged to be promising candidates for both sub-grid volume fraction advection as well as sub-grid surface tension effects. In this context, special attention has to be paid to the secondary filter. Owing to the nature of the quasi-singular surface tension term, surface-weighted filtering may be more appropriate and robust than standard volume filtering.

2D residual-based LES of flow around a pipeline close to a flat seabed

To extend the application of the variational multiscale method to ocean engineering, 2D residual-based large eddy simulations of flows around a circular cylinder close to a flat plane have been considered in the present study. A generalized-α method is applied to solving variational forms of the Navier–Stokes equations. Flows around a spanned marine pipeline at a subcritical Reynolds number 1.31×104 have been computed. The hydrodynamic forces are compared to the published experimental data by a PIV and numerical results by a RANS and a classical LES. Although the magnitude of the predicted hydrodynamic forces are over-predicted compared to the experimental data, they are in good agreement with those obtained by the classical LES. The effect of the gap distance between the cylinder and the seabed on the hydrodynamic forces are analyzed. It is found that the drag coefficients and the root-mean-square of the lift coefficients decrease as the gap distance decreases. The flow characteristics around the spanned pipeline are examined. It is demonstrated that the proximity of the cylinder to the seabed severely affects the wake flow of the cylinder, and that the vortex shedding is suppressed as G∕D<0.3, which is in agreement with the experimental and other numerical results. The pressure distributions along the seabed appear to coincide with the experimental data. It is significant that the present method results in more reliable shear stresses (friction velocities) along the seabed than the RANS, which is very important for the research of the sediment transportation.

An effective low dissipation method for compressible flows

An effective low-dissipation method for simulating compressible flows is proposed in this paper. The method adapts between the high order WENO-type schemes and the TVD reconstruction procedures with an indicator based upon the local fluid properties, such as the discontinuity sensor and the local Mach number. The method is capable of capturing shock waves without oscillations and resolving small scales without introducing excessive numerical dissipation. By choosing an appropriate method for different circumstances, the method is robust, low dissipative and high effective. The detailed construction of the method is presented and the numerical performances are validated with several numerical experiments. Finally, the dissipation's property of the method is investigated and quantified by conducting analyses on the Taylor Green vortex problem and the compressible isotropic turbulence problems. These results show that the proposed scheme is a promising candidate for the DNS (Direct Numerical Simulation) and the LES (Large Eddy Simulation) for compressible flows.

Channel width, bedform length and turbulence: numerical investigation of flow dynamics over laboratory-scale pool–riffle sequences

Spatial dimensions of bedforms relative to the flow depth are of great interest for both engineers and geoscientists, and continue to be an active area of research. These morphological features are of significant consequence for critical hydrodynamic parameters, which in turn has an impact on sediment and solute transport through the river system. In this study, we present results from three-dimensional large eddy simulation of flows over such bedforms in a straight channel. Rigid three-dimensional pool–riffle structures occupying the full span of the channel were considered as the macro-bedforms. The presence of pool–riffle affects the spatial heterogeneity of the flow parameters such as lateral flow concentration. We observed that the lateral flow concentration increases in convective deceleration (CDF) zone with peak values at the central plane in channels with narrower span. The peak values shift from the central plane towards sidewall in channels with larger width. Flow recovery is found to be faster in narrower channels in the conducted simulations. High-magnitude counter-rotating vortices (HCRVs) in CDF and pool transport the momentum of the flow towards the centre of the channel. Larger channel width ensures persistent presence of HCRV in CDF and pool. Longer riffle length in the streamwise direction is observed to further alter the lateral flow concentration compared to the shorter riffles. Instantaneous bed shear stress mapping reveals the presence of coherent high- and low-velocity streaks on riffles. The coherence of these structures is lost in CDF and pool accompanied with an increase in turbulence represented by an increase in turbulent kinetic energy and Reynolds stress.

On the self-similarity of wind turbine wakes in a complex terrain using large eddy simulation

Abstract. We perform large eddy simulation of flow in a complex terrain under neutral atmospheric stratification. We study the self-similar behavior of a turbine wake as a function of varying terrain complexity and perform comparisons with a flat terrain. By plotting normalized velocity deficit profiles in different complex terrain cases, we verify that self-similarity is preserved as we move downstream from the turbine. We find that this preservation is valid for a shorter distance downstream compared to what is observed in a flat terrain. A larger spread of the profiles toward the tails due to varying levels of shear is also observed.

A wavelet‐based variational multiscale method for the LES of incompressible flows in a high‐order DG‐FEM framework

SummaryThis work focuses upon the development of a wavelet‐based variant of the variational multiscale method (VMS) for accurate and efficient large eddy simulation (LES) called wavelet‐based VMS‐LES (WMS‐LES). This approach has been incorporated within the framework of a high‐order incompressible flow solver based upon the pressure‐stabilized discontinuous Galerkin finite element method (DG‐FEM). The VMS approach is designed to produce an a priori scale separation of the governing equations, in a manner which makes no assumptions on either the boundary conditions or the mesh uniformity. Using second‐generation wavelets (SGWs) elementwise for scale separation ensures, on one hand, the preservation of the computational compactness of the DG‐FEM scheme and, on the other hand, the ability to achieve scale separation in wavenumber space. The optimal space‐frequency localization property of the SGW provides an improvement over the commonly used Legendre polynomials. The suitability of the elementwise SGW scale‐separation operation as a tool for error indication has been demonstrated in an h‐adaptive computation of the reentrant corner test case. Finally, the DG‐FEM solver and the WMS‐LES method have been assessed through simulations upon the three‐dimensional Taylor‐Green vortex test case. Our results indicate that the WMS‐LES approach exhibits a distinct improvement over the monolevel LES approach. This effect is not produced by a change in the magnitude of the subgrid dissipation but rather by the redistribution of the subgrid dissipation in wavenumber space.

Large-eddy simulation study of combustion cyclic variation in a lean-burn spark ignition engine

Multi-cycle large-eddy simulation (LES) was performed to investigate combustion cyclic variability (CCV) in a single cylinder spark ignition engine with a homogeneous lean (λ = 1.25) isooctane-air mixture. The aim was to obtain physical insights into the early stage of combustion and its influence on CCV. Propagation of the flame was modeled by a transport equation for the filtered flame surface density within the LES framework. The ignition process was represented by the imposed stretch spark ignition model (ISSIM-LES). Ten consecutive cold flow LES cycles followed by two initialization cycles (12 cycles in total) were used to perform the reactive simulations concurrently. The simulation results were compared with experimental data. Although the number of computed cycles was fairly low, the LES was able to reproduce the cyclic variability observed in experiments both quantitatively and qualitatively. Firstly, validation of the simulation was done by comparing measured pressure traces. Secondly, correlations between the timing of the 10% fuel burnt mass fraction with early flame kernel growth and initial-to-turbulent transition period (in which there was an asymmetric flame kernel that persisted through the early development periods) were determined. Thirdly, calculated results of the flame propagation were analyzed at two cross-sections (in swirl and tumble planes) of the combustion chamber, which highlighted differences in instantaneous flame structures and propagation characteristics between the fastest and slowest cycles. Good overall agreement was obtained between the measurements and simulation data. The results revealed that the instantaneous velocity and fluctuation of flows around the spark vicinity affect growth of the early flame kernel and cause combustion cyclic variability.

Effects of nozzle-exit boundary-layer profile on the initial shear-layer instability, flow field and noise of subsonic jets

The influence of the nozzle-exit boundary-layer profile on high-subsonic jets is investigated by performing compressible large-eddy simulations (LES) for three isothermal jets at a Mach number of 0.9 and a diameter-based Reynolds number of $5\times 10^{4}$, and by conducting linear stability analyses from the mean-flow fields. At the exit section of a pipe nozzle, the jets exhibit boundary layers of momentum thickness of approximately 2.8 % of the nozzle radius and a peak value of turbulence intensity of 6 %. The boundary-layer shape factors, however, vary and are equal to 2.29, 1.96 and 1.71. The LES flow and sound fields differ significantly between the first jet with a laminar mean exit velocity profile and the two others with transitional profiles. They are close to each other in these two cases, suggesting that similar results would also be obtained for a jet with a turbulent profile. For the two jets with non-laminar profiles, the instability waves in the near-nozzle region emerge at higher frequencies, the mixing layers spread more slowly and contain weaker low-frequency velocity fluctuations and the noise levels in the acoustic field are lower by 2–3 dB compared to the laminar case. These trends can be explained by the linear stability analyses. For the laminar boundary-layer profile, the initial shear-layer instability waves are most strongly amplified at a momentum-thickness-based Strouhal number $St_{\unicode[STIX]{x1D703}}=0.018$, which is very similar to the value obtained downstream in the mixing-layer velocity profiles. For the transitional profiles, on the contrary, they predominantly grow at higher Strouhal numbers, around $St_{\unicode[STIX]{x1D703}}=0.026$ and 0.032, respectively. As a consequence, the instability waves rapidly vanish during the boundary-layer/shear-layer transition in the latter cases, but continue to grow over a large distance from the nozzle in the former case, leading to persistent large-scale coherent structures in the mixing layers for the jet with a laminar exit velocity profile.

Large Eddy Simulation of Flow over Wavy Cylinders with Different Twisted Angles at a Subcritical Reynolds Number

The deformation of the cylinder has been proved to greatly reduce the fluctuation of lift and the vortex-induced vibration. In this article, a new form of deformation mode for the smooth cylinder is proposed in order to reduce the vortex-induced vibrations, which can be applied to marine risers and submarine pipelines to ensure the working performance and safety of offshore platforms. Large eddy simulation (LES) is adopted to simulate the turbulent flow over wavy cylinders with three different twisted angles at a subcritical Reynolds number Re = 28,712. Comparing with the results of smooth cylinder, the maximum drag and lift reduction of wavy cylinder A3 with α = 40° can reach 17% and 84%, respectively, and the corresponding vortex formation length increases significantly, while the turbulence intensity decreases relatively. Meanwhile, the circumferential minimum pressure coefficient is greater than that of the smooth cylinder, which also provides a greater drag reduction for the cylinder. The surface separation line, turbulent kinetic energy distribution, and wake vortex structure indicate that the elongation of separated shear layer and wake shedding position is larger than that of the smooth cylinder, and the vorticity value in the near wake region decreases. A periodic vortex structure is generated along the spanwise direction, and a weaker and more stable Karman vortex street is reformed at a further downstream position, which ultimately leads to the reduction of drag and fluctuating lift of the wavy cylinder.