LES Results Research Articles

Modeling of Langmuir Circulation: Triple Decomposition of the Craik–Leibovich Model

Turbulent shear flows on shallow continental shelves (here shallow means that the interaction with the solid, no-slip bottom is important) are of great importance because of their role in vertical mixing as well as on the transport of sediment and bioactive material. The presence of a wavefield in these areas can lead to the appearance of Langmuir circulation which is known to strongly affect the dynamics of a turbulent flow. To investigate those dynamical effects within a RANS-type modeling framework, we apply a triple decompostion to the LES results of Langmuir circulation in order to further isolate the coherent structures from fluctuating velocity field. The results are compared to the classical double-decomposition. In contrast to the double-decomposition framework, the triple-decompostion more effectively educes the coherent structure field and quantifies the need to take into account the energy exchange between the coherent and random fluctuations as well as the overall impact of the coherent structures on the turbulence dynamics.

Study of Stall Development Around an Airfoil by Means of High Fidelity Large Eddy Simulation

Large Eddy Simulation of the bubble bursting process over a NACA-0012 airfoil at \(Re_{c}=10^{5}\) indicates that the flow at a fixed angle of attack below the critical stall value exhibits a short (with respect to Gaster’s criteria, Gaster, Number CP-4 in AGARD, 1966) Laminar Separation Bubble (LSB) at the leading edge of the airfoil. The airfoil is smoothly pitched-up through the static stall angle to reproduce the bursting process of the short LSB that initiates a leading edge stall typical of low Reynolds number airfoil. The temporal evolution of characteristic length scales is monitored during the transient flow. Particular attention is paid to the characteristic time involved during the growth and bursting of the LSB. A recent empirical bursting criterion is used to analyse the LES results.

The GASS/EUCLIPSE model intercomparison of the stratocumulus transition as observed during ASTEX: LES results

The GASS/EUCLIPSE model intercomparison of the stratocumulus transition as observed during ASTEX: LES results

The GASS/EUCLIPSE model intercomparison of the stratocumulus transition as observed during ASTEX: LES results

Large‐eddy simulations of a Lagrangian transition from a vertically well‐mixed stratocumulus‐topped boundary layer to a situation in which shallow cumuli penetrate an overlying layer of thin and broken stratocumulus are compared with aircraft observations collected during the Atlantic Stratocumulus Transition Experiment. Despite the complexity of the case and the long simulation period of 40 h, the six participating state‐of‐the‐art models skillfully and consistently represent the observed gradual deepening of the boundary layer, a negative buoyancy flux at the top of the subcloud layer and the development of a double‐peaked vertical velocity variance profile. The moisture flux from the subcloud to the stratocumulus cloud layer by cumulus convection exhibits a distinct diurnal cycle. During the night the moisture flux at the stratocumulus cloud base exceeds the surface evaporation flux, causing a net drying of the subcloud layer, and vice versa during daytime. The spread in the liquid water path (LWP) among the models is rather large during the first 12 h. From additional sensitivity experiments it is demonstrated that this spread is mainly attributable to differences in the parameterized precipitation rate. The LWP differences are limited through a feedback mechanism in which enhanced drizzle fluxes result in lower entrainment rates and subsequently a reduced drying at cloud top. The spread is furthermore reduced during the day as cloud layers with a greater LWP absorb more solar radiation and hence evaporate more.

Study of active flow control for a simplified vehicle model using the PANS method

Flow control has shown a potential in reducing the drag in vehicle aerodynamics. The present numerical study deals with active flow control for a quasi-2D simplified vehicle model using a synthetic jet (zero net mass flux jet). Recently developed near-wall Partially-Averaged Navier–Stokes (PANS) method, based on the ζ–f RANS turbulence model, is used. The aim is to validate the performance of this new method for the complex flow control problem. Results are compared with previous studies using LES and experiments, including global flow parameters of Strouhal number, drag coefficients and velocity profiles. The PANS method predicts a drag reduction of approximately 15%, which is closer to the experimental data than the previous LES results. The velocity profiles predicted by the PANS method agree well with LES results and experimental data for both natural and controlled cases. The PANS prediction showed that the near-wake region is locked-on due to the synthetic jet, and the shear layer instabilities are thus depressed which resulted in an elongated wake region and reduced drag. It demonstrates that the PANS method is able to predict the flow control problem well and is thus appropriate for flow control studies.

Numerical study of oscillating boundary layer flow over a flat plate using k–kL–ω turbulence model

Oscillating boundary layer flow over an infinite flat plate at rest was simulated using the k–kL–ω turbulence model for a Reynolds number range of 32⩽Reδ⩽10,000 ranging from fully laminar flow to fully turbulent flow. The k–kL–ω model was validated by comparing the predictions with LES results and experimental results for intermittently turbulent and fully turbulent flow regimes. The good agreement obtained between the k–kL–ω model prediction with the experimental and LES results indicate that the k–kL–ω model is able to accurately simulate transient intermittently turbulent flow and as well as accurately predict the onset of turbulence for such oscillatory flows.

Large-Eddy Simulation of Flow around a Horizontal-Axis Wind Turbine

Three-dimensional flow around a horizontal-axis wind turbine has been investigated with LES method coupled with sliding mesh and experimental measurement. The boundary conditions are set as the same as those of the experiment. The images of the pressure distribution, flow rate distribution, turbulent intensity, velocity vector and vortices of the wind turbine are presented to show the three-dimensional flow characteristics around the wind turbine. The relationship between flow and sound is further studied by analyzing the flow parameters pulsation spectrum to get the sound pressure level. LES results are compared with the wind-tunnel measurements collected with PIV, and good agreement is observed. The results serve as a reference for optimum design of the wind turbine with high functional performance and low level noise generation.

F071003 火花点火過程における点火安定性の予測(【F07100】エンジン開発に関わる最新計測・CAE技術,エンジンシステム部門企画,先端技術フォーラム)

In order to estimate spark-ignitability in a fuel-air mixture, turbulent Karlovitz number Ka is used, which is calculated based on the local velocity and equivalence ratio. This estimation is applied some LES or RANS calculation results of a natural-gas jet with impingement on a cavity wall. The local Ka is estimated from flame thickness, laminar burning velocity and characteristics time-scale of turbulence. For appling the LES result, the turbulence time scale is calculated by Smagorinski's model. For appling the RANS result, that is calculated based on k and ε. The result shows that the spark-ignition at the point of Ka<50 cause a stable combustion.

Large eddy simulations of coal gasification in an entrained flow gasifier

In this paper, we investigate multi-phase reacting flow in a coal-fed entrained flow gasifier using large-eddy simulations and Reynolds-averaged Navier Stokes models, respectively. An axial-flow type lab-scale gasifier is investigated. The simulations are performed using a Lagrangian–Eulerian method in which the coal particles are modeled using a Lagrangian approach and the gas phase is solved using an Eulerian approach. We compare the performance of LES and RANS results. The coal particle models include devolatilization, char consumption that uses heterogeneous chemistry and two-way coupling of mass, momentum and energy with the surrounding gas phase. The gas phase combustion is modeled using homogenous chemistry and the effect of turbulence on combustion and gasification is modeled using a partially stirred reactor approximation. Results show that LES captures the unsteady flow structures inside the gasifier. We show that modeling the unsteady mixing is critical to the accurate predictions of the gas phase species and carbon conversion in these gasifiers.

A 3D pseudospectral method for cylindrical coordinates. Application to the simulations of rotating cavity flows

The present work proposes a collocation spectral method for solving the three-dimensional Navier–Stokes equations using cylindrical coordinates. The whole diameter -R⩽r⩽R is discretized with an even number of radial Gauss–Lobatto collocation points and an angular shift is introduced in the Fourier transform that avoid pole and parity conditions usually required. The method keeps the spectral convergence that reduces the number of grid points with respect to lower-order numerical methods. The grid-points distribution densifies the mesh only near the boundaries that makes the algorithm well-suited to simulate rotating cavity flows where thin layers develop along the walls. Comparisons with reliable experimental and numerical results of the literature show good quantitative agreements for flows driven by rotating discs in tall cylinders and thin inter-disc cavities. Associated to a spectral vanishing viscosity [E. Séverac, E. Serre, A spectral vanishing viscosity for the LES of turbulent flows within rotating cavities, J. Comp. Phys. 226 (2007) 1234–1255], the method provides very promising LES results of turbulent cavity flows.

LES of turbulent jet in cross flow: Part 2 – POD analysis and identification of coherent structures

The paper presents results of a Proper Orthogonal Decomposition (POD) investigation of the LES based numerical simulation of the jet-in-crossflow (JICF) flowfield with Reynolds number based on the cross flow velocity and jet diameter Re=2400 and the velocity ratio of R=3.3. LES results are validated against pointwise time averaged Laser Doppler Anemometry (LDA) measurements in PART1 of this study. In PART2 of the presented study – a planar (2D) LES based snapshot POD analysis is first conducted on two mutually perpendicular planes located in the jet-to-crossflow entrance zone. The obtained results are directly compared and found to be in close agreement with results of a Particle Image Velocimetry (PIV) based planar (2D) snapshot POD analysis by Meyer et al. (JFM 583, p. 199–227, 2007), indicating that LES is able to predict the same large scale flow dynamics as that captured by PIV. Some differences are also observed, but appear to be directly connected to the differences in levels of the resolved turbulent kinetic energy (TKE) between LES and PIV datasets. Those differences proved to be linked to the process of filtering out the small-scale fluctuations implicit to the PIV measurement technique. Comparisons of TKE captured by the first POD modes showed that they are not affected by this implicit filtering.The LES based POD analysis was also conducted in 3D. The 3D POD analysis results based on the first two POD modes show a full ability to directly visualize details of the relationship between the counter-rotating vortex pair (CVP), the hanging vortex and the wake vortices. POD reconstruction shows that the CVP originates from the hanging vortex formed at the lateral sides of the jet. It also shows that the shedding process involving oscillation of the jet core is responsible for the creation of wake vortices and that the wake vortex originates from the hanging vortex, but grows quickly by “sucking up” the wall boundary layer fluid and vorticity.

LES of turbulent jet in cross-flow: Part 1 – A numerical validation study

The paper presents results of a LES based numerical simulation of the turbulent jet-in-cross-flow (JICF) flowfield, with Reynolds number based on cross-flow velocity and jet diameter Re=2400 and jet-to-cross-flow velocity ratio of R=3.3. The JICF flow case has been investigated in great detail, involving conduction of two independent precursor simulations, prior to the main JICF simulation, as the considered case has turbulent inflow conditions on both jet and cross-stream side. The LES results are directly compared to pointwise Laser Doppler Anemometry (LDA) measurements, showing a very good agreement on the level of various statistical quantities in all flow regions but the immediate jet-to-cross-flow exhaustion zone. Several LES computations involving grids of up to 15million grid points have been conducted, showing no improvement in the agreement between numerical results and measurements, possibly indicating a LDA measurement problem in this particular region.

CFD Modeling of Pollution Dispersion in Building Array: Evaluation of turbulent scalar flux modeling in RANS model using LES results

Modeling of turbulent scalar flux in CFD (Computational Fluid Dynamics) for near-field dispersion around buildings is examined by investigating both velocity and concentration fields obtained by two modeling approaches, i.e., RANS RNG k-ε and LES. A building array model with a point source located in between the central buildings is adopted as a target configuration. First, the prediction accuracy of LES is confirmed by comparing with the RNGcomputation and the results from an experiment conducted by the authors. LES gives better results than RNG, in terms of time-averaged velocity and concentration distribution in comparison with the wind tunnel experimental results. Next, the eddy viscosity and the eddy diffusivity are determined by LES data using a least square approach as suggested in the dynamic sub-grid scale model. Large differences can be observed between the distributions of the estimated eddy viscosity by using LES data and the eddy diffusivity obtained by RNG, since the eddy diffusivity is not always proportional to the eddy viscosity.

Large Eddy Simulation of Turbulent Flow in a T-Junction

A large eddy simulation at the conditions of experiments by Vattenfall [1] was performed in order to investigate the phenomenon of turbulent mixing affecting the thermal fatigue in a T-junction. In the present LES based on the dynamic Vreman model, mean velocity, turbulence intensity, and Reynolds shear stress profiles show good agreement with those measured in the Vattenfall experiment. With the present LES results verified, the characteristics of vortical structures in terms of turbulent mixing resulting in thermal fatigue was investigated. The energy spectra are shown to have a dominant frequency of around St = 0.5 agreeing with the value reported in experiments. On the other hand, the dominant frequency near the wall is different from that reported at the centerline (St ∼ 1), meaning that the thermal fatigue may not result from the wake structure, and is affected by the vortical structures existing near the wall. This result is consistent with one stated by Muramatsu [2] that the frequency characteristics near the wall are closely associated with arched vortices. In addition, the present LES results are used to provide useful insights in predicting this kind of flow using RANS computations.

LES of flow in the street canyon

Results of computer simulation of flow over a series of street canyons are presented in this paper. The setup is adapted from an experimental study by [4] with two different shapes of buildings. The problem is simulated by an LES model CLMM (Charles University Large Eddy Microscale Model) and results are analysed using proper orthogonal decomposition and spectral analysis. The results in the channel (layout from the experiment) are compared with results with a free top boundary.

Reprint of: Sources of high-speed jet noise: analysis of LES data and modeling

Results from recent LES studies of high-speed jet flows and its near and far-field noise (Mendez et al. AIAA-2010-271) are reviewed with an emphasis on validation of the LES results. Far-field noise is predicted using data collected on a conical FW-H surface. Very good agreement with the measurement reported by Bridges & Wernet (AIAA-2008-2834) is observed. Data analysis of the flow disturbances within the jet and in the near-field is used to better characterize the sound source processes. We analyze LES data of Bodony and Lele (Phys. Fluids, vol. 46, 2005) using a filter in the wavenumber-frequency domain. The supersonic components of pressure and radial velocity fluctuation show peaks within the jet and show the U8j scaling behavior consistent with the far-field radiated noise levels. The observed scaling is also consistent with a perturbation expansion of nearly incompressible turbulent fluctuations within the jet flow. Linear global mode analysis of the non-parallel jet mean flow is performed. Features of Mach wave radiation are evident in the global modes for supersonic convection. Large transient growth is found due to convective non-normality. Some implications of this analysis for jet noise modeling are discussed.

Sources of high-speed jet noise: Analysis of LES data and modeling

Results from recent LES studies of high-speed jet flows and its near and far-field noise (Mendez et al. AIAA2010-271) are reviewed with an emphasis on validation of the LES results. Far-field noise is predicted using data collected on a conical FW-H surface. Very good agreement with the measurement reported by Bridges & Wernet (AIAA-2008-2834) is observed. Data analysis of the flow disturbances within the jet and in the near-field is used to better characterize the sound source processes. We analyze LES data of Bodony and Lele (Phys. Fluids, vol. 46, 2005) using a filter in the wavenumber-frequency domain. The supersonic components of pressure and radial velocity fluctuation show peaks within the jet and show the Uj8 scaling behavior consistent with the far-field radiated j noise levels. The observed scaling is also consistent with a perturbation expansion of nearly incompressible turbulent fluctuations within the jet flow. Linear global mode analysis of the non-parallel jet mean flow is performed. Features of Mach wave radiation are evident in the global modes for supersonic convection. Large transient growth is found due to convective non-normality. Some implications of this analysis for jet noise modeling are discussed.

Influence of the Reynolds number and the spherical dimple depth on turbulent heat transfer and hydraulic loss in a narrow channel

The paper presents detailed numerical study of heat transfer enhancement by a spherical dimple placed on a wall in a narrow channel. RANS approach with MSST model is applied to investigate the influence of the depth to diameter ratio Δ and the Reynolds number on the flow and heat transfer. Numerical model was successfully validated using LDV measurements, pressure loss data and LES results. Special attention is paid to identification of the flow topology at different Δ and Reynolds numbers. Contribution of different heat exchanger surface parts to the heat transfer enhancement is analyzed. Application of entropy production as a criterion of heat exchanger efficiency is discussed. Detailed information gained from the present computations can be used to get a deep insight into flow physics over dimpled surfaces and as a benchmark for validation of numerical and experimental methods.

Large Eddy Simulations and Experiments of Flow Around Finite-Height Cylinders

The paper presents experimental and numerical results for the flow around a surface-mounted circular cylinder at the two height-to-diameter ratios of 2.5 and 5. The Reynolds number based on approach flow velocity and cylinder diameter is 43,000 and 22,000 for these two cases and the boundary layer of the approach flow has a thickness of about 10% of the cylinder height. The experiments comprise both flow visualizations with dye and laser Doppler velocimeter measurements of all mean velocity and fluctuation components. The numerical study is performed by an elaborate large eddy simulation on a staggered Cartesian grid using the immersed boundary method. The instantaneous flow behaviour including the shedding is analysed with information based on animations. For the long cylinder alternating shedding is found to occur over most of the height while for the shorter cylinder the shedding is observed mainly near the ground where it is also mostly alternating but intermittently also symmetrical. The mean-flow behaviour is analysed with the aid of streamlines and contour plots of mean-velocity and fluctuation components in various planes and a detailed comparison of LES and LDV results is provided, showing generally good agreement. The LES with very fine resolution near the free end allow a detailed study of the complex flow in this region with owl-face topology on the end wall previously observed in experiments. Behind the cylinder, the longitudinal recirculation region, the downstream development of tip vortices and the emergence of trailing vortices further downstream are analysed. The sum of the results, together with those from previous studies that were reviewed extensively, provides a comprehensive picture of the very complex flow behaviour.

Numerical simulation of flow past a square cylinder using Partially-Averaged Navier–Stokes model

In this study, we carry out a two-stage partially averaged Navier–Stokes (PANS) simulation past a square cylinder to examine the effect of a grid system on the predicted results. The important focus in PANS is how the control parameter which is the ratio of unresolved-to-total turbulent kinetic energy is determined in the flow field. In this work, the parameter is evaluated by using an equation that we propose. The model equation utilizes the turbulent length scale, the Kolmogorov length scale and the cut-off grid length. We first perform RANS simulation to approximately evaluate the turbulent length scale and the Kolmogorov length scale. Then we construct five different grid systems based on these length scales and perform PANS simulation in which the length scales to determine control parameter are also updated as the simulation proceeds. Simulation results are compared with experimental data and the other LES and DES results. The comparison suggests that PANS approach is very effective in simulating separated turbulent flows in the sense that accurate predictions can be obtained by using a much coarser grid system than is required for LES. We then suggest the grid spacing requirements for a proper PANS simulation.