Large-eddy Simulation Cases Research Articles

On the onset of wake meandering for an axial flow turbine in a turbulent open channel flow

AbstractLaboratory experiments have yielded evidence suggestive of large-scale meandering motions in the wake of an axial flow hydrokinetic turbine in a turbulent open channel flow (Chamorro et al., J. Fluid Mech., vol. 716, 2013, pp. 658–670). We carry out a large-eddy simulation (LES) of the experimental flow to investigate the structure of turbulence in the wake of the turbine and elucidate the mechanism that gives rise to wake meandering. All geometrical details of the turbine structure are taken into account in the simulation using the curvilinear immersed boundary LES method with wall modelling (Kang et al., Adv. Water Resour., vol. 34(1), 2011, pp. 98–113). The simulated flow fields are in good agreement with the experimental measurements and confirm the theoretical model of turbine wakes (Joukowski, Tr. Otdel. Fizich. Nauk Obshch. Lyub. Estestv., vol. 16, 1912, no. 1), yielding a near-turbine wake that consists of two layers: the tip vortex (or outer) shear layer that rotates in the same direction as the rotor; and the inner layer counter-rotating hub vortex. Analysis of the calculated instantaneous flow fields reveals that the hub vortex undergoes spiral vortex breakdown and precesses slowly in the direction opposite to the turbine rotation. The precessing vortex core remains coherent for three to four rotor diameters, expands radially outwards, and intercepts the outer shear layer at approximately the location where wake meandering is initiated. The wake meandering manifests itself in terms of an elongated region of increased turbulence kinetic energy and Reynolds shear stress across the top tip wake boundary. The interaction of the outer region of the flow with the precessing hub vortex also causes the rotational component of the wake to decay completely at approximately the location where the wake begins to meander (four rotor diameters downstream of the turbine). To further investigate the importance of turbine geometry on far-wake dynamics, we carry out LES under the same flow conditions but using actuator disk and actuator line parametrizations of the turbine. While both actuator approaches yield a meandering wake, the actuator line model yields results that are in better overall agreement with the measurements. However, comparisons between the actuator line and the turbine-resolving LES reveal significant differences. Namely, in the actuator line LES model: (i) the hub vortex does not develop spiral instability and remains stable and columnar without ever intercepting the outer shear layer; (ii) wake rotation persists for much longer distance downstream than in the turbine-resolving LES; and (iii) the level of turbulence kinetic energy within and the overall size of the far-wake meandering region are considerably smaller (this discrepancy is even more pronounced for the actuator disk LES case) compared with the turbine-resolving LES. Our results identify for the first time the instability mechanism that amplified wake meandering in the experiment of Chamorro et al., show that computational models that do not take into account the geometrical details of the turbine cannot capture such phenomena, and point to the potential significance of the near-hub rotor design as a means for suppressing the instability of the hub vortex and diminishing the extent and intensity of the far-wake meandering region.

LES case study on pedestrian level ventilation in two neighbourhoods in Hong Kong

Hong Kong is one of the most densely built-up and populated cities in the world. An adequate air ventilation at pedestrian level would ease the thermal stress in its humid subtropical climate, but the high-density city severely reduces the natural ventilation. This case study investigates pedestrian level ventilation in two neighbourhoods in Kowloon, downtown Hong Kong using the parallelized large-eddy-simulation (LES) model PALM. The LES technique is chosen here for a city quarter scale pedestrian comfort study despite of its high computational cost. The aims of the paper are a) to get a comprehensive overview of pedestrian level ventilation and a better understanding of the ventilation processes in downtown Hong Kong, b) to test the LES technique on this urban scale compared to the wind tunnel and c) to investigate how numerical/physical parameters influence ventilation. This case study is restricted to neutral stratification in order to allow a direct comparison with the wind tunnel. A sensitivity study quantifies the dependence of site-averaged ventilation on numerical and physical parameters and determines an appropriate urban LES set-up for two 1 km2 neighbourhoods in Kowloon (Tsim Sha Tsui, Mong Kok) that are investigated for prevailing E and SW wind. The results reveal the critical dependence of ventilation on the urban morphology. Air paths, street orientations, ground coverage, sites fronting the water, inter connectivity of spaces, building podium size and building heights can all affect the pedestrian wind environment. Isolated tall buildings may have a pronounced impact on ventilation both locally and downstream.

Effects of cloud horizontal inhomogeneity and drizzle on remote sensing of cloud droplet effective radius: Case studies based on large‐eddy simulations

This study investigates effects of drizzle and cloud horizontal inhomogeneity on cloud effective radius (re) retrievals from the Moderate Resolution Imaging Spectroradiometer (MODIS). In order to identify the relative importance of various factors, we developed a MODIS cloud property retrieval simulator based on the combination of large‐eddy simulations (LES) and radiative transfer computations. The case studies based on synthetic LES cloud fields indicate that at high spatial resolution (∼100 m) 3‐D radiative transfer effects, such as illumination and shadowing, can induce significant differences between retrievals ofre based on reflectance at 2.1 μm (re,2.1) and 3.7 μm (re,3.7). It is also found that 3‐D effects tend to have stronger impact onre,2.1 than re,3.7, leading to positive difference between the two (Δre,3.7−2.1) from illumination and negative Δre,3.7−2.1from shadowing. The cancellation of opposing 3‐D effects leads to overall reasonable agreement betweenre,2.1 and re,3.7 at high spatial resolution as far as domain averages are concerned. At resolutions similar to MODIS, however, re,2.1 is systematically larger than re,3.7when averaged over the LES domain, with the difference exhibiting a threshold‐like dependence on bothre,2.1and an index of the sub‐pixel variability in reflectance (Hσ), consistent with MODIS observations. In the LES cases studied, drizzle does not strongly impact reretrievals at either wavelength. It is also found that opposing 3‐D radiative transfer effects partly cancel each other when cloud reflectance is aggregated from high spatial resolution to MODIS resolution, resulting in a weaker net impact of 3‐D radiative effects onre retrievals. The large difference at MODIS resolution between re,3.7 and re,2.1 for highly inhomogeneous pixels with Hσ> 0.4 can be largely attributed to what we refer to as the “plane‐parallelrebias,” which is attributable to the impact of sub‐pixel level horizontal variability of cloud optical thickness onre retrievals and is greater for re,2.1 than re,3.7. These results suggest that there are substantial uncertainties attributable to 3‐D radiative effects and plane‐parallelre bias in the MODIS re,2.1retrievals for pixels with strong sub‐pixel scale variability, and theHσ index can be used to identify these uncertainties.

Simulating Flame Lift-Off Characteristics of Diesel and Biodiesel Fuels Using Detailed Chemical-Kinetic Mechanisms and Large Eddy Simulation Turbulence Model

Combustion in direct-injection diesel engines occurs in a lifted, turbulent diffusion flame mode. Numerous studies indicate that the combustion and emissions in such engines are strongly influenced by the lifted flame characteristics, which are in turn determined by fuel and air mixing in the upstream region of the lifted flame, and consequently by the liquid breakup and spray development processes. From a numerical standpoint, these spray combustion processes depend heavily on the choice of underlying spray, combustion, and turbulence models. The present numerical study investigates the influence of different chemical kinetic mechanisms for diesel and biodiesel fuels, as well as Reynolds-averaged Navier–Stokes (RANS) and large eddy simulation (LES) turbulence models on predicting flame lift-off lengths (LOLs) and ignition delays. Specifically, two chemical kinetic mechanisms for n-heptane (NHPT) and three for biodiesel surrogates are investigated. In addition, the renormalization group (RNG) k-ε (RANS) model is compared to the Smagorinsky based LES turbulence model. Using adaptive grid resolution, minimum grid sizes of 250 μm and 125 μm were obtained for the RANS and LES cases, respectively. Validations of these models were performed against experimental data from Sandia National Laboratories in a constant volume combustion chamber. Ignition delay and flame lift-off validations were performed at different ambient temperature conditions. The LES model predicts lower ignition delays and qualitatively better flame structures compared to the RNG k-ε model. The use of realistic chemistry and a ternary surrogate mixture, which consists of methyl decanoate, methyl nine-decenoate, and NHPT, results in better predicted LOLs and ignition delays. For diesel fuel though, only marginal improvements are observed by using larger size mechanisms. However, these improved predictions come at a significant increase in computational cost.

Impact of Surface Flux Formulations and Geostrophic Forcing on Large-Eddy Simulations of Diurnal Atmospheric Boundary Layer Flow

Abstract The impact of surface flux boundary conditions and geostrophic forcing on multiday evolution of flow in the atmospheric boundary layer (ABL) was assessed using large-eddy simulations (LES). The LES investigations included several combinations of surface boundary conditions (temperature and heat flux) and geostrophic forcing (constant, time varying, time and height varying). The setup was based on ABL characteristics observed during a selected period of the Cooperative Atmosphere–Surface Exchange Study—1999 (CASES-99) campaign. The LES cases driven by a constant geostrophic wind achieved the best agreement with the CASES-99 observations specifically in terms of daytime surface fluxes and daytime and nighttime profiles. However, the nighttime fluxes were significantly overestimated. The LES cases with the surface temperature boundary condition and driven by a time- and height-varying geostrophic forcing showed improved agreement with the observed nighttime fluxes, but there was less agreement with other observations (e.g., daytime profiles). In terms of the surface boundary condition, the LES cases driven by either surface temperature or heat fluxes produced similar trends in terms of the daytime profiles and comparisons with data from soundings. However, in reproducing the fluxes and nighttime profiles, the agreement was better with imposed temperature because of its ability to interact dynamically with the air temperature field. Therefore, it is concluded that surface temperature boundary condition is better suited for simulations of temporally evolving ABL flow as in the diurnal evolution of the ABL.

Large-Eddy Simulation of Separation Control for Flow over a Wall-Mounted Hump

This work describes an implicit large-eddy simulation (LES) for active control of flow over a wall-mounted hump. Both steady suction and oscillatory blowing and suction are compared to baseline conditions with no flow control. This simulation models an experiment conducted by NASA which was one of the test cases in their 2004 CFD Validation on Synthetic Jets and Turbulent Separation Control Workshop. A previous LES of the baseline case with the current scheme demonstrated significantly better agreement with the experimental flow physics than RANS in the separated region downstream of the hump. The current work concludes that this also holds for the cases using active control. The LES is accomplished using an implicit parallel flow solver that is based on an approximately-factored time-integration method using fourth-order spatial compact-dierence formulations and a high-order filtering strategy. To properly resolve the flow for this LES, the Reynolds number of 9.36 ◊ 10 5 used in the experiment was reduced to 2.0 ◊ 10 5 . Eects of lowering the Reynolds number were previously investigated using RANS. Flow features of the active control cases are compared with the baseline case, each other, and experimental data. The active control LES cases matches experimental data significantly better in the recirculation region than RANS. The baseline and steady suction LES separation reattachment lengths are within 2% and 4%, respectively, of the experimental locations. These simulations achieve very good agreement with the experimental surface pressure coecient, skin friction coefficient, mean velocity profiles, Reynolds stresses, and flow reattachment location. Because of the lower Reynolds number, the oscillatory blowing and suction flow control only has about 25% of the eectiveness of that observed in the experiment. The LES separation reattachment length is 10% longer than its experimental counterpart. Other flow quantities display favorable agreement with experimental data. Results demonstrate that both steady suction and oscillatory blowing and suction can be properly simulated by LES and eectively reduce the size of the separated flow region.

Assessment measures for URANS/DES/LES: an overview with applications

A survey of quality assessment methods is presented covering a range of possible errors in applications of CFD. Beside modeling and numerical errors further uncertainties in unsteady flow simulations are related to inlet conditions and statistical averaging. The main focus is on measures that can be applied to unsteady Reynolds averaged Navier–Stokes (URANS) solvers, hybrid large eddy simulations (HLES), detached eddy simulations (DES), and mostly LES applications. The quantification of uncertainty in LES (large eddy simulations) is difficult for two reasons: (i) substantial grid refinement is too expensive and time consuming; (ii) the modeling errors and discretization errors are convoluted in such a nonlinear manner that it is difficult to segregate one from the other. In hybrid URANS/LES this task is even more difficult because the transitional regions cannot be identified a priori. A brief review of attempts to quantify the accuracy of LES focusing on Richardson extrapolation (RE) is presented. Selected measures are applied to URANS, DES and LES cases from the literature. An attempt is made to separate the modeling contribution from the numerical errors in LES. The methods proposed for the quantification of LES and DES accuracy are still in their infancy stage. Early results are however, promising.

Shallow cumulus convection: A validation of large‐eddy simulation against aircraft and Landsat observations

AbstractLarge‐eddy simulation (LES) results of shallow cumulus convection are directly evaluated against in‐cloud aircraft measurements, as made during the Small Cumulus Microphysics Study (SCMS). To this purpose an LES case is first constructed, based on available observations (during SCMS). Then the simulations are directly compared with the in‐cloud measurements by using conditionally sampled fields. An advantage of the SCMS dataset is the combination of a range of different surface measurements, in‐cloud measurements by an aircraft at many levels in the cloud layer, and the availability of high‐resolution Landsat images.The results show that given the correct initial and boundary conditions the LES concept is capable of realistically predicting the bulk thermodynamic properties of temperature, moisture and liquid‐water content of the cumulus‐cloud ensemble as observed in SCMS. Furthermore, the vertical component of the in‐cloud turbulent kinetic energy and the cloud size distribution in LES were in agreement with the observations. These results support the credibility of cloud statistics as produced by LES in general, and encourage its use as a tool for testing hypotheses and developing parametrizations of shallow cumulus cloud processes.Several hypotheses which make use of conditionally sampled fields were tested on the SCMS data. The magnitudes and the decrease with height of the bulk entrainment rate following from the SCMS data confirm the typical values of the order of magnitude of 10−3 m−1 as reported in several other recent LES studies. An alternative formulation of the lateral entrainment rate as a function of the liquid‐water content and the mean lapse rate agrees well with the original form based on the conserved variables. Applying an often‐used simplified equation for the cloud vertical velocity to the aircraft measurements results in a reasonably closed budget. Copyright © 2003 Royal Meteorological Society

Vertical Mixing by Langmuir Circulations

Wind and surface wave frequently induce Langmuir circulations (LC) in the upper ocean, and the LC contribute to mixing materials down from the surface. In this paper we analyze large-eddy simulation (LES) cases based on surface-wave-averaged, dynamical equations and show that the effect of the LC is a great increase in the vertical mixing efficiency for both material properties and momentum. We provide new confirmation that the previously proposed K-profile parameterization (KPP) model accurately characterizes the turbulent transport in a weakly convective, wind-driven boundary layer with stable interior stratification. We also propose a modest generalization of KPP for the regime of weakly convective Langmuir turbulence. This makes the KPP turbulent flux profiles match those in the LES case with LC present fairly well, especially so for material properties being transported downwards from the ocean surface. However, some open issues remain about how well the present LES and KPP formulations represent Langmuir turbulence, in part because wave-breaking effects are not yet included.

A Comparison of Shear- and Buoyancy-Driven Planetary Boundary Layer Flows

Abstract Planetary boundary layer (PBL) flows are known to exhibit fundamental differences depending on the relative combination of wind shear and buoyancy forces. These differences are not unexpected in that shear instabilities occur locally, while buoyancy force sets up vigorous thermals, which result in nonlocal transport of heat and momentum. At the same time, these two forces can act together to modify the flow field. In this study, four large-eddy simulations (LESs) spanning the shear and buoyancy flow regimes were generated; two correspond to the extreme cases of shear and buoyancy-driven PBLs, while the other two represent intermediate PBLs where both forces are important. The extreme cases are used to highlight and quantify the basic differences between shear and convective PBLs in 1) flow structures, 2) overall statistics, and 3) turbulent kinetic energy (TKE) budget distributions. Results from the two intermediate LES cases are used to develop and verify a velocity scaling and a TKE budget mode...