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[en] Highlights: • The effect of vegetation on the evolution of a vertical buoyant jet is investigated using the LES model. • The spatio-temporal evolution of vortex structures of turbulent jet with vegetation is reproduced. • The existence of vegetation significantly increases the jet penetration height and dilution. • Spectral analysis is used to identify the Kelvin–Helmholtz frequency generated by the vegetation. - Abstract: Predicting the flow and dilution of a buoyant jet in vegetated regions is widely applied in ecology and engineering practices. Large eddy simulation is used to study a vertical buoyant jet that was injected into a cross-flow with emergent rigid vegetation. This simulation successfully reproduces the jet behaviour and the spatio-temporal evolution of vortex structures of turbulent jet with vegetation. The time-averaged velocity and temperature field are compared with the experimental results. The similarities and differences between the tests with and without vegetation are also studied. The existence of vegetation diminishes the channel velocity, thereby significantly increasing the jet penetration height and dilution. Spectral analysis is used to investigate the lengths of the vortices corresponding to the dominant frequency at different locations in the flow field.
[en] Matjiesfontein in the Karoo has been proposed as a suitable location for a new fundamental space geodetic observatory. On-site geodetic equipment will include a Lunar Laser Ranger (LLR). LLR requires sub-arcsecond optical seeing conditions for delivery of high quality and quantity data. Seeing conditions at the Matjiesfontein site will be evaluated by making use of an automated seeing monitor and by modelling atmospheric turbulence with Large Eddy Simulation Nansen Center Improved Code (LESNIC).
[en] FAST.Farm is a new midfidelity, multiphysics engineering tool for modeling the power performance and structural loads of wind turbines within a wind farm, including wake and array effects. Previous calibration of the tuneable model parameters of FAST.Farm has shown that its prediction of wake dynamics for a single wind turbine across different atmospheric stability conditions and nacelle-yaw errors matches well with high-fidelity large-eddy simulation at a small fraction of the computational expense. This paper presents a validation of FAST.Farm against large-eddy simulation for a series of cases - independent from those used to support the calibration - considering single-turbine and small wind-farm scenarios, which are both subject to variations in inflow and control. The validation has demonstrated that FAST.Farm reasonably accurately predicts: (1) thrust and power for individual turbines both in isolation and down the row of the small wind farm, (2) wake meandering behavior across different atmospheric conditions, and (3) averaged wake-deficit advection, evolution, and merging effects. As a result, the validation also highlights potential physics that could be improved in FAST.Farm in the future.
[en] We perform a direct numerical simulation (DNS) of forced homogeneous isotropic turbulence with a passive scalar that is forced by mean gradient. The DNS data are used to study the properties of subgrid-scale flux of a passive scalar in the framework of large eddy simulation (LES), such as alignment trends between the flux, resolved, and subgrid-scale flow structures. It is shown that the direction of the flux is strongly coupled with the subgrid-scale stress axes rather than the resolved flow quantities such as strain, vorticity, or scalar gradient. We derive an approximate transport equation for the subgrid-scale flux of a scalar and look at the relative importance of the terms in the transport equation. A particular form of LES tensor-viscosity model for the scalar flux is investigated, which includes the subgrid-scale stress. Effect of different models for the subgrid-scale stress on the model for the subgrid-scale flux is studied.
[en] Highlights: • Evaluation of the spectral entropy as a user-independent criterion to quantify the flow state. • Careful calibration by DNS for different flow states. • Various tests prove the robustness and the generality of the approach. • Retained threshold values are confirmed for two different CFD applications. • Same thresholds could be also applied for hybrid CFD simulations or for experimental studies. - Abstract: In many practical applications, the flow state (laminar, transitional, turbulent) might vary in space and/or in time for a given configuration. The aim of the current study is to show that the spectral entropy Sd, obtained from solving the eigenvalue problem for the temporal autocorrelation function, can be used in order to uniquely quantify the flow state and differentiate between laminar, transitional, or turbulent regimes; as such, it delivers a direct measure of turbulence level. Therefore, this quantity might support hybrid numerical simulations by determining the local flow state, identifying in this way the most suitable computational model and switching, e.g., from RANS to LES. The first test of the suggested approach relies on Direct Numerical Simulations (DNS) for decaying Homogeneous Isotropic Turbulence (HIT) performed for ten different Taylor Reynolds numbers. Results obtained by analyzing DNS indicate that Sd is an excellent candidate to quantify turbulence level and transition. To check the robustness of the corresponding analysis, the impact of different resolutions has been investigated, revealing that a correct state estimate is still obtained with a coarser spatial or temporal resolution. Finally, to check the generality of the approach, the entropy thresholds obtained from the DNS analysis have been used with the same algorithm to analyze 1) DNS results obtained for the Taylor-Green vortex benchmark at Re=1600 as well as 2) results obtained through Large Eddy Simulations in a blood nozzle, revealing in both cases a perfect agreement with a traditional, user-based analysis of the flow conditions. Hence, Sd appears to be an excellent quantitative indicator of laminar, transitional, or turbulent flow, allowing an automatic, user-independent analysis of the flow state for a variety of conditions. In principle, it could be used without modification to analyze experimental measurements as well.
[en] Here, a case study of persistent stratocumulus over the Azores is simulated using two independent large-eddy simulation (LES) models with bin microphysics, and forward-simulated cloud radar Doppler moments and spectra are compared with observations. Neither model is able to reproduce the monotonic increase of downward mean Doppler velocity with increasing reflectivity that is observed under a variety of conditions, but for differing reasons. To a varying degree, both models also exhibit a tendency to produce too many of the largest droplets, leading to excessive skewness in Doppler velocity distributions, especially below cloud base. Excessive skewness appears to be associated with an insufficiently sharp reduction in droplet number concentration at diameters larger than ~200 μm, where a pronounced shoulder is found for in situ observations and a sharp reduction in reflectivity size distribution is associated with relatively narrow observed Doppler spectra. Effectively using LES with bin microphysics to study drizzle formation and evolution in cloud Doppler radar data evidently requires reducing numerical diffusivity in the treatment of the stochastic collection equation; if that is accomplished sufficiently to reproduce typical spectra, progress toward understanding drizzle processes is likely.
[en] The application of the charge motion control valve is an effective and flexible way to reduce cycle-to-cycle variations (CCV) and improve engine performance. The objective of this research is to investigate the influence of the tumble flap (i.e., a specific type of charge motion control valve) placed in the intake port on the in-cylinder aerodynamics and CCV inside a non-reacting, direct injection spark ignition (DISI) engine by means of multi-cycle large-eddy simulation (LES) and proper orthogonal decomposition (POD). Extensive POD analyses involving the phase-dependent POD and phase-invariant POD were carried out. Furthermore, the POD quadrapole decomposition method was employed to gain more insight into the stochastic nature of in-cylinder flow. Results indicate that the tumble flap greatly affects the in-cylinder flow field and the first three subfields, but its effect on turbulent part is insignificant. With the closed tumble flap, more intense tumble motion, improved volumetric efficiency, increased energy portion of the mean part, decreased rate of energy decay, and reduced CCV could be achieved compared with the open tumble flap. It is also found that the phase-invariant POD is a useful tool to assess the effect of the tumble flap on the evolution characteristics. - Highlights: • LES is performed to analyze the flow in a DISI engine equipped with a tumble flap. • The effects of tumble flap on aerodynamics and CCV are assessed by two POD methods. • POD quadruple decomposition is used to quantify each subfield contribution to CCV. • Phase-invariant POD captures the evolution features of the main flow structures. • Closing the tumble flap increases the tumble intensity and reduces CCV
[en] Highlights: • DHRL simulation designed for scalar mixing and compared to IDDES. • Normal and inclined ethylene jet in Mach 2 crossflow with momentum ratio of 0.5. • Turbulent productions preserved for resolved and modeled portions of DHRL. • DHRL more consistent with experimental mean and variance ethylene concentration. • Inclined injection requires transfer of inflow RANS production into the interaction region. - Abstract: The objective of the study is to investigate the performance of a newly developed dynamic hybrid Reynolds-averaged Navier–Stokes/large-eddy simulation (DHRL) framework versus a traditional hybrid approach in the context of the crossflow mixing. An independent scalar-based transition parameter is introduced to partition the species turbulent production within DHRL. The models’ capability in resolving turbulent structures, agreeing with experimental measurements, and transferring turbulent production between modeled and resolved parts of the domain are discussed. Unsteady simulations corresponding to non-reacting ethylene injected in Mach 2 air crossflow are performed. The simulations correspond to both normal and low-angled injection with a momentum ratio of 0.5. Time-averaged fuel concentrations are compared with experimental Raman scattering data and with simulation results using a traditional improved delayed detached eddy simulation (IDDES) model. For the normal injection case, both DHRL and IDDES show reasonably accurate results, although DHRL is more consistent with the measured concentrations. The inclined jet case requires a large modeled contribution to the unsteady region since jet breakup and transition is less pronounced than the normal injection case. DHRL is able to couple Reynolds-averaged Navier–Stokes (RANS) and large-eddy simulation (LES) model terms more effectively and produce concentration fields consistent with the experiment. IDDES fails to predict unsteadiness and produces concentration distributions with large discrepancies from the experiment.
[en] Highlights: • A Detached Eddy Simulation model that mimics dynamic Smagorinsky formulation. • Adaptivity of model allows wall resolved eddy simulation on sufficient grids. • Ability to simulate natural and bypass transition is tested. - Abstract: A modification to the Adaptive-DES method of Yin et al. (2015) is proposed to improve its near-wall behavior. The modification is to the function (C_l_i_m) that imposes a lower limit on the dynamically evaluated coefficient (C_D_E_S). The modification allows Adaptive-DES to converge to wall-resolved eddy simulation, when grid resolution supports it. On coarse grids, or at high Reynolds number, it reverts to shielded DES — that is to DDES. The new formulation predicts results closer to wall-resolved LES than the previous formulation. It provides an ability to simulate transition: it is tested in both orderly and bypass transition. In fully turbulent, attached flow, the modification has little effect. Any improvement in predictions stem from better near-wall behavior of the adaptive method.
[en] Highlights: • The dynamic Vreman model is incorporated under the Lattice Boltzmann framework for the first time. • Results of turbulent channel flow are in good agreement with prior data by the direct numerical simulation. • The role of vertical thermal convection in the channel flow is investigated. - Abstract: In this article, the Vreman model with a dynamic procedure is applied for subgrid scale modeling of turbulent channel flow and heat transport, under the Lattice Boltzmann framework. Numerical simulations of channel flow at with a constant temperature difference at the upper and bottom boundaries are presented and verified via comparisons with existing data of direct numerical simulations and large eddy simulations with the dynamic Smagorinsky model. Additionally, in the same flow and thermal system, the effect of natural convection along the vertical direction is investigated both qualitatively and quantitatively. Results indicate that the bulk velocity is decreased whereas the heat transfer is substantially enhanced. Meanwhile, the vertical thermal convection also alters the profiles of first and second-order statistics, especially in that the temperature fluctuation is flattened in the central region of the channel. Overall, our work offers a useful extension of the current Lattice Boltzmann method in complex situations and will facilitate researches involving turbulent flow and heat transport.