[en] A three-dimensional plasma excitation model based on the phenomenological model was proposed and used as an excitation source of the jet. The large - eddy simulation method was used to simulate the flow field of a rectangular turbulent jet. The position deviation of the large eddy structures in the simulated flow field is within 8% compared with the experimental data. The model can accurately reflect the vortices distribution of the flow field and has a good accuracy. The variation trend of jet vortices is surfaces and vortices field is studied under different excitation voltages, which proves that the stream wise vortex is the key to the influence of the plasma exciter on the jet. The maximum value of velocity and vortices in the flow field enlarge with the voltage increasing while the turbulent position of the jet advances significantly, and the vortex shedding strength increases. The jet mixing characteristics are significantly enhanced. (paper)

[en] In this paper, we review recently developed closure-based and stochastic model approaches to subgrid scale modelling of eddy interactions motivated by advances in non-equilibrium statistical dynamical closure theory. We demonstrate how statistical dynamical closure models can be used to self-consistently calculate eddy damping and stochastic backscatter parameters, required in large eddy simulations (LESs), from higher-resolution simulations. A direct stochastic modelling scheme that is more generally applicable to complex models is then described and applied to LESs of quasigeostrophic turbulence of the atmosphere and oceans. We discuss the fundamental difference between atmospheric and oceanic LESs which is related to the difference in the deformation scales in the two classes of flows. We point out why the stochastic approach may be crucial when baroclinic instability is inadequately resolved. Finally, we discuss the application of inhomogeneous closure theory to the complex problem of flow over topography, and show that it can be used to understand the successes and limitations of currently used heuristic schemes and to provide a basis for further developments in the future. (comment)

[en] Highlights: • A novel data-based method to derive LES subgrid closure terms through neural networks. • A rigorous framework for definition of ideal/optimal LES and associated closure terms. • Successful approximation of the exact LES closure terms based on coarse grid data only. • A data-driven eddy viscosity closure model. -- Abstract: In this work, we present a novel data-based approach to turbulence modeling for Large Eddy Simulation (LES) by artificial neural networks. We define the perfect LES formulation including the discretization operators and derive the associated perfect closure terms. We then generate training data for these terms from direct numerical simulations of decaying homogeneous isotropic turbulence. We design and train artificial neural networks based on local convolution filters to predict the underlying unknown non-linear mapping from the coarse grid quantities to the closure terms without a priori assumptions. We show that selecting both the coarse grid primitive variables as well as the coarse grid LES operator as input features significantly improves training results. All investigated networks are able to generalize from the data and learn approximations with a cross correlation of up to 47% and even 73% for the inner elements, demonstrating that the perfect closure can indeed be learned from the provided coarse grid data. Since the learned closure terms are approximate, a direct application leads to stability issues. We show how to employ the artificial neural network output to construct stable and accurate models. The best results have been achieved with a data-informed, temporally and spatially adaptive eddy viscosity closure. While further investigations into the generalizability of the approach is warranted, this work thus represents a starting point for further research into data-driven, optimal turbulence models.

[en] Turbulent fluid flow due to the electromagnetic forces in induction crucible furnace (ICF) is modeled using k-ϵ, k-ω SST and Large Eddy Simulation (LES) turbulence models. Fluid flow patterns calculated by different turbulence models and their effects on the motion of non-metallic inclusions (NMI) in the bulk melt have been investigated. Results show that the conventional k-ϵ model cannot solve the transient flow in ICF properly. With k-ω model transient flow and oscillation behavior of the flow pattern can be solved, and the motion of NMI can be tracked fairly well. LES model delivers the best modeling result on both details of the transient flow pattern and motion trajectories of NMI without the limitation of NMI size. The drawback of LES model is the long calculation time. Therefore, for general purpose to estimate the dynamic behavior of NMI in ICF both k-ω SST and LES are recommended. For the precise calculation of the motion of NMI smaller than 10 μm only LES model is appropriate. (paper)

[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] Numerical simulations of the flow in a wind farm in north Texas have been performed with WRF (Weather Research and Forecasting model) and our in-house LES code. Five nested domains are solved with WRF to model the meso-scale variability while retaining a resolution of 50 meters in the wind farm region. The computational domain of our in-house LES code is nested into the inner most domain of the WRF simulation from where we get the inlet boundary conditions. The outlet boundary conditions are radiative and at this stage the coupling between the two codes is one-way. The turbines in WRF are mimicked using a modified Fitch approach, while in our in-house LES we have used a rotating actuator disk combined with immersed boundaries for tower and nacelle. Numerical results agree well with meteorological data from the met tower. The power production obtained numerically on each turbine compares well with SCADA data with an index of agreement ranging between 80% to 90%. The power production from the numerical results of our in-house LES code is slightly closer to SCADA data than that of WRF. (paper)

[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] In the framework of jet noise computation, a numerical simulation of a subsonic turbulent hot jet is performed using large-eddy simulation. A geometrical tripping is used in order to trigger the turbulence at the nozzle exit. In a first part, the validity of the simulation is assessed by comparison with experimental measurements. The mean and rms velocity fields show good agreement, so do the azimuthal composition of the near pressure field and the far field spectra. Discrepancies remain close to the nozzle exit which lead to a limited overestimation of the pressure levels in both near and far fields, especially near the 90"∘ angular sector. Two point correlation analyses are then applied to the data obtained from the simulation. These enable to link the downstream acoustic radiation, which is the main direction of radiation, to pressure waves developing in the shear layer and propagating toward the potential core end. The intermittency of the downstream acoustic radiation is evidenced and related to the coherent structures developing in the shear layer

[en] Highlights: • Large eddy simulations were carried out to predict the flow-induced noise generated on the centrifugal fan. • The explicit algebraic subgrid-scale stress model was employed for the large eddy simulations. • The flow-induced noise was predicted by using Lighthill's analogy and the method of Ffowcs Williams and Hawkings. • Experiments were performed using the optimally designed fan in a semi-anechoic chamber. -- Abstract: Large eddy simulations based on the explicit algebraic subgrid-scale stress model were carried out to predict the flow-induced noise generated on the centrifugal fan of a belt-driven starter generator using Lighthill's analogy and the method of Ffowcs Williams and Hawkings. The surrounding air was approximated by an ideal gas at fixed room temperature (T_in = 300 K), and the rotating velocity of the fan was considered to be 6000 rpm. The blade array angles were designed using the modulation method, and a large blade curvature was adopted. We identified several centrifugal fan design parameters that could minimize the flow-induced noise while also minimizing fan efficiency losses. Three design parameters: the top serrated edge (θ_t), the step leading edge (0.52 H_b) and the tail edge (d_b and r_b), played a critical role in preventing vortex generation and collision, significantly weakening the surface pressure fluctuations on the blade. The maximum sound pressure level at 800 Hz at a specific location was reduced by 5.5 dB (at the top serrated edge) and 6.8 dB (at the step leading edge) relative to the baseline case. The sound power, calculated over a hemisphere surface of 950 mm, was reduced by 77.3% (at the top serrated edge) and 61.0% (at the step leading edge) relative to the baseline whereas the mass flow rates were reduced by 5.2% and 10.6%, respectively. Experiments were performed using the optimally designed fan in a semi-anechoic chamber. The predicted sound pressure level and frequency were in good agreement with the experimentally measured values.

No abstract available