Results 1 - 10 of 164045
Results 1 - 10 of 164045. Search took: 0.073 seconds
|Sort by: date | relevance|
[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] 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] 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] Highlights: • Unsteady phenomenon of shock vector control. • Large eddy simulation. • Vortexes in the separation bubble. - Abstract: A detail description of the unsteady phenomena of three-dimensional shock vector control (SVC), including recirculation zones and shear layer regions, has been presented in this study. Shock vector control is a really efficient way to achieve flight direction control of high speed vehicle. Large eddy simulation (LES) has been applied to capture the unsteady characteristics of SVC method using bypass flow passage. Comparison of RANS and LES has been conducted in this study. LES model shows better results than others and it is able to capture the unsteady process very well. In this study, the separation bubble upstream of the injection port is the main source of flow unsteadiness. Large scale eddies in the whole flow field have been resolved by the LES model. Unsteady characteristics of SVC method at different nozzle pressure ratios (NPR) have been investigated. The time histories of thrust vector angle at different NPRs have been recorded by the LES model. The results indicate that it is possible to achieve SVC with the range of bypass mass flow ratio less than 7%. It is also revealed that nozzle pressure ratio has a strong effect on the unsteady phenomenon of SVC system.
[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] The very essence of large eddy simulation (LES) is that the LES-solution contains only scales of size ≥ δ, where δ is a user-chosen length scale. Therefore, in case the LES is based on an eddy viscosity model we determine the eddy viscosity such that any scales of size < δ are dynamically insignificant. In this paper, we address the following two questions: how much eddy diffusion is needed to (a) (counter)balance the production of scales of size smaller than δ; and (b) damp any disturbances having a scale of size smaller than δ initially. From this we deduce that the eddy viscosity νe has to depend on the invariants q=1/2tr(S2) and r=-1/3(S3) of the strain rate tensor S. The simplest model is then given by νe=3/2(δ/π)2|r|/q This model is successfully tested for a turbulent channel flow (Reτ = 590).
[en] We simulated a marine stratus deck sampled during the Marine Stratus/Stratocumulus Experiment (MASE) with a three-dimensional large eddy simulation (LES) model at different model resolutions. Various characteristics of the vertical velocity from the model simulations were evaluated against those derived from the corresponding aircraft in situ observations, focusing on standard deviation, skewness, kurtosis, probability density function (PDF), power spectrum, and structure function. Our results show that although the LES model captures reasonably well the lower-order moments (e.g., horizontal averages and standard deviations), it fails to simulate many aspects of the higher-order moments, such as kurtosis, especially near cloud base and cloud top. Further investigations of the PDFs, power spectra, and structure functions reveal that compared to the observations, the model generally underestimates relatively strong variations on small scales. The results also suggest that increasing the model resolutions improves the agreements between the model results and the observations in virtually all of the properties that we examined. Furthermore, the results indicate that a vertical grid size <10 m is necessary for accurately simulating even the standard-deviation profile, posing new challenges to computer resources.