[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] Here in this paper, a dynamic closure modeling approach has been derived to stabilize the projection-based reduced order models in the long-term evolution of forced-dissipative dynamical systems. To simplify our derivation without losing generalizability, the proposed reduced order modeling (ROM) framework is first constructed by Galerkin projection of the single-layer quasigeostrophic equation, a standard prototype of large-scale general circulation models, onto a set of dominant proper orthogonal decomposition modes. We then propose an eddy viscosity closure approach to stabilize the resulting surrogate model considering the analogy between large eddy simulation (LES) and truncated modal projection. Our efforts, in particular, include the translation of the dynamic subgrid-scale model into our ROM setting by defining a test truncation similar to the test filtering in LES. Finally, the a posteriori analysis shows that our approach is remarkably accurate, allowing us to integrate simulations over long time intervals at a nominally small computational overhead.

[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] Highlights: • Second and third order accurate local time stepping schemes are proposed. • The schemes are validated on a wide range of academical test cases. • The third order scheme is used to perform a 3D Large-Eddy Simulation. -- Abstract: In this paper, two local time stepping schemes of order two and three in time are proposed. By construction, they are not mass conservative but a correction stage is added to make them conservative. These schemes are compared with some local time stepping schemes existing in the literature (schemes of Constantinescu and Sandu). The comparisons are carried out on various test cases. They prove that our schemes are efficient and our third order local time stepping has a higher time accuracy than the schemes based on the strategy of Constantinescu and Sandu. Our third order local time stepping scheme is used to perform an industrial-like test case: a 3D Large-Eddy Simulation over an airfoil.

[en] The Note presents an alternative derivation and interpretation of the Germano identity and its error, showing that the Germano identity error directly estimates the residual of the LES equation (i.e., the misfit when evaluating the inexact equation for the exact solution) and therefore represents the source of errors in LES. The most prominent applications include a robust and easy to compute definition of the source of errors that can be used for grid/filter adaptation in LES or for estimating the coarse-graining related uncertainties, as well as offering a more robust explanation for the success of the dynamic procedure and the favorable characteristics that the dynamic models inherit.

[en] Efficient higher order interpolation schemes based on a multi-dimensional optimal order detection (MOOD) paradigm are developed coupled with the implicit time discretization scheme and further investigated for implicit large eddy simulation of compressible turbulence. The developed methodology utilizes higher-order either the upwind or the central interpolation to minimize numerical dissipation and meantime hybridizes shock capturing scheme that is also provided with higher-order interpolation to stabilize the solution. Simple and effective technique is proposed to apply the current method to an unsteady dual-time stepping scheme, which is to the best of the authors' knowledge the first time to develop the strategy for the MOOD application within an unsteady implicit time discretization framework. The resulting schemes are implemented in the simplified finite volume method that is constructed on non-uniform, curvilinear, multiblock structured grids. Numerical results for a comprehensive suite of both benchmark and practical problems demonstrate that the designed schemes simultaneously obtain the well-resolved broadband turbulence and the sharp shock profiles with considerable reduction in the computation cost.

[en] Large-eddy simulations (LES) using explicit filtering are performed to obtain grid-independent solutions of turbulent wake flow behind a circular cylinder at Re_D = 3900 on non-Cartesian type grids. A differential elliptic equation where the filter kernel is implicitly defined is discretized in an unstructured-grid solver to enable explicit filtering on non-Cartesian grids. The separation of filtering procedure from discretization is known to produce an LES solution of which error is mainly attributed to the capability of a sub-filter scale (SFS) model. Equipped with the differential filter and the Vreman SFS model, explicitly filtered LES on unstructured grids is shown to produce nearly grid-independent solutions for flow over a circular cylinder at a critical Reynolds number.

[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 S_d, 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 S_d 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, S_d 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] 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.