We present a subgrid-scale (SGS) scalar variance model based on the explicit algebraic subgrid scalar flux model, EASSFM (8). The EASSFM is a dynamic mixed nonlinear tensor eddy diffusivity model, which is derived from the modeled transport equation of the SGS scalar flux. The explicit algebraic form is obtained using the weak equilibrium assumption. The resulting model improves the direction of the predicted SGS flux vector and enables the prediction of shear-induced SGS fluxes, in contrast with the eddy diffusivity model. The EASSFM has been used for large eddy simulation (LES) of turbulent channel flow with and without system rotation (8; 9) and has been found to improve LES predictions over the dynamic eddy diffusivity model. A priori analysis of the EASSFM using the filtered DNS data from a reacting turbulent wall-jet has been performed recently (6; 7), which also showed favorable results. In this study, we evaluate our SGS scalar variance model using the filtered DNS database of a turbulent reacting wall-jet, which is an extension of our previous study on reactive turbulent wall-jet flows (5; 7) to a larger simulation domain. The results show a good agreement between the filtered DNS and our model predictions for the passive and active scalars. This indicates that acceptable predictions of the SGS scalar variance can be obtained using the EASSFM with the new SGS scalar variance model.

Aerodynamic simulations of a small horizontal-axis wind turbine, suitable for integration of wind energy in urban and peri-urban areas, are performed using the improved delayed detached-eddy simulation method. Simulations are carried out for three rotation rates and inlet conditions. Aerodynamic characteristics of the wind turbine such as forces, power production, pressure distribution as well as flow topologies are presented. The effect of different rotation rates as well as the effect of free stream turbulence on the turbine aerodynamics are discussed.

The major conclusion of this paper is that resolution requirements for large-eddy simulation (LES) of flow separation and reattachment can be significantly reduced using the anisotropy-capturing explicit algebraic subgrid-scale (SGS) stress model (EASSM) of Marstorp et al. (J. Fluid Mech., vol. 639, 2009, pp. 403–432), instead of the conventional isotropic dynamic eddy-viscosity model (DEVM). LES of flow separation in a channel with streamwise periodic hill-shaped constrictions and spanwise homogeneity is performed at coarse resolutions for which it is observed that flow separation cannot be predicted without a SGS model and cannot be correctly predicted by the DEVM, while reasonable predictions are obtained with the EASSM. It is shown that the lower resolution requirements by the EASSM, compared to the DEVM, is not only due its nonlinear formulation, but also due to the better formulation of its eddy-viscosity part. The improvements obtained with the EASSM have previously been demonstrated using higher-order numerical solvers for channel flows. In this study, it is observed that these improvements still remain using a low-order code with significant inherent numerical dissipation.

In this paper, aerodynamic and aero-acoustic simulations are performed for a small horizontal axis wind turbine, suitable for the integration of wind energy in urban and peri-urban areas. Detached-eddy simulation (DES) of compressible flow is performed to compute the flow field over the wind turbine. The far-field noise is computed using the Ffowcs - Williams and Hawkings acoustic analogy. Furthermore, the blade element momentum theory is used with a semi-empirical acoustic modeling approach to predict the wind turbine noise. The acoustic modeling approach is based on a semi-empirical formulation for airfoil self noise and an analytic formulation for inflow noise.

We investigate the heat-release effects on the characteristics of the subgrid-scale (SGS) stress tensor and SGS dissipation of kinetic energy and enstrophy. Direct numerical simulation data of a non-premixed reacting turbulent wall-jet flow with and without substantial heat release is employed for the analysis. This study comprises, among others, an analysis of the eigenvalues of the resolved strain rate and SGS stress tensors, to identify the heat-release effects on their topology. An assessment of the alignment between the eigenvectors corresponding to the largest eigenvalues of these two tensors is also given to provide further information for modelling of the SGS stress tensor. To find out the heat-release effects on the dynamics of the turbulent kinetic energy and enstrophy dissipation, probability density functions (PDFs) and mean values are analysed. The mean SGS shear stress and turbulent kinetic energy both slightly increase in the buffer layer and substantially decrease further away from the wall, due to the heat-release effects. Contrary to the kinetic energy, heat release decreases the mean SGS dissipation of enstrophy in the near-wall region. Moreover, differences in the shapes of the PDFs between the isothermal and exothermic cases indicate changes in the intermittency level of both SGS dissipations. Heat release also increases the SGS stress anisotropy in the near-wall region. Although, the structure of the mean resolved strain-rate tensor only marginally differs between the isothermal and exothermic cases in the near-wall region, substantial differences are observed in the jet area, where compressibility effects are important and heat-release effects are found to promote compression states. The differences in the relative alignment between the SGS stress and resolved strain-rate tensors in the isothermal and exothermic cases are discussed in connection with the differences in the SGS dissipation of kinetic energy.

Aerodynamic simulations of a small horizontal-axis wind turbine, suit- able for integration of wind energy in urban and peri-urban areas, are performed. Im- proved delayed detached-eddy simulation is used in the computations. Simulations are carried out for three rotation rates and inlet conditions. Aerodynamic charac- teristics of the wind turbine such as forces, power production, pressure distribution as well as flow topology are presented. The effect of different rotation rates on the turbine aerodynamics is discussed. 

In the FP7 SWIP project three small wind turbine designs have been considered. These include two Horizontal Axis machine with a rated power of 4 and 20 kW and a 2 kW Vertical axis configuration. These will be mounted at three pilot sites with supporting atmospheric modelling and measurements. This paper presents broadband aeroacoustic noise predictions. The prediction methods compare semi empirical predictions more usually used for large turbine applications) supported where appropriate by CFD flow calculations. In order to pursue mitigation measures for these scales of machines, the noise is analysed on a component basis and considered from the viewpoints of self noise and inflow noise. The vertical axis machine is shown to dominated by boundary layer noise whereas the horizontal axis machines are more susceptible to inflow noise. In this the modelling of the inflow turbulence spectrum is seen to be critical.

Large-eddy simulation (LES) of turbulent channel flow are performed with a new subgrid-scale (SGS) stress model. The simulations show that with this model we can well predict turbulent wall flows at coarse resolutions and moderately high Reynolds numbers. The commonly used dynamic Smagorinsky model fails at coarser resolutions. 

This study compares the conventional isotropic dynamic eddy viscosity model(DEVM) and anisotropy-resolving nonlinear explicit algebraic subgrid-scale(SGS) stress model (EASSM) of Marstorp et al. (J. Fluid Mech., vol. 639,2009, pp. 403–432) in large-eddy simulations (LESs) of flow separation in achannel with streamwise periodic hill-shaped constrictions and spanwise homogeneity(periodic hill flow). The results are validated with well-resolved LESdata of Breuer et al (Computers & Fluids, vol. 38, 2009, pp. 433-457). Threedifferent resolutions ranging from moderate to very coarse are used. LESs arecarried out with the Code Saturne, an unstructured collocated finite volumesolver for incompressible flows with a second-order central difference schemein space and a second-order discretisation in time. It has inherent numericaldissipation due to the low-order of the numerical method. LESs with no SGSmodel (NSM) are also carried out to analyse the influence of the SGS modelsin the presence of discretisation errors. LESs with the NSM show that the inherentnumerical dissipation is sufficient to give a reasonable prediction of themean velocity profiles at the finest resolution. The LES predictions of the meanvelocity and Reynolds stresses with the EASSM are found to be much moreaccurate than the ones with the DEVM at all resolutions. Although the SGSdissipation produced by the EASSM is found to be considerably lower than bythe DEVM, the EASSM predictions show appreciable improvements over theNSM, indicating the importance of the nonlinear part of the model. At thecoarsest resolution, where the SGS anisotropy is large, LES with the EASSMshows a reasonable prediction of the mean separation and reattachment points,whereas LES with the isotropic DEVM predicts a considerably delayed separationand early flow reattachment with a small separation bubble and the LESwith NSM does not display flow separation. At finer resolutions, the DEVMand NSM predict a shorter separation bubble than the EASSM, which has agood agreement with the well-resolved reference LES data. Hence, a correctprediction of the separation and reattachment by LES requires resolving the SGS anisotropy either by a fine grid or by an anisotropy-resolving SGS modelsuch as the EASSM.

The explicit algebraic subgrid-scale (SGS) stress model (EASM) of Marstorp et al. ["Explicit algebraic subgrid stress models with application to rotating channel flow," J. Fluid Mech. 639, 403-432 (2009)] and explicit algebraic SGS scalar flux model (EASFM) of Rasam et al. ["An explicit algebraic model for the subgrid-scale passive scalar flux,"J. Fluid Mech. 721, 541-577 (2013)] are extended with stochastic terms based on the Langevin equation formalism for the subgrid-scales by Marstorp et al. ["A stochastic subgrid model with application to turbulent flow and scalar mixing," Phys. Fluids 19, 035107 (2007)]. The EASM and EASFM are nonlinear mixed and tensor eddy-diffusivity models, which improve large eddy simulation (LES) predictions of the mean flow, Reynolds stresses, and scalar fluxes of wall-bounded flows compared to isotropic eddy-viscosity and eddy-diffusivity SGS models, especially at coarse resolutions. The purpose of the stochastic extension of the explicit algebraic SGS models is to further improve the characteristics of the kinetic energy and scalar variance SGS dissipation, which are key quantities that govern the small-scale mixing and dispersion dynamics. LES of turbulent channel flow with passive scalar transport shows that the stochastic terms enhance SGS dissipation statistics such as length scale, variance, and probability density functions and introduce a significant amount of backscatter of energy from the subgrid to the resolved scales without causing numerical stability problems. The improvements in the SGS dissipation predictions in turn enhances the predicted resolved statistics such as the mean scalar, scalar fluxes, Reynolds stresses, and correlation lengths. Moreover, the nonalignment between the SGS stress and resolved strain-rate tensors predicted by the EASM with stochastic extension is in much closer agreement with direct numerical simulation data.