A number of large-eddy simulations (LES) are performed for the calculation of the airflow over a horizontally homogeneous forest canopy for a wide range of thermal stability classes. For the first time, results from LES of a stably stratified canopy are also presented. Simulation results compare favourably to recent field measurements over a pine forest in south-eastern Sweden. The simple heat source model was found to perform adequately and to yield within-canopy heat-flux profiles typically observed for stable conditions in the field. Evidence was found for a layer of unstably stratified air in the canopy trunk space under stable stratification. The importance of a secondary wind-speed maximum is emphasized in stable conditions. Examination of the budget equation of turbulent kinetic energy (TKE) revealed, that during stable stratification, pressure transport plays an increasingly important role in supplying the canopy region with TKE.
Large-eddy simulations (LES) were used to predict the neutral atmospheric boundary layer over a sparse and a dense forest, as well as over grass-covered flat terrain. The forest is explicitly represented in the simulations through momentum sink terms. Turbulence data extracted from the LES served then as inflow turbulence for the simulation of the dynamic structural response of a generic wind turbine. In this way, the impact of forest density, wind speed and wind-turbine hub height on the wind-turbine fatigue loads was studied. Results show for example significantly increased equivalent fatigue loads above the two forests. Moreover, a comparison between LES turbulence and synthetically generated turbulence in terms of load predictions was made and revealed that synthetic turbulence was able to excite the same spectral peaks as LES turbulence but lead to consistently lower equivalent fatigue loads. * The quasi-steady state was assessed manually by following the time history of the velocity at several heights (most importantly at the top of the ABL). As soon as both wind direction and mean wind speed did not change in magnitude anymore, the quasi-steady state is reached. * Note that 40% of the FAST simulations failed for the lowest hub-height wind speed. Therefore, the results for the lowest wind speed are averaged over nine realizations, instead of 15.
Turbulence-resolving simulations have been performed using hybrid RANS-LES approaches for the turbulent flow around a three-element high-lift configuration. The main purpose is to explore the effect of some modeling-related numerical aspects on the simulation of resolved velocity and pressure fluctuations as potent noise-generating sources. Along with a presentation of resolved instantaneous and mean flow features, the impact of the time step and the spanwise extent of the computational domain is investigated. It is shown that the temporal resolution and the spanwise extension of the computational domain impose effects not only on the prediction of mean flow, but more significantly on the correlation of resolved turbulent structures, which may consequently affect the accuracy of flow-generated noise properties.
A hybrid RANS/LES modeling approach is used for simulating the turbulent flow around a three-element airfoil in high-lift configuration. A detailed analysis of the flow is made, based on the simulation outcome. A comprehensive aeroacoustic analysis involving all three elements of the airfoil is also presented. To provide input data for acoustic analogies, the results of the simulation are sampled at a permeable stationary surface near the airfoil and at the airfoil itself. The far-field noise signature of the high-lift airfoil is computed with the help of the Kirchhoff integral surface method, the Ffowcs-Williams and Hawkings method for a stationary, permeable surface, and the Curle method. The sound pressure level spectrum exhibits a broadbanded shape with several narrow-banded tonal peaks at low Strouhal numbers. The broad-banded peak at high Strouhal numbers, which is typically associated with vortex shedding behind the blunt slat trailing edge, was also captured. Using Curle's acoustic analogy, the noise emission pattern of the three elements is explored, isolated from each other, revealing that both slat and flap act as dipoles. By refining the used grid, the flow results are significantly improved in terms of slat shear layer instability and resolved turbulent content as compared to our previous work.
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