Showing papers by "Andreas Bechmann published in 2007"

Large-eddy simulation of atmospheric flow over complex terrain

Abstract: ii Acknowledgements iii

Identification of severe wind conditions using a Reynolds Averaged Navier-Stokes solver

Abstract: The present paper describes the application of a Navier-Stokes solver to predict the presence of severe flow conditions in complex terrain, capturing conditions that may be critical to the siting of wind turbines in the terrain. First it is documented that the flow solver is capable of predicting the flow in the complex terrain by comparing with measurements from two meteorology masts. Next, it is illustrated how levels of turbulent kinetic energy can be used to easily identify areas with severe flow conditions, relying on a high correlation between high turbulence intensity and severe flow conditions, in the form of high wind shear and directional shear which may seriously lower the lifetime of a wind turbine.

A CFD model of the wake of an offshore wind farm: Using a prescribed wake inflow

Abstract: An CFD model of the wake of an offshore wind farm, expanding existing measurements is proposed. The method is based on solving the Navier Stokes equation in a large domain downstream an offshore wind farm. The inflow of the domain is estimated using existing met mast measurements from both free stream and directly in-wake conditions. A comparison between the simulation results and measurements from a met mast are presented and the shortcomings of the methods are discussed.

Hybrid RANS/LES Method for High Reynolds Numbers, Applied to Atmospheric Flow over Complex Terrain

Abstract: The use of Large-Eddy Simulation (LES) to predict wall-bounded flows has presently been limited to low Reynolds number flows. Since the number of computational grid points required to resolve the near-wall turbulent structures increase rapidly with Reynolds number, LES has been unattainable for flows at high Reynolds numbers. To reduce the computational cost of traditional LES a hybrid method is proposed in which the near-wall eddies are modelled in a Reynolds-averaged sense. Close to walls the flow is treated with the RANS-equations and this layer act as wall model for the outer flow handled by LES. The well-known high Reynolds number two-equation k - turbulence model is used in the RANS layer and the model automatically switches to a two-equation k - subgrid-scale stress model in the LES region. The approach can be used for flow over rough walls. To demonstrate the ability of the proposed hybrid method, simulations of the wind flow over a complex terrain near Wellington in New Zealand are presented. Under certain conditions unsteady flow features have been measured at the site - flow features that could lead to high structural loads on a planned wind farm. These transient flow phenomena are reproduced with the new RANS/LES method. Additionally, the results from the hybrid method are compared with pure RANS results.

Atmospheric Flow over Terrain using Hybrid RANS/LES

Abstract: A hybrid RANS/LES model capable of simulating neutral atmospheric wind over natural terrain is presented. To reduce the computational cost of traditional LES the proposed method combines LES and a RANS wall model. Close to walls, where LES is computational expensive, the flow is treated with the RANS equations and this layer act as wall model for the outer flow handled by LES. The proposed model is based on the well-known two-equation k− ǫ RANS model and can either be run in LES-mode with wall modelling or in pure RANS mode. Calculations with RANS and the new LES model are presented for wind flow over the Askervein hill located in Scotland, and results are compared with measurements. Comparisons show that both RANS and the new LES model are able to capture the simple flow windward of the hill. In the complex wake region, however, only LES captures the high turbulence levels. The presented results are for a relative mild configuration of complex terrain, but the proposed method can also be used for highly complex terrain.