Showing papers by "Andreas Bechmann published in 2011"

The Bolund Experiment, Part II: Blind Comparison of Microscale Flow Models

Abstract: Bolund measurements were used for a blind comparison of microscale flow models. Fifty-seven models ranging from numerical to physical were used, including large-eddy simulation (LES) models, Reynolds-averaged Navier–Stokes (RANS) models, and linearized models, in addition to wind-tunnel and water-channel experiments. Many assumptions of linearized models were violated when simulating the flow around Bolund. As expected, these models showed large errors. Expectations were higher for LES models. However, of the submitted LES results, all had difficulties in applying the specified boundary conditions and all had large speed-up errors. In contrast, the physical models both managed to apply undisturbed ‘free wind’ boundary conditions and achieve good speed-up results. The most successful models were RANS with two-equation closures. These models gave the lowest errors with respect to speed-up and turbulent kinetic energy (TKE) prediction.

The Bolund Experiment, Part I: Flow Over a Steep, Three-Dimensional Hill

Abstract: We present an analysis of data from a measurement campaign performed at the Bolund peninsula in Denmark in the winter of 2007–2008. Bolund is a small isolated hill exhibiting a significantly steep escarpment in the main wind direction. The physical shape of Bolund represents, in a scaled-down form, a typical wind turbine site in complex terrain. Because of its small size the effect of atmospheric stratification can be neglected, which makes the Bolund experiment ideal for the validation of neutral flow models and hence model scenarios most relevant to wind energy. We have carefully investigated the upstream conditions. With a 7-km fetch over water, the incoming flow is characterized as flow over flat terrain with a local roughness height based on the surface momentum flux. The nearly perfect upstream conditions are important in forming a meaningful quantitative description of the flow over the Bolund hill. Depending on the wind direction, we find a maximum speed-up of 30% at the hill top accompanied by a maximum 300% enhancement of turbulence intensity. A closer inspection reveals transient behaviour with recirculation zones. From the wind energy context, this implies that the best site for erecting a turbine based on resource constraints unfortunately also imposes a penalty of high dynamic loads. On the lee side of Bolund, recirculation occurs with the turbulence intensity remaining significantly enhanced even at one hill length downstream. Its transient behaviour and many recirculation zones place Bolund in a category in which the linear flow theory is not applicable.

CFD simulations of the MEXICO rotor

Abstract: The wake behind a wind turbine model is investigated using Computational Fluid Dynamics (CFD), and results are compared with measurements. The turbine investigated is the three-bladed test rotor (D = 4.5 m) used in the Model Experiments in Controlled Conditions (MEXICO) wind tunnel experiment. During the MEXICO experiment, particle image velocimetry measurements of the induction upstream and downstream of the rotor were performed for different operating conditions, giving a unique dataset to verify theoretical models and CFD models. The present paper first describes the efforts in reproducing the experimental results using the Reynold-Averaged Navier-Stokes method. Second, three-dimensional airfoil characteristics are extracted that allow simulations with simpler wake models. Copyright © 2011 John Wiley & Sons, Ltd.

3D CFD computations of transitional flows using DES and a correlation based transition model

Abstract: The present article describes the application of the correlation based transition model of Menter et al. in combination with the Detached Eddy Simulation (DES) methodology to two cases with large degree of flow separation typically considered difficult to compute. Firstly, the flow is computed over a circular cylinder from Re = 10 to 1 × 106 reproducing the cylinder drag crisis. The computations show good quantitative and qualitative agreement with the behaviour seen in experiments. This case shows that the methodology performs smoothly from the laminar cases at low Re to the turbulent cases at high Re. Secondly, the flow is computed over a thick airfoil at high angle of attack, in this case the DU-96-W351 is considered. These computations show that a transition model is needed to obtain correct drag predictions at low angle of attack, and that the combination of transition and the DES method improve agreement in the deep stall region. Copyright © 2010 John Wiley & Sons, Ltd.

Hybrid RANS/LES applied to complex terrain

Abstract: Large Eddy Simulation (LES) of the wind in complex terrain is limited by computational cost. The number of computational grid points required to resolve the near-ground turbulent structures (eddies) are very high. The traditional solution to the problem has been to apply a wall function that accounts for the whole near-wall region. Recently, a hybrid method was proposed in which the eddies close to the ground were modelled in a Reynolds-averaged sense (RANS) and the eddies above this region were simulated using LES. The advantage of the approach is the ability to use shallow cells of high aspect ratio in the RANS layer and thereby resolve the mean near-wall velocity profile. The method is applicable to complex terrain and the benefits of traditional LES are kept intact. Using the hybrid method, simulations of the wind over a natural complex terrain near Wellington in New Zealand are demonstrated. 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 method. Additionally, the results from the hybrid method are compared with RANS results. Copyright © 2010 John Wiley & Sons, Ltd.

MEXICO Wind Tunnel and Wind Turbine modelled in CFD

Abstract: The MEXICO Experiment is reproduced in CFD, including the geometry of the wind tunnel and the wind turbine rotor. The wind turbine is modelled both as a full rotor and as an actuator disc. Various questions regarding the wind tunnel effects on the measurements are investigated. As in a previous work carried out without modelling the wind tunnel, the CFD methods are found to give satisfying agreement with the axial velocity deficit in the wake. However, confirming the previous work, the blade loadings estimated from CFD are found to be consistently larger than the one estimated from measurements. In order to investigate further this issue, the loadings estimated from measurement are used with an actuator disc model. This approach gives a too small wake deficit in comparison with the measurements, which tends to agree with the full rotor computation results.

CFD Simulations of the Mexico Wind Tunnel and Wind Turbine

Abstract: The MEXICO Experiment is reproduced in CFD, including the geometry of the wind tunnel and the wind turbine rotor. The wind turbine is modelled both as a full rotor and as an actuator disc. Various questions regarding the wind tunnel effects on the measurements are investigated. As in a previous work carried out without modelling the wind tunnel, the CFD methods are found to give satisfying agreement with the axial velocity deficit in the wake. However, confirming the previous work, the blade loadings estimated from CFD are found to be consistently larger than the one estimated from measurements. In order to investigate further this issue, the loadings estimated from measurement are used with an actuator disc model. This approach gives a too small wake deficit in comparison with the measurements, which tends to agree with the full rotor computation results.