Ferme (agriculture) | EPFL Graph Search

thumb|Ferme de Belgique. Une ferme est, au sens propre, une exploitation agricole exploitée sous le régime d'une location de longue durée avec un loyer annuel fixe, ou fermage. Il s'agit d'un contrat de louage entre le propriétaire et le fermier qui porte sur un domaine comportant selon les cas des terres, des forêts, des étendues d'eau, des bâtiments d'exploitation et d'habitation, parfois avec le cheptel et le matériel d'exploitation, des droits de marque et d'appellation d'origine, des servitudes, etc. Le terme de « ferme » a été utilisé en Angleterre dès le pour désigner des domaines agricole, ainsi que dans les colonies anglaises d'Amérique. Il n'a été adopté en France qu'à partir de la Révolution française pour remplacer les mots manse (mesnil), censive, borie (boriage), etc. qui étaient des concessions perpétuelles abolies en même temps que les institutions féodales pour être remplacées par des contrats de location de gré à gré. Il s'est généralisé pour désigner toute explo

Flow adjustment inside large finite-size wind farms approaching the infinite wind farm regime

Fernando Porté Agel, Ka Ling Wu

Due to the increasing number and the growing size of wind farms, the distance among them continues to decrease. Thus, it is necessary to understand how these large finite-size wind farms and their wakes could interfere the atmospheric boundary layer (ABL) dynamics and adjacent wind farms. Fully-developed flow inside wind farms has been extensively studied through numerical simulations of infinite wind farms. The transportation of momentum and energy is only vertical and the advection of them is neglected in these infinite wind farms. However, less attention has been paid to examine the length of wind farms required to reach such asymptotic regime and the ABL dynamics in the leading and trailing edges of the large finite-size wind farms. Large eddy simulations are performed in this study to investigate the flow adjustment inside large finite-size wind farms in conventionally-neutral boundary layer with the effect of Coriolis force and free- atmosphere stratification from 1 to 5 K/km. For the large finite-size wind farms considered in the present work, when the potential temperature lapse rate is 5 K/km, the wind farms exceed the height of the ABL by two orders of magnitude for the incoming flow inside the farms to approach the fully-developed regime. An entrance fetch of approximately 40 times of the ABL height is also required for such flow adjustment. At the fully-developed flow regime of the large finite-size wind farms, the flow characteristics match those of infinite wind farms even though they have different adjustment length scales. The role of advection at the entrance and exit regions of the large finite- size wind farms is also examined. The interaction between the internal boundary layer developed above the large finite-size wind farms and the ABL under different potential temperature lapse rates are compared. It is shown that the potential temperature lapse rate plays a role in whether the flow inside the large finite-size wind farms adjusts to the fully-developed flow regime. The flow characteristics of the wake of these large finite-size wind farms are reported to forecast the effect of large finite-size wind farms on adjacent wind farms. A power deficit as large as 8% is found at a distance of 10 km downwind from the large finite-size wind farms.

2017

Wake flow in a wind farm during a diurnal cycle

Mahdi Abkar, Fernando Porté Agel, Ahmad Sharifi

The atmospheric boundary layer (ABL) undergoes substantial changes in its structure and dynamics in the course of a day due to the transient nature of forcing factors such as the surface fluxes of heat and momentum. The non-stationary nature of the mean wind and turbulence in the ABL, associated with the diurnal cycle, can in turn affect the structure of wind turbine wakes and their effects on power losses within wind farms. In this research, large-eddy simulation is used to study the evolution of the turbine wakes and their effects on power losses inside an idealised finite-size wind farm in the course of a full diurnal cycle. The simulation results show a strong effect of atmospheric stability on the wind farm wakes and associated power losses. During the day, the positive buoyancy flux and associated turbulence production lead to a relatively high turbulence level in the background ABL flow, which enhances turbulent mixing and wake recovery. In contrast, during the night, the relatively low turbulence intensity of the ambient ABL flow results in a relatively slow rate of entrainment of momentum into the wake and, consequently, a slow wake recovery. As a result, the averaged power deficit in the wind farm is found to increase with increasing thermal stability. In particular, the averaged power deficit increased from 28% under the most convective condition to about 57% under the most stable condition. It is also found that the low-level jet formed before dawn is weakened owing to the energy extraction by the turbines and deflected upward over the farm. In addition, strong vertical wind veer at that time, associated with the Coriolis force and shallow boundary layer, causes important lateral wake stretching effects, which in turn significantly impacts the performance of the waked turbines.

Taylor & Francis2016

Simulation of Turbulent Flow Inside and Above Wind Farms: Model Validation and Layout Effects

Fernando Porté Agel, Yu-Ting Wu

A recently-developed large-eddy simulation framework is validated and used to investigate turbulent flow within and above wind farms under neutral conditions. Two different layouts are considered, consisting of thirty wind turbines occupying the same total area and arranged in aligned and staggered configurations, respectively. The subgrid-scale (SGS) turbulent stress is parametrized using a tuning-free Lagrangian scale-dependent dynamic SGS model. The turbine-induced forces are modelled using two types of actuator-disk models: (a) the 'standard' actuator-disk model (ADM-NR), which calculates only the thrust force based on one-dimensional momentum theory and distributes it uniformly over the rotor area; and (b) the actuator-disk model with rotation (ADM-R), which uses blade-element momentum theory to calculate the lift and drag forces (that produce both thrust and rotation), and distributes them over the rotor disk based on the local blade and flow characteristics. Validation is performed by comparing simulation results with turbulence measurements collected with hot-wire anemometry inside and above an aligned model wind farm placed in a boundary-layer wind tunnel. In general, the ADM-R model yields improved predictions compared with the ADM-NR in the wakes of all the wind turbines, where including turbine-induced flow rotation and accounting for the non-uniformity of the turbine-induced forces in the ADM-R appear to be important. Another advantage of the ADM-R model is that, unlike the ADM-NR, it does not require a priori specification of the thrust coefficient (which varies within a wind farm). Finally, comparison of simulations of flow through both aligned and staggered wind farms shows important effects of farm layout on the flow structure and wind-turbine performance. For the limited-size wind farms considered in this study, the lateral interaction between cumulated wakes is stronger in the staggered case, which results in a farm wake that is more homogeneous in the spanwise direction, thus resembling more an internal boundary layer. Inside the staggered farm, the relatively longer separation between consecutive downwind turbines allows the wakes to recover more, exposing the turbines to higher local wind speeds (leading to higher turbine efficiency) and lower turbulence intensity levels (leading to lower fatigue loads), compared with the aligned farm. Above the wind farms, the area-averaged velocity profile is found to be logarithmic, with an effective wind-farm aerodynamic roughness that is larger for the staggered case.

Springer Netherlands2013