Sandrine Aubrun

Is the Actuator Disc Concept Sufficient to Model the Far-Wake of a Wind Turbine?

In order to study the wind turbine wake and its eventual interactions with neighbouring wind turbines, several numerical and physical modelling approaches are used. Some model the wind turbine with the simplest model, that is the actuator disc concept, adding a drag source (i.e. pressure loss) within the surface swept by the blades (numerical [2], physical [1]). Some use the Blade Element Momentum Theory, which takes into account the blade rotation effect on the wake and the aerodynamic features of the blades [3]. Some use Large Eddy Simulation to compute the unsteady flow around the entire rotor [5]. In a wind resource assessment context, the latter one is not practical enough to be used since the computation times are extremely long.

Influence of the Slant Angle of 3D Bluff Bodies on Longitudinal Vortex Formation

This paper presents the experimental results and analytical arguments concerning simplified geometries of the rear window and windscreen of automotive vehicles in order to contribute to a better understanding of the swirling structure formation and vortex bursting processes. Static pressure distributions and skin friction line visualizations on both sides of the edge where the swirling structure is generated on the rear window of an Ahmed body are presented for different slant angles. Results show the influence of the slant angle on the swirling structure formation and further show that the vortex bursting process can be promoted by small rear window angles. These results are then extrapolated with the help of analytical demonstrations to the windscreen configuration to demonstrate that large windscreen slopes would have the same disintegration effect.

Unsteady analyses of the flow separation on the rea r window of a simplified ground vehicle model.

This paper presents the results of an experimental investigation of the separated flow over the 25 deg. inclined rear slanted surface of an Ahmed body. Experiments performed using hot wire anemometry and PIV concentrate on the symmetry plane of the geometry. Results show an average separated flow as well as a low Strouhal number spectral activity. POD analysis applied on the PIV instantaneous velocity fields show that the lower order mode correspond to a “flapping” behavior of the separation on the rear inclined surface, oscillating between a fully separated and a quasi-fully attached configurations.

Evaluation of RANS/actuator disk modelling of wind turbine wake flow using wind tunnel measurements

Wake modelling plays a central role in wind farm planning through the evaluation of losses, the prediction of the energy yield, and the estimation of turbine loads. These models must be reasonably accurate – to minimise financial risk – and yet economical so that many configurations can be tested within reasonable time. While many such models have been proposed, an especially attractive approach is based on the solution of the Reynolds-averaged Navier-Stokes equations with two-equation turbulence closure and an actuator disk representation of the rotor. The validity of this approach and its inherent limitations however remain to be fully understood. To this end, detailed wind tunnel measurements in the wake of a porous disk (with similar aerodynamic properties as a turbine rotor) immersed in a uniform flow are compared with the predictions of several closures. Agreement with measurements is found to be excellent for all models. This unexpected result seems to derive from a fundamental difference in the tu...

Is the Meandering of a Wind Turbine Wake Due to Atmospheric Length Scales

This study is about the meandering phenomenon of a wind turbine wake. The atmospheric boundary layer is reproduced in a wind tunnel as well as wind turbines using porous discs. Hot wire anemometry is used to carry out time investigations through the study of space-time correlations. Particle Image Velocimetry is also used to observe the spatial development of the wake and especially the horizontal oscillations characterising the meandering.

Comparison of flow modification induced by plasma and fluidic jet actuators dedicated to circulation control around wind turbine airfoils

In order to reduce the aerodynamic load fluctuations on wind turbine blades by innovative control solutions, strategies of active circulation control acting at the blade airfoil trailing edge are studied, allowing lift increase and decrease. This study presents a comparison of results obtained by performing surface plasma and continuous fluidic jet actuation on a blade airfoil designed with a rounded trailing edge. In the present study, both actuator types are located at the trailing edge. Plasma actuators act uniformly in the spanwise direction, whereas fluidic jets blow through small squared holes distributed along the span, and therefore, provide a three-dimensional action on the flow. Load and velocity field measurements were performed to assess the effectiveness of both actuators and to highlight the flow mechanisms induced by both actuation methods for lift-up configurations. Results are presented for a chord Reynolds number of 2. 105 and for a lift coefficient increase of 0.06.

Experimental Characterization of the Unsteady Flow Over the Rear Slant of an Ahmed Body

This study presents results of an experimental analysis of the unsteady features of the flow around the rear part of an Ahmed body with a rear slant angle of 25°. This analysis focuses on the half elliptic separation bubble that developps on the rear slanted surface and brings new information, improving the understanding of the flow unsteadiness. Flow investigations are carried out using hot wire probe measurements for velocity fluctuations in the plane of symmetry above the rear slanted surface and five unsteady flush mounted pressure taps (Kulite transducers) simultaneously acquiring static pressure fluctuations along the middle line of the slanted surface. Spectral analysis and Proper Orthogonal Decomposition of the output signal show the emergence of a low frequency unsteadiness and high frequency activities which, in accordance with bibliography about separated and reattaching flow configurations, is related to a global flapping of the separated shear layer and a large scale vortices shedding. Characteristic frequencies of both instabilities is given and physical effects of the low frequency unsteadiness is related with the flapping motion of the separated shear layer.Copyright

Numerical study of how stable stratification affects turbulence instabilities above a forest cover: application to wind energy

Forest areas are of increasing interest for the wind energy industry. However, they induce complex flows with strong shear and high turbulence levels. Stably stratified atmospheric conditions, typical during nighttime and especially in winter, add to the challenge of accurately estimating wind resources. Such conditions typically imply strong wind shear and cause larger structural fatigue loads to wind turbines. In this work, large-eddy simulations are performed in neutral and stable conditions over a forest to analyze the influence of the combined effect of forest and thermal stabilities on the unsteady characteristics of the wind flow. Taking advantage of the unsteady resolution provided by the simulations, turbulent characteristics of each thermal stability including the organization of turbulent structures are presented. The resulting comparison between the two cases is put into perspective for wind energy applications.

Turbulent large-scale structure effects on wake meandering

This work studies effects of large-scale turbulent structures on wake meandering using Large Eddy Simulations (LES) over an actuator disk. Other potential source of wake meandering such as the instablility mechanisms associated with tip vortices are not treated in this study. A crucial element of the efficient, pragmatic and successful simulations of large-scale turbulent structures in Atmospheric Boundary Layer (ABL) is the generation of the stochastic turbulent atmospheric flow. This is an essential capability since one source of wake meandering is these large - larger than the turbine diameter - turbulent structures. The unsteady wind turbine wake in ABL is simulated using a combination of LES and actuator disk approaches. In order to dedicate the large majority of the available computing power in the wake, the ABL ground region of the flow is not part of the computational domain. Instead, mixed Dirichlet/Neumann boundary conditions are applied at all the computational surfaces except at the outlet. Prescribed values for Dirichlet contribution of these boundary conditions are provided by a stochastic turbulent wind generator. This allows to simulate large-scale turbulent structures - larger than the computational domain - leading to an efficient simulation technique of wake meandering. Since the stochastic wind generator includes shear, the turbulence production is included in the analysis without the necessity of resolving the flow near the ground. The classical Smagorinsky sub-grid model is used.The resulting numerical methodology has been implemented in OpenFOAM. Comparisons with experimental measurements in porous-disk wakes have been undertaken, and the agreements are good. While temporal resolution in experimental measurements is high, the spatial resolution is often too low. LES numerical results provide a more complete spatial description of the flow. They tend to demonstrate that inflow low frequency content - or large- scale turbulent structures - is an important parameter when simulating wake meandering and plays a significant role.

Predicting wake meandering in real-time through instantaneous measurements of wind turbine load fluctuations

The present study deals with the real-time estimation of the wind turbine wake meandering and the determination of the best indicators to measure in order to build a real-time wake meandering model. Results are obtained through experiments performed in an atmospheric boundary layer wind tunnel, where a specific set-up enables to measure simultaneously the incoming lateral velocity fluctuations, the lateral force fluctuations applied to the wind turbine model and to track in real-time the lateral position of its wake. It is an extension of a previous work about the determination of good candidates to build some real-time predictors of the wake meandering. The strong correlations between the incoming transverse velocity fluctuations, the lateral force fluctuations and the lateral position of the wake farther downstream are quantified, confirming that the large-scale turbulent eddies impact directly the wake meandering. Three different versions of a wake meandering predictor model are compared. It leads to the conclusion that the monitoring of the global force fluctuations applied to a wind turbine could be used to predict in real-time the meandering of the generated wake.