Jose Lebron

Experimental study of the horizontally averaged flow structure in a model wind-turbine array boundary layer

When wind turbines are deployed in large arrays, their ability to extract kinetic energy from the flow decreases due to complex interactions among them, the terrain topography and the atmospheric boundary layer. In order to improve the understanding of the vertical transport of momentum and kinetic energy across a boundary layer flow with wind turbines, a wind-tunnel experiment is performed. The boundary layer flow includes a 3×3 array of model wind turbines. Particle-image-velocity measurements in a volume surrounding a target wind turbine are used to compute mean velocity and turbulence properties averaged on horizontal planes. Results are compared with simple momentum theory and with expressions for effective roughness length scales used to parametrize wind-turbine arrays in large-scale computer models. The impact of vertical transport of kinetic energy due to turbulence and mean flow correlations is quantified. It is found that the fluxes of kinetic energy associated with the Reynolds shear stresses a...

Optimizing the Unrestricted Placement of Turbines of Differing Rotor Diameters in a Wind Farm for Maximum Power Generation

This paper presents a new method (the Unrestricted Wind Farm Layout Optimization (UWFLO)) of arranging turbines in a wind farm to achieve maximum farm efficiency. The powers generated by individual turbines in a wind farm are dependent on each other, due to velocity deficits created by the wake effect. A standard analytical wake model has been used to account for the mutual influences of the turbines in a wind farm. A variable induction factor, dependent on the approaching wind velocity, estimates the velocity deficit across each turbine. Optimization is performed using a constrained Particle Swarm Optimization (PSO) algorithm. The model is validated against experimental data from a wind tunnel experiment on a scaled down wind farm. Reasonable agreement between the model and experimental results is obtained. A preliminary wind farm cost analysis is also performed to explore the effect of using turbines with different rotor diameters on the total power generation. The use of differing rotor diameters is observed to play an important role in improving the overall efficiency of a wind farm.Copyright

Response Surface Based Cost Model for Onshore Wind Farms Using Extended Radial Basis Functions

This paper develops a cost model for onshore wind farms in the U.S.. This model is then used to analyze the influence of different designs and economic parameters on the cost of a wind farm. A response surface based cost model is developed using Extended Radial Basis Functions (E-RBF). The E-RBF approach, a combination of radial and non-radial basis functions, can provide the designer with significant flexibility and freedom in the metamodeling process. The E-RBF based cost model is composed of three parts that can estimate (i) the installation cost, (ii) the annual Operation and Maintenance (O&M) cost, and (iii) the total annual cost of a wind farm. The input parameters for the E-RBF based cost model include the rotor diameter of a wind turbine, the number of wind turbines in a wind farm, the construction labor cost, the management labor cost and the technician labor cost. The accuracy of the model is favorably explored through comparison with pertinent real world data. It is found that the cost of a wind farm is appreciably sensitive to the rotor diameter and the number of wind turbines for a given desirable total power output.Copyright

Experimental study of the kinetic energy budget in a wind turbine streamtube

The classical streamtube analysis for a wind turbine couples the Bernoulli equation with the actuator-disk model to obtain the maximum possible power extraction at the wind turbine based on the difference between inlet and outlet fluxes of kinetic energy. However, the classical analysis neglects turbulence, which can play a crucial role in the various interactions of the streamtube with the turbulent Atmospheric Boundary Layer (ABL). The present study aims at examining the fluxes of kinetic energy on a wind turbine streamtube including the effects of turbulence, based on an existing Particle Image Velocimetry (PIV) data set obtained previously in a wind tunnel study of a model wind farm. The data are used to evaluate the most relevant fluxes of kinetic energy through the various parts of the control surface bounding the stream tube. It is found that the flux due to mean axial velocity, the turbulence-induced flux across the periphery of the streamtube, and the power extracted at the wind turbine are all o...

Interaction Between a Wind Turbine Array and a Turbulent Boundary Layer

This paper describes further data analysis and discussion of an experimental study discussed in reference [1], in which a scaled down wind farm is placed in a turbulent boundary layer in a wind-tunnel. A 3x3 array of wind turbine models were built and inflow conditions were tailored to reproduce conditions expected to hold in the field under neutral flow. Hot-wire anemometry was used to characterize the inflow properties. Measurements using Stereo-Particle Image Velocimetry (S-PIV) were performed in 18 planes surrounding the center wind turbine at the third downstream row. The gathered data was interpolated in the x and z directions to generate cross-stream planes used for the estimation of the wind turbine induction factor. Assuming an axisymmetric streamtube shape, the resulting induction factor is 0.087, which corresponds to a lightly loaded wind turbine. Results are then compared to direct mechanical power measurements. Moreover, the streamtube is visualized in order to gain insight into the flow and to test the axisymmetric assumption used for the calculation of the induction factor. Results show that the streamtube is indeed close to axisymmetric, but exhibits some slight distortions due to strong tower effects and shear from the wall.

Effects of free-stream turbulence on rough surface turbulent boundary layers

Several effects of nearly isotropic free-stream turbulence in transitionally rough turbulent boundary layers are studied using data obtained from laser Doppler anemometry measurements. The free-stream turbulence is generated with the use of an active grid, resulting in free-stream turbulence levels of up to 6.2 %. The rough surface is characterized by a roughness parameter k + ≈ 53, and measurements are performed at Reynolds numbers of up to Re θ = 11 300. It is confirmed that the free-stream turbulence significantly alters the mean velocity deficit profiles in the outer region of the boundary layer. Consequently, the previously observed ability of the Zagarola & Smits (J. Fluid Mech., vol. 373, 1998, p. 33) velocity scale U ∞ δ*/δ to collapse results from both smooth and rough surface boundary layers, no longer applies in this boundary layer subjected to high free-stream turbulence. In inner variables, the wake region is significantly reduced with increasing free-stream turbulence, leading to decreased mean velocity gradient and production of Reynolds stress components. The effects of free-stream turbulence are clearly identifiable and significant augmentation of the streamwise Reynolds stress profiles throughout the entire boundary layer are observed, all the way down to the inner region. In contrast, the Reynolds wall-normal and shear stress profiles increase due to free-stream turbulence only in the outer part of the boundary layer due to the blocking effect of the wall. As a consequence, there is a significant portion of the boundary layer in which the addition of nearly isotropic turbulence in the free-stream, results in significant increases in anisotropy of the turbulence. To quantify which turbulence length scales contribute to this trend, second-order structure functions are examined at various distances from the wall. Results show that the anisotropy created by adding nearly isotropic turbulence in the free-stream resides mostly in the larger scales of the flow. Furthermore, by analysing the streamwise Reynolds stress equation, it can be predicted that it is the wall-normal gradient of (u 2 v) term that is responsible for the increase in (u 2 ) profiles throughout the boundary layer (i.e. an efficient turbulent transport of turbulence away from the wall). Furthermore, a noticeable difference between the triple correlations for smooth and rough surfaces exists in the inner region, but no significant differences are seen due to free-stream turbulence. In addition, the boundary layer parameters δ*/δ 95 , H and c f are also evaluated from the experimental data. The flow parameters δ*/δ 95 and H are found to increase due to roughness, but decrease due to free-stream turbulence, which has significance for flow control, particularly in delaying separation. Increases in c f due to high free-stream turbulence are also observed, associated with increased momentum flux towards the wall.

Streamwise development of the wind turbine boundary layer over a model wind turbine array

The streamwise development of turbulence statistics and mean kinetic energy in a model wind farm consisting of 3 × 5 wind turbines is studied experimentally in a wind tunnel. The analysis uses planar Particle Image Velocimetry data obtained at the centerline plane of the wind farm, covering the inflow as well as four planes in between five downstream wind turbines. The data analysis is organized by dividing these measurement planes into three regions: the above-rotor, rotor-swept, and below-rotor regions. For each field, flow development is quantified using a properly defined relative difference norm based on an integration over each of the regions. Using this norm, it is found that the mean streamwise velocity approaches a fully developed state most rapidly, whereas the flow development is more gradual for the second-order statistics. The vertical entrainment flux of the mean kinetic energy by the Reynolds shear stress, ⟨U⟩⟨u′v′⟩, is observed to develop at a rate similar to that of the Reynolds shear stress rather than the mean streamwise velocity component. Its development is slowest in the layer nearest to the ground. Analysis of various terms in the mean kinetic energy equation shows that the wind turbine boundary layer has not yet reached fully developed conditions by the fifth turbine but that it is approaching such conditions. By comparing the vertical entrainment flux with the horizontal flux due to the mean flow, it is found that the former increases, whereas the latter decreases, as function of downstream distance, but that the former is already an important contributor in the developing region.

Free-Stream Turbulence Effects on the Flow around an S809 Wind Turbine Airfoil

Two-dimensional Particle Image Velocimetry (2-D PIV) measurements were performed to study the effect of free-stream turbulence on the flow around a smooth and rough surface airfoil, specifically under stall conditions. A 0.25-m chord model with an S809 profile, common for horizontal-axis wind turbine applications, was tested at a wind tunnel speed of 10 m/s, resulting in Reynolds numbers based on the chord of Re c ≈ 182,000 and turbulence intensity levels of up to 6.14%. Results indicate that when the flow is fully attached, turbulence significantly decreases aerodynamic efficiency (from L/D ≈ 4.894 to L/D ≈ 0.908). On the contrary, when the flow is mostly stalled, the effect is reversed and aerodynamic performance is slightly improved (from L/D ≈ 1.696 to L/D ≈ 1.787). Analysis of the mean flow over the suction surface shows that, contrary to what is expected, free-stream turbulence is actually advancing separation, particularly when the turbulent scales in the free-stream are of the same order as the chord. This is a result of the complex dynamics between the boundary layer scales and the free-stream turbulence length scales when relatively high levels of active-grid generated turbulence are present.

Isotropic Free-stream Turbulence Promotes Anisotropy in a Turbulent Boundary Layer

The study of how external conditions affect turbulent boundary layers is important since such effects are often present in common engineering applications. Earlier investigations on surface roughness have shown its effect on the mean velocity and Reynolds stress profiles. Similarly, the effects of free-stream turbulence have been well documented [1, 2, 3]. However, the results available until now are limited to low Reynolds numbers. Hence, the aim of this investigation is to study the effects of high free-stream turbulence on rough surface turbulent boundary layers, at relatively high Reynolds numbers. This investigation focused on the penetration mechanisms of free-stream turbulence into the boundary layer, identifying the length scales that dominate these mechanisms and studying the effects on the resulting turbulence anisotropy [4]. These effects will also be studied in turbulent boundary layers subject to favorable pressure gradients.

Effect of Isotropic Free-stream Turbulence in Favorable Pressure Gradient Turbulent Boundary Layers over a Rough Surface

Laser Doppler and Hotwire anemometry measurements were performed to study the effect of various conditions, namely free-stream turbulence (FST), favorable pressure gradient, and surface roughness, on turbulent boundary layers. Measurements were carried out at Re θ ≤ 4,300 and free-stream turbulence levels of up to 7%, generated using an active grid. Results show that with the addition of FST, classical scaling laws are not able to collapse the profiles of mean velocity. Moreover, boundary layer parameters, including skin friction coefficient, confirm a complex interaction between the external conditions and the inner/outer flow. The discrepancy in the behavior of the stream-wise and wall-normal variances due to the presence of free-stream turbulence suggests that the addition of nearly isotropic free-stream turbulence promotes anisotropy in the body of the boundary layer. Second-order structure functions are examined to identify and quantify which turbulence length-scales contribute mostly to creating this discrepancy. The analysis demonstrates that the effect of FST resides in a wide range of length scales, and is not limited to the largest scales of the flow as in the case of ZPG flows.