Understanding Wind-Turbine Wake Breakdown Using Computational Fluid Dynamics

Abstract

This work explores the breakdown of the wake downstream of the Model Experiments in Controlled Conditions Project (known as the MEXICO project) wind-turbine rotor and assesses the capability of computational fluid dynamics in predicting its correct physical mechanism. The wake is resolved on a fine mesh able to capture the vortices up to eight rotor radii downstream of the blades. At a wind speed of 15 m∕s, the main frequency present in the computational fluid dynamics signals for up to four radii was the blade-passing frequency (21.4 Hz), where the vortex cores fall on a perfect spiral. Between four and five radii downstream, higher-frequency content was present, which indicated the onset of instabilities and results in vortex pairing. The effect of modeling a 120 deg azimuthally periodic domain and a 360 deg three-bladed rotor domain was studied, showing similar predictions for the location of the onset of instabilities. An increased frequency content was captured in the latter case. Empirical and wake models were also explored, they were compared with computational fluid dynamics, and a combination of kinematic and field models was proposed. The obtained results are encouraging and suggest that the wake instability of wind turbines can be predicted with computational fluid dynamics methods, provided adequate mesh resolution is used. Nomenclature= rotor radius, m R = residual Re = Reynolds number r = radial position. m S = source vector in Navier-Stokes equations T = thrust, N Tu = turbulence intensity, % u = axial velocity, m∕s u ∞ = freestream velocity, m∕s V = cell volume v = radial velocity, m∕s x = axial position, m x, y, z = Cartesian coordinates λ = tip speed ratio ϕ = pitch angle, deg ω = vorticity, 1∕s

Wind-Turbine Wake Encounter by Light Aircraft

“…The LIDAR measurements were firstly compared with the full CFD results [16], indirectly, as the full CFD method was applied to the MEXICO rotor in a uniform inflow [16]. The CFD results revealed that wake instability started at a position of about 2.5D downstream of the rotor and the breakdown occurred further downstream between 3D and 4D.…”

“…Recently, full CFD methods [15,16] were used to study the wake development and breakdown on a three-bladed rotor model of 4.5 m diameter. This rotor model was used in the MEXICO project [17], where blade surface pressure and wake velocity measurements with particle image velocimetry (PIV) were carried out in the DNW wind tunnel.…”

Demonstration of a coupled floating offshore wind turbine analysis with high-fidelity methods

“…The pressure and PIV data of the MEXICO project (Carrión et al (2014)) have also been used for validation, where the wake was resolved on a fine mesh capable to capture and preserve the vortices downstream the rotor (Figure 2b), which enabled the prediction of the onset of wake instabilities (Carrión et al (2015)). …”

CFD Investigation of a Complete Floating Offshore Wind Turbine

“…The pressure and PIV data of the MEXICO project Snel 2007, 2012) have also been used for validation (Carrión et al 2014b), where the wake was resolved on a fine mesh capable to capture and preserve the vortices downstream the rotor (Fig. 17.2), which enabled the prediction of the onset of wake instabilities (Carrión et al 2015).…”

“…Based on the free-stream condition and the size of the first cell, the y C parameter was estimated to be y C D 1.2. It must be noted that the grid was relatively coarse as compared with the one used by Carrión et al (2015) to capture the wake of the MEXICO rotor. However, a grid convergence study showed that this density is sufficient to produce meaningful, grid-independent results.…”