D. Baldacchino

Experimental parameter study for passive vortex generators on a 30% thick airfoil: Experimental parametric vortex generator study

Passive vane-type vortex generators (VGs) are commonly used on wind turbine blades to mitigate the effects of flow separation. However, significant uncertainty surrounds VG design guidelines. Understanding the influence of VG parameters on airfoil performance requires a systematic approach targeting wind energy-specific airfoils. Thus, the 30%-thick DU97-W-300 airfoil was equipped with numerous VG designs, and its performance was evaluated in the Delft University Low Turbulence Wind Tunnel at a chord-based Reynolds number of 2×106. Oil-flow visualizations confirmed the suppression of separation as a result of the vortex-induced mixing. Further investigation of the oil streaks demonstrated a method to determine the vortex strength. The airfoil performance sensitivity to 41 different VG designs was explored by analysing model and wake pressures. The chordwise positioning, array configuration, and vane height were of prime importance. The sensitivity to vane length, inclination angle, vane shape, and array packing density proved secondary. The VGs were also able to delay stall with simulated airfoil surface roughness. The use of the VG mounting strip was detrimental to the airfoils performance, highlighting the aerodynamic cost of the commonly used mounting technique. Time-averaged pressure distributions and the lift standard deviation revealed that the presence of VGs increases load fluctuations in the stalling regime, compared with the uncontrolled case.

Towards integral boundary layer modelling of vane-type vortex generators

This paper details an experimental investigation of submerged vane-type vortex generators (VGs) on a flat plate turbulent boundary layer inside a boundary layer wind tunnel. Spanwise planes at various positions downstream of the VG trailing edges are measured using stereoscopic particle image velocimetry (SPIV). The analysis focuses on the streamwise and spanwise development of the boundary layer, in light of the boundary layer integral parameters. The high spatial SPIV resolution enables an accurate resolution of the boundary layer with and without vortex generators. Through the study of the measured wake flow, insights towards an integral boundary layer modelling approach of vortex generators is gained through the verification of the vortex development properties as well as the net effect on the encompassing boundary layer.

Wind turbine wake stability investigations using a vortex ring modelling approach

In the present study, a simple inviscid vortex ring (VR) modelling approach is used to represent the developing rotor wake. This allows a straightforward investigation and comparison of the impact of uniform, yawed and sheared flow conditions on the development of the rotor wake, with the additional possibility of including ground effect. The effect of instabilities on the development of the wake is manually introduced in the form of perturbations of strength, ring position and size. The phenomenon of vortex filament interaction or leapfrogging, could play a role in the observation of unsteady phenomena and is therefore also addressed. Such a study is hence performed in light of recent conflicting views on the causes of wake meandering: is the observed dynamic wake behaviour a result of large scale turbulent forcing or do more subtle and intrinsic wake instabilities play a role? This study concludes that the presence of the ground and external perturbations, most notably changes in the wake pitch and the rotor thrust coefficient, can significantly affect the steady development of the wake. The mutual vortex pairing instability, whilst displaying interesting periodic behaviour, does not correlate with periodic wake behaviour reported by Medici et al. [1]. However, in the absence of unsteady inflow, it is shown that the wake of a Horizontal Axis Wind Turbine (HAWT) is certainly prone to displaying unstable, dynamic behaviour caused by these additional factors.

Point vortex modelling of the wake dynamics behind asymmetric vortex generator arrays

In this work, we present a simple inviscid point vortex model to study the dynamics of asymmetric vortex rows, as might appear behind misaligned vortex generator vanes. Starting from the existing solution of the infinite vortex cascade, a numerical model of four base-vortices is chosen to represent two primary counter-rotating vortex pairs and their mirror plane images, introducing the vortex strength ratio as a free parameter. The resulting system of equations is also defined in terms of the vortex row separation and the qualitative features of the ensuing motion are mapped. A translating and orbiting regime are identified for different cascade separations. The latter occurs for all unequal strength vortex pairs. Thus, the motion is further classified by studying the cyclic behaviour of the orbiting regime and it is shown that for small mismatches in vortex strength, the orbiting length and time scales are sufficiently large as to appear, in the near wake, as translational (non-orbiting). However, for larger mismatches in vortex strength, the orbiting motion approaches the order of the starting height of the vortex. Comparisons between experimental data and the potential flow model show qualitative agreement whilst viscous effects account for the major discrepancies. Despite this, the model captures the orbital mode observed in the measurements and provides an impetus for considering the impact of these complex interactions on vortex generator designs.

Coupling of a free wake vortex ring near-wake model with the Jensen and Larsen far-wake deficit models

This paper presents a simple physical model to improve the currently used far-wake deficit models in the wind industry. The main improvement is deemed on the determination of the wake deficit in the near-wake. A Vortex Ring Model (VRM) is used to calculate the induced velocities in the near-wake, which are then coupled to the Jensen far-wake model and the Larsen far-wake model based on the concept of Eddy Viscosity (EV). The inviscid near-wake VRM is based on the shedding of discrete tip vortex rings released from a uniformly loaded actuator disc. The model is validated against wind tunnel measurements from experiments with a two- bladed turbine and a circular metal mesh with a uniform porosity to represent an actuator disc. The VRM shows a good agreement with the experimental data with respect to the wake deficit evolution. The VRM is coupled with two well-known engineering type far-wake models: the Jensen and Larsen wake deficit models. The results of the coupling of the VRM and the more elaborated Larsen far-wake model are compared against a 3D Large Eddy Simulation (LES) CFD model. This comparison shows the effect of different near-wake models on the development of centreline velocities in the far-wake. The centreline velocity deficit predicted by the VRM-Larsen model more closely matches LES calculations in comparison with the reference Larsen model.