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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Aerodynamics and rotor design' taking place on Wednesday, 12 March 2014 at 09:00-10:30. The meet-the-authors will take place in the poster area.

Giuseppe Tescione TUDelft, The Netherlands
Co-authors:
Giuseppe Tescione (1) F P Daniele Ragni (1) Carlos Simão Ferreria (1) Gerard van Bussel (1)
(1) TUDelft, Delft, The Netherlands

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Abstract

Vertical axis wind turbine tip vortex dynamics

Introduction

The study of tip vortices is of high interest in wind energy. Previous studies demonstrated connections between the instability of the helical tip vortex system of a horizontal axis wind turbine and the wake recovery. Vertical axis wind turbines have a different tip vortex system which needs detailed analysis and proper models. Their dynamics may enhance the wake recovery of such turbines. A stereoscopic particle image velocimetry analysis on the tip vortex system of a vertical axis wind turbine is presented, with a phenomenological discussion and a comparison with a numerical model based on vortex elements.

Approach

An experimental campaign has been conducted in the Open Jet Facility of TUDelft on a model of aVertical Axis Wind Turbine (VAWT). The model is a two-bladed, H-shaped rotor, with a diameter of 1 m, a solidity of 0.11, and an aspect ratio of 1.0. The turbine operated at a tip speed ratio of 4.5. The blades are a straight extrusion of a NACA0018 airfoil, operating at an averaged chord-based Reynolds number of 170’000. Fig. 1 shows schematics of the model used, while Fig. 2 shows schematics of the blade motion seen from top with the system of reference used and the conventions.


The measurements were taken with Stereoscopic Particle Image Velocimetry (SPIV) on seven vertical planes, aligned with the flow, and on the horizontal plane at the turbine mid-span. Several fields of view have been combined to cover the rotor and near wake region from 1 diameter upstream to 3 diameters downstream the turbine axis. Ensembles of phase-locked SPIV images were processed to extract the three components of the averaged and RMS velocity field and the out-of-plane vorticity field.
A numerical analysis with a 3D unsteady potential flow solver, based on a panel code and a free vortex wake, has been conducted on a simplified model of the experiment. The model includes only the 2 blades and their motion. The position of the wake vortex elements is stored for each time step and the vorticity field is then obtained by interpolation while the velocity fields are obtained by direct integration.
The results from experiments are discussed and compared with the simulations.

Main body of abstract

The experimental data on the vertical planes highlight some interesting basic features of the system of vortices released by the tip of the blades of a VAWT. Particularly the vorticity plots (Fig. 3) provide a visualization of the coherent structures, their motion and relative interaction and diffusion. The plots display phase-locked averaged fields on the seven vertical planes, spanning the rotor area in cross-stream direction (y-axis).
The roll-up of the vortex sheet released by the trailing edge of the blade is clearly visible for the upwind passage of the blade and partially for the downwind passage (see the plot of y/R = 0 in Fig. 3). The resulting tip vortices present a sensible different peak vorticity, with the upwind being stronger. This is in line with the observation that the tip vortex strength is proportional to the blade bound circulation and the fact that the upwind blade experiences higher angles of attack and relative velocities than the downwind blade, operating in a partially developed wake of the turbine. At the position of the release of the downwind part of the tip vortex tube, the two structures interact with a mutual induction which leads to a rapid roll-over, a sensible deformation and an increased diffusion of the coherent structures which finally merge in a continuous, disorganized vorticity region. Coherent structures are no longer detectable after x/R > 3 (x-axis being in the stream-wise direction).
The vertical motion of the tip vortices, and how it changes in the cross-stream direction, is also visible in Fig. 3 and in Fig. 4 (showing the vertical component of the velocity).
The tip vortex tube moves inboard (negative vertical velocity) in the inner part of the wake (-0.8 < y/R < 0.4) and outboard in the windward (y/R > 0.4) and leeward (y/R < -0.8) part. This motion is highly asymmetric with the maximum inboard motion located on the leeward side (-0.4 < y/R < 0) and the maximum outboard motion at the windward side (y/R = 1). Fig. 5 shows the resultant wake geometry in the xy plane (top) and yz plane (bottom) of the only upwind portion of the tip vortex tube. The consequence of such induction field is visible in Fig. 6 (showing the stream-wise component of the velocity) where it can be observed how the wake of a VAWT experiences a vertical contraction in the middle part and a more pronounced wake deficit to the windward side.
Results from the numerical simulations showed a similar behaviour of the wake in terms of kinematics in the first diameters downwind the rotor. The asymmetric wake evolution is well captured, with the vertical motion of the tip vortices showing the same trend of the experimental data. Fig. 7 shows a comparison between numerical and experimental results on the stream-wise velocity. Quantitatively the strengths of the tip vortices are overestimated by the numerical model. The absence of the struts and the tower in the numerical simulation may be addressed as responsible for these discrepancies, as well as the lack of a viscous diffusion scheme. The result is an overestimation of the vertical motion and of the wake recovery.






Conclusion

The experimental campaign with SPIV on the flow around a model VAWT provided visualization and a measure of the dynamics of the VAWT near wake. The motion and evolution of the system of tip vortices was discussed. Measurements showed a vertical motion of the tip vortices inside the wake in the middle part with a consequent contraction of the wake region. On the leeward and windward side the tip vortices move outboard with consequent wake expansion in the vertical direction. Such motion is asymmetric in the cross-flow direction with a more enhanced divergence on the windward side. The wake deficit follows the same trend. Results also showed the interaction of the segment of the tip vortex tube released by the upwind passage of the blade with the segment from the downwind passage. This interaction leads to a mutual induction with a roll-over, a significant stretching and a following diffusion of the two structures which are no longer detectable after 3 radius downstream.
The present research contributes to the understanding of some physical phenomena characterizing the dynamics of the wake of a VAWT, which showed significant differences with the one of a HAWT. The understanding of the underlying physics is fundamental for the development of proper models.
The research also compared results from a numerical analysis based on a panel-body, free vortex wake model. They showed agreement on the wake and tip vortices dynamics, with a tendency of an over estimation of the vorticity of the structures and their subsequent induction and motion. The use of vortex elements proved to be a promising tool for predicting the evolution of flows with high content of vorticity such as the wake of a VAWT. Quantitatively the discrepancies between the simulations and the experiments are addressed to the lack of the struts and the tower in the numerical simulation and the absence of a viscous diffusion models, which have to be included in future developments.



Learning objectives
The present research aims to contribute to the understanding of the physical phenomena underlying the VAWT wake flows.
VAWT wake presents significant differences with the one of HAWT, the present research focused on the tip vortices.
The differences in the VAWT wake require different models for wind farm planning. Vortex element models proved to be able to well capture the vortex dynamics of VAWT wakes.



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