Coupled Vortex Effect

Dr. Ion Paraschivoiu (with comments by Robert N Thomas, Dr. Farooq Saeed, and Norbert V. Dy), IOPARA Inc., 18 May 2009

In this work it is shown that both CARDAAV code and CFD simulations can predict accurate estimates of the power coefficient with and without the vortex effect. The effect of vortex augmentation is predicted with a good degree of accuracy using CFD compared to the field test data. The comparison between single rotor and 3-rotor models clearly demonstrates that the proximity of the rotors increases the power coefficient. The analysis also shows that the increase in the torque magnitude or velocity amplification is greater on the advancing blades, i.e., blades moving against the free stream direction as well as when the tip-to-tip distance is decreased.

Dr. Ion Paraschivoiu, IOPARA Inc,  2 November 2009

In this work it was shown that CFD and CARRDAV calculate very close values of the power coefficient for vertical axis wind turbines, nevertheless the computational time is very long for CFD runs. We have shown that the “coupled vortex effect” is also increasing the coefficient of power for three WHI 3000 wind turbines places in a linear array. The optimal location and closeness of the turbines has not been investigated. We note that the turbines can be placed even closer. Furthermore, we recommend a study of the crosswind to evaluate the decrease in power coefficient that can result if the wind changes direction.

Analysis of the Coupled Vortex Effect

Ion Paraschivoiu, Memo to WHI, 3 Feb 2010

Wind turbines must withstand harsh environments that induce many stress cycles into their components. A numerical analysis package is used to illustrate the sobering variability in predicted fatigue life with relatively small changes in inputs. The variability of the input parameters is modeled to obtain estimates of the fatigue reliability of the turbine blades.

CFD Analysis of Vertical Axis Wind Turbines in Close Proximity

Marius Paraschivoiu, Chad X. Zhang, Selvanayagam Jeyatharsan, Norbert V. Dy, Farooq Saeed, Robert N. Thomas, Ion Paraschivoiu, IOPARA Inc. 2011

This paper presents an analysis based on computational fluid dynamics of vertical axis wind turbines when placed in close proximity in a linear array. It has been noticed that VAWTs placed close to each other with counter rotation motions have a higher coefficient of power than a single turbine. This was termed the “coupled vortex effect”. Two mechanisms have been identified to cause this increase in efficiency: the stream tube contraction effect and the vortex effect. The first is due to the blockage effect from neighboring turbines while the later is related to the neighboring turbine acting as a vortex that induces an increased flow field. This paper analyzes each of these effects and studies the influence of the turbine size and the rotation speed. The change of torque on each blade due to these effects is investigated for two different sizes of wind turbines.

This project, carried out by IOPARA Inc. as a contract project for Wind Harvest International Inc. was completed successfully and the experience gained in the specific nature of the unique simulations was invaluable.  Among the different rotor solidities analyzed as part of the solidity parametric study, a rotor solidity of 16.5 % would provide the highest power coefficient (CP =49.38 % at TSR = 3.01, with coupled vortex effect). One should also mention that the performances of the different rotor solidities of the WHI 3000 were calculated using the same interference factors, calibrated for the rotor solidity of 16.5 %. Calibration of the interference factors for the other rotor solidities would require additional CFD simulations.

WHI CEC Final Report

Robert Thomas and Kevin Wolf, Wind Harvest International, 12 February 2012

WHI hired Iopara Inc. to create an aerodynamic model of its vertical axis wind turbine using data from an existing array of its VAWTs. The modeling showed that lower solidity turbines than what WHI had been testing (33% solidities and a 32% Cp max) would achieve much higher efficiencies (Cp max). All three versions (12.375%, 16.5%, and 24.75% solidities) are expected to achieve a Cp max of between 47% and 49.5% with the 16.5% solidity being the most efficient. The distance rotors are placed apart from one another and their orientation to the wind can benefit turbine energy output but more research is needed. The analysis of the Levelized Cost of Energy shows that the best LCOE can be achieved by the 12.375% solidity turbine because it has fewer blades which results in lower costs that make up for its slightly lower Cpmax.

Nenuphar Research

Data Analysis from Experimental Measurements on a Vertical Axis Wind Turbine

Raphael Coneu, KTH School of Industrial Engineering and Management, August 2017

This thesis presents the analysis of the data measured during the test campaign of a vertical axis wind turbine prototype developed by the company Nenuphar Wind in France. Three studies are presented: the study of the loads measured during the test campaign, the study of the vibrations, and the study of the stall conditions on the blades. Focus is put on the methodology of these analyses rather than on their detailed results. The preliminary processing of data is presented in more details, in particular the determination of selection levels asserting the quality of the data, an analysis of the drift of the zero of the loads sensor and an analysis of the cross-talk effects on the same loads sensors. With reference to the experience gained on previous prototypes, the quality of the data acquired on this prototype was greatly improved, allowing the use of stricter selection levels and a better quality of the results. The drift of the zero of the strain gauges was identified to be caused by temperature effects and was corrected on most of the sensors, leading to a significant decrease of the uncertainty on the loads measurements. The analysis of the cross-talks led to the implementation of a new and more precise way of calculation of the uncertainty due to these effects. Finally, the study of the dynamic stall on the blades of the new prototype developed by Nenuphar is described at the end of the report. The comparison with the previous prototype showed similar stall angular ranges, but less stalled rotations were observed on the new prototype than on the previous one.

Efficiency Improvement of Vertical-Axis Wind Turbines with Counter-Rotating Lay-Out

Nicolas Parneix, Rosalie Fuchs, Alexandre Immas, Frederic Silvert, Paul Deglaire, 2016

Improving the performance of Vertical Axis Wind Turbines is the key to make VAWTs commercially successful. Scientists have investigated several interesting concepts for that purpose: increasing the swept area, especially to the limit the losses due to 3D effects, or using flap and pitch systems to control the flow around the blades. Nenuphar, with the Twinfloat concept, propose to take advantage of all these concpets combined with an aerodynamic effect called counter-rotating effect. The turbine is made of two 2.5MW VAWT on only on floater, to reach a rated power of 5MW. The proximity of the two rotors generates a contraction of the streamtubes that flow in the area between the rotors, thus increasing the air flow rate going through both the swept areas and thereby the performance of the VAWT.

Experimental Validation of Pharwen Code Using Data from VAWT Wind Tunnel Tests with Imposed Motions

Marianne Dupont, Pitance Denis, Joanna Kluczewska-Bordier, Alexandre Immas, Paul Deglaire, WindEurope Conference, At Amsterdam, November 2017

NENUPHAR, in collaboration with IRT Jules Verne and ECN (Ecole Centrale de Nantes) has performed extensive testing in a large wind tunnel (CSTB, Nantes) of a scaled-model of a two-bladed VAWT in the frame of the "MOQUA" project. The model was mounted on a 6DOF robot (hexapod) capable of imposing simple and combined motions to the structure. The wind turbine was operated at a TSR (tip speed ratio) of about 4 with simulated floater motions (surge, sway, heave, roll, pitch, yaw, and combinations of those). Measurements were carried out using several internal balances placed at the junctions of the rotor elements. Main tests objectives consisted in the validation of the unsteady aerodynamic behavior of the VAWT and in the validation of the PHARWEN numerical code, NENUPHAR's simulation tool. The experimental validation strategy and results from the comparison between the experiments and the results of numerical simulations are presented in the paper, with a focus on the loads variation over one full rotor revolution that are specific to VAWTs.

Wind tunnel experiments of a pair of interacting vertical-axis wind turbines

A Vergaerde, T De Troyer, J Kluczewska-Bordier, N Parneix, F Silvert and MC Runacres, Journal of Physics: Conference Series, 2018

Vertical-axis wind turbines (VAWTs) have received a renewed interest in the wind energy research community, mainly for off-shore applications. One advantage is that installing a pair of counter rotating VAWTs on the same floating platform would result in thrust reduction and potential cancellation of the mooring yaw moment. In addition, such configurations could benefit from increased power output and reduced wake losses.In this article, we report on wind tunnel experiments to study the mechanical power output of a reference VAWT scale model, tested individually and in a closely-spaced pair of VAWTs.The power output of the individual VAWT configuration is compared with a pair of VAWTs spaced 1.3 diameters apart for two counter rotating directions. A net power increase in the power coefficient for the paired configuration of up to 17.0 % compared with two individual rotors has been observed.

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