Airfoil data sensitivity analysis for actuator disc simulations used in wind turbine applications

To analyse the sensitivity of blade geometry and airfoil characteristics on the prediction of performance characteristics of wind farms, large-eddy simulations using an actuator disc (ACD) method are performed for three different blade/airfoil configurations. The aim of the study is to determine how the mean characteristics of wake flow, mean power production and thrust depend on the choice of airfoil data and blade geometry. In order to simulate realistic conditions, pre-generated turbulence and wind shear are imposed in the computational domain. Using three different turbulence intensities and varying the spacing between the turbines, the flow around 4-8 aligned turbines is simulated. The analysis is based on normalized mean streamwise velocity, turbulence intensity, relative mean power production and thrust. From the computations it can be concluded that the actual airfoil characteristics and blade geometry only are of importance at very low inflow turbulence. At realistic turbulence conditions for an atmospheric boundary layer the specific blade characteristics play an minor role on power performance and the resulting wake characteristics. The results therefore give a hint that the choice of airfoil data in ACD simulations is not crucial if the intention of the simulations is to compute mean wake characteristics using a turbulent inflow

The impact of wind direction in atmospheric BL on interacting wakes at Horns Rev wind farm

Large eddy simulations of the Navier-Stokes equations are performed to simulate the Horns Rev off shore wind farm 15 km outside the Danish west coast. The aim is to achieve a better understanding of the wake interaction inside the farm. The simulations are performed by combining the in- house developed computer code Ellip-Sys3D with the actuator-disc methodology. In the actuator-disc method the blades are represented by a disc at which body forces representing the aerodynamic loading are introduced. The body forces are determined by computing local angles of attack and tabulated aerofoil coefficients. The ad- vantage of using the actuator-disc technique is that it is not necessary to resolve blade boundary layers since the computational resources are devoted to simulating the dynamics of the flow structures. In the present study approximately 13.6 million mesh points are used to resolve the wake structure in the park. The results from the CFD simulations are evaluated and the downstream evolution of the velocity field is depicted. Special interest is given to what extent the production is de- pendent on the inflow angle and turbulence level. The study shows that the applied method captures the main production variation within the wind farm. The result further demonstrates that levels of production correlate well with measurements. However, in some cases the variation of the measurement data is caused by variation of measurement conditions with inflow angles. The study also shows that the wind veer has a significant impact on the wake interaction and power losses of downstream turbine positions

A fast-running physics-based wake model for a semi-infinite wind farm

This paper presents a new generation of fast-running physics-based models to predict the wake of a semi-infinite wind farm, extending infinitely in the lateral direction but with finite size in the streamwise direction. The assumption of a semi-infinite wind farm enables concurrent solving of the laterally-averaged momentum equations in both streamwise and spanwise directions. The developed model captures important physical phenomena such as vertical top-down transport of energy into the farm, variable wake recovery rate due to the farm-generated turbulence, and also wake deflection due to turbine yaw misalignment and Coriolis force. Of special note is the model's capability to predict and shed light on the counteracting effect of Coriolis force causing wake deflections in both positive and negative directions. Moreover, the impact of wind-farm layout configuration on the flow distribution is modelled through a parameter called the local deficit coefficient. Model predictions were validated against large-eddy simulations extending up to 45 kilometres downstream of wind farms. Detailed analyses were performed to study the impacts of various factors such as incoming turbulence, wind-farm size, inter-turbine spacing, and wind-farm layout on the farm wake

Comparison between experiments and Large-Eddy Simulations of tip spiral structure and geometry

International audienceResults from Large-Eddy Simulations using the actuator line technique have been validated against experimental results. The experimental rotor wake, which forms the basis for the comparison, was studied in a recirculating free-surface water channel, where a helical vortex was generated by a single-bladed rotor mounted on a shaft. An investigation of how the experimental blade geometry and aerofoil characteristics affect the results was performed. Based on this, an adjustment of the pitch setting was introduced, which is still well within the limits of the experimental uncertainty. Excellent agreement between the experimental and the numerical results was achieved concerning the circulation, wake expansion and pitch of the helical tip vortex. A disagreement was found regarding the root vortex position and the axial velocity along the centre line of the tip vortex. This work establishes a good base for further studies of more fundamental stability parameters of helical rotor wakes

Viraalimarkkinointi : vaihtoehto massamarkkinoinnille

Tämä opinnäytetyö käsittelee viraalimarkkinointia. Viraalimarkkinointi on perinteisestä word of mouth -markkinoinnista kehittynyt markkinointimuoto, jossa markkinointiviestit leviävät sähköisessä ympäristössä kuluttajalta toiselle kasvokkaisviestinnän sijaan. Tekniikan kehittyminen ja erilaisten sosiaalisten verkkoyhteisöjen alati kasvava suosio ovat tehneet viraalimarkkinoinnista yrityksien keskuudessa suositun ja kustannustehokkaan vaihtoehdon massamarkkinoinnille. Opinnäytetyön tavoitteena on koota manuaali, jolla voi selventää viraalimarkkinointia markkinointimuotona asiasta tietämättömille henkilöille. Opinnäytetyö on jaettu kolmeen lukuun, joista ensimmäinen käsittelee teoreettisesti word of mouth -markkinointia. Toinen luku käsittelee teoreettisesti varsinaista aihetta, eli viraalimarkkinointia. Kolmas luku esittelee eri asiantuntijoiden ja tutkijoiden neuvoja viraalikampanjan toteuttamiseen sekä konkreettisen ohjeen luoda yksinkertainen viraalikampanja.This thesis focuses on viral marketing. Viral marketing is a developed form of traditional word-of-mouth marketing. In viral marketing the marketing message spreads through electronical environment from one consumer to another, instead of face-to-face communication as in traditional word-of-mouth marketing. The tremendous development of technology and continuously crescent popularity of social web communities have made viral marketing a popular and cost efficient alternative for massmarketing among enterprises. The research problem of the thesis is to assemble a manual that would clarify viral marketing as a marketing form to persons who are not familiar with it. The thesis is divided into three different parts. The first, the theoretically part, focuses on word-of- mouth marketing. The second theoretical section focuses on viral marketing, the primary subject of the thesis. And finally, the third part introduces advice for executing actual viral campaigning by experts and researchers. The third part also introduces a concrete guideline to create a simple viral campaign

Determination of Wind Turbine Near-Wake Length Based on Stability Analysis

A numerical study on the wake behind a wind turbine is carried outfocusing on determining the length of the near-wake based on the instability onset ofthe trailing tip vortices shed from the turbine blades. The numerical model is based onlarge-eddy simulations (LES) of the Navier-Stokes equations using the actuator line(ACL) method. The wake is perturbed by applying stochastic or harmonic excitations inthe neighborhood of the tips of the blades. The flow field is then analyzed to obtain thestability properties of the tip vortices in the wake of the wind turbine. As a mainoutcome of the study it is found that the amplification of specific waves (travelingstructures) along the tip vortex spirals is responsible for triggering the instabilityleading to wake breakdown. The presence of unstable modes in the wake is related tothe mutual inductance (vortex pairing) instability where there is an out-of-phasedisplacement of successive helix turns. Furthermore, using the non-dimensional growthrate, it is found that the pairing instability has a universal growth rate equal to π/2.Using this relationship, and the assumption that breakdown to turbulence occurs once avortex has experienced sufficient growth, we provide an analytical relationship betweenthe turbulence intensity and the stable wake length. The analysis leads to a simpleexpression for determining the length of the near wake. This expression shows that thenear wake length is inversely proportional to thrust, tip speed ratio and the logarithmicof the turbulence intensit

Mutual inductance instability of the tip vortices behind a wind turbine

Two modal decomposition techniques are employed to analyse the stability of wind turbine wakes. A numerical study on a single wind turbine wake is carried out focusing on the instability onset of the trailing tip vortices shed from the turbine blades. The numerical model is based on large-eddy simulations (LES) of the Navier-Stokes equations using the actuator line (ACL) method to simulate the wake behind the Tj ae reborg wind turbine. The wake is perturbed by low-amplitude excitation sources located in the neighbourhood of the tip spirals. The amplification of the waves travelling along the spiral triggers instabilities, leading to breakdown of the wake. Based on the grid configurations and the type of excitations, two basic flow cases, symmetric and asymmetric, are identified. In the symmetric setup, we impose a 120 degrees symmetry condition in the dynamics of the flow and in the asymmetric setup we calculate the full 360 degrees wake. Different cases are subsequently analysed using dynamic mode decomposition (DMD) and proper orthogonal decomposition (POD). The results reveal that the main instability mechanism is dispersive and that the modal growth in the symmetric setup arises only for some specific frequencies and spatial structures, e.g. two dominant groups of modes with positive growth (spatial structures) are identified, while breaking the symmetry reveals that almost all the modes have positive growth rate. In both setups, the most unstable modes have a non-dimensional spatial growth rate close to pi/2 and they are characterized by an out-of-phase displacement of successive helix turns leading to local vortex pairing. The present results indicate that the asymmetric case is crucial to study, as the stability characteristics of the flow change significantly compared to the symmetric configurations. Based on the constant non-dimensional growth rate of disturbances, we derive a new analytical relationship between the length of the wake up to the turbulent breakdown and the operating conditions of a wind turbine