Wake Effect of a WT Wind Turbulence

7/29/2019 Wake Effect of a WT Wind Turbulence

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Fluent NEWS spring 2002 5

environmental

M any companies through-out the world have beenapplying their skills andexpertise to the development of renewable energy sources. Thenumber of companies involved in theproduction of clean and sustainableenergy will undoubtedly increase inthe near future due in part to a com-mitment to the Kyoto Protocol(1997), which calls for sweeping reduc-tions in man-made green-house gasemissions, and in part to an increasedawareness of the environment.

One of the most abundantsources of renewable energy iswind, and technology exists todayfor the efficient extraction of ener-gy from wind for power generation.The efficiency of wind power is tiedto a number of factors, one of whichis the positioning of wind turbines

near other wind turbines or structures.

Decreased distances give rise to wakeeffects for the downstream units, whichcan lead to changeable wind loads,reduced energy yield, and vibrationinduced fatigue on the rotors andpotentially on nearby power lines.

One popular operation conceptfor wind turbines allows for adjust-ments in the blade pitch to deliver a reasonably constant power outputwhen there are variations in the windspeed. The wake behind these so-called pitch-regulated wind turbinesdepends on a number of parame-ters, such as blade geometry, pitchangle, and rotor speed on the hard-ware side and wind velocity, turbu-lence characteristics, and windgradients on the environmentalside. The large number of govern-ing parameters makes it difficult tojudge whether wake influences will

lead to loads not considered during

the original construction process. Ina recent series of simulations at TVNord e.V., FLUENT has been used toexamine the wakes behind wind tur-bines of this type on the basis of their geometry and operating character-istics.

TV Nord e.V. is one of GermanysTechnical Inspection Agencies andhas the goal of protecting human-ity, the environment, and propertyagainst detrimental effects caused bytechnical installations and systems of every kind. To this end, it promotesthe economic installation or manu-facture and use of technical equip-ment, production, and operatingfacilities.

In a typical simulation, approxi-mately 650 data points are used tocreate the geometry of a single rotor blade. A fine grid on the whole rotor

surface is used to create a volume

In the Wake of a

Wind Turbineby Thomas Hahm and Jrgen Krning, TV Nord e.V., Hamburg, Germany

Velocity contours behind one turbine show the wake effect on a second, smaller turbine


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6 Fluent NEWS spring 2002

mesh of about 750,000 cells that grad-ually coarsens as the distance fromthe blades increases. The dimensionsof the flow domain are adjusted tosuit the needs of the specific prob-lem. Downstream distances of six toten times the rotor diameter have beenmodeled so far. The multiple refer-

ence frames (MRF) model is used toaccount for the rotation of the blades.Blade pitch, wind speed and direc-tion, turbulence intensity and lengthscale, and rotor speed are input for each simulation.

To validate the CFD model,wake measurements behind a 55 kWpitch-regulated turbine were takenfrom the literature [Ref. 1]. Despitesome inconsistencies in the measuredwind velocities, good agreement

between the measurements and cal-culated values was obtained. In addi-tion, calculations presented inReference 1, based on a simpler modelthat did not use the blade geome-try, were not able to predict flow detailsthat were captured by the 3DFLUENT runs. In particular, the

enhancement of wind velocity at theedges of the wake could only be pre-dicted by the CFD calculations, eventhough the magnitude of theenhancement was larger than themeasured value.

Once the model was validated,it was used for several investigationsof wake effects. On the previous page,one wind turbine is shown operat-ing in the wake of a second, larger turbine. A wind velocity of 12.5 m/sec,

with a turbulence intensity of 13%,was imposed upstream of the frontturbine. Filled contours of constantmean velocity in the plane of the small-er turbine, four diameters behind thefront turbine, show that the veloc-ity field is nonuniform and not cen-tered on the hub. Line contours in

the plane containing the two turbinesillustrate the decay in the wake as afunction of distance behind the tur-bine. These results were used to helpanalyze the special wake loadsexperienced by the rear turbine.

In another example, the excita-tion of vibrations in a power line wasstudied. Wind speeds in the rangeof 1 to 7 m/s and normal to the direc-tion of the power line are most like-ly to cause these vibrations [Ref. 2].

The geometry (front) and typical surface mesh (back)of a turbine rotor and hub

Velocity magnitude slightly downstream of the rotor plane

environmental

Velocity magnitude in the wake of a wind turbine


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If there is a considerable shift in thewind speeds due to wake loadings

on the power line, the installationof vibration dampers on the power lines might be indicated. In the casestudied, where the power line runs25 m above the ground, well belowthe turbine hub, the wake passes over the power line without causing anyinterference.

Currently, there is little dataavailable for the turbulence intensi-ty in the vicinity of installed wind tur-bines, and this point requires further investigation. Today, different empir-

ical models are used to predict tur-bulence intensity in the wake of windturbines [Ref. 3]. Since these mod-els only predict single averaged val-ues along the wake axis and differ from one another, they cannot beused to validate the CFD calculations.The distribution of turbulence inten-sity computed by FLUENT in the wakeregion is in reasonably good agree-

ment with theory. Absolute values,however, fall well below measured

turbulence intensities due to effectsnot captured in the current model(e.g. tip vortices and wake mean-dering). Nonetheless, the flexibilityand increased rigor of the CFD cal-culations, when compared to the sim-pler models, suggests that thismethodology can offer improvedinsight into the efficient productionof wind energy in the years to come.

In summary, given the rotor geom-etry and operating characteristics, CFDcalculations are able to predict the

wind velocities inside the wake of awind turbine. Specific operating con-ditions, such as pitch angle and rotor speed, can easily be analyzed.Three-dimensional simulations of windturbines can also be extended toinclude landscape topography (seepage 8) and other objects locatedin or near the wake.s

Path lines through the turbine colored by velocity magnitude

References1. Beyer, H.G. et. al.;Messungen von Windgeschwindigkeit und Turbulenz in der

Nachlaufstrmung eines 55 kW Windenergiekonverters mit variabler Drehzahl (Measurement of windspeed profiles and turbulence in the flow after a 55 kW wind enconverter with variable speed); DEWEK '92, Deutsche Windenergie-Konferenz 1992;

Wilhelmshaven 1993.2. Degener, T.; Kieling, F.; Tzschoppe, J.;Mindestabstand zwischen Windenergieanlagen

und Freileitungen (Minimum distance between wind energy plants and overhead lines;Elektrizittswirtschaft Jg. 98 (1999), No. 7, p. 32-35.

3. Dekker, J.W.M.; Pierik, J.T.G. (Eds); European Wind Turbine Standards II; Petten, ThNetherlands: ECN Solar & Wind Energy, 1998.

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Wake Effect of a WT Wind Turbulence