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Layout design for a venturi to encase a wind turbine integrated in a high rise

Beller, Christina

Published in:Conference proceedings (online)

Publication date:2008

Link back to DTU Orbit

Citation (APA):Beller, C. (2008). Layout design for a venturi to encase a wind turbine integrated in a high rise. In Conferenceproceedings (online) European Wind Energy Association (EWEA).

Layout Design for a Venturi to encase a Wind Turbineintegrated in a High Rise

C. Beller

Risø National Laboratory for Sustainable Energy Stiftungslehrstuhl WindenergieTechnical University of Denmark University of StuttgartDK-4000 Roskilde, Denmark D-70550 Stuttgart- Vaihingen, Germanychristina.beller@risoe.dk swe@ifb.uni-stuttgart.de

Abstract:

In this work a general CFD supported investiga-tion of the potential of Venturi shaped turbine cas-ings, compared to a constant in diameter tubing,is carried out. Inspired by and based on theskyscraper design ’Castle House’, including tur-bines integrated into the buildings structure, a pa-rameter study about encased turbines in urban ar-eas is developed. The simulation approach is val-idated by modelling a DIN EN ISO 5167-3:2003Venturi device in the CFD environment of FIDAP.Starting with this standardized Venturi geometry,combined with characteristic wind tunnel designrules, three tubing geometry are constructed. Thetop floor of the building is simplified as a ’coin’model with a hole in the middle, wherein the tur-bine is placed later on. This is to simplify themodel to be axis- symmetric. Contractions of 1.00,2.25, 4.00, and 9.00 are simulated. Analyses ofthe flow through the single contractions are car-ried out and by comparison the most effective ge-ometry is chosen to work on further with. In thenext step, a turbine, represented by an actuatordisc, is implemented by body forces. A focus istaken on positioning the turbine with varying thrustcoefficients at three different characteristic Venturidepths. The resulting flow behaviour is qualitativedescribed and a discussion follows.Key words: encased, Venturi, urban wind energy

1 Introduction

Urban landscape changes due to new architec-tures. Buildings are built much higher, thereforeroofs are reaching in high wind speed layers,planned urban structure is applied, which can beused to direct mainstreams, and consciousnessabout environmental friendly systems is moreand more present, which increases the toleranceand even stimulates desire of private renewableenergy. Few ideas of urban turbines, encased [1],

with vertical axis [2] and moreover unconventionalconcepts [3], [4] are realized. Some designs areof the category ’house hold turbines’ and aremeant to supply the house, with no respect to theintegration of the wind turbine into the flow situ-ation around the building itself [5]. Some othersare already considering the upwind on a housefaçade and are placed on logical places alignedto the main wind direction [6], [7]. All of them wereso far fixed on an already existing building. Thepotential of considering a turbine already in theconceptual house design pays of in saving thestructure for a turbine tower, in increased windvelocity and directing by smart blockage of thebuilding and with this a higher power harvesting.Especially, the so long controversial encasedturbine contra arguments can be cancelled out-no additional heavy structure is necessary, be-cause it can be integrated in the architecture. It isdefinitely a process of compromises, on the sideof the architectural freedom and the side of thetechnical efficiency.

A new project, ’Castle House’, is planned forthe year 2009. The high riser shall be erectedin London, surrounded by low buildings (fig. 1).The new way of turbines integration in build-

Figure 1: ’Castle House’, conceptual design [8]

ings structure by encasing them offers someinteresting new parameters in terms of noisereduction, concentrating and directing the wind,

tip loss reduction and more, which need to bewell investigated. The conceptual design showsstraight ducts for the turbine encasings andsmooth transitions from the facade to the inletwere not taken into consideration. The idea is toprevent or at least diminish separation at the inletand accelerate the air by application of a Venturishaped tube. With the mass conservation law,

dm

dt i= ρAivi = const., (1)

two cross sections in a tube are related in the fol-lowing way,

v2 = v1A1

A2, (2)

while the inlet cross section over throat section ra-tio states the contraction,

Co =Ainlet

Athroat. (3)

Thus, the higher the contraction, the higher is thevelocity amplification and therefore the power am-plification. Available power defined as,

Pi =12dm

dt iv2

i =12ρAiv

3i , (4)

leads with (1) to the power ratio,

P2

P1=

12

dmdt 2

v22

12

dmdt 1

v21

=A2v

32

A1v31

, (5)

and therefore to

P2

P1= Co2. (6)

How this statement proves true or not is shownin section 3. To keep the simulation simple anddevelop a basic study but orientated on a currentproject, the problem is rotationally symmetric de-fined. With the assumption of an axis-symmetricalproblem the house is reduced to its upper part andthen represented by a coin shaped model (fig. 2).The outer radius is one of the constant parame-

Figure 2: Coin model, Co = 1

ters for the four investigated contractions. For the

inlet and outlet diameter a constant value of 9mis settled. Another fixed parameter is the length,which is determined by the Venturi design laws ex-plained in section 2. The mean wind velocity, vw,is stated with 6.00m/s. Starting up from this basis,three inlay shapes with the contractions 2.25, 4.00and 9.00 are found, inserted into the coin model,Co1.00, to be examined and compared. Next tothe interest which contracting curvature for the in-lay is most advantegous in respect to power ampli-fication lies the interest how the turbine positioninginfluences the flow behaviour. Fig. 3 shows suchan inlay, here for the contraction 2.25. It consists

Figure 3: Coin model inlay, Co = 2.25

of the contracting, smooth shaped inlet, the con-stant diameter section called the throat and thediffuser expanding from the throat diameter backto the inlet diameter. Due to the Venturi effectthe highest velocity occurs in the section with thesmallest diameter and therefore in the throat. Ger-ald J. W. Van Bussel [9] did in his one dimensionalanalysis not consider the turbine location. Withinthis simulation viscous effects are considered andthe velocity profile inside the tube is consequentlydepending on radial and axial coordinates. Threedistinctive positions along the axis are chosen tofind the most attractive variant in terms of the in-teraction of the blockage effect of the contractedduct itself and the blockage of the turbine. Posi-tion one, pos1, is chosen at the interface of theinlet and throat. Because losses due to viscouseffects are not negligible, the highest amount ofenergy can be found at this position and the rel-atively long throat following gives the flow the op-portunity to develop to a more uniform profile andtherefore reduces the risk of separation in the dif-fuser. Separation in the diffuser causes significantpressure losses. The second position, pos2, isdefined in the throat, where a developed veloc-ity profile exists. Because the critical Reynoldsnumber is far exceeded, the velocity profile is ablunt turbulent one with mainly axial velocity com-ponents. With an almost constant core velocity,the flow approach towards the turbine is compa-rable to the situation for a bare turbine in a free

field and in consideration of a suggested conven-tional turbine desirable. To attend the theoreticalstreamline tube (fig. 4) of a bare turbine, posi-tion three, pos3, was found. By decelerating the

Figure 4: Ideal streamline tube, unducted turbine[10]

flow the streamline tube enlarges in diameter. Ofcourse, the situation for an encased turbine is dif-ferent by all means, but the velocity is deceleratedby the turbine as well and the diffuser allows orsupports, respectively, the expansion and so con-tributes a smooth flow through the device. Anotheridea, but not accomplished within this work, is thepositioning in half the depth of the diffuser, to in-clude the whole stream tube shown in fig. 4 anduse the inlet and throat as kind of wind accelerator,supporting the stream tube system. The contra ar-gument is the bigger diameter and the therewithlinked higher financial costs.

2 Venturi Layout

With the help of the Venturi standard [11] the pa-rameter settings within FIDAP are validated. Ithazards the guess, that the geometry given in thestandard for a Venturi is most investigated and de-veloped over the years to be optimized in mattersof losses due to friction but also separation. Touse the standard geometry would be recommend-able, as long as it is meant to accelerate a fluid.But as soon as an obstacle is implemented in thethroat, the application is changed and the stan-dard geometry does not constitute an optimumanymore. The configuration of an Eiffel wind tun-nel with closed model section is redolent of a Ven-turi tube but with an obstacle in the throat. Insteadof the natural wind, a fan is generating the airflow through the tunnel. Wind tunnel tests demandalso low turbulence intensity in the flow and lossesmight be reduced. Therefore based on wind tun-nel designs [12] certain laws for the Venturi geom-etry meant to encase a turbine are derived, as

§1 high curvature should take place in big diam-eter

§2 a constant area duct before the rotor sectionshould be employed

§3 the throat length behind the rotor should besufficient long

§4 diffusers opening angle has to be sufficientsmall

and are discussed following. It turns out that theISO standard fulfils the conditions. With respect tominimize losses due to friction, implicit minimizingthe all over length, the nozzle, throat and diffuserlength should be kept short as possible, butwithout incurring flow separation. Due to the lawof continuity for an incompressible fluid, the losswill not be reflected in the dynamic pressure, butin the static pressure, while the loss coefficientis defined on the basis of total pressure anddynamic pressure. Hence power losses areproportional to power two of the velocity in thecross section. This conclusion leads to the firstdesign law, concerning the inlet.

inlet: Curvatures of the structure and there-fore deflections of the flow are linked to losses,while the losses are proportional to the squaredvelocity.

§1 high curvature should take place in big diam-eter

The smaller radius is placed at the inlet mouthand is passing over tangential to the bigger radiusarc, meaning a high curvature by the small radius,hence in the section with lower velocities and alower curvature in the section, leading to the max-imum velocity (fig. 5).

Figure 5: Inlet nozzle consisting of two tangentialmerged circles

throat: It is in general desirable to reduce thelength of the whole Venturi device, thus an idea isto skip the whole throat section and join the noz-zle directly to the diffuser (fig. 6). Also, the powerlosses in the throat are sizable. Power could besaved by keeping it short. But design law numbertwo and three tell us: Contractions do not delivera uniform velocity distribution to the beginning ofthe throat, so

§2 a constant area duct before the rotor sectionshould be employed

Behind the rotor the flow might be separated andall separated flow zones shall be close before thebeginning of the diffuser, therefore

§3 the throat length behind the rotor should besufficient long

Figure 6: Straight section in front and behind therotor

diffuser: Since the power losses at any pointin the tunnel are expected to vary as the speedcubed, the purpose of the diffuser is to reduce thespeed with as little energy loss as possible. Min-imum energy loss corresponds to maximum pres-sure recovery. It is in general desirable to reducethe speed in the shortest possible distance withoutincurring flow separation.

§4 diffusers opening angle has to be sufficientsmall

Because of the throat segment behind the rotorplane, the separated flow zones are expected tobe closed again. Therefore a bigger opening an-gle is applicable. Here the diffuser opening angleis limited to max. ϕ = 30◦. With the lenght as aconstant parameter the model geometries are de-termined and listed in tab. 1.

Co 1.00 2.25 4.00 9.00d[m] 9.00 6.00 4.50 3.00

Linlet[m] − 1.57 2.32 3.07Lthroat[m] − 4.20 3.15 2.10Ldiffuser[m] − 10.28 10.87 11.20

ϕ2 [◦] 0.00 8.21 12.04 15.00

Table 1: d: throat diameter; Linlet: inlet length;Lthroat: throat length; Ldiffuser: diffuser length;ϕ2 : half diffuser opening angle

3 Model without Turbine

In this chapter the turbine is not implementedyet to find out about the blocking and separationbehaviour of the single contractions structuresthemselves and to choose later the most rea-sonable models to insert turbines at differentpositions. The computational domain is set up asseen in fig. 7, while the parameter settings arechosen in the following way:

PROBLEM: nonlinear, turbulent, axis-symmetric;VISCOSITY MODEL: Boussinesq-RNG (twoequation); SOLUTION: segregated solver,upwinding; ACCURACY: velc = 1 ∗ 10−5;resc = 1 ∗ 10−5; FLUID: ρ = 1.225kg/m3;µ = 1.8375 ∗ 10−5Ns/m2; INITIAL CON-DITIONS: k = 0.003; ε = 0.00045; INLET:radial velocity = 0m/s; axial velocity = 6m/s;k = 0.003; ε = 0.00045; SYMMETRY AXIS:radial velocity = 0m/s; STRUCTURE:wall velocity = 0m/s.

Figure 7: Computational domain set up, barestructure

With this settings interesting observations aremade, startig with the contraction Co = 1.00.

Co1.00: The streamline plot (fig. 8) showsa separation bubble inside the device, whichforms the so called ’Vena Contracta’ and thereforeworks like a Venturi constriction. In the inlet is

Figure 8: Streamline plot; Co = 1.00, bare

a mean velocity of 5.35m/s calculated. Windtunnel measurements carried out within the’Castle House’ feasibility study carried out by

the group of consultancy including Norwin A/Sfound an increase of the undisturbed wind velocitytaking place through the tubing, while the housemodel was additionally blocking the wind. Thestrong upwind, generated at the house facadeconcentrated the wind towards the upper part andtherefore the building blockage forced more flowgoing through the holes. This parameter studyhere decouples the effect of the building andtherefore the skewed inflow velocity componentsand concentrates on the inlay shaping only.

Co2.25: In comparison to the tube withoutcontraction, no separation takes place in the inletfor the contraction 2.25 (fig. 9). Especially the

Figure 9: Streamline plot; Co = 2.25, bare

diffuser in a Venturi shape turns out to be themost critical part, but also here the flow keepsattached. Remarkable is the all over shapeof the structure added up with the separationbubbles. It is redolent of a common profile shape.Fig. 10 shows the axial velocities around thestructure Co2.25. The field of interest is the throatsection where the highest velocities occur. Atthe throat inlet and outlet close to the wall theflow is deflected most and therefore must beaccelerated most as well, resulting in suction andvelocity peaks, respectively. The axial velocity

Figure 10: Velocity plot; Co = 2.25, bare

profiles are plotted at the important positionsalong the device axis; device inlet at 0m, throatinlet at 1.6m, position where the velocity profileis expected to be developed, 3.4m, throat outlet

at 5.8m and the device outlet at 16.3m. Usuallya turbine blade is exposed to a regular windvelocity profile. By rotation the blade experiencesa combined velocity. The more far out, towardsthe tip, the smaller gets the angle of attack. Togain the best lift over drag ratio geometrical twistis applied. An argument to position a turbine inthe cross sections with these distinctive peaks,throat inlet and throat outlet, is that the bladetwist could be reduced. Additionally would theresulting velocity at the tip be very high and withthis a high lift force could be obtained at a positiongenerating moment most effective. For the inletand the outlet mean velocity a value of 4.23m/s iscalculated. Through the throat a mean velocity of9.52m/s is determined, while the velocity profilesdiffers from each other in significant way. Fig.11 shows the high pressure area around thestagnation point and its spacious influence. It is

Figure 11: Static pressure plot; Co = 2.25, bare

important to consider, that the streamline throughthe stagnation point is representing the borderbetween streamlines entering the structure andstreamlines surrounding the structure. The influ-ence of the stagnation point position is obvious,but no analytical relationship is derived within thiswork.

Co4.00: On the outer surface of the devicethe separation bubble has grown compared tothe contraction 2.25, but it still reattaches to therear (fig. 12). In contrast to the flow through the

Figure 12: Streamline plot; Co = 4.00, bare

diffuser in Co2.25, flow separation is detected inthis case, which leads to higher pressure lossesthrough the diffuser but also to reduced efficiencyof the whole device. The stagnation point ofthe contraction of 4.00 is positioned already onthe curvature of the inlet (xstCo4.00 = 0.019m;ystCo4.00 = 4.187m), which reflects the blockingeffect of the higher contraction. Negative axial ve-

locity components exist in the very entrance of thedevice, resulting in a smaller ’effective’ inlet andthus contraction (fig. 13). The mean velocity in

Figure 13: Streamline through stagnation point

the inlet is with 2.67m/s far below the undisturbedwind velocity. Such a strong contraction of theflow increases the drag intensely. Directly linkedto that is a reduced mass flow through the device.The high pressure region around the stagnationpoint is shutting down the inlet.

Co9.00: For the contraction 9.00 the streamlineplot shows big separation areas (fig. 14). The

Figure 14: Streamline plot; Co = 9.00, bare

bubble on the outer surface has grown over therear edge and mingles with the outflow of thedevice. Inside the diffuser the flow is almost notattached at all. The stagnation point lies withxstCo9.00 = 0.19 m; ystCo9.00 = 3.45 m deep inthe nozzle and hazards the guess that the highamplification failed. Indeed, the throat velocitylies with 8.95m/s even below the contraction4.00 throat velocity. The low velocity area behindthe structure is deflected towards the axis andgenerates therein a kind of shielding. Alreadyfor contraction 4.00 this deflecting trend can beobserved. At the same time the pressure shield atthe front face is getting more rigorous (fig. 15).

Figure 15: Static pressure plot; Co = 9.00, bare

Comparison: Two properties are influencingthe structures in their quality to increase thepotential energy in the throat. One aspect to lookat is the blockage by the contraction. Anotherone is the velocity amplification. With increasingcontraction the velocity amplification increases,but the resistance increases as well, so that massflows arround the structure instead of flowingthrough it to get accelerated. The optimumbetween mass reduction and mass accelerationhas to be found. The blockage is defined by thevelocity in the throat, which would be achievedwith an inlet velocity equal to the wind velocity,over the actual lower throat velocity, due to masssurrounding the structure.

blockage =vthroattubed

vthroatCF D

(7)

The velocity amplification is defined by the actualthroat velocity over the undisturbed wind velocity.

velocity amplification =vthroatCF D

vw(8)

In terms of taking into account the two aspectsmentioned above, a contraction of approximately2.25 constitutes the maximum in effectiveness,where velocity amplification over blockage ratio isbiggest. A contraction of one is then more effec-tive than a contraction of four. A closer look on theenergy flux shows something different. Air flowingthrough a cross section in the size of the inlet, withthe velocity of the undisturbed wind has an energyflux potential of

dEw

dt=

12dm

dtv2

w =12ρAinletv

3w. (9)

The actual energy fluxes in the single throat sec-tions are calculated corresponding and listed in ta-ble 2. To keep the application of the geometriesfeasible, focus is taken on the contraction 2.25.With a throat diameter of six meters this struc-ture offers space for a small turbine. Besides thismore practical aspect the fact of the highest en-ergy amplification and no separation through thewhole Co2.25 device makes it most attractive. Byinserting a turbine, the blockage will be increasedeven more and therefore the mass flow throughthe throat reduced. In the following chapter thiscircumstance is examined for the contraction 2.25,while the turbine is positioned at three differentsignificant throat depths.

4 Model with turbine

With the help of body forces implementation a tur-bine modelled as an actuator disc can be simu-

definition Co1.00 Co2.25 Co4.00 C09.00vthroatCF D

[m/s] − 5.35 9.52 10.67 8.95vinletCF D

[m/s] − 5.35 4.23 2.67 1.00vthroattubed

[m/s] Ainlet

Athroatvw 6.00 13.50 24.00 54.00

dmdt CFD

[kg/s] Athroatρvthroat 416.95 329.68 207.82 77.51blockage [−] (7) 1.12 1.42 2.25 6.03

velocity amplification [−] (8) 0.89 1.59 1.78 1.49energyflux amplification [−] dE/dtthroatCF D

dE/dtw0.71 1.77 1.41 0.37

Table 2: Flow characteristics overview, models without turbine

lated. The constant distributed body forces consti-tute the thrust. For the free standing turbine, thethrust coefficient is defined [13] as

CT =T

12ρv

2wArotor

, (10)

referring to the undisturbed wind velocity flowingthrough an area in the size of the rotor, while T isthe turbines thrust. The power coefficient is theratio of harvested power over available power in across section equal to the area swept by the rotorand can also be written as

CP =TvAD

12ρv

3wArotor

, (11)

with vAD as the retarded wind velocity in the rotorplane. Inserting (10) in (11) the power coefficientcan be defined as

CP = CTvAD

vw, (12)

whileT = ∆pArotor. (13)

Assuming the ideal conditions from Froude- Rank-ine theorem a Betz maximum CPf

= 1627 and a

CTf= 8

9 are expected for a bare turbine. Prereq-uisite of an ideal power harvesting is an axial in-duction factor of one third, which shall be reflectedin a velocity in the actuator disc of two third of theundisturbed wind velocity, hence vAD = 4m/s.Fig. 16 shows the resulting values for the pres-sure and the velocity along the symmetry axis. Assoon as the rotor is encased it must be clarifiedto which area the dimensionless coefficients re-fer to. For the first consideration the coefficients,CP and CT , shall refer to the actual rotor sweep-ing area.This is the conventional way of looking atit, but makes a comparison difficult. Mostly, in in-vestigations done before, the increased mass flowthrough the turbine caused by a diffuser or shroudwas referred to the rotor cross section and com-pared to a bare rotor of the same size. Here isthe inlet area kept constant and given is the same

Figure 16: Velocity and static pressure plot alongsymmetrical axis, AD in free field, Betz optimum

initial situation, but the rotor diameter varies cor-responding to the contractions. Which inner ge-ometry combined with a turbine makes most outof the offered wind entering the same inlet is as-certained. In general the power coefficient is theratio of harvested power over power contained inthe wind. That is why in a second definition thepower coefficient refers to the inlet area, C∗p .

C∗P =∆pvADArotor

12ρv

3wAinlet

=CP

Co, (14)

Values for the thrust coefficient referring tothe rotor area are CT0 = 0.24, CT1 = 0.30,CT2 = 0.0.36, CT3 = 0.44, CT4 = 0.53,CT5 = 0.59, CT6 = 0.62, CT7 = 0.64, CT8 = 0.65,CT9 = 0.69.The settings considering the geom-etry, turbulence modeling, wind properties andboundary conditions are not changed comparedto the bare structure simulations. But the settingshere are added up with an actuator disc. Thesystems, casing and turbine, are named in the fol-lowing way Co2.25pos1, Co2.25pos2, Co2.25pos3and Co1.00pos1, Co1.00pos2, Co1.00pos3,respectively. In case of the contraction 1.00 ispos1 positioned directly at the device inlet, pos2 ata location where the velocity profile is expected tobe developed without the influence of the applied

thrust and pos3 directly at the outlet of the device.Pos1 is located directly at the throat inlet forthe contraction 2.25, while pos2 is located in thethroat where the velocity profile is expected to bedeveloped in case of a bare structure and pos3is positioned at the outlet of the throat, beforethe diffuser starts.A detailed examination of thesimulation results for Co2.25AD (Co = 2.25;actuator disk implemented) is performed, whilecase Co1.00AD (Co = 1.00; actuator diskimplemented) is run only until a thrust coefficientof CT = 0.53.

Co2.25AD: The results for CP at pos1, pos2and pos3 are plotted over the applied CT in fig.17. For the lower pressure sinks due to the actu-

Figure 17: CP over CT plot, Co2.25AD, pos1,pos2, pos3

ator disk the behaviour of the flow does not differdistinctively. At CT3 = 0.44 the first characteristicbranching occurs. The turbine at pos1 is blockingthe flow less than the turbines placed at pos2 andpos3, although the same thrust is affecting theflow. The static pressure sink due to the turbinehas the typical pressure maximum in front ofthe AD and the minimum behind approachingasymptotic the atmospheric pressure again. Torecall this pressure development, please see fig.16. This pressure maximum upstream the turbineis reaching more out of the structure inlet in caseone, than in two or three. So the pressure peakhas more space compared to the pressure peaksin front of the turbines positioned at pos2 or pos3.Because of the more restricted space for the pres-sure peak the ’pressure density’ is increasing andtherefore the blockage is increasing as well. Thiscircumstance is reflected by the decreasing inletvelocity and with it the smaller mass flow throughthe structure (fig. 18). In this figure it can also beseen, that the static pressure maximum along theaxis for the AD at pos1 is located 3.14m, for theAD at pos2 2.42m and for the AD at pos3 1.97m

in front of the structure inlet. By implementing

Figure 18: Static pressure plotted along the axis;structure inlet at 0m; Co2.25AD, CT = 0.53

CT5 = 0.59 a new effect starts to take over. Sincethe velocity is decelerated by the turbine, but incontrast to the pressure change in a continuousmanner, the lowest speed for the turbine wakeat position two is closer situated to the diffuserend, than for the turbine at position three. Whathappens is that the turbine wake with very lowvelocities grows and marches upstream until itacts at the diffuser outlet like a blocking area. Inthe mixing area of the two flow streams behind thestructure, the outer stream is deflected towardsthe axis due to the ’over expanded’ jet situationthere. At this point, additionally, the wake of thestructure with its separation bubble on the outersurface and the rear face starts to interact. Theoutflow has to find its way between the turbinewake and the structure wake (fig. 19). A barrier

Figure 19: Superposition of axial velocity andstreamline plot; Co2.25pos3, CT = 0.59

from behind is generated vice versa to the one atthe front face. An advantage of the turbine wakecould be the deflection of the devices inner flowtowards the diffuser wall and therein it contributesto the prevention of separation. Because the wakeis oscillating and therefore this effect is unstable,an idea could be to place a solid structure behindthe device to create an artificial but constantdeflection in the wake area. Further investigationsin this field are necessary, but not done withinthis study. If more energy is extracted, the flowthrough the Venturi device is decelerated more,while the flow around the structure is increasingin velocity, mass and deflection angle until thetwo wake areas close the structure from behind.The turbine at pos3 with the biggest disadvantage

concerning the blockage at the inlet operateswith the highest thrust coefficient and withstandsthe highest pressure drop, respectively. Butthe highest power coefficients among the threepositions is reached by the turbine at pos1. Fig.20 show a storyboard for the flow developmentwith the turbine implemented at position one.

Figure 20: Streamline, AD velocity profile, pres-sure and axial velocity plots; Co2.25pos1, CT =0.53, CT = 0.59, CT = 0.62, CT = 0.66

Co1.00AD: With the streamline plots forCo1.00pos1 (fig. 21) the growing separationbubble on the outer surface of the model is visi-ble. Remarkable is the streamline tube wideningbehind the structure, resembling the expectedstreamline tube for a bare turbine. The typicalstreamline tube seems to be shortly interruptedby the structure and continues afterwards. Thebehaviour that can be observed is, the higherthe deceleration of the wind velocity is the morenarrows the ’supporting’ streamline tube. ForCT = 0.3, for example, the inlet cross sectionof the streamline tube far in front of the struc-ture is around 9m2 larger than for the case ofCT = 0.53. This also reflects the decreasingmass flow through the structure and actuatordisc, respectively. However, the turbines positionseems not to influence the performance in asignificant way.Comparison: The qualitative description of the

Figure 21: Streamline plots; Co1.00AD, CT =0.30, CT = 0.36, CT = 0.44, CT = 0.53

untypical system Co2.25AD flow behaviour andthe explanation by the ’shield’ at the front face andthe ’blocking’ at the rear face is not analyticallydefined. It could be the influence of the more orless deep located turbine with its pressure peaks.In contrast to the contraction of 2.25, the modelwith no Venturi shaping inside shows a veryregular behaviour with no significant differencesamong the three turbine positions. Probablyhigher thrust coefficients would enforce moredistinctive characters. For the Co2.25AD model,pos1 turns out to be the most effective position,therefore its results are compared in the followingwith the Co1.00pos1 results. As mentionedearlier, to compare the two cases with the help ofthe power coefficient is difficult, because the rotordiameters are not the same, but the inlet diameter.In case of referring the power coefficient to therotor area, the Betz maximum of 16

27 is exceededwith CP4(Co2.25pos1) = 0.69 by 16.68%, whilethe thrust coefficient, referring to the rotor areaas well, lies with CT4(Co2.25pos1) = 0.53 farbelow 8

9 . Which value can be directly used forcomparison is the power itself.

P = CP12ρv3

wArotor. (15)

The derived values are listed in tab. 3. An idealbare turbine and its operation, one rotor with inletdiameter, big bare, and one rotor with throat di-ameter, small bare, is analytically calculated [13]and inserted in the same table. Although theCo1.00AD model is not run for higher thrust co-efficients, higher power values could be achievedcertainly. But comparing the power maximum forthe contraction 2.25, turbine positioned at pos1,with the power value achieved by the model with-out constriction but same pressure drop, showsthe potential of the shaping combined with theright turbine positioning. To compare the turbine

CT1 CT2 CT3 CT4

Co1.00CP [−] 0.23 0.25 0.30 0.31

power [W ] 1953 2115 2486 2592Co2.25CP [−] 0.40 0.47 0.55 0.69C∗P [−] 0.18 0.21 0.25 0.31

power [W ] 1506 1756 2067 2586big bareCP [−] 0.28 0.33 0.38 0.45

power [W ] 2319 2747 3233 3767small bareCP [−] 0.28 0.33 0.38 0.45

power [W ] 1031 1221 1437 1674

Table 3: Flow characteristics overview, modelswithout turbine

encased by the constricted tubing with the oneencased by a constant diameter tubing the C∗Pseems to be a convenient coefficient, while thefact of the power harvesting by a smaller turbineis not reflected with it. Comparing the big barerotor power with the power gained by the Co1.00encased turbine, it performs better. Of course, ithas to be remarked, that effects like tip loss reduc-tion, varying thrust distribution etc. are not consid-ered here and might change the conclusion. Def-initely the comparison between a small bare rotorand the Co2.25 encased rotor is significant. Thepower amplification is 54.24%.

5 Conclusion

Within a comparison with bare turbines, the en-cased turbine, nine meters in diameter, producedless power than the bare turbine the same size.Reviewing the bare Co1.00 structure, alreadyblocking the wind, a mean velocity of 5.35m/s wasfound in the device, which lies below the 6m/s forthe approaching wind velocity. In contrast to thatthe 6m in diameter turbine, placed inside the Ven-turi shaped duct, seemed to be able to produce54.24% more power than a bare turbine the samesize. Regards to the different tip loss situationsmight lead to another result, but were not consid-ered in this work. It also has to be mentioned, thatthis comparison is only valid for a constant winddirection. Thus comes a Venturi shaped casingcompared to a straight duct with the advantages ofa smaller rotor and with it lower production, trans-portation etc. costs and especially installation onhigh buildings with corresponding erection equip-ment might be more feasible or less complicated.Also important is due to the contracting inlet, the

lowered turbulence intensity in the flow approach-ing the turbine. Furthermore is the Venturi effectamplifying the wind velocity and so the air veloc-ity in the throat might be sufficient high to operatethe turbine, while the big rotor experiencing veloc-ities even below wind velocity still stands still. Onthe other hand is it very important to consider thechanged loads situation on the building structure,which might needs to be strengthened, plannedstiffer or may more flexible at certain locations. Ingeneral is an encasing for turbines in urban ar-eas recommendable, e.g. in case of a blade lossthe centrifugal energy will be first caught by thecasing. Also frequently moving shadows, cast onbuildings, could be reduced, due to the more hid-den turbine. Lower adequate noise levels, de-manded in towns, could be fulfilled by the possi-bility to integrate damping material in the encas-ing structure and probably directing of the soundwaves. But these are already some topics whichhave to be investigated in further studies. It isnot possible to integrate the Venturi shape withoutchanging the appearance dramatically (fig. 22).

Figure 22: Venturi shape integrated in ’CastleHouse’

6 Future work

It was a challenge to restrict the parameter studyand find constant parameter, respectively. There-fore a wide field for further investigations alreadyexists only for the parameter study itself. So itwould be interesting to know more about the flowbehaviour with different wind velocities, influenceof skewed inflow, regards to the tip loss situa-tion, structure to prevent human hazard in case of

blade loss, considering of turbine fixing and na-celle structure and many more. The next stepcould be to go from the decoupled coin modelback to the Venturi integrated in a building andtake the whole structure geometry exposed to awind velocity profile in urban areas into account.Probably a vertical skewed device build in anglecould catch the upwind better to prevent separa-tion in the inlet.

7 Acknowledgments

This work is dedicated to my beloved family.

References

[1] www.enflo-windtec.ch; 21stMarch2008

[2] www.pacwind.net; 21stMarch2008

[3] www.loopwing.co.jp; 21stMarch2008

[4] www.aeroliftpatent.com; 21stMarch 2008

[5] www.hushenergy.com; 21stMarch 2008

[6] www.avinc.com; 21stMarch 2008

[7] www.aerotecture.com; 21stMarch 2008

[8] www.intelligenttravel.typepad.com;21stMarch 2008

[9] Van Bussel G. The science of making moretorque from wind: Diffuser experiments andtheory revisited; Conference Series 75. Jour-nal of Physics, 2007

[10] Kühn M. Windenergienutzung I; lecturenotes; Universität Stuttgart, 2006

[11] DIN EN ISO 5167-3:2003; Durchflussmes-sung von Fluiden mit Drosselgerätenin voll durchströmten Leitungen mitKreisquerschnitt- Teil3: Düsen und Ven-turidüsen; DIN Deutsches Institut für Nor-mung e. V. Berlin, Deutschland, 2003

[12] Barlow JB, Rae WHJr, Pope A. Low-SpeedWind Tunnel Testing; John Wiley & Sons,Inc., third edition, 1999

[13] Hansen MOL. Aerodynamics of Wind Tur-bines; James & James (Science Publishers)Ltd., London 2001