leeward flow visualization of a multi-mw wind turbine

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14th International Conference on Wind Engineering – Porto Alegre, Brazil – June 21-26, 2015

Leeward Flow Visualization of a multi-MW Wind Turbine Tower 1:75 scale

Anabel Apcarian1, Jorge Luis Lässig

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1Department of Civil Eng., Faculty of Engineering, Universidad Nacional del Comahue, Neuquén, Argentina

2Department of Applied Mechanics, Faculty of Engineering, Universidad Nacional del Comahue, Neuquén, Argentina

email: [email protected], [email protected]

ABSTRACT: In the Northern part of the Neuquén Province in Argentina an Aeolic Energy Generation Park is currently under

design. The winds blowing across this territory are mainly characterized by their great intensity and turbulence. The wind

turbines to be installed will be multi-MW type, for which the most usual support structure typology features a series of tubular

tapered cylinder-shaped towers. The flow pattern of these structures, usually exposed to local winds, has not yet been studied.

Thus, prior to the structural design, it is necessary to determine its characterization. The main purpose of this study was to

visualize the leeward flow of a tower for a multi-MW aerogenerator subjected to a wind profile of atmospheric boundary layer,

and the rotor detained on feathered pitch position. The test setup was performed with an open-loop indraft wind tunnel that

supports uniform flow and atmospheric boundary layer flow conditions. Standard-tower scale models of 1:75 were built for a

3MW turbine. Visualizations of flow with smoke and weather vanes for Reynold numbers based on a mean diameter between

8,7x103 and 4,4x10

4 were made. Vortex shedding shown in the tower being tested is three-dimensional phenomenon and the

flow pattern of the wake features variations related to the incidental flow velocity. For Reynold numbers between 3,1x103 and

4,4x104, lateral and leeward vertical flow currents are shown. Additionally, a cyclical variation in the flow angle of incidence for

wind velocities from 5m/s to 15 m/s was observed. The methods chosen for visualization provide a first approach in the

qualitative description of the wake flow. Future research should aim to find correlation between the vortex shedding patterns

observed in this study and spectral analysis from the fluctuating components of wind velocity.

KEY WORDS: Tapered Cylinders; Oblique Vortex Shedding; Wind Turbine Tower; Vortex Patterns

1 INTRODUCTION

In the Northern part of the Neuquén Province in Argentina an Aeolic Energy Generation Park is currently under design. The

winds blowing across this territory are mainly characterized by their great intensity and turbulence [1].

The wind turbines to be installed will be multi-MW type, for which the most usual support structure typology features a series

of tubular tapered cylinder-shaped towers.

The non-uniformity of the inflow or the tower geometry may cause three-dimensional effects in the vortex shedding such as

oblique vortex shedding, cellular vortex shedding, vortex dislocations and vortex splitting [2][3]. Currently, these effects are not

totally understood and during the last years empirical models have been created in order to explain them. Most of these records

can be found in the Williamson and Govardhan Review [4].

A permanent aerodynamic instability such as vortex shedding may cause structural deflections, instigating an aeroelastic

phenomenon. Fluid-elastic interaction is a function of the structural characteristics and the fluid. Even with moderate wind

velocities great oscillations can appear on the structure, subjecting it to a great number of load cycles that may wear out the

material. Fatigue may cause structural collapse before the material reaches its ultimate failure and this is why structural design

should rely on a realistic description of the vortex shedding pattern.

The main purpose of this study was to visualize the leeward flow of a tower for a multi-MW aerogenerator subjected to a wind

profile of atmospheric boundary layer, and the rotor detained on feathered pitch position.

2 METODOLOGY

2.1 Test setup

The test setup was performed with an open-loop indraft wind tunnel that supports uniform flow and atmospheric boundary

layer flow conditions. The tunnel is located in the “Boundary Layer & Fluid Dynamic Environmental Laboratory” at the

Department of Aeronautics, School of Engineering of the National University of La Plata. The tunnel is 24 m long and has a

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constant cross-sectional area of 2.60 m by 1.80 m. It operates by aspiration and consists of nine fans of 1.25 m diameter and15

HP power each. The maximum speed for tensile testing is 15 m/s.

The airflow conditions in the wind tunnel were similar to the ones of the local region under study. As prototype, a standard

sized tower for a 3MW wind turbine was chosen (Figure 1). The height of the tower was 80 m and its minor and major

diameters were 3 m and 6 m respectively. Wood-based 1:75 scale models were constructed to be used during the testing (Figure

2).

Figure 1: Prototype dimensions

Figure 2: Test setup

2.2 Weather vanes visualization

Weather vanes made with 1cm-length white threads were placed in the middle of the tower as shown in figure 3. Movements

for flow velocities of 10 m/s, 12 m/s, 13 m/s and 15 m/s were observed. These speed rates were correlated to Reynolds numbers

between 3.1x104 and 4.4 x10

4 based on the tower mean diameter.

2.3 Smoke visualization

Smoke tests at increasing wind velocities were performed to visualize the flow structure around the tower. Reynolds numbers

for these tests were between 8,7x103 and 4,4x10

4.

2.4 Record and data process

Tests were recorded with the use of 60 fps and 30 fps cameras. Videos were analyzed and photograms were extracted in order

to examine in detail the types of vortexes being released and also to perform measurements over the images graphically.

Ø90,00m

4,18m

80,0

0m

2,36m

3


Figure 3: Weather vanes

3 RESULTS

Weather vanes visualizations showed lateral and leeward vertical flow currents on the tower coming from different directions

(see figures 4 to 7). Their locations slightly varied with the wind velocity.

Smoke tests showed turbulent upstream flow, which confirms the reproduction of real conditions of the prototype. Wake

vortex patterns were three-dimensional and varied with the flow velocity (Figure 8).

Vortex types 2S, 2P (Williamson & Roshko Classification) and hybrid modes were identified [5]. Vortex movements in

spanwise direction were observed in the wake, which seemed more notorious at lower flow velocities.

Vortex shedding frequency varies along the spam due to the local diameter variation. It was also observed that the greater the

flow velocity, the lower the wake amplitude and the smaller the vortex formation region. A cyclical variation in the flow angle

of incidence for velocities from 5m/s to 15 m/s was observed.

Since it was not possible to simultaneously observe the flow across the entire span, conclusions over the presence of vortex

release cells cannot be withdrawn. However, a stochastic phenomenon of dislocation and vortex adhesion was observed. Some

2S and 2P vortexes bend with respect to the model axis. When they are dragged downstream the current, they change their

configuration rolling up and linking to another type of structure usually present in the adjacent layers of the fluid. This new

configuration is manifested in the shape of a hairpin vortex and induces perturbations in the wake that are lately transmitted

upstream. An example of this process is shown in figure 9.

In other cases, a vortex starts rolling up in one plane and continues its movement in a transversal direction, tracing a three-

dimensional path, as it can be seen in figure 10.

4 CONCLUSIONS

Vortex shedding shown in the tower being tested is three-dimensional phenomenon and the flow pattern of the wake features

variations related to the incidental flow velocity.

For Reynolds numbers between 3,1x103 and 4,4x10

4, lateral and leeward vertical flow currents are shown. Additionally, a

cyclical variation in the flow angle of incidence for wind velocities from 5m/s to 15 m/s was observed.

Vortex release phenomenon type 2S and 2P was detected. Downstream the current, the formation of fork vortexes that cause

perturbation upstream was observed. Test kind and scope could not show the presence of release cells; however a stochastic

phenomenon of dislocation and vortex adhesion was noted.

The methods chosen for visualization provide a first approach in the qualitative description of the wake flow. Future research

should aim to find correlation between the vortex shedding patterns observed in this study and spectral analysis from the

fluctuating components of wind velocity.

14th International Conference on Wind Engineering

Figure 4: Weather Vanes visualization for 1



: Weather Vanes visualization for 10 m/s incoming flow (Re =2.9x104). Time frame between each photogra

is 0.02s

Weather Vanes visualization for 12 m/s incoming flow (Re =3.5x104). Time frame between each photogra

is 0.02s

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Time frame between each photogram






Weather Vanes visualization for 13 m/s incoming flow (Re =3.8x104). Time frame between each photogra

is 0.02s

Weather Vanes visualization for 15 m/s incoming flow (Re =4.4 x104). Time frame between each photogra

is 0.02s

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Figure

Figura 9: Secuence of vortex splits and adhesions for Re =


Figure 8: Smoke visualization for 5 m/s incoming flow

ortex splits and adhesions for Re = 1.45 x 104, based on mean diameter.

each photogram is 0.09 seconds.

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diameter. Time frame between


Figura 10: Structural 3D evolution in

ACKNOWLEDGMENTS

The authors gratefully acknowledge the support

Department of Aeronautics, School of Engineering of the National University of La Plata

contribution.

REFERENCES

[1] Lässig, J. L., Palese, C., & Apcarian, A.

Meteorológica , 36 (2), 83-93, 2011.

[2] Vallès, B., Andersson, H.I. and Jenssen, J.B., Oblique Vortex Shedding behind Tapered Cylinders. Journal of Fluids and

Structures, Vol. 16, pp. 453-463, 2002.

[3] Narasimhamurthy, Vagesh D. et al., Simulation of Unsteady Flow pa

Boundary Method, European Conference on Computational Fluid Dynamics, Delft, 2006.

[4] Williamson, C.H.K. and Govardhan, R., A brief review of recent results in vortex

Engineering and Industrial Aerodynamics, 96, pp. 713

[5] Williamson, C.H.K. and Roshko, A., Vortex formation in the wake of an oscilating cylinder,

Structures, Vol. 2, pp. 355-381,1998.


evolution in the wake, at Re = 1.45 x 104, based on mean diameter.

photogram is 0.06 seconds.

gratefully acknowledge the support of the “Boundary Layer & Fluid Dynamic Environmental Laboratory” at the

Department of Aeronautics, School of Engineering of the National University of La Plata, and

Lässig, J. L., Palese, C., & Apcarian, A. Vientos Extremos en la Provincia de Neuquén. (C. A. Meteorólogos, Ed.)

] Vallès, B., Andersson, H.I. and Jenssen, J.B., Oblique Vortex Shedding behind Tapered Cylinders. Journal of Fluids and

] Narasimhamurthy, Vagesh D. et al., Simulation of Unsteady Flow past Tapered Circular Cylinders u

Boundary Method, European Conference on Computational Fluid Dynamics, Delft, 2006.

Govardhan, R., A brief review of recent results in vortex-induced vibrations, Journal of Wind

Engineering and Industrial Aerodynamics, 96, pp. 713-735, 2008.

] Williamson, C.H.K. and Roshko, A., Vortex formation in the wake of an oscilating cylinder,

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, based on mean diameter. Time frame between each

“Boundary Layer & Fluid Dynamic Environmental Laboratory” at the

, and Msc. Lynn Van Broock’s

Vientos Extremos en la Provincia de Neuquén. (C. A. Meteorólogos, Ed.)

] Vallès, B., Andersson, H.I. and Jenssen, J.B., Oblique Vortex Shedding behind Tapered Cylinders. Journal of Fluids and

st Tapered Circular Cylinders using and Immersed

induced vibrations, Journal of Wind

] Williamson, C.H.K. and Roshko, A., Vortex formation in the wake of an oscilating cylinder, Journal of Fluids and

leeward flow visualization of a multi-mw wind turbine

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