Keywords: dynamic stall, free stream turbulence, pitching airfoil

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1 Applied Mechanics and Materials Vol. 225 (2012) pp Online available since 2012/Nov/29 at (2012) Trans Tech Publications, Switzerland doi: / Experimental Study of Free Stream Turbulence Effects on Dynamic Stall of Pitching Airfoil by using Particle Image Velocimetry T.S. Leu 1,a, J. M. Yu 2, C. C. Hu 3, J. J. Miau 4, S. Y. Liang 5, J.Y. Li 6, J. C. Cheng 7 and S. J. Chen 8 1,2,4,5,6 National Cheng Kung University, Taiwan 3 Kao Yuan University, Taiwan 7 National Formosa University, Taiwan 8 Temple University, USA a tsleu@mail.ncku.edu.tw Keywords: dynamic stall, free stream turbulence, pitching airfoil Abstract. The unsteady flow fields above NACA 0015 airfoil pitching with/without upstream turbulence generator are investigated in a water tunnel by mean of particle image velocimetry (PIV). The turbulence was generated by a square bar mesh situated at the inlet of the test section. The airfoil pitching waveform is performed under the condition calculated from the angle of attack histogram of a vertical axis wind turbine (VAWT). By using PIV, the instantaneous vortex structures above the pitching airfoil can be revealed. It allows us to study the free stream turbulence effects on dynamic stall over an airfoil at pitching waveform the same as VAWT. It is found that the free stream turbulence intensity has significant impacts on the dynamic stall process. The dynamic stall process is delayed to higher incidence angles on increasing the turbulence intensity. Introduction The unsteady flow phenomena of dynamic stall above a pitching airfoil attracted a significant attention to aircraft during the last decade [1-3]. The most available data were devoted to low turbulence. Recently, a great deal of attention has been paid to investigate the aerodynamic performance of vertical-axis wind turbines (VAWT) [4-6]. VAWT often operates inside an atmospheric turbulent boundary layer. The purpose of this study is to investigate the turbulence effects on dynamic stall by using Particle Image Velocimetry (PIV) techniques. Particular attention is given to the variation of dynamic stall phenomena and associated vortex structures in the presence of free stream turbulence intensity. By investigation of the dynamic stall in the pitching airfoil experiments, one can have more insights what would occur for VAWT. Experimental Setup The experiments were conducted in the water tunnel with a test section of 600cm x 60cm x 250cm. Fig. 1 shows the apparatus for the experiments. The airfoil, NACA 0015, used in this study with chord length C=3cm and a wingspan of 18cm was horizontally installed in the center of the test section. The turbulence generator is constructed by a wooden square bar mesh located at the inlet of the test section. The cross section of wooden square bars is 10 mm by 10 mm with mesh size of 20 mm. The free stream turbulence intensity at the center of test section is about 6.9%. The airfoil driving mechanism is capable of pitching the model with a predefined waveform by a servo motor. The pitching rotation axis is fixed at the quarter chord of an airfoil. Airfoil Pitching Waveform. Fig. 2 shows the airfoil pitching waveform in the current experiments. The airfoil pitching waveform is calculated from the angle of attack (AOA) histogram of VAWT at tip speed ratio λ=2 where pitching amplitude of the waveform is between ±30 o. The relation between the incidence angle (α) of the flow and the tip speed ratio (λ) of VAWT can be described as follows [6]: All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, (ID: /12/12,03:28:09)

2 104 AEROTECH IV sinωt α = tan 1 ( ) (1) cosωt+ λ where α is the incidence angle of attack (AOA) related to the blade chord, ω is the angular speed of a VAWT. The tip speed ratio λ of VAWT with rotational radius R and free stream velocity U is defined as [6]: Rω λ = (2) U k=0.09 α (Degree) Fig. 1 Schematic sketch of the experimental setup Time (Sec.) Fig. 2 The waveform of pitching airfoil In Fig. 2, the incidence angle (α) increases from α=0 o to α=30 o during the time t=0~2.6 sec, then decreases from α=30 o to α=-30 o during the time t=2.6~5.4 sec. We define airfoil is in an upstroke period if the incidence angle of the airfoil is increasing (t=0~2.6 & 5.4~7.8 sec) and down stroke period when the airfoil is pitching downward (t=2.6~5.4 sec). All the experiments were conducted at free stream velocity U =13.7 cm/s, which corresponds to Reynolds number Re( = U C/ υ) = The phenomenon of dynamic stall is investigated by using particle image velocimetry (PIV) techniques. For PIV measurement, tracer particles are hollow glass microspheres of 13µm diameter with a specific gravity of about 1.0. The illumination was made by laser-light-sheet to visualize the two dimensional flow phenomena in the vertical plane at the test section. The laser used in the present study is an argon laser with energy 5 W and wavelength 488nm. The thickness of the laser sheet is about 1mm. The observation was made by a monochrome high speed digital CMOS camera (Photron FASTCAM SA-X) having image size and resolution of 1280 pixel x 1024 pixel with 12 bit. The digital images are analyzed with commercial software (PIV View) to obtain the velocity vector and vorticity distributions. Results and Discussion Fig. 3 presents sequential plots of instantaneous velocity vector with streamline patterns and vorticity distribution around the airfoil at different incidence angles when the airfoil is pitching with the waveform shown in Fig. 2. For better comparison, the left plots in Fig. 3 shows the flow structures without an upstream turbulence generator and the right plots depict the flow field with an upstream turbulence generator. For flow fields without turbulence generator (left-side plots of Fig. 3), the flow remains attached to the airfoil at incidence angles below α<10 o. At incidence angle α=11 o (left plot of Fig. 3a), the flow separates from the airfoil. The separation region grows in size from α=14 o to α=17 o (Fig. 3b~Fig. 3d). At incidence angle α=17 o (Fig. 3d), the thickening of the vertical region that occurs from mid-chord length to trailing edge is due to the presence of a strong organized dynamic stall vortex above the airfoil. The dynamic stall vortex becomes stronger and the vortex remains above the airfoil for the incidence angle increasing from α=20 o to the maximum α=30 o. After the incidence angle α=30 o, the dynamic stall vortex starts shedding downward when the airfoil is pitching down from α=30 o to α=20 o (Fig. 3k~Fig. 3m).

3 Applied Mechanics and Materials Vol (a) α=11 o upstroke (b) α=14 o upstroke (c) α=15 o upstroke (d) α=17 o upstroke (e) α=20 o upstroke

4 106 AEROTECH IV (f) α=22 o upstroke (g) α=23 o upstroke (h) α=24 o upstroke (i) α=26 o upstroke (j) α=28 o upstroke

Applied Mechanics and Materials Vol. 225 107 (k) α=30 o upstroke (l) α=25 o down stroke (m) α=20 o down stroke Fig.

5 Applied Mechanics and Materials Vol (k) α=30 o upstroke (l) α=25 o down stroke (m) α=20 o down stroke Fig. 3 Instantaneous flow velocity vector, streamline and vorticity distribution around the airfoil at various incidence angles during pitching (left plots are the flow without turbulence generator, right plots are the flow with turbulence generator) About the flow fields with turbulence generator (right-side plots of Fig. 3), they show different dynamic stall evolution process from the cases without turbulence generator. Unlike flow separation at α=11 o in the cases without turbulence generator, no obvious flow separation occurs at an incidence angle below α<17 o (Fig. 3a ~ Fig. 3d). At incidence angle α=20 o, a small separation bubble near the leading edge appears clearly in Fig. 3e. Fig. 3e shows the beginning of dynamic stall with a clockwise (negative) vorticity region occurred near the leading edge. One feature of the turbulence effects on the dynamic stall is the delay of dynamic stall to angles beyond the cases without turbulence generator. The leading edge vortex (LEV) grows in size and becomes an organized dynamic stall vortex from α=20 o to α=24 o (Fig. 3e~Fig. 3f). At incidence angle α=24 o (Fig. 3f), the organized dynamic stall vortex attaches on the airfoil. After the incidence angle α=26 o, the dynamic stall vortex still grows, but starts to move downward when the airfoil is pitching up from α=26 o to maximum α=30 o (Fig. 3i~Fig. 3k). In the down stroke period from α=30 o to α=20 o, the vortex seems to move back behind the airfoil at α=25 o and sheds downstream at α=20 o. The significant feature of free stream turbulence effects can be studied by careful comparison between the flow structures with and without a turbulence generator. For instance, the flow field begins to separate at α=11 o for the cases without turbulence generator. Instead of separation from leading edge, flow separates near mid-chord length and moves toward leading edge for flow field without turbulence generator. Dynamic stall processes happen from α=14 o to α=30 o. In the down stroke period, vortex shedding occurs. However, the flow structures with turbulence generator still keep attached until α=17 o and a small leading edge separation bubble occurs at α=20 o, the shear

6 108 AEROTECH IV layer region of turbulent free stream is relatively smaller than the cases without turbulence generator. This is due to the higher momentum transfer capability of turbulence. With higher turbulence intensity in the free stream, the momentum transfer across the shear layer is enhanced. Thus, the dynamic stall is significantly delayed until α=20 o. Dynamic stall process evolutes from α=20 o upstroke to α=25 o down stroke for the cases with turbulence generator. Concluding Remarks The unsteady flow fields above a NACA 0015 airfoil pitching under the waveform condition calculated from the angle of attack (ΑΟΑ) histogram of a vertical axis wind turbine (VAWT) at tip speed ratio λ=2 are investigated in a water tunnel by mean of PIV. The flow patterns above the suction side of the airfoil and formation of stall vortices have been observed. It is found that the free stream turbulence intensity has significant impacts on the dynamic stall process. Without an upstream turbulence generator, dynamic stall vortex occurs at α=17 o and evolutes from α=17 o to α=30 o upstroke. With the turbulence generator at upstream, the dynamic stall processes happen at higher incidence angle than the cases without turbulence generator. With an upstream turbulence generator, dynamic stall vortex occurs at α=20 o and dynamic stall process evolutes from α=20 o upstroke to α=25 o down stroke. Acknowledgement Funding support of National Science Council, Taiwan, under the project number P and E for this work is gratefully acknowledged. References [1] M. Raffel, J. Kompenhans, and P. Wernert, Investigation of the unsteady flow velocity field above an airfoil pitching under deep dynamic stall conditions, Experiment in Fluids, Vol. 19, pp (1995) [2] P. Wernert, G. Koerber, F. Wietrich, M. Raffel, and J. Kompenhans, Demonstration by PIV of the non-reproducibility of the flow field around an airfoil pitching under deep dynamic stall conditions and consequences thereof, Aerospace Science and Technology, Vol. 2, pp (1997) [3] H. Oshima, and B. R. Ramaprian, Velocity Measurements over a Pitching Airfoil, AIAA [4] Journal, Vol. 35, No. 1, pp (1997) [5] R. E. Sheldahl, Comparison of Field and Wind Tunnel Darrieus Wind Turbine Data, SAND (1981) [6] N. Fujisawa, and M. Takeuchi, Flow Visualization and PIV measurements of Flow Field around a Darrieus Rotor in Dynamic Stall, Journal of Visualization, Vol. 1, No. 4, pp (1999) [7] M. Islam, D. S. K. Ting, and A. Fartaj, Aerodynamic Models for Darrieus-Type Straight-Bladed Vertical Axis Wind Turbines, Renewable & Sustainable Energy Reviews, Vol. 12, No. 4, pp (2008)

7 AEROTECH IV / Experimental Study of Free Stream Turbulent Effects on Dynamic Stall of Pitching Airfoil by Using Particle Image Velocimetry /

Dynamic Stall For A Vertical Axis Wind Turbine In A Two-Dimensional Study

Abstracts of Conference Papers: TSBE EngD Conference, TSBE Centre, University of Reading, Whiteknights, RG6 Dynamic Stall For A Vertical Axis Wind Turbine In A Two-Dimensional Study R. Nobile 1,*, Dr M.