Vortex-Induced Vibration of an Airfoil Placed in Crossflow · Vortex-Induced Vibration of an...
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Vortex-Induced Vibration of an Airfoil Placed in Crossflow Bridget M. Benner, Daniel W. Carlson, Banafsheh Seyed Aghazadeh, and Yahya Modarres-Sadeghi Fluid-Structure Interactions Laboratory, University of Massachusetts, Amherst Introduction Vertical-axis wind turbines have the potential to be the future of offshore wind energy. My project focuses on experimental study of the response of a vertical-axis turbine blade experiencing flow-induced vibration at varying angles of attack in the range of α=0°-90° and a reduced velocity range of U*=0-12. The results will be used to better understand the type of flow- induced oscillations the airfoil experiences; i.e., vortex-induced vibrations, which can have disastrous consequences for the structure. Conclusions During operation, vertical-axis turbines are known to encounter every possible angle of attack. Thus, for certain flow velocities and structural natural frequencies, vertical axis turbines are shown to experience VIV. Due to symmetry, this VIV lock-in range would be experienced twice per revolution for each blade, having a deleterious effect on the fatigue life of the turbine as a whole. Results • Oscillations were observed for angles of attack between 55° and 90°. Figure 3. • When oscillations occurred, the frequency of the structure at that particular reduced velocity and angle, matched the structure’s natural frequency at the same angle of attack. Figure 4. • Flow visualization tests were conducted to study the vortex shedding patterns in the wake of the airfoil. The shedding patterns concluded the structure was experiencing VIV. Figure 5. Experimental Design • Experiments were conducted in a recirculating water tunnel, using a scaled NACA airfoil (NACA 0021). • The airfoil was completely submerged in flow and the set-up holding the airfoil in the test section constrained the oscillations of the airfoil to one degree of freedom in the direction normal to the incoming flow (crossflow). Figure 1 and 2. Acknowledgements: This work is supported in part by the National Science Foundation under NSF award number 1460461. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect those of the National Science Foundation. http://cleantechnica.com What is Vortex-Induced Vibration? •A bluff body that is either flexible or flexibly-mounted is placed in water or air flow. •A wake forms that is unstable, producing vortices that induce unsteady forces on the body. •The frequency of vortex shedding locks in with the system’s natural frequency, producing large amplitude oscillations. Figure 2. Schematic of the experimental set-up. Figure 1. Flow direction and the angle of attack. Figure 5. Video. Downstream view of airfoil oscillating and flow visualization at α=70°, U*=6.96. Top: Figure 3. Dimensionless amplitude of the crossflow oscillations (A*=A/D) versus the reduced velocity (U*=U/(nf*D)) for each angle of attack, α. Bottom: Figure 4. Frequency ratio (f*=f/nf) versus the reduced velocity, U*, for each angle of attack where oscillations were observed. Reference: B. Seyed-Aghazadeh, D.W. Carlson, Y. Modarres-Sadeghi (2016). Vortex-Induced Vibration and Galloping of Prisms with Triangular Cross- Sections Placed in Water Flow.
Transcript of Vortex-Induced Vibration of an Airfoil Placed in Crossflow · Vortex-Induced Vibration of an...
Vortex-Induced Vibration of an Airfoil Placed in Crossflow
Bridget M. Benner, Daniel W. Carlson, Banafsheh Seyed Aghazadeh, and Yahya Modarres-Sadeghi Fluid-Structure Interactions Laboratory, University of Massachusetts, Amherst
Introduction Vertical-axis wind turbines have the potential to be the future of offshore wind energy. My
project focuses on experimental study of the response of a vertical-axis turbine blade experiencing flow-induced vibration at varying angles of attack in the range of α=0°-90° and a reduced velocity range of U*=0-12. The results will be used to better understand the type of flow-induced oscillations the airfoil experiences; i.e., vortex-induced vibrations, which can have disastrous consequences for the structure.
Conclusions During operation, vertical-axis turbines are known to
encounter every possible angle of attack. Thus, for certain flow velocities and structural natural frequencies, vertical axis turbines are shown to experience VIV.
Due to symmetry, this VIV lock-in range would be experienced twice per revolution for each blade, having a deleterious effect on the fatigue life of the turbine as a whole.
Results • Oscillations were observed for angles of attack
between 55° and 90°. Figure 3. • When oscillations occurred, the frequency of the
structure at that particular reduced velocity and angle, matched the structure’s natural frequency at the same angle of attack. Figure 4.
• Flow visualization tests were conducted to study the vortex shedding patterns in the wake of the airfoil. The shedding patterns concluded the structure was experiencing VIV. Figure 5.
Experimental Design
• Experiments were conducted in a recirculating water tunnel, using a scaled NACA airfoil (NACA 0021).
• The airfoil was completely submerged in flow and the set-up holding the airfoil in the test section constrained the oscillations of the airfoil to one degree of freedom in the direction normal to the incoming flow (crossflow). Figure 1 and 2.
Acknowledgements: This work is supported in part by the National Science Foundation under NSF award number 1460461. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect those of the National Science Foundation.
http://cleantechnica.com
What is Vortex-Induced Vibration?
• A bluff body that is either flexible or flexibly-mounted is placed in water or air flow. • A wake forms that is unstable, producing vortices that induce unsteady forces on the body. • The frequency of vortex shedding locks in with the system’s natural frequency, producing large amplitude oscillations.
Figure 2. Schematic of the experimental set-up.
Figure 1. Flow direction and the angle of attack.
Figure 5. Video. Downstream view of airfoil oscillating and flow visualization at α=70°, U*=6.96.
Top: Figure 3. Dimensionless amplitude of the crossflow oscillations (A*=A/D) versus the reduced velocity (U*=U/(nf*D)) for each angle of attack, α. Bottom: Figure 4. Frequency ratio (f*=f/nf) versus the reduced velocity, U*, for each angle of attack where oscillations were observed.
Reference: B. Seyed-Aghazadeh, D.W. Carlson, Y. Modarres-Sadeghi (2016). Vortex-Induced Vibration and Galloping of Prisms with Triangular Cross-Sections Placed in Water Flow.
