Search

Search Results

• 1. [Article] A Parametric Study on Power Variation for Model Wind Turbine Arrays

  This thesis presents the results of wind tunnel experiments performed for various model wind turbine arrays. The aim is to understand how siting affects power output. To optimize wind farm efficiency the ...

  Citation × Citation Citation Abstract Copy Title: A Parametric Study on Power Variation for Model Wind Turbine Arrays Author: DeLucia, Dominic Year: 2013 This thesis presents the results of wind tunnel experiments performed for various model wind turbine arrays. The aim is to understand how siting affects power output. To optimize wind farm efficiency the experiments vary the parameters of the model wind turbines and the layout of the wind turbine array. The parameters include the alignment, height, spacing, and the rotational direction of the model wind turbines. These experiments employ mechanical torque sensors to simultaneously measure the torque and rotor angular velocity, which yields a direct measurement of the fluid mechanical power extracted by the turbine at multiple locations. For a 4 × 3 array, the power is calculated at the center turbine in each of the rows. Variations in wind farm efficiency ranging from 55% to 90% are observed between the 13 different layouts tested. Modifications to the layout of the wind turbine array clearly affects the power output of the wind turbines downstream. The results of such experiments highlight the importance of studying the relationship between wind farm layout and power output. Title: A Parametric Study on Power Variation for Model Wind Turbine Arrays Author: DeLucia, Dominic Year: 2013 This thesis presents the results of wind tunnel experiments performed for various model wind turbine arrays. The aim is to understand how siting affects power output. To optimize wind farm efficiency the experiments vary the parameters of the model wind turbines and the layout of the wind turbine array. The parameters include the alignment, height, spacing, and the rotational direction of the model wind turbines. These experiments employ mechanical torque sensors to simultaneously measure the torque and rotor angular velocity, which yields a direct measurement of the fluid mechanical power extracted by the turbine at multiple locations. For a 4 × 3 array, the power is calculated at the center turbine in each of the rows. Variations in wind farm efficiency ranging from 55% to 90% are observed between the 13 different layouts tested. Modifications to the layout of the wind turbine array clearly affects the power output of the wind turbines downstream. The results of such experiments highlight the importance of studying the relationship between wind farm layout and power output. Close

• 2. [Article] Distribution of Mean Kinetic Energy Around an Isolated Wind Turbine and a Characteristic Wind Turbine of a Very Large Wind Farm

  An isolated wind turbine and a very large wind farm are introduced into large-eddy simulations of an atmospheric boundary layer. The atmospheric flow is forced with a constant geostrophic wind and a time-varying ...

  Citation × Citation Citation Abstract Copy Title: Distribution of Mean Kinetic Energy Around an Isolated Wind Turbine and a Characteristic Wind Turbine of a Very Large Wind Farm Author: Cortina, Gerard, Calaf, Marc, Cal, Raul Bayoan Year: 2016 An isolated wind turbine and a very large wind farm are introduced into large-eddy simulations of an atmospheric boundary layer. The atmospheric flow is forced with a constant geostrophic wind and a time-varying surface temperature extracted from a selected period of the CASES-99 field experiment. A control volume approach is used to directly compare the transfer of mean kinetic energy around a characteristic wind turbine throughout a diurnal cycle considering both scenarios. For the very large wind farm case, results illustrate that the recovery of mean kinetic energy around a wind turbine is dominated by the vertical flux, regardless of atmospheric stratification. Contrarily, for an isolated wind turbine, the recovery is dependent on the background atmospheric stratification and it is produced by a combination of advection, vertical flux, and pressure redistribution. The analysis also illustrates that during the unstable stratification periods vertical entrainment of mean kinetic energy dominates, whereas during the stable regime horizontal entrainment is predominant. Finally, it is observed that in both scenarios, the single wind turbine and the large wind farm cases, turbulent mixing is driven by the background convective stratification during the unstable period and by the effect of the wind turbine during the stable regime. Title: Distribution of Mean Kinetic Energy Around an Isolated Wind Turbine and a Characteristic Wind Turbine of a Very Large Wind Farm Author: Cortina, Gerard, Calaf, Marc, Cal, Raul Bayoan Year: 2016 An isolated wind turbine and a very large wind farm are introduced into large-eddy simulations of an atmospheric boundary layer. The atmospheric flow is forced with a constant geostrophic wind and a time-varying surface temperature extracted from a selected period of the CASES-99 field experiment. A control volume approach is used to directly compare the transfer of mean kinetic energy around a characteristic wind turbine throughout a diurnal cycle considering both scenarios. For the very large wind farm case, results illustrate that the recovery of mean kinetic energy around a wind turbine is dominated by the vertical flux, regardless of atmospheric stratification. Contrarily, for an isolated wind turbine, the recovery is dependent on the background atmospheric stratification and it is produced by a combination of advection, vertical flux, and pressure redistribution. The analysis also illustrates that during the unstable stratification periods vertical entrainment of mean kinetic energy dominates, whereas during the stable regime horizontal entrainment is predominant. Finally, it is observed that in both scenarios, the single wind turbine and the large wind farm cases, turbulent mixing is driven by the background convective stratification during the unstable period and by the effect of the wind turbine during the stable regime. Close

• 3. [Article] Blade Number Effects in a Scaled Down Wind Farm

  Two 3 × 4 scaled down wind farms were analyzed to understand differences in wind turbine boundary layers when turbines operating at identical power coefficients have two or three blades. Mean streamwise ...

  Citation × Citation Citation Abstract Copy Title: Blade Number Effects in a Scaled Down Wind Farm Author: Newman, A. Jensen, Cal, Raúl Bayoán, Castillo, Luciano Year: 2015 Two 3 × 4 scaled down wind farms were analyzed to understand differences in wind turbine boundary layers when turbines operating at identical power coefficients have two or three blades. Mean streamwise velocities in two bladed turbine near wakes ranged between 10 and 100% larger than those in the three bladed case with large differences just behind the nacelle. In the rotor swept region of far wakes, mean velocity differences between the two arrays were about 10% (max) and became smaller with increasing streamwise direction. Contrary to these findings, regions above and below rotors become less similar deep in the array. Incoming flow to downstream turbines was shown to have greater Reynolds streamwise normal stress for three bladed rotors. Percentage differences ranged between about 30% for the second turbine down to 10% for the fourth turbine. Additionally, there is qualitative evidence that suggests incoming streamwise Reynolds normal stress becomes similar between the two types of turbines, indicating that asymptotically two and three bladed turbines could have similar fatigue loading properties. These results show that use of two bladed turbines would have the most impact when used in a wind farm's first two rows. Title: Blade Number Effects in a Scaled Down Wind Farm Author: Newman, A. Jensen, Cal, Raúl Bayoán, Castillo, Luciano Year: 2015 Two 3 × 4 scaled down wind farms were analyzed to understand differences in wind turbine boundary layers when turbines operating at identical power coefficients have two or three blades. Mean streamwise velocities in two bladed turbine near wakes ranged between 10 and 100% larger than those in the three bladed case with large differences just behind the nacelle. In the rotor swept region of far wakes, mean velocity differences between the two arrays were about 10% (max) and became smaller with increasing streamwise direction. Contrary to these findings, regions above and below rotors become less similar deep in the array. Incoming flow to downstream turbines was shown to have greater Reynolds streamwise normal stress for three bladed rotors. Percentage differences ranged between about 30% for the second turbine down to 10% for the fourth turbine. Additionally, there is qualitative evidence that suggests incoming streamwise Reynolds normal stress becomes similar between the two types of turbines, indicating that asymptotically two and three bladed turbines could have similar fatigue loading properties. These results show that use of two bladed turbines would have the most impact when used in a wind farm's first two rows. Close

• 4. [Article] Focused-Based Multifractal Analysis of the Wake in a Wind Turbine Array Utilizing Proper Orthogonal Decomposition

  Hot-wire anemometry measurements have been performed in a 3×3 wind turbine array to study the multifractality of the turbulent kinetic energy dissipation. A multifractal spectrum and Hurst exponents are ...

  Citation × Citation Citation Abstract Copy Title: Focused-Based Multifractal Analysis of the Wake in a Wind Turbine Array Utilizing Proper Orthogonal Decomposition Author: Ali, Naseem, Falih Kadum, Hawwa, Cal, Raúl Bayoán Year: 2016 Hot-wire anemometry measurements have been performed in a 3×3 wind turbine array to study the multifractality of the turbulent kinetic energy dissipation. A multifractal spectrum and Hurst exponents are determined at nine locations downstream of the hub height, bottom and top tips. Higher multifractality is found at 0.5D and 1D downstream of the bottom tip and hub height. The second order of the Hurst exponent and combination factor shows the ability to predict the flow state in terms of its development. Snapshot proper orthogonal decomposition (POD) is used to identify the coherent and incoherent structures and to reconstruct the stochastic velocity signal using a specific number of the POD eigenfunctions. The accumulation of the turbulence kinetic energy in the top tip location exhibits fast convergence compared with the bottom tip and hub height. The dissipation of the large and small scales is determined using the reconstructed stochastic velocities. The higher multifractality is shown in the dissipation of the large scale compared with small scale dissipation showing consistency with the behavior of the original signals. Multifractality of turbulent kinetic energy dissipation in the wind farm is examined and the effect of the reconstructed flow field via proper orthogonal decomposition on the multifractality behavior is investigated. Findings are relevant in wind energy as multifractal parameters identify the variation between the near- and far-wake regions. Title: Focused-Based Multifractal Analysis of the Wake in a Wind Turbine Array Utilizing Proper Orthogonal Decomposition Author: Ali, Naseem, Falih Kadum, Hawwa, Cal, Raúl Bayoán Year: 2016 Hot-wire anemometry measurements have been performed in a 3×3 wind turbine array to study the multifractality of the turbulent kinetic energy dissipation. A multifractal spectrum and Hurst exponents are determined at nine locations downstream of the hub height, bottom and top tips. Higher multifractality is found at 0.5D and 1D downstream of the bottom tip and hub height. The second order of the Hurst exponent and combination factor shows the ability to predict the flow state in terms of its development. Snapshot proper orthogonal decomposition (POD) is used to identify the coherent and incoherent structures and to reconstruct the stochastic velocity signal using a specific number of the POD eigenfunctions. The accumulation of the turbulence kinetic energy in the top tip location exhibits fast convergence compared with the bottom tip and hub height. The dissipation of the large and small scales is determined using the reconstructed stochastic velocities. The higher multifractality is shown in the dissipation of the large scale compared with small scale dissipation showing consistency with the behavior of the original signals. Multifractality of turbulent kinetic energy dissipation in the wind farm is examined and the effect of the reconstructed flow field via proper orthogonal decomposition on the multifractality behavior is investigated. Findings are relevant in wind energy as multifractal parameters identify the variation between the near- and far-wake regions. Close

• 5. [Article] Streamwise Evolution of Statistical Events in a Model Wind-Turbine Array

  Hot-wire anemometry data, obtained from a wind-tunnel experiment containing a 3×3 3 × 3 model wind-turbine array, are used to conditionally average the Reynolds stresses. Nine profiles at the centreline ...

  Citation × Citation Citation Abstract Copy Title: Streamwise Evolution of Statistical Events in a Model Wind-Turbine Array Author: Viestenz, Kyle, Cal, Raul Bayoán Year: 2016 Hot-wire anemometry data, obtained from a wind-tunnel experiment containing a 3×3 3 × 3 model wind-turbine array, are used to conditionally average the Reynolds stresses. Nine profiles at the centreline behind the array are analyzed to characterize the turbulent velocity statistics of the wake flow. Quadrant analysis yields statistical events occurring in the wake of the wind farm where quadrants 2 and 4 produce ejections and sweeps, respectively. The scaled difference between these two events is expressed via the ΔR0 Δ R 0 parameter and is based on the ΔS0 Δ S 0 quantity as introduced by M. R. Raupach (J Fluid Mech 108:363–382, 1981). ΔR0 Δ R 0 attains a maximum value at hub height and changes sign near the top of the rotor. The ratio of quadrant events of upward momentum flux to those of the downward flux, known as the exuberance, is examined and reveals the effect of root vortices persisting to eight rotor diameters downstream. These events are then associated with the triple correlation term present in the turbulent kinetic energy equation of the fluctuations where it is found that ejections play the dual role of entraining mean kinetic energy while convecting turbulent kinetic energy out of the turbine canopy. The development of these various quantities possesses significance in closure models, and is assessed in light of wake remediation, energy transport and power fluctuations, where it is found that the maximum fluctuation is about 30% 30 % of the mean power produced. Title: Streamwise Evolution of Statistical Events in a Model Wind-Turbine Array Author: Viestenz, Kyle, Cal, Raul Bayoán Year: 2016 Hot-wire anemometry data, obtained from a wind-tunnel experiment containing a 3×3 3 × 3 model wind-turbine array, are used to conditionally average the Reynolds stresses. Nine profiles at the centreline behind the array are analyzed to characterize the turbulent velocity statistics of the wake flow. Quadrant analysis yields statistical events occurring in the wake of the wind farm where quadrants 2 and 4 produce ejections and sweeps, respectively. The scaled difference between these two events is expressed via the ΔR0 Δ R 0 parameter and is based on the ΔS0 Δ S 0 quantity as introduced by M. R. Raupach (J Fluid Mech 108:363–382, 1981). ΔR0 Δ R 0 attains a maximum value at hub height and changes sign near the top of the rotor. The ratio of quadrant events of upward momentum flux to those of the downward flux, known as the exuberance, is examined and reveals the effect of root vortices persisting to eight rotor diameters downstream. These events are then associated with the triple correlation term present in the turbulent kinetic energy equation of the fluctuations where it is found that ejections play the dual role of entraining mean kinetic energy while convecting turbulent kinetic energy out of the turbine canopy. The development of these various quantities possesses significance in closure models, and is assessed in light of wake remediation, energy transport and power fluctuations, where it is found that the maximum fluctuation is about 30% 30 % of the mean power produced. Close

• 6. [Article] Conditional Averaging and Classification in the Near Wake of a Wind Turbine Array Boundary Layer

  Flow perturbation induced by interaction with a turbine rotor produces considerable turbulence which can alter the productivity of subsequent units within a wind farm. Previous methods have characterized ...

  Citation × Citation Citation Abstract Copy Title: Conditional Averaging and Classification in the Near Wake of a Wind Turbine Array Boundary Layer Author: Smith, Sarah E. Year: 2017 Flow perturbation induced by interaction with a turbine rotor produces considerable turbulence which can alter the productivity of subsequent units within a wind farm. Previous methods have characterized near wake vorticity of a single turbine as well as recovery distance for various turbine array configurations. This study aims to build from previous methods of analysis from the perspective of the rotor well within a turbine array and develop a model to examine points of significant imposition in relation to rotational effects. Hot wire anemometry was employed downstream of a turbine located in the middle of the third row in a 3x3 array with data points selected in a plane parallel to the turbine rotor. Data was collected in two separate periods capturing simultaneous streamwise and wall normal velocity components and simultaneous streamwise and transverse velocity components. Contour plots of the data were constructed and analyzed in relation to rotor position. The proposed modeling method was successful in that Reynolds stresses induced by the rotor in the near wake region aligned with rotor position were visible and well-defined in expected locations. Furthermore, examination of stresses in terms of streamwise and transverse velocity components clearly depicted rotational effects induced by the rotor. Title: Conditional Averaging and Classification in the Near Wake of a Wind Turbine Array Boundary Layer Author: Smith, Sarah E. Year: 2017 Flow perturbation induced by interaction with a turbine rotor produces considerable turbulence which can alter the productivity of subsequent units within a wind farm. Previous methods have characterized near wake vorticity of a single turbine as well as recovery distance for various turbine array configurations. This study aims to build from previous methods of analysis from the perspective of the rotor well within a turbine array and develop a model to examine points of significant imposition in relation to rotational effects. Hot wire anemometry was employed downstream of a turbine located in the middle of the third row in a 3x3 array with data points selected in a plane parallel to the turbine rotor. Data was collected in two separate periods capturing simultaneous streamwise and wall normal velocity components and simultaneous streamwise and transverse velocity components. Contour plots of the data were constructed and analyzed in relation to rotor position. The proposed modeling method was successful in that Reynolds stresses induced by the rotor in the near wake region aligned with rotor position were visible and well-defined in expected locations. Furthermore, examination of stresses in terms of streamwise and transverse velocity components clearly depicted rotational effects induced by the rotor. Close