The distinctive wake characteristics and aerodynamic loads of two side-by-side wall-mounted cylinders were experimentally studied under turbulent boundary layer flows with various gaps. Time-resolved particle image velocimetry was used to analyze the mean and unsteady wake features, whereas a high-resolution load cell was applied to measure the characteristics of lift and drag forces. The results show that the decrease in gap between two cylinders can effectively delay the wake recovery and suppress both the downwash and upwash flows near the top and bottom ends. Overall, with smaller gaps, the turbulence intensity near the top end becomes higher due to the stronger local velocity shear. The distribution of integral time scales indicates that the velocity fluctuations in the near wake region along the middle cylinder span are highly influenced by the local recirculation flows, whereas those near the top end are dominated by the mixing of boundary layer flows. By accounting the equivalent incoming velocity along the cylinder span, both lift and drag coefficient present a similar trend compared to the “infinite length” cylinder cases from previous works. Interestingly, different from cylinders with “infinite length,” no clear intermittency of aerodynamic loads was observed in the current work. This can be attributed to the suppression of two-dimensional vortex shedding due to the three-dimensional flow effects and strong background turbulence. The joint distribution of the lift and drag forces reveals that the lift fluctuations increase significantly with the growth of cylinder gaps, whereas that of drag force remains nearly constant.

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The objective of this study is to investigate the influence of winglet pitching as an aero-brake on the performance of a model wind turbine by wind tunnel experiments. Time-resolved particle image velocimetry, force sensor, and datalogger were used to characterize the coupling between wake statistics, aerodynamic loads, and rotation speed. Results highlighted that, for a winglet with 4% of the rotor diameter length, the increase of its pitching angle can significantly reduce the turbine rotation speed up to ∼28% and thrust coefficient of ∼20%. The winglet pitching induced minor influence on the velocity deficit in the very near wake regions, while its influence on accelerating the wake recovery become clear around three diameters downstream the turbine rotor. The turbulence kinetic energy exhibited a distinctive increase under large pitching angles in the near wake region at the turbine hub height due to the strong vertical flow fluctuations. Further investigation on the spectra of wake velocities revealed that the pitching of winglet can suppress the high-pass filtering effects of turbines on wake fluctuations; such large-scale turbulence facilitated the flow mixing and accelerated the wake transport.

This work describes the results from wind tunnel experiments performed to maximize wind plant total power output using wake steering via closed loop yaw angle control.The experimental wind plant consists of nine turbines arranged in two different layouts; both are two dimensional arrays and differ in the positioning of the individual turbines. Two algorithms are implemented to maximize wind plant power: Log-of-Power Extremum Seeking Control (LP-ESC) and Log-of-Power Proportional Integral Extremum Seeking Control (LP-PIESC). These algorithms command the yaw angles of the turbines in the upstream row. The results demonstrate that the algorithms can find the optimal yaw angles that maximize total power output. The LP-PIESC reached the optimal yaw angles much faster than the LP-ESC. The sensitivity of the LP-PIESC to variations in free stream wind speed and initial yaw angles is studied to demonstrate robustness to variations in wind speed and unknown yaw misalignment.extremum seeking control, log-of-power feedback, model-free wind farm power maximization, wake steering, wind tunnel experiment, yaw control * | INTRODUCTIONComplex wake interactions in wind farms can lead to significant power losses, which reduce the annual energy production (AEP) and revenue. 1,2 Wake steering using nacelle yaw angle control solutions have been proposed to reduce power losses in waked conditions. [3][4][5] The idea is to introduce an intentional yaw misalignment between the turbine rotor and the incoming wind direction, which can deflect the wake laterally by an amount depending on the yaw misalignment. [6][7][8][9] Using this idea on the upstream turbines one can steer their wakes away from the trailing turbines and hence avoid or mitigate wake interactions, leading to an overall increase in power.In this approach, the overall wind farm power is increased by decreasing the power capture of selected upstream turbines, which in turn would increase the power capture of downstream turbines. 10 The output power of the downstream turbines would increase because of the lateral maneuvering of the incoming wake. However, the power of the upstream turbines, that is, intentionally yawed turbines, will decrease as only the

The incipient sediment suspension downstream of a thin square tab with various inclination angles was fundamentally described using experiments and theoretical arguments. Volumetric particle tracking velocimetry and particle image velocimetry were applied to measure the sediment particle distributions and wake flows under various incoming velocities and obstacle inclination angles. Results show that suspension of sediments occurred downstream of the obstacles due to the enhanced local turbulence intensities. Although the sediment volumetric fraction was mostly determined by the variation of wake turbulence kinetic energy under low inclination angles (i.e., obstacles nearly parallel to incoming flow), this was not the case under high inclination angle counterparts where distinct downward velocities dominated the wake region and suppressed the sediment suspension. This led to the decrease of sediment volumetric fraction with the growth of tab inclination at high inclination angles. Supported by flow measurements, a revised Rouse number was introduced which allows to quantitatively integrate the coupled influence of turbulence mixing load and vertical flows across various incoming velocities and inclination angles.