During the last decades the exploitation of energy from the wind has become one of the most promising renewable energy technologies. The main strive in today’s development of wind turbines is to increase the efficiency of the turbine and to build bigger rotors that are able to extract more power out of the wind. When it comes to the planning and designing of a wind park, also the aerodynamic interactions between the single turbines must be taken into account. The flow in the wake of the first row turbines is characterized by a significant deficit in wind velocity and by increased levels of turbulence. Consequently, the downstream turbines in a wind farm cannot extract as much power from the wind anymore. Furthermore, the additional turbulence in the wake could be a reason for increased material fatigue through flow-induced vibrations at the downstream rotor. The main focus of this experimental study is to investigate the local velocity deficit and the turbulence intensities in the wake behind an array of two model wind turbines. For two different turbine separation distances, the wake is scanned at three different downstream positions. The experiments are performed at the wind tunnel (1.9m x 2.7m cross section) at NTNU Trondheim using two model wind turbines with a rotor diameter of 0.9m. A hot wire probe is used to scan the wake behind the model turbines in defined positions. Moving axially downstream the velocity deficit in the wake gradually recovers and the turbulence intensity levels slowly decrease. Furthermore, a gentle expansion of the wake can be observed. The wake profiles measured in close distances behind the rotor are characterized by evident asymmetries. Further downstream in the wake turbulent diffusion mechanisms cause a more uniform and more symmetrical flow field. Moreover, the turbulence intensity behind the second wind turbine is found to be significantly higher than behind one unobstructed turbine. Also, considerably higher velocity deficits are found in the near wake behind the second turbine compared to the wake behind one unobstructed turbine. However, the velocity profile at five rotor diameters downstream in the wake behind the second turbine is already very similar to the velocity distribution behind the first turbine. Furthermore, the velocity field and turbulence intensity distribution in the wake behind the second turbine is more symmetrical and more uniform than behind the first turbine.

Yes / This research article analyzed the self-preserving behaviour of wall-wake region of a circular pipe mounted horizontally over a flat rigid sand bed in a shallow flow in terms of mean velocity, RSS, and turbulence intensities. The study aims to investigate self-preservation using appropriate length and velocity scales.in addition to that wall-normal distributions of the third-order correlations along the streamwise direction in the wake region are analyzed. An ADV probe was used to record the three-dimensional instantaneous velocities for four different hydraulic and physical conditions corresponding to four cylinder Reynolds numbers. The results revealed that the streamwise velocity deficits, RSS deficits, and turbulence intensities deficits distributions displayed good collapse on a narrow band when they were non-dimensionalized by their respective maximum deficits. The wall-normal distance was non-dimensionalized by the half velocity profile width for velocity distributions, while the half RSS profile width was used in the case of the RSS deficits and turbulence intensities deficits distributions. The results indicate the self-preserving nature of streamwise velocity, RSS, and turbulence intensities in the wall-wake region of the pipe. The third-order correlations distributions indicate that sweep is the dominant bursting event in the near-bed zone. At the same time, ejection is the dominant bursting event in the region above the cylinder height.