Wake meandering of a model wind turbine operating in two different regimes

Abstract

The flows behind a model wind turbine under two different turbine operating regimes (Region 2 for turbine operating at optimal condition with the maximum power coefficient and 1.4 degrees of pitch angle, and Region 3 for turbine operating at sub-optimal condition with a lower power coefficient and 7 degrees of pitch angle) are investigated using wind tunnel experiments and large-eddy simulations (LES). Measurements from the model wind turbine experiment reveal that the power coefficient and turbine wake are affected by the operating regime. Simulations employing a new class of actuator surface methods which parameterize both the turbine blades and nacelle with and without a nacelle model are carried out for each operating condition to study the influence of the operating regime and nacelle on the formation of the hub vortex and wake meandering. Flow field statistics and energy spectra of the simulated wakes are in good agreement with the measurements. For simulations with a nacelle model, the mean flow field is composed of an outer wake, caused by energy extraction from the incoming wind by turbine blades, and an inner wake directly behind the nacelle, while for the simulations without a nacelle model, the central region of the wake is occupied by a jet. The simulations with the nacelle model reveal an unstable helical hub vortex expanding outwards towards the outer wake; while the simulations without a nacelle model show a stable and columnar hub vortex. Because of the different interactions of the inner region of the wake with the outer region of the wake, a region with higher turbulence intensity is observed in the tip shear layer for the simulation with a nacelle model. The hub vortex for the turbine operating in Region 3 remains in a tight helical spiral and intercepts the outer wake a few diameters further downstream than for the turbine operating in Region 2. Wake meandering, a low frequency large-scale motion of the wake, commences in the region of high turbulence intensity for all simulations with and without a nacelle model indicating that neither a nacelle model nor an unstable hub vortex is a necessary requirement for the existence of wake meandering. However, further analysis of the wake meandering and instantaneous flow field using a filtering technique and dynamic mode decomposition show that the unstable hub vortex energizes the wake meandering. The turbine operating regime affects the shape and expansion of the hub vortex altering the location of the onset of the wake meandering and wake meander oscillating intensity. Most importantly, the unstable hub vortex promotes a high amplitude energetic meandering which cannot be predicted without a nacelle model. arXiv:1802.03836v1 [physics.flu-dyn]

Effect of turbine nacelle and tower on the near wake of a utility-scale wind turbine

“…According to Magnusson (1999), this near-wake flow pattern is associated with a reduction in lift, and correspondingly axial induction, at the blade roots. This accelerated flow region has been observed in wind tunnel studies where the model blade geometries cause a similar reduction in lift at the roots (Hancock & Pascheke 2014, Foti et al 2018, and in LES studies where the turbine geometry accurately represents a fieldscale turbine (Schulz et al 2017). Gallacher & More (2014) observed this phenomenon in the average sense using nacelle-mounted lidar on an offshore wind turbine.…”

A Review on the Meandering of Wind Turbine Wakes

“…An actuator surface model for the nacelle was then developed in [3], which can predict the interaction of the nacelle-induced hub vortex with the tip shear layer on relatively coarse grids, as shown in Figure 5c. The effect of nacelle on wake meandering for different operational regimes was also examined [66]. It was found that the amplitudes of wake meanders are underpredicted for the simulation without a nacelle model.…”

“…In [71], Heisel et al studied the spectral characteristics of wake meandering using both wind tunnel and field-scale measurements. It is noted that, in this review, we refer to turbines such as the EOLOS turbine of 96 m rotor diameter as utility scale, the SWiFT turbine of 27 m rotor diameter and above as field scale, and turbines employed in wind tunnels (e.g., the G-1 wind turbine model of rotor diameter 1.1 m [66] and the miniature model turbine of rotor diameter 0.13 m [64]) as laboratory scale. A deficit of the energy of the large-scale eddies (as shown in Figure 7a,b for the field-scale measurements) is observed in both measurements, which is caused by the transfer of energy from the large-scale eddies to the small scales in the turbine wake, which was also observed in numerical simulations, as shown in Figure 6.…”

“…The downwind variations of the standard deviations of the wake center positions for the downwind turbine, which are in the range of 0.2D-0.4D, are more complex than that of the upwind turbine. The wake meandering behind the G-1 wind turbine model of rotor diameter 1.1 m operating in Regime 2 and 3 was investigated using wind tunnel experiment and LES [66]. The wake meanders, as shown in Figure 9a-d, are identified using a spatiotemporal filtering technique developed in [64,66].…”

“…The wake meandering behind the G-1 wind turbine model of rotor diameter 1.1 m operating in Regime 2 and 3 was investigated using wind tunnel experiment and LES [66]. The wake meanders, as shown in Figure 9a-d, are identified using a spatiotemporal filtering technique developed in [64,66]. Wake meandering of frequency around 0.3 is observed for both operational regimes.…”