The impact of stable atmospheric boundary layers on wind-turbine wakes within offshore wind farms

Dörenkämper, Martin; Witha, Björn; Steinfeld, Gerald; Heinemann, Detlev; Kühn, Martin

doi:10.1016/j.jweia.2014.12.011

Cited by 124 publications

(108 citation statements)

References 28 publications

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“…In the current case, the maximal directional change is approximately 2 • near the end of the farm. Dörenkämper et al (2015) also observed a slight deviation to the left for a wind farm under stable atmospheric conditions, and attributed this deflection to the decrease in Coriolis force. Other studies reported wind farm wakes turning away from the pressure gradient towards the geostrophic wind direction (Van der Laan et al 2015; Volker et al 2015), and they attributed this effect to turbulent transport of momentum from above.…”

Section: Allaerts and J Meyersmentioning

confidence: 89%

Boundary-layer development and gravity waves in conventionally neutral wind farms

2017

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While neutral atmospheric boundary layers are rare over land, they occur frequently over sea. In these cases they are almost always of the conventionally neutral type, in which the neutral boundary layer is capped by a strong inversion layer and a stably stratified atmosphere aloft. In the current study, we use large-eddy simulations (LES) to investigate the interaction between a large wind farm that has a fetch of 15 km and a conventionally neutral boundary layer (CNBL) in typical offshore conditions. At the domain inlet, we consider three different equilibrium CNBLs with heights of approximately 300 m, 500 m and 1000 m that are generated in a separate precursor LES. We find that the height of the inflow boundary layer has a significant impact on the wind farm flow development. First of all, above the farm, an internal boundary layer develops that interacts downwind with the capping inversion for the two lowest CNBL cases. Secondly, the upward displacement of the boundary layer by flow deceleration in the wind farm excites gravity waves in the inversion layer and the free atmosphere above. For the lower CNBL cases, these waves induce significant pressure gradients in the farm (both favourable and unfavourable depending on location and case). A detailed energy budget analysis in the turbine region shows that energy extracted by the wind turbines comes both from flow deceleration and from vertical turbulent entrainment. Though turbulent transport dominates near the end of the farm, flow deceleration remains significant, i.e. up to 35 % of the turbulent flux for the lowest CNBL case. In fact, while the turbulent fluxes are fully developed after eight turbine rows, the mean flow does not reach a stationary regime. A further energy budget analysis over the rest of the CNBL reveals that all energy available at turbine level comes from upwind kinetic energy in the boundary layer. In the lower CNBL cases, the pressure field induced by gravity waves plays an important role in redistributing this energy throughout the farm. Overall, in all cases entrainment at the capping inversion is negligible, and also the work done by the mean background pressure gradient, arising from the geostrophic balance in the free atmosphere, is small.

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Section: Allaerts and J Meyersmentioning

confidence: 89%

Boundary-layer development and gravity waves in conventionally neutral wind farms

2017

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“…More recently, Churchfield et al (2016) and Abkar and Porté-Agel (2016) have shown similar results using LES of wakes in a stable ABL. By contrast, Dörenkämper et al (2015) reported a small anticlockwise deflection in a LES of a wind farm wake in a stable ABL. Dörenkämper et al (2015) generated the ABL with a precursor simulation using a different roughness length compared to the one applied in the wind farm simulation in order to model coast effects on an offshore wind farm.…”

Section: Introductionmentioning

confidence: 85%

Why the Coriolis force turns a wind farm wake clockwise in the Northern Hemisphere

Laan

Sørensen²

2017

Wind Energ. Sci.

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Abstract.The interaction between the Coriolis force and a wind farm wake is investigated by Reynoldsaveraged Navier-Stokes simulations, using two different wind farm representations: a high roughness and 5 × 5 actuator disks. Surprisingly, the calculated wind farm wake deflection is the opposite in the two simulations. A momentum balance in the cross flow direction shows that the interaction between the Coriolis force and the 5 × 5 actuator disks is complex due to turbulent mixing of veered momentum from above into the wind farm, which is not observed for the interaction between the Coriolis force and a roughness change. When the wind farm simulations are performed with a horizontally constant Coriolis force in order to isolate the effect of the wind veer, the wind farm wake deflection of the 5 × 5 actuator disks simulation remains unchanged. This proves that the present wind veer deflects the wind farm wake and not the local changes in the Coriolis force in the wake deficit region. An additional simulation of a single actuator disk, operating in a shallow atmospheric boundary layer, confirms that the Coriolis force indirectly turns a wind turbine wake clockwise, as observed from above, due to the presence of a strong wind veer.

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“…Thus, the axial and tangential velocity at the rotor disk U n and U t are selected as the unknown parameters instead of a and a [31][32][33]. In this study, a coupled approach is adopted for the ADM-R Energies 2018, 11, 665 6 of 24 model in the yawed condition, in which U n and U t are directly obtained from the CFD simulation result of local velocities on the rotor disk U x , U y , U z by using the following transformation:…”

Section: = Cosmentioning

confidence: 99%

A New Analytical Wake Model for Yawed Wind Turbines

2018

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Abstract:A new analytical wake model for wind turbines, considering ambient turbulence intensity, thrust coefficient and yaw angle effects, is proposed from numerical and analytical studies. First, eight simulations by the Reynolds Stress Model are conducted for different thrust coefficients, yaw angles and ambient turbulence intensities. The wake deflection, mean velocity and turbulence intensity in the wakes are systematically investigated. A new wake deflection model is then proposed to analytically predict the wake center trajectory in the yawed condition. Finally, the effects of yaw angle are incorporated in the Gaussian-based wake model. The wake deflection, velocity deficit and added turbulence intensity in the wake predicted by the proposed model show good agreement with the numerical results. The model parameters are determined as the function of ambient turbulence intensity and thrust coefficient, which enables the model to have good applicability under various conditions.

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The impact of stable atmospheric boundary layers on wind-turbine wakes within offshore wind farms

Cited by 124 publications

References 28 publications

Boundary-layer development and gravity waves in conventionally neutral wind farms

Boundary-layer development and gravity waves in conventionally neutral wind farms

Why the Coriolis force turns a wind farm wake clockwise in the Northern Hemisphere

A New Analytical Wake Model for Yawed Wind Turbines

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