Wind tunnel study on natural instability of the normal force on a full‐scale wind turbine blade section at Reynolds number 4.7 · 10⁶

Abstract

Wind turbines are exposed to the turbulent wind of the atmospheric boundary layer. Consequently, the aerodynamic forces acting on the rotor blades are highly complex. To improve the understanding, a common practice is the experimental or numerical investigation of 2d (wind turbine) blade sections. In these investigations, the flow around the 2d blade section is assumed to be two‐dimensional; however, 3d effects are known to occur. Therefore, we combine 2d CFD simulations and experimental investigations in a wind tunnel with a 2d wind turbine rotor blade section at full‐scale (i.e., chord length c=1.25m and chord‐based Reynolds number of Rec=4.7·106). In the wind tunnel, the inflow turbulence intensity is TI≈1.5%. To avoid wall effects biasing the results, the profile does not span the whole test section. The profile was equipped with two rows of pressure taps around the airfoil, close to the center, to monitor the time‐resolved aerodynamic response as well as the flow around the airfoil. The normal force, cp curves, and the separation point are analyzed. While 2d simulations and experiments match well, in the experiments, we find natural instabilities, that is, local and temporal variations of the flow separation point at angles of attack close to the maximum lift that are not triggered externally, for example, by inflow variations.

Reynolds effects on wall pressure fluctuations at stall inception of a wind turbine blade section

“…The highly intermittent flow is studied over an airfoil shape corresponding to the 2D extrusion of a 2MW turbine at 80% of the rotor diameter. Evidence of intermittent flow separation and reattachment near maximum lift conditions was first provided by Neunaber et al [7] at a full scale chord-based Reynolds number of Re c > 3 × 10 6 . In the present study, the influence of the Reynolds number was determined on the intermittency phenomenon by considering a wide range of chord-based Reynolds numbers Re c from 3 × 10 5 to 4.2 × 10 6 .…”

“…From figure 5, the pressure distribution has the same pattern for all Re c : a peak of pressure coefficient at the leading edge that reaches -3.3, a zone of high suction in the first half of the chord, then a decrease of the suction until the trailing edge also called the recovery region. When the mean pressure distribution reaches a plateau, it indicates the steady separation point location as defined by [1,7]. While comparing different Re c , no effect was shown on the leading edge peak in term of its location or its amplitude.…”

“…The maximum thickness, 20%, is located at around 33% of the chord while the maximum camber, 4%, is located at approximately 49% of the chord. Previous studies from [1,7] have shown that this blade shape has a trailing edge stall behaviour according to Gault's classification [3]. The airfoil is 5m wide and has the full scale chord length of c = 1.25m.…”

“…Moreover, wind turbines operate over a large range of Reynolds numbers from stand still to high velocities and intermittent gust. Near stall, the flow becomes more three-dimensional and highly unsteady which induces important load fluctuations ( [1,5,7,8]) that are not considered in dynamic stall models. The stall inception is also found to greatly depend on the…”

Developing a Digital Twin Framework for Wind Tunnel Testing: Validation of Turbulent Inflow and Airfoil Load Applications

“…The inlet velocity is 25ms − 1. The airfoil shape was derived from scans of a 2MW wind turbine blade section, at 82% of its length (see also Neunaber et al (2022)). It has been scaled-down to 1/10th of the original chord length, so that the chord length is c = 0.125 m and the chord-based Reynolds number is Re c = 2.0 × 10 5 .…”