Progress in the validation of rotor aerodynamic codes using field data

Abstract

Abstract. Within the framework of the fourth phase of the International Energy Agency (IEA) Wind Task 29, a large comparison exercise between measurements and aeroelastic simulations has been carried out featuring three simulation cases in axial, sheared and yawed inflow conditions. Results were obtained from more than 19 simulation tools originating from 12 institutes, ranging in fidelity from blade element momentum (BEM) to computational fluid dynamics (CFDs) and compared to state-of-the-art field measurements from the 2 MW DanAero turbine. More than 15 different variable types ranging from lifting-line variables to blade surface pressures, loads and velocities have been compared for the different conditions, resulting in over 250 comparison plots. The result is a unique insight into the current status and accuracy of rotor aerodynamic modeling. For axial flow conditions, a good agreement was found between the various code types, where a dedicated grid sensitivity study was necessary for the CFD simulations. However, compared to wind tunnel experiments on rotors featuring controlled conditions, it remains a challenge to achieve good agreement of absolute levels between simulations and measurements in the field. For sheared inflow conditions, uncertainties due to rotational and unsteady effects on airfoil data result in the CFD predictions standing out above the codes that need input of sectional airfoil data. However, it was demonstrated that using CFD-synthesized airfoil data is an effective means to bypass this shortcoming. For yawed flow conditions, it was observed that modeling of the skewed wake effect is still problematic for BEM codes where CFD and free vortex wake codes inherently model the underlying physics correctly. The next step is a comparison in turbulent inflow conditions, which is featured in IEA Wind Task 47. Doing this analysis in cooperation under the auspices of the IEA Wind Technology Collaboration Program (TCP) has led to many mutual benefits for the participants. The large size of the consortium brought ample manpower for the analysis where the learning process by combining several complementary experiences and modeling techniques gave valuable insights that could not be found when the analysis is carried out individually.

Technical modeling challenges for large idling wind turbines

“…The code has been improved over the time accommodating several latest development in the field. Several publications documented verification studies of the Bladed code performance [8][9][10][11].…”

ExaWind: Open‐source CFD for hybrid‐RANS/LES geometry‐resolved wind turbine simulations in atmospheric flows

“…The time stepping scheme in ( 10)-( 16) can be summarized as a predictor-corrector approach wherein a lagged pressure in (10) is used to predict the velocity u * which is not guaranteed to be divergence free, followed by a projection to enforce the divergence-free constraint on the velocity field and update the pressure (and pressure gradient). The alternative to this predictor-corrector approach would be an implicit method in which we solve for the velocity and pressure in the same step.…”

“…For the purpose of this study, the turbine geometry was simplified by ignoring the tower, hub, and nacelle structures, and only the three blades were modeled. We use the grid generated by DTU for this turbine as part of the International Energy Agency (IEA) Wind Task 29 10 . Figure 13 presents the near‐body grid for the NM80 rotor.…”

“…With the advection term computed, the intermediate velocity u * in ( 10) is advanced by solving the Helmholtz problem discretized using a cell-centered finite-difference method forming a seven-point stencil in 3D. 60 Note, for the Helmholtz solve in (10), the already computed advection term is treated as a constant. The intermediate velocity vector u * is not guaranteed to be divergence-free.…”

“…Given the balance of accuracy and computational cost accorded by actuator methods, these techniques continue to be valid for many design load cases, and their place in the wind community is unquestionable. However, the next‐generation of actuator methods is expected to be informed by higher fidelity geometry‐resolved CFD simulations 10,13 …”