Previous research has revealed the need for a validation study that considers several wake quantities and code types so that decisions on the trade-off between accuracy and computational cost can be well informed and appropriate to the intended application. In addition to guiding code choice and setup, rigorous model validation exercises are needed to identify weaknesses and strengths of specific models and guide future improvements. Here, we consider 13 approaches to simulating wakes observed with a nacelle-mounted lidar at the Scaled Wind Technology Facility (SWiFT) under varying atmospheric conditions. We find that some of the main challenges in wind turbine wake modeling are related to simulating the inflow. In the neutral benchmark, model performance tracked as expected with model fidelity, with large-eddy simulations performing the best. In the more challenging stable case, steady-state Reynolds-averaged Navier-Stokes simulations were found to outperform other model alternatives because they provide the ability to more easily prescribe noncanonical inflows and their low cost allows for simulations to be repeated as needed. Dynamic measurements were only available for the unstable benchmark at a single downstream distance. These dynamic analyses revealed that differences in the performance of time-stepping models come largely from differences in wake meandering. This highlights the need for more validation exercises that take into account wake dynamics and are able to identify where these differences come from: mesh setup, inflow, turbulence models, or wake-meandering parameterizations. In addition to model validation findings, we summarize lessons learned and provide recommendations for future benchmark exercises.

Abstract. A new analytical wind turbine wake model, based on a super-Gaussian shape function, is presented. The super-Gaussian function evolves from a nearly top-hat shape in the near wake to a Gaussian shape in the far wake, which is consistent with observations and measurements made on wind turbine wakes. Using such a shape function allows to recover the mass and momentum conservation that is violated when applying a near-wake regularization function to the expression of the maximum velocity deficit of the Gaussian wake model. After a brief introduction of the theoretical aspects, an easy-to-implement model with a limited number of parameters is derived. The super-Gaussian model predictions are compared to wind tunnel measurements, full-scale measurements and a LES simulation, showing a good agreement and an improvement compared with predictions based on the Gaussian model.

Abstract. A new analytical wind turbine wake model, based on a super-Gaussian shape function, is presented. The super-Gaussian function evolves from a nearly top-hat shape in the near wake to a Gaussian shape in the far wake, which is consistent with observations and measurements of wind turbine wakes. Using such a shape function allows the recovery of the mass and momentum conservation that is violated when applying a near-wake regularization function to the expression of the maximum velocity deficit of the Gaussian wake model. After a brief introduction of the theoretical aspects, an easy-to-implement model with a limited number of parameters is derived. The super-Gaussian model predictions are compared to wind tunnel measurements, full-scale measurements, and a large-eddy simulation (LES), showing a good agreement and an improvement compared with predictions based on the Gaussian model.

Observations of swell dissipation across oceans reveal a significant loss of energy that can be the result of many of processes. Among these candidate mechanisms, this paper examines the properties of the viscous air-sea boundary layer driven by swells in order to characterize the induced atmospheric flow regime and its associated viscous dissipation over swells. A series of 3-D numerical experiments is carried out with a RANS model and appropriate turbulence closure. These experiments reveal a laminar to turbulent transition in the near free-surface region for a common range of characteristic amplitudes and periods of swells under stationary conditions. At low Reynolds number, laminar conditions prevail and computed decay rates conform to the analytical formulation l m of the Stokes interfacial boundary layer for this problem. The turbulent regimes are characterized as well, and the new decay rates follow a nondimensional relation l51:42l m Re 1:5310 5 0:41 above Re 51:5310 5 (e.g., amplitude larger than 1.1 m for a 14 s monochromatic wave period). Typical decay rates are up to 4 times above the laminar values, which is a factor 10 less than the largest rates estimated for oceanic conditions. A sensitivity analysis is finally conducted to evaluate the influence of the stationary hypothesis. It demonstrates a short setup length and low relative variations of the unsteady decay rates for laminar, transitioning and developed turbulent conditions, which confirms the evaluation of steady decay rates.