Sasan Sarmast

A survey of modelling methods for high-fidelity wind farm simulations using large eddy simulation

Large eddy simulations (LES) of wind farms have the capability to provide valuable and detailed information about the dynamics of wind turbine wakes. For this reason, their use within the wind energy research community is on the rise, spurring the development of new models and methods. This review surveys the most common schemes available to model the rotor, atmospheric conditions and terrain effects within current state-of-the-art LES codes, of which an overview is provided. A summary of the experimental research data available for validation of LES codes within the context of single and multiple wake situations is also supplied. Some typical results for wind turbine and wind farm flows are presented to illustrate best practices for carrying out high-fidelity LES of wind farms under various atmospheric and terrain conditions. This article is part of the themed issue ‘Wind energy in complex terrains’.

Validation of the actuator line and disc techniques using the New MEXICO measurements

Actuator line and disc techniques are employed to analyse the wake obtained in the New MEXICO wind turbine experiment. The New MEXICO measurement campaign done in 2014 is a follow-up to the MEXICO ...

Comparison of actuator disc and Joukowsky rotor flows, to explore the need for a tip correction

In van Kuik & Lignarolo (2015, Wind Energy accepted with modifications) potential flow solutions have been obtained for classical actuator discs (axisymmetric, uniform load) presenting a model for the non-uniformity of the axial velocity at the disc. Current rotor design models proceed from a disc with a uniform axial velocity, modified by a tip correction like the one of Prandtl-Glauert-Shen. A comparison shows that this correction leads to a similar distribution as obtained from the potential flow solutions. A next comparison is made with the velocity distribution at the blade position of a Joukowsky rotor with constant bound circulation, calculated by an Actuator Line and a Lifting Line model. The resulting distributions correspond reasonably well to the potential flow disc distribution, in magnitude as well as shape. This implies that this non-uniform distribution is relevant for a rotor with a finite number of blades, and could be the basis for rotor design instead of the uniform but tip-corrected distribution.

Comparison between experiments and Large-Eddy Simulations of tip spiral structure and geometry

Results from Large-Eddy Simulations using the actuator line technique have been validated against experimental results. The experimental rotor wake, which forms the basis for the comparison, was studied in a recirculating free-surface water channel, where a helical vortex was generated by a single-bladed rotor mounted on a shaft. An investigation of how the experimental blade geometry and aerofoil characteristics affect the results was performed. Based on this, an adjustment of the pitch setting was introduced, which is still well within the limits of the experimental uncertainty. Excellent agreement between the experimental and the numerical results was achieved concerning the circulation, wake expansion and pitch of the helical tip vortex. A disagreement was found regarding the root vortex position and the axial velocity along the centre line of the tip vortex. This work establishes a good base for further studies of more fundamental stability parameters of helical rotor wakes.

Validation of the actuator disc approach in PHOENICS using small scale model wind turbines

In this study two wind turbine setups are investigated numerically: (a) the flow around a single model wind turbine and (b) the wake interaction between two in-line model wind turbines. This is don ...

Instability of the Helical Tip Vortices behind a Single Wind Turbine

A numerical study on a single wind turbine wake has been carried out focusing on the instability properties of the trailing tip vortices shed from the turbine blades. The numerical model is based on large-eddy simulations (LES) of the Navier-Stokes equations together with the actuator line method to simulate the wake behind the Tjaereborg wind turbine. The wake is perturbed by low amplitude stochastic excitations located in the neighborhood of the tip spiral, giving rise to spatially developing instabilities. Dynamic mode decomposition (DMD) is then utilized for identification of the coherent flow structures. The DMD results indicate that the amplification of specific waves along the spiral is responsible for triggering the instability leading to wake breakdown. Two types of dynamic structures dominates the flow; low and high frequency groups. Examination of these structures reveals that the dominant modes have the largest spatial growth.

Actuator line simulations of a Joukowsky and Tjæreborg rotor using spectral element and finite volume methods

The wake structure behind a wind turbine, generated by the spectral element code Nek5000, is compared with that from the finite volume code EllipSys3D. The wind turbine blades are modeled using the ...