The Science of Making Torque from Wind (TORQUE 2018)

Authors:
Michael Alletto, Jamal Adib, Alexander Radi, Jidith Langner, Carlos Peralta, Andree Altmikus, Marcus Letzel

Abstract:
A new industrial methodology for CFD-based site assessment is presented. It is based on a steady state two equation RANS model and includes the effect of Coriolis force, forests and buoyancy in the equations for turbulence. The computational results are validated with Monin-Obukhov similarity theory, the Høvsøre meteorological mast and one real site experiment. The validation evaluates the profile quality over homogenous terrain, with focus on profile consistency and shape. Different heat fluxes and roughness lengths are considered. In the second validation section the well-known Askervein hill is simulated in order to demonstrate the applicability of the model to a real site.

Authors:
Niranjan S Ghaisas, Aditya Ghate, Sanjiva K. Lele

Abstract:
A novel wind turbine conguration comprising of four identical rotors mounted on a single tower is studied using large-eddy simulation (LES) with turbine forces modeled using an actuator drag-disk model. The characteristics of the wakes of the multi-rotor turbine are compared to those of the wake of a conventional turbine comprised of a single rotor per tower. The single-rotor turbine has twice the diameter and the same thrust coecient as the rotors of the multi-rotor turbine. Several multi-rotor congurations, with varying horizontal and vertical spacings between the four rotors, are evaluated. The multi-rotor turbine wakes are found to recover faster, while the turbulence intensity in the wake is smaller, compared to the wake of the conventional turbine. The mean velocity proles obtained from the LES results are predicted accurately by a semi-analytical model assuming Gaussian radial proles of the velocity deficits, and either linear or quadratic superposition of multiple wakes. The interaction between multiple multi-rotor turbines is contrasted with that between multiple single-rotor turbines by considering wind farms with five turbine units aligned perfectly with each other and the wind direction and separated by four single-rotor diameters. The wake losses are found to be significantly smaller in such a wind farm comprising of multi-rotor turbines as compared to single-rotor turbines. The analytical model is employed to determine the thrust coefficient and axial spacings for which multi-rotor turbines lead to smaller wake losses than the conventional single-rotor turbine. These results suggest that a larger planform energy flux can be achieved without significantly increased fatigue loads by using multi-rotor turbines instead of conventional, single-rotor turbines.

Authors:
Anup Kc

Abstract:
Small wind turbines (SWT) that are designed based on the IEC 61400-2 standard suffer structural and operational complexities when operated in built environment, because such environment impose stochastic variations in wind speed and turbulence that are difficult to estimate. The wind conditions in open terrain and built environment are compared for turbulence intensity (TI) and intermittency using 1-pt and 2-pt statistics respectively. The urban wind data was found to not conform to the IEC standard; the TI in the built environment wind field was 21%, which was higher than the NTM indicated in the IEC standard. The TI in the open terrain was below 18% for all mean wind speeds. Similarly, for three chosen wind speed bins, the urban wind field had higher intermittency for smaller timescale but resulted to smaller intermittency as the time lag increased.  The small scale fluctuations in the wind flow field resulting in intermittency was found to be independent of the mean wind speed, however, they may cause extreme load changes on torque and whole energy conversion process.

Authors:
Roberto Aurelio Chavez-Arroyo

Abstract:
The different terms of momentum and energy budget simulated by the Weather Research and Forecasting model (WRF) are examined at sites that depict diverse climate and orographic environments. The analysis focuses both on particular weather episodes of one-to-few days duration and in one-year statistics useful for the estimation of the annual wind climate of a particular location. To estimate their accuracy, a validation exercise is performed in the Cabauw Experimental Site for Atmospheric Research (CESAR) in the Netherlands, where a fair agreement is found by comparing the mean surface pressure gradient term extracted from WRF with the pressure-derived geotrophic winds observed by the CESAR project. This study is carried out under the New European Wind Atlas (NEWA) project as part of the effort to develop a model-chain system that connects mesoscale with microscale wind flow models with different ranges fidelity for wind site assessment applications. Therefore the analysis deepens in the horizontal and vertical variability of the tendencies in each of the sites with the ultimate goal of providing some guidelines of their best usage as large-scale forcing of the microscale models.

Authors:
Nicolas Coudou

Abstract:
We investigate several algorithms to detect in real-time the centerline of a wind turbine wake. First, we apply these methods during Large Eddy Simulations of an isolated wind turbine subject to a uniform (TI = 0%) and a synthetic turbulent inflow (TI = 10%). The simulations are performed using a vortex-particle mesh method with the blades modelled using immersed lifting lines. The most robust algorithm is then applied to investigate the wakes positions inside a wind farm.