Low wind spells characterization over Europe as seen from ERA5 reanalysis

<p>Light wind conditions can be a very relevant feature from the wind power perspective. If such values are below a certain threshold (fixed or relative to some percentile), from the renewable energy production perspective, the amount of such energy is then strongly reduced or even suppressed. Frequency and intensity of such conditions is therefore of high interest, and a characterization of how these conditions can remain in time (during several hours, or days) can be even more important. From a climatic perspective, those episodes could be named as spells. This is the case of dry or wet ones, when referring to precipitation and its absence, or hot or cold ones, when focusing on temperatures. There is plenty of literature focused on that extreme conditions, for example in the set of indices to define extreme events developed by the ETCCDI (the CCl/CLIVAR/JCOMM Expert Team (ET) on Climate Change Detection and Indices: http://etccdi.pacificclimate.org/). However, no mention is made to wind there. Here, we will explore the application of those indices for temperature and precipitation, but now applied to wind values, when they remain below normal values during a certain period of time. Several considerations will first be made to define light wind thresholds. Then, the statistical characterization of the persistence of those conditions will be inspected. ERA5 reanalysis over Europe for the last 40 years is used as the database to perform such analysis, at a resolution of 0.25 degrees for the whole region. From ERA5 time frequency, we are able to analyze hourly scales, due to the high time variability of wind, which can be also of quite relevant interest from the energetic resource perspective. We also analyze daily scales, which is more typical from a climatic focus, and see how these results can be related to mean wind conditions at each point. Time climatic variability and spatial obtained patterns are also studied. Results from this work will be useful to advance in a more systematic and rigorous climatic description of such wind conditions, that would be very important from the energy perspective, for example. In particular, we are interested in exploring the recently developed concept of energy droughts (Raynaud et al., 2018).</p>

Research on combination of passive magnetic bearing and mechanical bearing for vertical axis wind turbine

Aiming at the “light wind start, light wind power generation” of vertical axis wind turbine, a new T-shaped radial passive magnetic bearing with high suspension characteristics is proposed. Passive magnetic bearings used in vertical axis wind turbines usually have small bearing capacity and difficult magnetization. The new T-shaped radial PMB can improve the radial bearing capacity, and the three magnetic rings all adopt simple axial magnetization. The new T-shaped radial PMB is combined with mechanical auxiliary bearing to form the suspension system of wind turbine. In the stable state, the suspension system can realize radial and axial stable suspension. The structure and working principle of the suspension system are briefly described. Through the finite element simulation, the characteristics of the new T-shaped radial PMB, the traditional double-ring PMB and the T-shaped PMBs are compared. Taking the high bearing capacity and high stiffness of the new T-shaped radial PMB as the optimization objective, the multi-objective optimization of the new T-shaped radial PMB was carried out by changing its geometric parameters (inner diameter, magnetization length and air gap). The research results show that: Under the same bearing capacity, the volume of the new T-shaped radial PMB is reduced by about 78.64%. Under the same volume, its bearing capacity increased by about 30.7%, and its stiffness increased by about 96.1%. After optimization, its radial bearing capacity increased to 101.38 N, and its stiffness increased to 202.76 N/mm.

Correlation Between Strong Wind and Leisure Craft Grounding in Croatian Waters

This paper examines the correlation between strong wind and the frequency of small leisure craft grounding by analysing the available data on maritime accidents in the Adriatic. The primary goal of this study was to verify the hypothesis from prior research that strong wind is the prime cause of groundings in certain areas of the Adriatic. Contrary to the conclusions of the prior research, the new analysis indicates a far more uniform spatial distribution of wind-caused grounding accidents across all the examined areas of Croatian Adriatic waters. Furthermore, the analysis indicates that most grounding accidents occur in light wind conditions, suggesting that groundings can predominantly be attributed to factors other than strong wind. Several important drawbacks of the analysis stemming from the lack of accurate data on accidents in Croatian waters are discussed and suggestions given for the improved collection thereof that would greatly contribute to the future research on this topic. The inability to determine the exact causes of particular accidents from available data makes it impossible to accurately establish the number of grounding accidents caused by strong wind. In the future, more detailed statistical data could improve our understanding of the correlation between adverse weather conditions and recreational vessel accidents in the Adriatic.

The Transition from Downward to Upward Air–Sea Momentum Flux in Swell-Dominated Light Wind Conditions

Fifteen hours of consecutive swell data from the experiment Flux, État de la Mer, et Télédétection en Condition de Fetch Variable (FETCH) in the Mediterranean show a distinct upward momentum flux. The characteristics are shown to vary systematically with wind speed. A hysteresis effect is found for wave energy of the wind-sea waves when represented as a function of wind speed, displaying higher energy during decaying winds compared to increasing winds. For the FETCH measurements, the upward momentum transfer regime is found to begin for wind speeds lower than about U = 4 m s−1. For the lowest observed wind speeds U < 2.4 m s−1, the water surface appears to be close to dynamically smooth. In this range almost all the upward momentum flux is accomplished by the peak in the cospectrum between the vertical and horizontal components of the wind velocity. It is demonstrated that this contribution in turn is linearly related to the swell significant wave height Hsd in the range 0.6 < Hsd < 1.4 m. For Hsd < 0.6 m, the contribution is zero in the present dataset but may depend on the swell magnitude in other situations. It is speculated that the observed upward momentum flux in the smooth regime, which is so strongly related to the cospectral peak at the dominant swell frequency, might be caused by the recirculation mechanism found by Wen and Mobbs in their numerical simulation of laminar flow of a nonlinear progressive wave at low wind speed.