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

2018 ◽  
Vol 75 (8) ◽  
pp. 2579-2588 ◽  
Author(s):  
Ulf Högström ◽  
Erik Sahlée ◽  
Ann-Sofi Smedman ◽  
Anna Rutgersson ◽  
Erik Nilsson ◽  
...  
Keyword(s):  
Wind Speed ◽  
Momentum Flux ◽  
Water Surface ◽  
Sea Waves ◽  
Progressive Wave ◽  
Wind Speeds ◽  
Low Wind Speed ◽  
Light Wind ◽  
Wind Sea ◽  
Almost All

Abstract 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.

Effect of Swell on Wind Stress for Light to Moderate Winds

2020 ◽  
Vol 77 (11) ◽  
pp. 3759-3768
Author(s):  
Charles L. Vincent ◽  
Hans C. Graber ◽  
Clarence O. Collins
Keyword(s):  
Wind Speed ◽  
Gulf Of Mexico ◽  
Inflection Point ◽  
Friction Velocity ◽  
Flow Transition ◽  
Wind Speeds ◽  
Impact Stress ◽  
Low Wind Speed ◽  
Wind Sea ◽  
Low Wind Speeds

AbstractBuoy observations from a 1999 Gulf of Mexico field program (GOM99) are used to investigate the relationships among friction velocity u*, wind speed U, and amount of swell present. A U–u*sea parameterization is developed for the case of pure wind sea (denoted by u*sea), which is linear in U over the range of available winds (2–16 m s−1). The curve shows no sign of an inflection point near 7–8 m s−1 as suggested in a 2012 paper by Andreas et al. on the basis of a transition from smooth to rough flow. When observations containing more than minimal swell energy are included, a different U–u* equation for U < 8 m s−1 is found, which would intersect the pure wind-sea curve about 7–8 m s−1. These two relationships yield a bilinear curve similar to Andreas et al. with an apparent inflection near 7–8 m s−1. The absence of the inflection in the GOM99 experiment pure wind-sea curve and the similarity of the GOM99 swell-dominated low wind speed to Andreas et al.’s low wind speed relationship suggest that the inflection may be due to the effect of swell and not a flow transition. Swell heights in the range of only 25–50 cm may be sufficient to impact stress at low wind speeds.

scholarly journals Strategies for Enhancing the Low Wind Speed Performance of H-Darrieus Wind Turbine—Part 1

2019 ◽  
Vol 1 (1) ◽  
pp. 185-204 ◽  
Author(s):  
Palanisamy Mohan Kumar ◽  
Krishnamoorthi Sivalingam ◽  
Teik-Cheng Lim ◽  
Seeram Ramakrishna ◽  
He Wei
Keyword(s):  
Wind Speed ◽  
Poor Performance ◽  
Vertical Axis ◽  
Speed Ratio ◽  
Wind Speeds ◽  
Low Wind Speed ◽  
Viable Solution ◽  
Speed Performance ◽  
Low Wind Speeds ◽  
Starting Characteristics

Small wind turbines are key devices for micro generation in particular, with a notable contribution to the global wind energy sector. Darrieus turbines, despite being highly efficient among various types of vertical axis turbines, received much less attention due to their starting characteristics and poor performance in low wind speeds. Radically different concepts are proposed as a potential solution to enhance the performance of Darrieus turbine in the weak wind flows, all along the course of Darrieus turbine development. This paper presents a comprehensive review of proposed concepts with the focus set on the low wind speed performance and critically assessing their applicability based on economics, reliability, complexity, and commercialization aspects. The study is first of its kind to consolidate and compare various approaches studied on the Darrieus turbine with the objective of increasing performance at low wind. Most of the evaluated solutions demonstrate better performance only in the limited tip speed ratio, though they improve the low wind speed performance. Several recommendations have been developed based on the evaluated concepts, and we concluded that further critical research is required for a viable solution in making the Darrieus turbine a low speed device.

Impact of wind speeds on turbulent transport of carbon dioxide (CO2) fluxes at a tropical location in Ile - Ife, southwest Nigeria

2020 ◽  
Author(s):  
Theodora Bello ◽  
Adewale Ajao ◽  
Oluwagbemiga Jegede
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Land Surface ◽  
Stable Boundary Layer ◽  
Turbulent Transport ◽  
Strong Wind ◽  
Stable Boundary ◽  
Wind Speeds ◽  
Low Wind Speed ◽  
Tropical Area

&lt;p&gt;The study investigates impact of wind speeds on the turbulent transport of CO&lt;sub&gt;2 &lt;/sub&gt;fluxes for a land-surface atmosphere interface in a low-wind tropical area between May 28&lt;sup&gt;th&lt;/sup&gt; and June 14&lt;sup&gt;th&lt;/sup&gt;, 2010; and May 24&lt;sup&gt;th&lt;/sup&gt; and June 15&lt;sup&gt;th&lt;/sup&gt;, 2015. Eddy covariance technique was used to acquire turbulent mass fluxes of CO&lt;sub&gt;2&lt;/sub&gt; and wind speed at the study site located inside the main campus of Obafemi Awolowo University, Ile &amp;#8211; Ife, Nigeria. The results showed high levels of CO&lt;sub&gt;2 &lt;/sub&gt;fluxes at nighttime attributed to stable boundary layer conditions and low wind speed. Large transport and distribution of CO&lt;sub&gt;2 &lt;/sub&gt;fluxes were observed in the early mornings due to strong wind speeds recorded at the study location. In addition, negative CO&lt;sub&gt;2 &lt;/sub&gt;fluxes were observed during the daytime attributed to prominent convective and photosynthetic activities. The study concludes there was an inverse relationship between turbulent transport of CO&lt;sub&gt;2 &lt;/sub&gt;fluxes and wind speed for daytime period while nighttime CO&lt;sub&gt;2&lt;/sub&gt; fluxes showed no significant correlation.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;Keywords&lt;/strong&gt;: CO&lt;sub&gt;2 &lt;/sub&gt;fluxes, Wind speed, Turbulent transport, Low-wind tropical area, Stable boundary layer&lt;/p&gt;

scholarly journals 1980–2010 Variability in U.K. Surface Wind Climate

2013 ◽  
Vol 26 (4) ◽  
pp. 1172-1191 ◽  
Author(s):  
Nick Earl ◽  
Steve Dorling ◽  
Richard Hewston ◽  
Roland von Glasow
Keyword(s):  
Wind Speed ◽  
Decadal Variability ◽  
Surface Wind ◽  
Rayleigh Distribution ◽  
Daily Maximum ◽  
International Studies ◽  
Atlantic Sector ◽  
Wind Speeds ◽  
Low Wind Speed ◽  
Wind Climate

Abstract The climate of the northeast Atlantic region comprises substantial decadal variability in storminess. It also exhibits strong inter- and intra-annual variability in extreme high and low wind speed episodes. Here the authors quantify and discuss causes of the variability seen in the U.K. wind climate over the recent period 1980–2010. Variations in U.K. hourly mean (HM) wind speeds, in daily maximum gust speeds and in associated wind direction measurements, made at standard 10-m height and recorded across a network of 40 stations, are considered. The Weibull distribution is shown to generally provide a good fit to the hourly wind data, albeit with the shape parameter k spatially varying from 1.4 to 2.1, highlighting that the commonly assumed k = 2 Rayleigh distribution is not universal. It is found that the 10th and 50th percentile HM wind speeds have declined significantly over this specific period, while still incorporating a peak in the early 1990s. The authors' analyses place the particularly “low wind” year of 2010 into longer-term context and their findings are compared with other recent international studies. Wind variability is also quantified and discussed in terms of variations in the exceedance of key wind speed thresholds of relevance to the insurance and wind energy industries. Associated interannual variability in energy density and potential wind power output of the order of ±20% around the mean is revealed. While 40% of network average winds are in the southwest quadrant, 51% of energy in the wind is associated with this sector. The findings are discussed in the context of current existing challenges to improve predictability in the Euro-Atlantic sector over all time scales.

scholarly journals An Experimental Field Dataset with Buoyant, Neutral, and Dense Gas Atmospheric Releases and Model Comparisons in Low–Wind Speed (Diffusion) Conditions

2010 ◽  
Vol 49 (9) ◽  
pp. 1805-1817
Author(s):  
Veronica E. Wannberg ◽  
Gustavious Williams ◽  
Patrick Sawyer ◽  
Richard Venedam
Keyword(s):  
Wind Speed ◽  
Field Measurements ◽  
Release Rates ◽  
Wind Speeds ◽  
Low Wind Speed ◽  
Current State ◽  
Practice Models ◽  
Atmospheric Releases ◽  
Low Wind Speeds ◽  
Atmospheric Concentrations

Abstract A unique field dataset from a series of low–wind speed experiments, modeling efforts using three commonly used models to replicate these releases, and statistical analysis of how well these models were able to predict the plume concentrations is presented. The experiment was designed to generate a dataset to describe the behavior of gaseous plumes under low-wind conditions and the ability of current, commonly used models to predict these movements. The dataset documents the release and transport of three gases: ammonia (buoyant), ethylene (neutral), and propylene (dense) in low–wind speed (diffusion) conditions. Release rates ranged from 1 to 20 kg h−1. Ammonia and ethylene had five 5-min releases each to represent puff releases and five 20-min releases each to represent plume releases. Propylene had five 5-min puffs, six 20-min plumes, and a single 30-min plume. Thirty-two separate releases ranging from 6 to 47 min were conducted, of which only 30 releases generated useful data. The data collected included release rates, atmospheric concentrations to 100 m from the release point, and local meteorological conditions. The diagnostics included nine meteorological stations on 100-m centers and 36 photoionization detectors in a radial pattern. Three current state-of-the-practice models, Aerial Locations of Hazardous Atmospheres (ALOHA), Emergency Prediction Information code (EPIcode), and Second-Order Closure Integrated Puff (SCIPUFF), were used to try to duplicate the measured field results. Low wind speeds are difficult to model, and all of the models had difficulty replicating the field measurements. However, the work does show that these models, if used correctly, are conservative (overpredict concentrations) and can be used for safety and emergency planning.

Detection and Characterization of Microscale Breaking Waves

2003 ◽  
Author(s):  
M. H. Kamran Siddiqui ◽  
Mark R. Loewen
Keyword(s):  
Wind Speed ◽  
Water Surface ◽  
Air Entrainment ◽  
Breaking Waves ◽  
Skin Layer ◽  
Wave Crest ◽  
Gas Transfer ◽  
Near Surface ◽  
Wind Speeds ◽  
The Waves

Microscale breaking waves are short wind-generated waves that break without air entrainment. At low to moderate wind speeds microscale breaking waves play an important role in enhancing air-water heat and gas transfer. We report on a series of experiments conducted in a wind-wave flume at Harris Hydraulics Laboratory (University of Washington, Seattle) designed to investigate the importance of microscale breaking waves in generating near-surface turbulence and in enhancing air-sea gas and heat transfer rates. Non-invasive experiments were performed at wind speeds ranging from 4.5 m/s to 11 m/s and at a fetch of 5.5 m. The skin-layer or water surface temperature was measured using an infrared (IR) imager and digital particle image velocimetry (DPIV) was used to obtain simultaneous measurements of the two-dimensional velocities immediately below the water surface. Analysis of the simultaneous DPIV and infrared datasets revealed that microscale breaking waves generate strong vortices in their crests that disrupt the cool skin layer at the water surface and create thermal wakes that are visible in the infrared images. While non-breaking waves do not generate strong vortices and hence do not disrupt the skin layer. We developed a scheme based on the magnitude of vorticity in the wave crest to identify microscale breaking waves. The results show that at a wind speed of 4.5 m/s, 11% of the waves broke. The percentage of breaking waves increased with wind speed and at a wind speed of 11 m/s, 91% of the waves were microscale breaking waves. Comparison of different geometric and flow properties of microscale breaking and non-breaking waves revealed that microscale breaking waves are steeper, larger in amplitude and generate more turbulent kinetic energy compared to non-breaking waves.

scholarly journals Experimental and Numerical Investigation on the Performance of Straight-Bladed Vertical Axis Wind Turbine by Adding Cavities to it Structure

2020 ◽  
Vol 26 (4) ◽  
pp. 64-79
Author(s):  
Ahmed Saadi AlJarakh ◽  
Hussain Yousif Mahmood
Keyword(s):  
Wind Speed ◽  
Vertical Axis ◽  
Power Coefficient ◽  
Vertical Axis Wind Turbine ◽  
Wind Speeds ◽  
Low Wind Speed ◽  
Low Performance ◽  
New Policies ◽  
Energy Harnessing

As the prices of the fuel and power had fluctuated many times in the last decade and new policies appeared and signed by most of the world countries to eliminate global warming and environmental impact on the earth surface and humanity exciting, an urgent need appeared to develop the renewable energy harnessing technologies on the short-term and long-term and one of these promising technologies are the vertical axis wind turbines, and mostly the combined types. The purpose of the present work is to combine a cavity type Savonius with straight bladed Darrieus to eliminate the poor self-starting ability for Darrieus type and low performance for Savonius type and for this purpose, a three-bladed Darrieus type with symmetrical S1046 airfoil was tested experimentally and numerically at different wind speeds (4.5 m/s, 8 m/s and 10 m/s) and it showed a poor self-starting ability at low wind speed although its higher performance at high wind speed. However when adding the cavities in two setup configuration and testing it at the same conditions, it was found that when adding the cavities as reversed cups in the core of the turbine, the performance increased and the power coefficient reached a maximum value at 10 m/s wind speed and it was observed to be 0.0914 , but when the solidity increased by adding three cavities, the performance was higher at low wind speed (4.5 m/s) but it tragically decreased at higher wind speed which indicates that the performance depends on the solidity and the turbine configuration. On the other hand, the numerical simulation showed a good match with the experimental results although it under-predicted the performance.

Wind Energy Harnessing System for Low and High Wind Speeds

2019 ◽  
Author(s):  
Majid Rashidi ◽  
Jaikrishnan R. Kadambi ◽  
Renjie Ke
Keyword(s):  
Wind Speed ◽  
Wind Energy ◽  
Wind Turbine ◽  
High Speed ◽  
Power Transmission ◽  
Vertical Axis ◽  
Geographic Locations ◽  
Wind Speeds ◽  
Low Wind Speed ◽  
Energy Harnessing

Abstract This work presents the design and analysis of a novel wind energy harnessing system that makes use of wind defecting structures to increase the ambient wind speed at geographic locations with relatively low wind speed. The system however reacts to highspeed wind conditions by altering the profile of the wind defecting structure in order to eliminate wind speed amplification attribute of the system, thereby protecting the wind turbine assembly at high speed wind conditions. Although increasing the wind speed is advantageous at geographic locations that the wind speed is typically low; however, from times to time, there could be sustained high-speed wind conditions at the same locations that may damage the wind turbine systems that take advantage of the wind defecting structures. The present work disclosed a wind deflecting structure formed by at least two sail-like partial cylindrical structures that are supported atop of a tower-like foundation in a symmetric arrangement, where one or more wind turbines can be installed in the space between the two partial cylinders. The two partial cylinders, each substantially in form a quarter cylinder is made of plurality of parallel ribbed-like bars, hereafter referred to as “bars” with a flexible thin material that are mechanically supported by the bars. The bars are oriented in a direction perpendicular to the ground; allowing the wing deflecting structures to accept horizonal axis or vertical axis turbines in the space between them. The function of the bars is to allow the thin material, attached to them, to assume a curved configuration substantially in the form of a quarter cylinder. The apparatus is equipped with wind speed monitoring devices, and power source and power transmission means, such as cable-pulleys, chain-sprockets, gears, or mechanical linkages that all work in concert to deploy or stow the thin material along the vertical rods depending to the magnitude of the prevailing wind speed. Preliminary computational fluid dynamics analyses have shown that the wind deflecting structure proposed here in amplifies the wind speed by a factor of 1.65.

Numerical Investigation of Hot Air Recirculation in an Air-Cooled Steam Condenser Under Ambient Conditions

2011 ◽  
Author(s):  
Weifeng He ◽  
Yiping Dai ◽  
Qingzhong Ma ◽  
Danmei Xie
Keyword(s):  
Wind Speed ◽  
Power Plant ◽  
Ambient Air ◽  
Finned Tube ◽  
Ambient Conditions ◽  
Navier Stokes Equation ◽  
Wind Speeds ◽  
Low Wind Speed ◽  
Hot Air ◽  
Air Flows

Air-cooled steam condensers (ACSCs) have been extensively utilized to reject heat in modern power plant. Hot air recirculation, which implies that the heated air from the exchangers is again drawn back into the axial fans influence the performance of the ACSC. Hot air recirculation under different wind speeds and directions is numerically simulated in an ACSC of a 2×600MW air-cooled power plant with the commercial Computational Fluid Dynamics (CFD) code, FLUENT, and the performance of the ACSC is investigated. Fan boundary is applied to simulate the fan characteristics when the ambient air flows through the rotor and the source term is added to the Navier-Stokes equation to simulate the pressure loss when the air flows through the exchangers. Phase transition is involved in the simulation because the turbine exhaust condensates in the finned tube exchangers while the ambient air flows outside. As a result, user define function based on the actual steam property is applied to simulate the heat transfer course between the exhaust and the ambient air. Two different mechanisms of hot air are simulated: one is based on wind speed and the other is based on wind direction. The simulation result shows that when the wind blows in the front of the ACSC, the hot air from the heat exchanger flow out free at low wind speed while it flows into the fan in the A-frame, and reverse irrigation occurs. Recirculation rate reaches its peak value at α = 135° under the obstacle effect of the turbine and boiler houses. The hot air recirculation under ambient conditions is systematically studied in the paper, and the research results provide the reference for the design and operation of the power plant.

Shrub canopy interception of diaspores dispersed by wind

2020 ◽  
pp. 1-9
Author(s):  
Xuanping Qin ◽  
Zhimin Liu ◽  
Minghu Liu ◽  
Wei Liang ◽  
Carol C. Baskin ◽  
...  
Keyword(s):  
Wind Speed ◽  
Projection Area ◽  
Canopy Interception ◽  
Wind Speeds ◽  
Final Destination ◽  
Plant Canopies ◽  
Low Wind Speed ◽  
Large Influence ◽  
Shrub Canopy ◽  
Canopy Type

Abstract Interception by plant canopies during wind dispersal can affect the final destination of diaspores. However, how the interaction of wind speed, canopy type and diaspore attributes affects interception of diaspores by the plant canopy has rarely been studied. We investigated canopy interception for 29 species with different diaspore attributes, six canopy types and six wind speeds in controlled experiments in a wind tunnel. Shrub canopy interception of diaspores were controlled by wind speed and diaspore attributes, but the latter had a greater influence on canopy interception than the former. At low wind speed, diaspore wing loading had a large influence on canopy interception, whereas at high wind speed, diaspore projection area had a large influence. The chance of canopy interception at a particular wind speed was additionally affected by the type of canopy. This study increases our knowledge of the dispersal process, corrects the previous understanding of diaspore dispersal potential and improves the theoretical basis for predicting spatial pattern and dynamics of plant populations.

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