Study on the Wind Profiles and Turbulent Characteristics in the Baroclinic Convective Boundary Layer
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Abstract Based on the data of STORM‐FEST (STorm‐scale Operational and Research Meteorology‐Front Experiment System Test) carried out near the boundary between Kansas and Nebraska, U. S. in 1992, the vertical distributions of temperature, humidity and wind velocity in the baroclinic convective boundary layer were analysed. The results showed that the temperature and humidity were well mixed in the convective boundary layer. The wind velocity was also well mixed, but there existed wind shear sometime. Under conditions with and without wind shear the turbulent kinetic energy budget was calculated. Finally. The possible reasons of wind shear formation in the convective boundary layer were discussed.Keywords:
Convective Boundary Layer
Thermal wind
Log wind profile
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Some methods for the numerical prediction of the velocity and direction of wind were investigated and they were summerized as follows.(1) Gradient wind is easily calcuiated by giving the pressure and temperature distribution and a nomogram to calculate the wind velocity from the pressure gradient, latitude and radius of curvature is made assuming the air density to be 1.1×10-3gr/cm3.(2) The relations between surface and gradient wind are functions of pressure distribution as well as latitude of the station, and the mean values of the ratio of the velocity of the surface wind to that of the gradient wind and the angle between the surface wind and the isobar are calculated for some standard pressure distribution.(3) The diurnal variation of the wind velocity is caused by diuranal change of the stability of the lower layer due to solar radiation and following similitude law between the air temperature and the wind velocity is proved theoretically and confirmed by the results of the observation where V0 and _??_0 are the velocity and the temperature in the upper layer.(4) Diurnal change of wind velocity V, such as land and sea breaze, mountain and vallcy wind, is theoretically proved to be given by V=b √ΔT where b is a contant, ΔT the amplitude of diurnal change of air temperature.(5) The wind velocity in the area of cyclone or typhoon is given by following empirical formula V=α√760-p where α is a constant, p pressure at the center of depression.(6) The wind velocity at cold front is conservative at least during 12 hours, hence it is predicted by mere extrapolation of the motion of the front.
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Thermal wind
Typhoon
Pressure gradient
Roughness length
Density of air
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This paper presents a new method for determining the interaction between a wind farm and the prevailing wind for wind energy siting studies plus first insights on the modification of the wind profile as obtained with this method. It is shown that neutral planetary boundary layer flow with wind farming essentially is steady and two-dimensional; and that the convective forces, the Coriolis forces and the vertical and spanwise gradients of the turbulent momentum fluxes all have the same order of magnitude. A numerical representation in the form of backward differences allows for an implicit solution of the two horizontal velocity components in vertical direction, iterating on the turbulent viscosity, in combination with a marching solution in the horizontal directions. The continuity equation is sat-isfied by employing the Lagrange multiplier method to the velocity components that satisfy the continuity equation. Resolved profiles show how most of the wind speed change occurs in the lower part of the boundary layer whereas most of the wind direction change occurs in the upper part, and that the thinner the boundary layer or the larger the surface roughness, the larger the wind direction change. Near a 5 MW wind turbine with a rotor diameter of 100 m operating at full load the ve-locity deficit is of the order of 5%, the wind direction change is increased with 1 ... 2 deg, and the velocity recovery distance is 20 rotor diameters. For a wind farm with 22 of these turbines these numbers are 15%, 2 ... 3 deg, and 2 wind farm length scales.
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Roughness length
Thermal wind
Supersonic wind tunnel
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Analytical model of fully developed wind farms in conventionally neutral atmospheric boundary layers
The wind energy industry relies on computationally efficient engineering-type models to design wind farms. Typically these models do not account for the effect of atmospheric stratification in either the boundary layer or the free atmosphere. This study proposes a new analytical model for fully developed wind-turbine arrays in conventionally neutral atmospheric boundary layers frequently encountered in nature. The model captures the effect of the free-atmosphere stratification, Coriolis force, wind farm layout and turbine operating condition on the wind farm performance. The model is developed based on the physical insight derived from large-eddy simulations. We demonstrate that the geostrophic drag law (GDL) for flow over flat terrain can be extended to flow over fully developed wind farm arrays. The presence of a vast wind farm significantly increases the wind farm friction velocity compared with flow over flat terrain, which is modelled by updated coefficients in the GDL. The developed model reliably captures the vertical wind speed profile inside the wind farm. Furthermore, the power production trends observed in simulations are reliably reproduced. The wind farm performance, normalized by the geostrophic wind speed, decreases as the free-atmosphere thermal stability increases or the Coriolis force decreases. In addition, we find that the optimal wind farm performance is obtained at a lower thrust coefficient than the Betz limit, which indicates that optimal operating conditions for turbines in a wind farm are different than for a single turbine.
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Thermal wind
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Abstract Large, rapid, and intermittent changes in wind direction were commonly observed in low–wind speed conditions in the very stable boundary layer during the phase 2 of the Project Sagebrush field tracer study. This paper investigates the occurrence and magnitude of these wind direction changes in the very stable boundary layer and explores their associated meteorological factors. The evidence indicates that these wind direction changes occur mainly at wind speeds of less than 1.5 m s −1 and are associated with momentum and sensible heat fluxes approaching zero in low–wind shear conditions. This results in complete vertical decoupling. They are only weakly dependent on the magnitude of turbulence. The magnitude of the wind direction changes is generally greatest near the surface, because of the greater prevalence of low wind speeds there, and decreases upward.
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Thermal wind
Decoupling (probability)
Wind Stress
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Based on computational fluid dynamics and large-eddy simulation technologies, a numerical simulation method of a moving thunderstorm field is established by considering the impinging jet model. In addition, the influence of movement speed of the storm center and jet velocity changes on the characteristics of the wind field is presented. By drawing horizontal wind speed radial profile curves, the vertical profile curves of horizontal wind velocity, radial wind profile curves of vertical wind speed, and transient wind speed characteristics of a moving thunderstorm downburst are illustrated. The results demonstrate that the wind field of a moving thunderstorm exhibits clear asymmetric characteristics. As the height from the ground increases, the horizontal wind speed first increases and then decreases. The extreme value of the horizontal wind speed appears at the position of (0.01–0.03) Djet (where Djet is the jet spout diameter) near the ground, which is consistent with the characteristics of a static thunderstorm wind field. Simultaneously, the distribution characteristics of wind pressure on the surface of wind turbines under moving thunderstorms is further studied. By considering variations of the wind attack angle, jet velocity, and moving speed, the results of wind pressure on the tower and blades of the wind turbine are presented. The coupling of the moving speed and jet velocity has a significant influence on the wind field and pressure on the wind turbine surface.
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Thermal wind
Maximum sustained wind
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Abstract Most large‐eddy simulation studies related to wind energy have been carried out either by using a fixed pressure gradient to ensure that mean wind direction is perpendicular to the wind turbine rotor disk or by forcing the flow with a geostrophic wind and timely readjusting the turbines' orientation. This has not allowed for the study of wind farm characteristics with a time‐varying wind vector. In this paper, a new time‐adaptive wind turbine model for the large‐eddy simulation framework is introduced. The new algorithm enables the wind turbines to dynamically realign with the incoming wind vector and self‐adjust the yaw orientation with the incoming wind vector similar to real wind turbines. The performance of the new model is tested first with a neutrally stratified atmospheric flow forced with a time‐varying geostrophic wind vector. A posteriori, the new model is used to further explore the interaction between a synthetic time‐changing thermal atmospheric boundary layer and an embedded wind farm. Results show that there is significant potential power to be harvested during the unstable time periods at the cost of designing wind turbines capable of adapting to the enhanced variance of these periods. Stable periods provide less power but are more constant over time with an enhanced lateral shear induced by an increased change in wind direction with height. Copyright © 2015 John Wiley & Sons, Ltd.
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Log wind profile
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Here we use accurate observations of the wind speed vector to analyze the behavior with height of the wind direction. The observations are a combination of tall meteorological mast and long-range wind lidar measurements covering the entire atmospheric boundary layer. The observations were performed at the Høvsøre site in Denmark, which is a flat farmland area with a nearly homogeneous easterly upstream sector. Therefore, within that sector, the turning of the wind is caused by a combination of atmospheric stability, Coriolis, roughness, horizontal pressure gradient and baroclinity effects. Atmospheric stability was measured using sonic anemometers placed at different heights on the mast. Horizontal pressure gradients and baroclinity are derived from outputs of a numerical weather prediction model and are used to estimate the geostrophic wind. It is found, for these specific and relatively short periods of analysis, that under both barotropic and baroclinic conditions, the model predicts the gradient and geostrophic wind well, explaining for a particular case an 'unusual' backing of the wind. The observed conditions at the surface, on the other hand, explain the differences in wind veering. The simulated winds underpredict the turning of the wind and the boundary-layer winds in general.
Thermal wind
Log wind profile
Pressure gradient
Anemometer
Maximum sustained wind
Atmospheric instability
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Combined LiDAR/cup anemometer observations performed in the summer of 2006 of wind speed profiles up to 161 m have been analyzed within an open sea sector at the Horns Rev offshore wind farm. The influence of atmospheric stability on the surface layer wind shear is studied by using a bulk formulation of the Richardson number to derive the Obukhov length from 10 minutes mean temperature and wind speed measurements. The influence of the boundary layer height on the wind speed profile gives a strong over-prediction of the wind speed in stable atmospheric conditions. A length scale model is suggested where the boundary layer height is taken into account. The resulting wind profile agrees well compared to the combined LiDAR/mast profiles in and beyond surface layer.
Log wind profile
Maximum sustained wind
Anemometer
Thermal wind
Atmospheric instability
Surface layer
Mast (botany)
Wind Stress
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A model of wind and turbulence has been described for the surface boundary layer. The wind structure in the surface layer is considered to be a function of the surface parameters, stability, and height. The surface parameters considered are: (1) the surface roughness length; (2) the surface friction velocity; and (3) the zero plane displacement height. The stability parameter, Z/L, where L is the Monin-Obukov stability length, describes the thermal effect on the wind profile. The logarithmic wind profile is used to describe the mean wind field in the neutral boundary layer, and a logarithmic profile with a stability defect is used to describe the stable and unstable atmospheric conditions. For the very stable conditions, the logarithmic wind law does not hold. Under this condition, the layers of the atmosphere become disconnected and large scale frontal motions are the predominate factor in defining the wind profile. Figures are presented which represent some typical wind profiles in the very stable condition.
Log wind profile
Roughness length
Thermal wind
Surface layer
Shear velocity
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Thermal wind
Log wind profile
Geostrophic current
Pressure gradient
Potential temperature
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