Wind shear

About: Wind shear is a research topic. Over the lifetime, 8023 publications have been published within this topic receiving 185373 citations.

Spatial study of the wake meandering using modelled wind turbines in a wind tunnel

Abstract: The present paper presents a study of the wake meandering phenomenon with the help of physical modelling in atmospheric boundary layer (ABL) wind tunnel. The wind turbine is modelled at a geometric scale of 1:400 with a static porous disk (actuator disk concept). The main objective of the present work was to supply quantitative information on the meandering process. The instantaneous wake width and its horizontal and vertical swerves from the mean wake axis are quantified using specific image processing of instantaneous velocity fields obtained by particle image velocimetry downstream of the modelled wind turbine. We observe that the wind turbine wake displacements from the mean streamwise axis are very important when the turbulence length scales are larger than the wake width. We also observe that, in ABL conditions, horizontal displacements are higher than the vertical ones with a proportion in agreement with the ratio between the transverse and vertical turbulence intensities σv∕σw ≈ 3∕2. It is finally concluded that the instantaneous wake width remained nearly constant downstream of the wind turbine model whatever the flow conditions and that the extent of the mean wind turbine wake is dominantly due to the meandering process. This confirms that it is necessary to include the meandering process in the wake analysis in order to estimate the fatigue loading on wind turbines within wind farms properly. Copyright © 2011 John Wiley & Sons, Ltd.

On the Generation of African Squall Lines

Abstract: Squall lines (SLs) form an important component of the meteorology of northern Africa, and in particular, contribute substantially to rainfall totals. Their generation requires the existence of a potentially unstable low-level supply of moisture overlain by dry desert air and vertical wind shear beneath the midlevel African easterly jet. The instability may be released (and an SL initiated) by factors such as surface heating, topography, African waves, or surface evaporation. The relative importance of each of these factors and the means by which they impact on SL generation is reviewed. This is followed by a detailed analysis of one month of satellite imagery and surface data for August 1985 over a portion of central northern Africa. The novelty of our study lies in the temporal resolution of the satellite imagery, which with 21 images per day allows the identification of a large number of short-lived SLs (4-h duration or less). On the southern fringes of the Sahara these are likely to contribute...

A three‐dimensional numerical investigation of the idealized planetary boundary layer

Abstract: The nonlinear equations of motion are integrated numerically in time for a region of x‐y‐z space of volume 3h × h × h, where h turns out to be a height slightly above the level where the wind first attains the geostrophic flow direction. Only the ideal case is treated of a horizontal lower boundary, neutral stability, horizontal homogeneity of all dependent mean variables except the mean pressure, and statistically steady state. The resulting flow patterns are turbulent and the eddies transport required amounts of momentum vertically. Topics which are investigated include the relative directions of stress, wind shear and wind; differences in Ekman wind spirals for the neutral numerical case and a stable atmospheric case; profiles of dimensionless turbulence statistics; effect of allowing the mean density to be either constant or to decrease with height; effect of the wind direction or latitude upon the turbulence intensities; and characteristic structure of the eddies in the planetary boundary layer.

Factors Responsible for Precipitation Efficiencies in Midlatitude and Tropical Squall Simulations

Abstract: Different definitions of storm precipitation efficiency were investigated from numerical simulators of convective systems in widely varying ambient conditions using a two-dimensional cloud model with sophisticated ice microphysics. The model results indicate that the vertical orientation of the updrafts, which is controlled by the vertical wind shear, and the ambient moisture content are important in determining storm efficiency. In terms of rainfall divided by condensation, simulated efficiencies ranged from 20%–35% for convective systems that tilted strongly against the low-level shear (upshear), to 40%–50% for erect storms. Changes in environmental moisture produced smaller variations in efficiency that were less than 10%. Upright convection allows for effective collection of cloud condensate by precipitation, whereas lower efficiencies in upshear storms are due to greater evaporation of cloud at middle levels and evaporation of rain at lower levels. Development of trailing stratiform precipit...

Numerical simulation of Martian dust devils

Abstract: [1] Large eddy simulations of vertical convective vortices and dust devils in the Martian convective boundary layer are presented, employing a version of the Mars MM5 mesoscale model, adapted to use periodic boundary conditions and run at resolutions of 10 to 100 m. The effects of background horizontal wind speed and shear on dust devil development are studied in four simulations, each extending over the daytime portion of one Martian day. The general vorticity development in all cases is similar, with roughly equal positive and negative vorticity extrema. Two dust devils were found to develop in the highest wind speed case and in a case run without background wind. The dust devil structures were found to agree well qualitatively with terrestrial dust devil observations, including the prediction of greatly diminished vertical velocities in the vortex core. Thermodynamic scaling theory of dust devils was found to provide good prediction of the relationship between central pressure and temperature in the modeled vortices. Examination of the turbulent kinetic energy budgets suggests balance between buoyancy generation and loss through dissipation and transport. The vorticity for the dust devils is provided by twisting of horizontal vorticity into the vertical. The horizontal vorticity originates from horizontal variations in temperature at the lower boundary (thermal buoyancy). While the horizontal winds generated by the modeled dust devils were likely insufficient to lift dust, this study provides a solid starting point for dynamic modeling of what may be an important component of the Martian dust cycle.