High Intensity Winds Research Articles

Downburst versus boundary layer induced wind loads for tall buildings

Downbursts are transient phenomena that produce wind profiles that are distinctly different from synoptic boundary layers. Wind field data from Computational Fluid Dynamics (CFD) simulations of isolated downburst-like impinging jets, are used to investigate structural loads of tall buildings due to these high intensity winds. The base shear forces and base moments of tall buildings of heights between 120 and 250 m produced by downburst winds of various scales are compared with the forces from the equivalent boundary layer gust winds, with matched 10-metre wind velocity. The wind profiles are mainly functions of the size of the downburst and the radial distance from the centre of the storm. Wind forces due to various downburst profiles are investigated by placing the building at different locations relative to the storm center as well as varying the size of the downburst. Overall it is found that downbursts larger than approx. 2,000 m in diameter might produce governing design wind loads above those from corresponding boundary layer winds for tall buildings.

Behaviour of guyed transmission line structures under downburst wind loading

Past experience indicates that the majority of failures of electrical transmission tower structures occurred during high intensity wind events, such as downbursts. The wind load distribution associated with these localized events is different than the boundary layer wind profile that is typically used in the design of structures. To the best of the authors` knowledge, this study represents the first comprehensive investigation that assesses the effect of varying the downburst parameters on the structural performance of a transmission line structure. The study focuses on a guyed tower structure and is conducted numerically using, as a case study, one of the towers that failed in Manitoba, Canada, during a downburst event in 1996. The study provides an insight about the spatial and time variation of the downburst wind field. It also assesses the variation of the tower members` internal forces with the downburst parameters. Finally, the structural behaviour of the tower under critical downburst configurations is described and is compared to that resulting from the boundary layer normal wind load conditions.

Characterisation of bio-aerosols during dust storm period in N–NW India

Bio-investigations for pollen and spores were performed on dry free-fall dust and PM 10 aerosol samples, collected from three different locations separated by a distance of 600 km, situated in dust storm hit region of N–NW India. Presence of pollen of trees namely Prosopis ( Prosopis juliflora and Prosopis cinearia), Acacia, Syzygium, Pinus, Cedrus, Holoptelea and shrubs namely Ziziphus, Ricinus, Ephedra and members of Fabaceae, Oleaceae families was recorded but with varying proportions in the samples of different locations. Poaceae, Chenopodiaceae/Amaranthaceae, Caryophyllaceae, Brassicaceae and Cyperaceae (sedges) were some of the herb pollen identified in the samples. Among the fungal spores Nigrospora was seen in almost all samples. Nigrospora is a well known allergen and causes health problems. The concentration of trees and shrubs increases in the windward direction just as the climate changes from hot arid to semiarid. The higher frequency of grasses (Poaceae) or herbs could either be a result of the presence of these herbs in the sampling area and hence the higher production of pollen/spores or due to the resuspension from the exposed surface by the high-intensity winds. But we cannot ascertain the exact process at this stage. The overall similarity in the pollen and spore assemblage in our dust samples indicates a common connection or source(s) to the dust in this region. Presence of the pollen of the species of Himalayan origin in our entire samples strongly point towards a Himalayan connection, could be direct or indirect, to the bioaerosols and hence dust in N–NW India. In order to understand the transport path and processes involved therein, present study needs further extension with more number of samples and with reference to meteorological parameters.

Numerical simulations of impinging jets with application to downbursts

Numerical simulations are employed to investigate the macro-flow dynamics and scale (Reynolds number) dependency of impinging jets with applications to downburst related high intensity winds. The flow is quasi-periodic with vortex rings, generated by the initial jet instability, impinging on the surface where they determine an unsteady separation-reattachment of the boundary layer. The maximum velocities are encountered at less than 0.05 jet diameters from the surface immediately after the touch down of the main ring-vortex. Due to the unstable separation-reattachment of the boundary layer the flow is Reynolds number (scale) dependent. The maximum in the mean velocity profiles increases and tends towards the wall as the Reynolds number increases. The unsteady Reynolds Averaged Navier–Stokes simulations provide time series which when fitted to the RFD full-scale data, (Gast, K.D., Schroeder, J.L., 2005. Extreme wind events observed in the 2002 thunderstorm outflow experiment. In: Proceedings of the 10th Americas Conference on Wind Engineering (10 ACWE), Baton Rouge, LA, USA), are used to estimate both the length and the velocity scales of the full-scale event. Moreover, these simulations provide a first spatio-temporal flow model for wind loading on structures due to downbursts.

Finite element modeling of transmission line under downburst wind loading

Despite the fact that extensive research has been carried out on transmission lines subjected to normal wind loads, their behaviour under high intensity wind loads (HIW), such as downburst, is poorly defined. This paper describes a detailed numerical model that can be used to predict the structural performance of a transmission towers as part of a transmission line system under downburst loading. The time history of the downburst wind data is based on a previously developed and validated computational fluid dynamic model. The procedure used to scale the velocity wind data and to transform them to forces is described. Three-dimensional linear elastic frame elements are used to model the members of the towers while two-dimensional curved beam elements with geometric non-linearity included are used to model the conductors and the ground wires. A transmission line that suffered previously from significant damage due to a downburst event is then considered as a case study. Comparison between the results of the downburst analysis and those due to a normal wind that are typically used in the design reveals the importance of considering HIW loads when attempting the structural design of transmission towers.

Factors limiting phytoplankton productivity in 49 shallow reservoirs of North Côte d'Ivoire (West Africa)

Abstract Several hundred ranching reservoirs are scattered in North Côte d'Ivoire, where they have an increasing economical importance as water resources during the dry season. To obtain a synoptic view of their limnological characteristics and to identify key factors limiting algal productivity, 49 of these shallow reservoirs were sampled once in 1997 at the end of the dry season. They showed low conductivity (range 40-230 µS cm-1 ) and low pH (range 5.1-7.3), and lakes deeper than 2 m were stratified. Most lakes had low nutrient concentrations (median: 0.09, 3.49 and 0.44 µM, for NO3-N, NH4-N and PO4-P, respectively). They were very turbid, with most of Secchi depths ranging between 0.1 and 0.4 m, and had high seston weights (median, 45 mg l-1). Many lakes had high chlorophyll concentrations (median, 106 µg l-1 ), and some of them featured algal blooms, while many lakes had primary production value <1000 mg C m-2 d-1. Algae of size >10 µm represented about 50 percent of the chlorophyll biomass and phytoplankton production. Classification on hydrological and particle data identified five groups based on depth, turbidity and productivity. During the dry season, the initial nutrient pulse linked to the annual flood is exhausted. At this time of the year, phytoplankton production was nitrate limited in some lakes, while in some others, phosphorus was limiting. Deep sites (>2 m) were less turbid and less productive than shallow ones (<2 m). In shallow unstratified lakes showing a nutrient-rich bottom layer, easy vertical mixing allow a regular enhancement of the algal productivity. In deeper and stratified lakes, nutrient enrichment only occurs when high intensity winds induce vertical mixing, and their productivity is directly related to these episodic nutrient pulses.

Modelling of tornado and microburst-induced wind loading and failure of a lattice transmission tower

Many transmission line and tower failures worldwide are attributed to high intensity winds (HIW) associated with tornadoes and microbursts. This paper describes models for the wind velocity time–histories of transient tornado and microburst events and the resulting loading on a lattice tower. A dynamic structural analysis has been undertaken for two HIW events, predicting a tornado-induced shear failure, as observed in the field. However, the microburst does not produce failure, due to its lower intensity and longer duration. Indeed, such failures, if they occur in practice, are likely to be associated with a more broad-fronted microburst loading in which conductor loads contribute.

A risk model for design of transmission line systems against thunderstorm downburst winds

In this paper we describe a probabilistic model for the design of transmission line systems for local high-intensity winds of the downburst type, usually associated with thunderstorms. The downburst damage ‘footprint’ has been assumed to be roughly rectangular in shape, and the transmission line as a line ‘target’. With a number of other assumptions, a practical model for estimating the risk of a strike of damaging downburst on a line of specified length and orientation has been developed, and applied to high-voltage lines in Australia.

Influence of Atchafalaya River Discharge and Winter Frontal Passage on Suspended Sediment Concentration and Flux in Fourleague Bay, Louisiana

Suspended sediment concentrations and fluxes between Fourleague Bay, Louisiana and the northern Gulf of Mexico were sampled every 3h for 3 months to examine the importance of atmospheric cold fronts and riverine forcing on the functioning of this estuarine system. A cold front index was developed and used to identify major winter frontal passages likely to have the largest effects on material concentrations and transport. Suspended sediment concentrations ranged from 11 to 1527mg l−1; the highest values occurred during winter frontal passages and the lowest during calm periods. High concentrations are generated by a continuous source of sediment from the Atchafalaya River and resuspension of benthic sediment via high intensity winds associated with cold fronts along with sufficient duration to keep the sediment in suspension. Spring peak discharge of the Atchafalaya River increased water levels and sediment concentrations in the bay leading to strong seasonal net exports of water (1·02×109m3) and sediment (1·72×108kg) into the Gulf of Mexico through Oyster Bayou over the 89-day study. Net fluxes associated with tidal forcing were nearly balanced with a small net export due to freshwater input. The combination of high volumes of water originating from the northern bay and the restricted outlet to the Gulf often cause increased water levels and inundation of the surrounding marshes and potential advection of sediments onto the marsh surface. The results suggest that marsh drainage often increases the particulate organic carbon export as a result of marsh flushing.

Forest disturbance in hurricane-related downbursts in the Appalachian mountains of North Carolina

We characterized five 0.2–1.1 ha gaps created by downbursts during Hurricane Opal in xeric oak forest at the Bent Creek Experimental Forest, Asheville, NC. Direction of windthrow was nonrandom in four of the five gaps, but differed among gaps suggesting that each was caused by an independent downburst. Windthrows reduced tree density by 19–39% and basal area (BA) by 30–53% within gaps. Most windthrows were uprooted (17–38% of all trees) versus broken below 1.8 m height (0–3%). Most species were uprooted in proportion to their abundance regardless of canopy position. Red oaks ( Quercus coccinea, Quercus rubra and Quercus velutina) were disproportionately uprooted, while Nyssa sylvatica and Acer rubrum were resistant to uprooting. As a group, Quercus lost 27–47% of individuals and 41–50% of BA. Q. coccinea lost ≥44% of trees and >55% of BA in sites where it occurred. Only minor shifts in canopy species dominance were evident. For several species, significantly more large-diameter individuals uprooted than their smaller counterparts. No relationship between dbh and number uprooted was detected for the red oaks, however. Canopy position appeared to have little bearing on this relationship. Uprooting disturbed 1.6–4.3% of the ground area and displaced 130–587 m 3 of root–soil–rock masses (rootmasses) per gap. We suggest that episodic, high-intensity wind is not uncommon, and has a substantial influence on forest structure, species composition, regeneration and microtopography of the southern Appalachian mountains at variable scales.

High intensity wind belts as massive energy sources

Coal and breeders will not be ready in time to replace the shortfall in oil and natural gas. Calculations of the year-average energy available from wind generators involves a factor which relates the cube of the mean annual wind ( A) to the mean of the cubes of the instantaneous wind velocities ( B). B A is 2·7. Allowance is made for a number of efficiency factors. The practical equation for electricity obtained after conversion to hydrogen, passage and reconversion to electricity, is: P elec≈2 × 10 −4 v 3 MW , for a rotor of 100 m in a location where the mean annual wind is v k.p.h. Thus, for v = 30, P elec ⋍ 5 MW per rotor. Below latitude 35, v = 30 is available over a front of about 3000 km on the sea. In many regions of the world, v = 25 ( P elec = 3·1 MW ) is available at sea. For the total energy consumption (including those now provided by oil and natural gas) for 10 6 people, about 2500 rotors located in a wind of 25–30 k.p.h. would suffice. The practicality of wind rotors of 100 m or equivalent radius need proving. Designs are proposed. Electrolysis of seawater evolves chlorine and its reconversion to oxygen is not a difficulty, but an extra cost. Hydrogen transfer up to 4000 km would be economic. Environmental considerations are optimal. The sea is the best location. Undersea storage is preferable. A projected cost of electricity at 1000 miles from the rotor is 8 mils kWh −1. The concept of large sea-borne rotors in high velocity wind belts with long distance hydrogen transmission offers a more readily attainable (and more environmentally acceptable) prospect than atomic or solar, possibilities.