Eric Savory

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 tornadoinduced 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 WIND TUNNEL INVESTIGATION OF THE INFLUENCE OF SOLAR-INDUCED WALL-HEATING ON THE FLOW REGIME WITHIN A SIMULATED URBAN STREET CANYON

A wind tunnel study has been undertaken to assess theinfluence of solar-induced wall heating on the airflowpattern within a street canyon under low-speed windconditions. This flow is normally dominated by large-scalevortical motion, such that the wind moves downwards at thedownstream wall. In the present work the aim has been toexamine whether the buoyancy forces generated at this wallby solar-induced heating are of sufficient strength tooppose the downward inertial forces and, thereby, changethe canyon flow pattern. Such changes will also influencethe dispersion of pollutants within the street. In theexperiments the windward-facing wall of a canyon has beenuniformly heated to simulate the effect of solar radiation.Four different test cases, representing different degreesof buoyancy (defined by a test Froude number, Fr), havebeen examined using a simple, 2-D, square-section canyonmodel in a wind tunnel. For reference purposes, the neutralcase (no wall heating), has also been studied. The approachflow boundary layer conditions have been well defined, withthe wind normal to the main canyon axis, and measurementshave been taken of canyon wall and air temperatures andprofiles of mean velocities and turbulence intensities.Analysis of the results shows clear differences in the flowpatterns. As Fr decreases from the neutral case there arereductions of up to 50% in the magnitudes of the reverseflow velocities near the ground and in the upward motionnear the upstream wall. A marked transition occurs at Fr ≈ 1, where the single dominant vortex, existing at higher Fr values, weakens and moves upwards whilst a lower region of relatively stagnant flow appears. This transition hadpreviously been observed in numerical model predictions butat a Fr at least an order of magnitude higher.

Influence of Geometry on the Mean Flow within Urban Street Canyons – A Comparison of Wind Tunnel Experiments and Numerical Simulations

A comparison between numerical simulations and wind tunnel modelling has been performed to examine the variation with streamwise aspect ratio (width/height, W/H) of the mean flow patterns in a street canyon. For this purpose a two-dimensional (2-D) cavity was subjected to a thick turbulent boundary layer flow perpendicular to its principal axis. Five different test cases, W/H = 0.3, 0.5, 0.7, 1.0 and 2.0, have been studied experimentally with flow measurements taken using pulsed-wire anemometry. The results show that the skimming flow regime, with a large vortex in the canyon, occurred for all the cases investigated. For the cavities with W/H ≤ 0.7 a weaker secondary circulation developed beneath the main vortex. The narrower the canyon, the smaller the wind speed close to the cavity ground, giving increasingly poor ventilation qualities. The corresponding numerical results were obtained with the Computational Fluid Dynamics (CFD) code CHENSI that uses the standard k-ε model. The intercomparison showed good agreement in terms of the gross features of the mean flow for all the geometries examined, although some detailed differences were observed.

Hemisphere and hemisphere-cylinders in turbulent boundary layers

Abstract The work described is an experimental investigation of the flow around hemispheres and hemisphere-cylinders immersed in three different turbulent boundary layers. The effect on the near wake regime of changes in the approach flow or model geometry are examined using pulsed-wire anemometer measurements supplemented by flow visualisation studies. In addition, the influence of model surface roughness on the mean pressure distributions and on the critical Reynolds number is assessed, with reference to the flows around other highly curved bodies such as cylinders, spheres and semi-cylindrical barrel vaults.

The flow regime in the turbulent near wake of a hemisphere

The work presented is a wind tunnel study of the near wake region behind a hemisphere immersed in three different turbulent boundary layers. In particular, the effect of different boundary layer profiles on the generation and distribution of near wake vorticity and on the mean recirculation region is examined. Visualization of the flow around a hemisphere has been undertaken, using models in a water channel, in order to obtain qualitative information concerning the wake structure.

Wind tunnel studies on a dome in turbulent boundary layers

Abstract The work described is an initial investigation into the flow past a hemispherical dome immersed in two thick turbulent boundary layers of different turbulence intensity and velocity profiles. Pressure distributions over the surface of the dome are presented together with mean velocity and turbulence intensity profiles measured in the near wake using single hot-wire and pulsed-wire anemometers. It is shown that with increased turbulence intensity in the boundary layer, both separation region and reattachment point move downstream. The width of the shear layer and the maximum turbulence intensity are also shown to depend on the turbulence in the approaching boundary layer

Spatial Variability and Source-Receptor Relations at a Street Intersection

A wind tunnel study of dispersion at a simple urban intersection comprising two perpendicular streets is described. Concentration and flow field measurement were undertaken to determine the importance of the exchange of pollutants between the streets and to investigate source-receptor relationships at the intersection. The results showed that only in a symmetrical situation were exchanges negligible and that small departures from symmetry, brought about in the experiments through an off-set in the street alignment or a change of orientation relative to the wind, were sufficient to establish significant exchanges. The results also showed that significant structure appeared in the concentration fields in the streets as a result. Examples are shown where concentrations on one side of a street are entirely due to emissions from the perpendicular street, whereas on the opposite side concentrations depend on emission upwind in the same street as the receptor. The results imply that exchanges between street systems are likely to be the norm in practice and that the consequences of such exchanges are not confined to the immediate vicinity of the intersection.

Influence of street obstructions on flow processes within urban canyons

This paper is concerned with modelling of the motion of air within the urban environment and is directed towards a deeper understanding and controlling of pollutant dispersion in individual street canyons. Some possible methodologies by which the roughness associated with obstacles, such as small buildings, kiosks, trees and stationary vehicles within the canyon may be described for incorporation into an overall canyon model, are discussed. This is illustrated by results from a simple wind tunnel experiment utilising an idealised two-dimensional street canyon in which the internal roughness is represented by an array of vertical cylinders. These reveal the presence of complex flow patterns within the roughness layer. Integrated and averaged mean velocity and turbulence intensity parameters are presented to indicate the extent of the effect of the presence of the internal roughness upon the canyon flow regime.

The separated shear layers associated with hemispherical bodies in turbulent boundary layers

Abstract The work presented is a wind tunnel investigation of the highly curved, three-dimensional shear layers separating from the surfaces of hemispherical bluff bodies immersed in three different turbulent boundary layers. The thickness and growth rate of each shear layer is examined, over most of the region from separation to reattachment on the ground plane, and the influence of upstream conditions is assessed. In addition, the distributions of normal and shear stresses throughout the near wake region are reported.

Large-Scale Structures over a Single Street Canyon Immersed in an Urban-Type Boundary Layer

An analysis of the dynamics of the flow over a street canyon immersed in an atmospheric boundary layer is presented, using particle image velocimetry measurements in a wind tunnel. Care was taken to generate a 1:200 model scale urban type boundary layer that is correctly scaled to the size of the canyon buildings. Using proper orthogonal decomposition (POD) of the velocity field and conditional averaging techniques, it is first shown that the flow above the opening of the canyon consists of a shear layer separating from the upstream obstacle, animated by a coherent flapping motion and generating large-scale vortical structures. These structures are alternately injected into the canyon or shed off the obstacle into the outer flow. It is shown that unsteady fluid exchanges between the canyon and the outer flow are mainly driven by the shear layer. Finally, using POD, the non-linear interaction between the large-scale structures of the oncoming atmospheric boundary layer and the flow over the canyon is demonstrated.