Cw Letchford

Pressure distributions on a cube in a simulated thunderstorm downburst—Part A: stationary downburst observations

Abstract Thunderstorms are responsible for a large amount of wind-induced damage around the world. It is known that the wind characteristics in thunderstorms, particularly downbursts, differ significantly from those of synoptic scale boundary layer winds. This paper describes a study aimed at simulating the flow structure in a downburst and obtaining the pressure field on a cube immersed in such a flow. Part A presents the data obtained from a stationary wall jet simulation of a thunderstorm downburst, while Part B presents the data from a moving downburst simulation. The pressure distributions on the cube are compared with data from uniform and boundary layer flows.

Thunderstorms—their importance in wind engineering (a case for the next generation wind tunnel)

This paper attempts a state-of-the-art summary of research into thunderstorm wind fields from an engineering perspective. The characteristics of thunderstorms and the two extreme wind events-tornadoes and downbursts-spawn by thunderstorms are described. The significant differences from traditional boundary layer flows are highlighted. The importance of thunderstorm gusts in the worldwide database of extreme wind events is established. Physical simulations of tornadoes and downbursts are described and discussed leading to the recommendation that Wind Engineering needs to focus more resources on the fundamental issue - What is the flow structure in the strongest winds?

The application of the Quasi-steady Theory to full scale measurements on the Texas Tech Building

Full scale wind velocity and pressure measurements at the Texas Tech Field Research Laboratory have been used to evaluate the performance of the Quasi-steady Theory. As expected the pressure fluctuations deviate away from the theorys predictions in regions of flow separation, although the incorporation of non-linear terms improves the match. Significantly, the area-averaged loads over substantial parts of the building have high correlation with the approach flow and the probability density functions, as well as the rms and peak pressure coefficients, are well predicted by the Quasi-steady Theory.

Wind loads on rectangular signboards and hoardings

Drag or normal force coefficients on a range of rectangular signboards or hoardings with varying aspect ratios, clearance ratios and porositys are presented for a range of wind directions and compared with various design data. Whereas, there is reasonably good agreement for data for walls on ground, significant discrepancies exist in normal force coefficients for cases as the panel becomes elevated in the boundary layer. Discrepancies in codification have been highlighted and revised loading data recommended.

Wind loads on free-standing walls in turbulent boundary layers

Pressure measurements on walls immersed in turbulent boundary layer flow from two different wind tunnel facilities are presented and compared. A variety of wall configurations, wind directions and shielding arrangements were studied. Some of this data has been incorporated into codes of practice for the design of free-standing walls. Differences in the most fundamental case, that of a wall completely spanning the wind tunnel, are attributed to differences between the two wind tunnel flow simulations over the height of the wall.

The distribution and correlation of fluctuating pressures on the Texas Tech Building

Full scale pressure measurements at the Texas Tech Field Research Laboratory have been used to investigate the distribution and correlation of fluctuating pressures on a low rise building. Eigenvector analysis of the covariances indicate that longitudinal and lateral turbulence are responsible for the majority of pressure fluctuations. However, a significant second eigenvector for flow normal to the roof edges corresponds with the rollup and discrete shedding of the separation bubble as a whole. Conditional sampling of peak pressures shows that they are better correlated spatially than the time averaged fluctuations.

Mean wind loads on porous canopy roofs

Mean overall lift and drag forces on a range of canopy or open roof forms with varying porosities are presented. In general, lift forces decrease while for low roof pitches (α < 15°) drag forces increase as porosity is increased in the range 0-23%. Resolution of these forces into equivalent net roof pressures reveals that wind load may be transferred from the leeward to the windward areas, leading to potential overloading of the supporting structure. Mean and fluctuating pressure measurements were undertaken to confirm the inferred pressure distributions on the roofs.

Net pressures on a low-rise full-scale building

Abstract The quasi-steady design approach is used by many wind loading standards such as AS1170.2 1989, to determine the design net (i.e. external–internal) pressures on cladding elements and fixtures of low-rise buildings. External and internal pressures measured on the WERFL test building at Texas Tech were used to determine the net pressures at selected points, representative of cladding elements and fixtures on the walls and roof of the building, when nominally sealed and when containing large openings. The mean and fluctuating internal pressures were small in the nominally sealed building and the net pressures derived from AS1170.2 were conservative. In the case of the building with a dominant windward opening, the internal pressure closely follows the external pressure at the opening, and the net pressures derived from AS1170.2 were smaller compared to the measured pressures in some areas near the roof windward edge.

On the influence of v and w component turbulence on roof pressures beneath conical vortices

Wind tunnel experiments have been conducted to examine the influence of v and w component turbulence on roof pressures beneath conical vortices. Mean pressure coefficients for a range of pitch (vertical) and azimuth (horizontal) flow angles on a specially constructed model were obtained and combined with turbulence intensity measurements to predict rms fluctuating pressures using the quasi-steady theory. Velocity/pressure cross correlations were also obtained. Incorporation of the w component turbulence term improved the comparison with measured values but discrepancies remain as a result of the fundamental failure of the quasi-steady theory to deal with flow distortion and building generated turbulence. Simultaneous velocity and pressure data collected and analysed revealed that extremes in pressure were associated with large excursions in the lateral wind vector.

Wind loads on planar canopy roofs, Part 2: fluctuating pressure distributions and correlations

Fluctuating pressure distributions on planar canopy roofs are presented based 1:100 scale wind tunnel measurements obtained in a simulated suburban atmospheric boundary layer. For a range of roof pitch angles and wind directions, point and area-averaged pressure measurements were obtained with the worst loads experienced adjacent to lines of flow separation. For two particular roof pitches the cross correlations of area-averaged pressures were obtained and the covariance integration technique used to estimate the total fluctuating loads on the roof for a range of wind directions. Significant reductions in overall uplift and racking loads, were obtained using this method when compared with the traditional quasi-steady approach. This is because the partial correlation of fluctuating pressures is accounted for in covariance integration.