Ben L. Sill

The aylesbury comparative experiment: A final report

Abstract The Aylesbury Comparative Experiment has sought to evaluate the consistency in wind tunnel modelling of low-rise buildings. Seventeen labs worldwide reported results for identical models at 1:100 scale. Analysis indicated that errors in reference pressure are greater than those measured at each of the taps. For the sample of seventeen labs, the lab-to-lab variation in pressure coefficient was above 40 percent.

Aerodynamic roughness length: Correlation with roughness elements

Abstract This paper examines the relationship between the aerodynamic roughness length, z o , and the size, shape, and spacing of the roughness elements in both natural and controlled situations. Correlations are given to allow prediction of z o . Preliminary results of a wind tunnel study with random height roughness elements indicate that errors up to 30% can occur in the measured pressure coefficients on a low rise structure when related to a prototype situation.

On the failure of single-story wood-framed houses in severe storms

Abstract Wind-tunnel pressure measurements are used to define the wind forces at the roof to wall interface of typical single-story, wood-framed houses. These forces are then combined with the weight of the roof, connector capacities and the effects of internal pressure to estimate the most likely wind speeds at which a roof would separate from the walls. This is then extended to predict the risk of serious structural damage in a typical design hurricane, taking into account the form of construction, amount of shelter and the degree to which the windows are protected. Conclusions are drawn regarding the suitability of damage observations to estimate wind speeds and the susceptibility of various forms of construction to damage in severe storms.

Effects of surface roughness element spacing on boundary-layer velocity profile parameters

Abstract Turbulent boundary-layer velocity profiles over rough surfaces depend on the size, shape and spacing of the roughness elements. In this study, simple theoretical equations to predict the aerodynamic roughness length and displacement height as functions of element spacing density are developed when cubes are used as roughness elements. These parameters are also evaluated from wind-tunnel tests for various element spacing densities. The experimental results agree well with the theoretical equations. Dependence of the shear velocity (or surface drag coefficient), turbulence intensity and longitudinal turbulent length scale on the element spacing density are also evaluated from the experiments.

IAWE Aylesbury Comparative Experiment — Preliminary results of wind tunnel comparisons

Abstract The Aylesbury Comparative Experiment seeks to validate techniques for modelling low-rise buildings at large scale factors in wind tunnels by comparing the results from a 1: 100 scale standard model in wind tunnels around the world. In this paper a summary of the preliminary results from the first 11 laboratories is presented. Linear regression analysis shows that the regression scatter is quite small, but there are consistent differences between laboratories in zero level and gain, each in the range ± 15% . These are attributed to differences in the method of acquisition and the position of the reference static and dynamic pressures.

Turbulent boundary layer profiles over uniform rough surfaces

Abstract Historically, atmospheric boundary layer profiles have been most commonly expressed by logarithmic and power law forms. This paper examines several ways in which the wind profile can be “derived” and shows that all the methods lead either to logarithmic or power law expressions. The logarithmic form is seen to be best near the surface, with the power law a better fit at greater heights or over a larger range of elevations. Results show that the power law exponent is a function of the boundary layer thickness and the surface roughness. The various expressions obtained are compared with the log-polynomial profile proposed by Deaves and Harris (CIRIA Rep. 76, 1978) since it has been verified with many atmospheric profiles measured to as much as half the boundary layer thickness. When compared over the entire boundary layer, the power law agrees with the Deaves and Harris model to within a maximum of 5% and generally much less; a weighted log-power law expression agrees with the Deaves and Harris expression to within 1%. The velocity defect forms of the several expressions are examined, and all show similar values and variation.

Pressures on a cube embedded in a uniform roughness field of variable spacing density

Abstract It is well known that surrounding obstacles can alter the pressure cladding loads on low-rise structures exposed to strong winds. The present study examines some of these changes for a cube, surrounded by cubes of the same size for varying spacing densities. The changes in the pressure coefficients on the top and windward faces are reduced to dependency on spacing, wind direction, and location on the cube. Some simple comparisons are made with “exposure” adjustments suggested by one building code.

The Aylesbury comparative experiment: A status report

Abstract To date, the mean and 1 sec rms pressures from 11 facilities have been incorporated into the spreadsheet. Additional results have been recently obtained, bringing the total number of participants to 15. Work has begun to include all of these in the spreadsheet. In addition, the data for the extremes are also being incorporated into the spreadsheet. Even though 15 laboratories from throughout the world have conducted the ACE tests, not a single facility from the United States has participated. Many labs from the U.S. will be contacted and asked to conduct the test program. Even if a lab is not asked, it is requested that you volunteer. The goal is to have the testing program complete in the next year to 15 months. There is every reason to believe that between 20 to 30 labs will have tested the model at the conclusion of the test program. This is an excellent opportunity to evaluate the capabilities of a wind tunnel facility against many others world wide. Interested individuals should contact the senior author or one of the regional coordinators.

Entrainment models and their application to jets in a turbulent cross flow

Abstract In this study, simple flows, such as buoyant jet and line thermal discharged into a still, uniform environment, are modelled mathematically by using integral equations of mass, momentum and thermal energy conservation. The governing equations are solved for entrainment rate by using an assumed growth rate model, the coefficients of which are fixed empirically. Entrainment models obtained by this method predict just as well as do models developed by other researchers who used different approaches. Our entrainment models are then applied to the problem of a turbulent jet discharged into a turbulent cross flow including both the near, intermediate, and far fields in a single model. It is assumed that the entrainment rate for this case can be obtained as a sum of terms representing near field entrainment of a buoyant jet and a line thermal, and far field mixing due to ambient turbulence. The predicted trajectories, velocities, and dilution rates agree satisfactorily with experimental results.

Pressure distribution on a low-rise building model subjected to a family of boundary layers

Abstract In natural environments, terrain roughness over a sizeable area is almost always in a random pattern. The roughness elements on the ground, e.g., trees and buildings, usually are not uniform in size, shape, or in the spacing between them. However, simulations conducted in the wind tunnel for such terrain conditions often simplify, if not completely overlook, the random nature of the problem. The randomness is typically replaced by a uniform roughness field in which the roughness elements are uniform in size, shape, and spacing. In fact, uniform roughness fields have become a standard tool in wind tunnel simulations. As a result, the prototype terrain condition is not modeled geometrically and the consequences of leaving out randomness in a simulation have not been examined thoroughly. To address this issue, several random roughness fields and one uniform roughness field were tested. Boundary layers generated by these roughness fields as well as the pressure distribution on a low-rise building model were recorded. Comparison of the resulting data showed significant differences between random roughness and uniform roughness simulations, even though all the boundary layer profiles fell within a single terrain roughness category.