P. Saathoff

Dilution of exhaust from a rooftop stack on a cubical building in an urban environment

Abstract The dispersion of a tracer gas emitted from a short stack on a cubical building was investigated using field and wind tunnel experiments. Air samples were obtained on the roof and the leeward side of the building. Dilution data were compared with estimates of minimum dilution ( D min ) obtained with design formulas of Wilson/Chui/Lamb and Halitsky. The Halitsky model produced conservative predictions of D min . Estimates of D min obtained with the Wilson/Chui/Lamb model were in reasonable agreement with the field data. Wind tunnel dilution values were usually within a factor of two of the field data. The accuracy of the wind tunnel data improved as distance from the source increased. Wind tunnel dilution was found to be strongly dependent on the ratio of exhaust speed to wind speed, M , particularly for M values in the range of 2–4. This dependence is believed to be associated with the wind tunnel modelling of the stack exhaust, and is thus probably not a feature of the full-scale situation.

Wind pressure on roofs of various geometries

Abstract This paper refers to the work carried out to determine local pressure coefficients appropriate for the design of multi-span gabled (folded) roofs of low buildings. In particular, a wind tunnel study was conducted to investigate the influence of roof slope and number of spans on local pressure coefficients measured on gabled roofs. Single-span, double-span and multi-span models were tested for roof slopes of 4:12 and 12:12. For both roof slopes, the values of mean and peak pressure coefficients measured in regions of the double-span and multi-span roofs were generally slightly greater than corresponding values for the single-span roof. However, maximum suctions measured near the ridge-edge of the 4:12 single-span model exceeded the double-span and multi-span data. On the other hand, suctions measured in the trough of the double-span and multi-span 12:12 models were more than 50% larger than corresponding data obtained with the single-span model. The single-span data generally agree with values or pressure coefficients specified by the National Building Code of Canada (1985) but some discrepancies have also been identified.

Effects of freestream turbulence on streamwise pressure measured on a square-section cylinder

Abstract Wind tunnel experiments were performed to study the effects of freestream turbulence on the flow around a square cylinder. The results indicate that the pressure fluctuations on streamwise surfaces are produced mainly by periodic vortex shedding when turbulence intensity, σ u / U , is less than 12%. As σ u / U approaches 20%, vortex shedding weakens significantly and pressure fluctuations occur over a broad range of frequency. Flow visualization experiments show that extreme negative pressure peaks in highly turbulent flow are associated with intermittent reattachment of separated shear layers. The results also show that an increase in turbulence scale causes peak pressure coefficients to increase.

Codification of Wind pressure coefficients for sawtooth roofs

Abstract Codified pressure coefficients for sawtooth roofs are proposed for use in wind standards and codes of practice. Design pressures are based on boundary layer wind tunnel data obtained using an open country exposure. Local and area-averaged pressure coefficients were measured on models with one, two and four spans and a roof angle of 15 degrees. The proposed design pressure coefficients are expected to be applicale for a range of roof angles from 10 to 30 degrees. In most regions of the roof, critical values of mean and peak suction obtained with the double-span and multi-span models were similar in magnitude to dat obtained with the single-span (mono-sloped roof) model. However, suctions measured near the roof edge in the trough of the double-span and multi-span models were more than 50% larger than corresponding data obtained with the single-span model. A similar trend was also found for positive pressures. Local peak pressure coefficients obtained in the present study are compared with design values specified in the Australian Standard AS 1170.2 (1989). Peak suctions and positive pressures obtained for particular wind directions were generally larger than the values given in AS 1170.2.