Influence of neighbouring structures on building façade pressures: Comparison between full-scale, wind-tunnel, CFD and practitioner guidelines

H Gough,M.-F King,P Nathan,C.S.B Grimmond,A Robins,C.J Noakes,Z Luo,J.F Barlow

doi:10.1016/j.jweia.2019.03.011

Abstract

Façade pressure coefficients are widely used to determine wind-driven ventilation potential for buildings at the design stage. Over nine months we measured façade pressure coefficients under full-scale conditions using the 6 m Silsoe cube in isolation and in a staggered array. Results are compared against a 1:300 wind-tunnel model, a time-dependent computational fluid dynamics (CFD) model at full-scale and to published pressure coefficients in ventilation design guidance for a range of wind angles.Across all wind angles, wind-tunnel, CFD and published models tended to underestimate full-scale experimental pressure coefficients in magnitude but replicated trends well for a single face on the isolated cube. Agreement was weaker for the array; pressure coefficients are asymmetric with wind direction and results sensitive to model set up and measurement strategies. Differences in pressure coefficient across the building compared well in both isolated and array cases, suggesting this is a more robust parameter for models than individual facet data.It is recommended that building symmetry and surrounding areas should be considered when relying on ventilation guidelines. Scale and computational models are effective to support design for more complex cases; however, it is important to ensure measurement locations are representative and that uncertainties are quantified.

Highlights

Predicting ventilation potential in naturally ventilated buildings is a key component in designing buildings that effectively support occupant health and comfort (Wang et al, 2008)
To ensure that the calculated pressure coefficients are unaffected by instrument sensitivity, only full-scale 30 min averages with near neutral stability (À0.1 < jz/Lj < 0.1, where z/L is the Obukhov stability parameter) and a reference wind speed at 6 m Uref > 3 m sÀ1 are used in line with limitations of the pressure tap response time (Richards and Hoxey, 2012)
For wind tunnel measurements the standard error of the averaged data is taken and for Computational Fluid Dynamics (CFD) data errors are based on the standard deviation over the sampling period of the measurement

Summary

Introduction

Predicting ventilation potential in naturally ventilated buildings is a key component in designing buildings that effectively support occupant health and comfort (Wang et al, 2008). The resulting pressure coefficients (Cp) can be used to calculate the flow through ventilation openings at different locations on the façade, using the orifice equation to relate flow rate to wind speed (CIBSE, 2018) Such approaches are used to produce generic Cp data for different building geometries that are published in the design guidance (CIBSE, 2005) and can be used to carry out ventilation calculations on simpler buildings. These scale-model approaches rely on several assumptions and uncertainties are not always given (Chiu and Etheridge, 2007; Chu et al, 2009). CFD and scale models need to be tested against full-scale data to ensure that the model is representative of the full-scale flow and captures all flow features

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