Area-averaged Pressure Coefficients Research Articles

Requirements for partial turbulence simulations using nondimensional turbulence energy contributions

Quantifying turbulence effects is crucial for understanding building aerodynamics and for ensuring accurate wind tunnel test methods. This is especially important in wind tunnel methods that require post-experiment adjustments because approximate wind fields are used, such as the Partial Turbulence Simulation (PTS) approach. Understanding and analyzing these effects enables load adjustments since the PTS method only requires matching the high frequency portions of the upstream spectra of the longitudinal velocity component in model and full-scale. However, the limits for which the PTS method is applicable are unclear in terms of the allowable range of wind field characteristics that can be used in the wind tunnel simulation. To address this, the paper utilizes two nondimensional parameters, one representing the small-scale turbulence energy, ES, and the other the large-scale turbulence energy, EL, to elaborate the aerodynamic effects of turbulence intensity and integral length scales in the upstream wind. The results show that the maximum allowable mismatch ratio of integral length scales and of Jensen numbers between model and full-scale simulations depend on the target small-scale turbulence energy and the maximum allowable deviation of small-scale energy. By quantifying the effects of ES and EL on area-averaged pressure coefficients, the allowable limits are identified for wind tunnel test parameters that lead to negligible differences in the resultant pressure coefficient statistics in regions of separated-reattaching flow on the roof of a low-rise building.

Large-Scale Wind Testing on Roof Overhangs for a Low-Rise Building

Roof overhangs are prone to wind damage because they are subject to wind load at both the upper and bottom surfaces. Wind standards assume that the pressure at the bottom covering of a roof overhang will be the same as the external pressure coefficient on the adjacent wall surface. A large-scale experimental campaign was carried out at the Wall of Wind (WOW) Research Experimental Facility to investigate the validity and possible limitations of such assumptions. The experimental setup considered two 1:10 scaled models [0.61 m (2 ft) and 1.83 m (6 ft) inclined overhangs with a soffit] of a low-rise hip roof building with roof slope 4:12, eave height of 7.5 m (24 ft), and horizontal dimensions of 12.2 m (40 ft)× 15.24 m (50 ft). The two models were tested for open terrain for 40 wind directions (WDs). The study provided information on pressure variations at the top and bottom surfaces of overhangs, adjacent roof areas, and underneath walls. Pressure and correlation coefficients were generated between soffits and underneath walls to quantify the effect of overhang width. The research showed that the 0.61 m (2 ft) overhang experienced higher suction coefficients at the edges compared to the 1.83 m (6 ft) overhang. In addition, the results confirmed that, for both configurations, soffit positive pressure coefficients may be assumed to be equal to the adjacent wall external pressure, as stated by a common standard, while this might not be applicable for negative pressure coefficients. Correlation and regression analyses between soffit pressure taps and wall upper taps show that the 1.83 m (6 ft) soffit appeared to be less correlated with the wall upper taps, compared to the 0.61 m (2 ft) soffit. Finally, area-averaged pressure coefficients for overhangs and adjacent roof areas were compared to the provisions in one standard for each specified zone, and differences were found.

A method to estimate peak pressures on low-rise building models based on quasi-steady theory and partial turbulence analysis

A new method for estimating peak area-averaged pressure coefficients on the roof of a low-rise building model is developed. The method relies on the well-known quasi-steady vector model to account for the large-scale, low-frequency fluctuations of the upstream wind. The novel approach is to account for small-scale and body-generated turbulence effects using a stochastic model. Wind tunnel data for a 1/50-scale low-rise building model for six different upstream turbulence conditions are used for the analysis. The fluctuating pressure component induced by the small-scale and body-generated turbulence is obtained by subtracting the quasi-static pressure component from the original pressure signal. It is observed that the small-scale pressure component is highly dependent on turbulence level of the upstream flow, such that normalization by the small-scale components of the upstream turbulence kinetic energy leads to a self-similar distribution. A Monte Carlo simulation was used to combine the small-scale and quasi-steady pressure components. The new model provides good predictions of peak area-averaged pressure coefficients that cannot be captured by the quasi-steady approach.

An Examination of the Gust Effect Factor for Rigid High-Rise Buildings

In order to systematically investigate the gust effect factor for rigid buildings, the derivation of the gust effect factor in ASCE 7–16 is carefully reviewed and scale model pressure tests were carried out for rectangular-plan high-rise buildings with plan aspect ratios ranging from 0.11 to 9. The gust effect factor and the aerodynamic admittance function (AAF) for area-averaged pressure coefficients and base drag coefficients were obtained and discussed in detail. The results show that the AAF has direct influence on the value of the gust effect factor, depending on whether effects of non-contemporaneous gust actions or body-generated turbulence are playing a leading role. The ASCE 7–16 gust effect factor for rigid buildings underestimates the measured values for individual walls due to differences in the AAF, peak factors, and the employment of the 3 s moving average filter. However, the ASCE 7–16 gust factor for overall drag is estimated within 5% or better.

Comparative study of full-scale and model-scale wind pressure measurements on a gable roof low-rise building

A full-scale experimental low-rise building with gable roof was constructed in a typhoon-prone area in China, aiming to study the wind effects on a typical low-rise building during tropical cyclones. Meanwhile, it was also intended to provide reliable full-scale measurements for verification of widely used simulation techniques, such as wind tunnel testing and numerical simulation. This paper presents a detailed comparative study of the wind effects on the experimental building between the full-scale measurements during six tropical cyclones and the wind tunnel test results on a 1:50 scaled model of the low-rise building, including point pressure coefficients, area-averaged pressure coefficients and wind pressure probability density functions. The model-scale and full-scale mean pressure coefficients exhibits a good agreement. However, significant differences are found for the root-mean-square (RMS) and peak negative pressure coefficients on the gable end roof under oblique winds. The probable causes for these discrepancies including inadequacy of large-scale turbulence, mismatch of Reynolds number and elevation angles (vertical wind angle of attack) in the wind tunnel test are explored based on the field measurements.

Comparison of high resolution pressure measurements on a high-rise building in a closed and open-section wind tunnel

Wind tunnel testing represents an established technique for the assessment of wind-induced pressure on cladding systems. Nonetheless, some physical events, such as the pressure peaks that occur on a building’s lateral facades, are not fully understood. To enable detailed analysis of the nature of these pressure peaks, we performed high-resolution pressure measurements on a high-rise building in two different wind tunnels: the closed-circuit wind tunnel of Politecnico di Milano, and the open-circuit Wall of Wind facility at Florida International University. The objective of the paper is to present the experimental set-up and the high-resolution pressure data, and to investigate the characteristics of the extreme suction events at individual pressure taps and their relevance for cladding design. We first compare the two atmospheric boundary layers, and subsequently present the pressure coefficients statistics. Then, we present probability density functions of the local and area-averaged pressure coefficients and visualize the space-time characteristics of two peak events to investigate their relevance for cladding design. The experiments provide consistent results and exhibit two types of suction events: one is characterized by an extremely short duration and spatial extension, while the other impacts a larger portion of the facade.

Aerodynamic Mitigation of Wind Uplift on Low-Rise Building Roof Using Large-Scale Testing

During strong wind events such as hurricanes and thunderstorms, building roofs are subjected to high wind uplift forces (suctions), which often lead to severe roofing component damage and possibly, water intrusion. It is therefore crucial to accurately estimate peak suctions (negative pressures) on roofs for design purposes and to develop mitigation devices to reduce wind uplift and possible damage. Past research suggests that mitigation devices, in various forms and configurations, can significantly reduce wind effects on buildings’ roofs. Perforated parapets have been shown by many researchers to be one of the most effective and low-cost mitigation devices for reducing roof suction. However, such devices contain perforations of critical functionality that may be Reynolds number Re dependent. Therefore, it is essential to study their functionality and possible Re dependency in wind tunnel testing. This paper focuses on investigation of Reynolds number Re similitude aspects of porous devices and large-scale model testing of discontinuous porous parapets to estimate their roof uplift reduction efficacy that would be representative of the prototype (full-scale) conditions. Large scale (1:8) experiments were implemented at the 12-Fan Wall of Wind Experimental Facility (WOW) at Florida International University (FIU) to achieve adequate local Reynolds number Re to ensure realistic results. In this study, a relatively smaller ratio of parapet height to roof height has been considered as compared to previous studies. The paper delineates the step-by-step design of the parapets based on kinematic similitude requirements cited in the literature. The goal was to achieve a discontinuous porous parapet design that provides similar uplift reduction efficacy as compared to those reported in the literature, but with lower height and smaller length to ensure cost-effectiveness, ease of installation, and architectural aesthetics of the retrofitted building. The effects of perforated parapets on both individual tap and area-averaged pressure coefficients were explored. The results indicated a maximum of 45% reduction in individual tap peak pressure coefficients at the corner roof and a 40% reduction in area-averaged peak pressure coefficients. The study showed that low-height ratio perforated parapets could be as effective as those reported in the literature with higher height ratios.

Wind Loads on Low-Profile, Tilted, Solar Arrays Placed on Large, Flat, Low-Rise Building Roofs

The author examined wind loads on low-profile, roof-mounted solar arrays, placed on large, low-rise buildings with nearly flat roofs by using scale models in a boundary layer wind tunnel. The author also examined the effects of building size and array geometry on enveloping curves of area-averaged pressure coefficients, typical of use for design. It was found that wind loads on the array increase with building size; normalizing the effective wind area by the building wall size leads to enveloping curves that collapse onto a single curve for each array geometry. For tilt angles less than 10°, there is an approximate linear increase in the pressure coefficients as the tilt angle increases. For arrays with tilt angles of 10° or more, the wind loads do not depend significantly on the tilt angle and are relatively constant. Roof zones for wind loads on solar arrays are larger than roof zones for bare roofs and depend on the array tilt angle.

Wind pressures on multi-level flat roofs of medium-rise buildings

Since there is limited information on design of wind pressures on multi-level flat roofs from previous studies and in wind loading codes, a wind tunnel experiment was carried out on 1:67 scaled models of a series of medium-rise buildings. Using an assembled cubic plexiglass model and uniformly distributed pressure taps, multi-level flat roofs with different configurations were easily fabricated and pressures on the roofs measured. Mean and fluctuating, local peak and area-averaged pressure coefficients were mainly investigated and stochastic characteristics of wind pressures on multi-level flat roofs were determined. Parameters of step geometry such as the effects of step dimension and configuration were considered. The results showed no significant difference between minimum values of negative peak pressures on simple flat roofs and on multi-level flat roofs, while negative pressure distributions and maximum values of positive peak pressures on the low-roof of multi-level flat roofs were highly dependent on step parameters. A comparison with ASCE/SEI7-10 indicates that negative values with small tributary areas are larger than those in related contents in the code, while negative values in other zones and positive values agree closely with the code. Gaussian and negative-skewed non-Gaussian PDFs, as well as positive-skewed non-Gaussian ones, were measured for multi-level flat roofs.

Wind-Induced Cladding and Structural Loads on Low-Wood Building

A test building was constructed in Fredericton, New Brunswick and was used to collect pressure and force coefficient data. A 1:200 scale model was also fabricated and tested for a total of 15 angles of wind attack at the Building Aerodynamics Laboratory of Concordia University. Mean and peak local and area-averaged pressure coefficients were measured and finite-element analysis via a 3D linear model was performed in order to compute force coefficients for the selected wind directions. Pressure distribution comparisons between the full-scale and the wind tunnel results showed generally good agreement with small discrepancies attributed to the direction fluctuations of the full-scale records. Force coefficient comparison between the full-scale and the finite element analysis results showed good agreement for the mean values. However, significant overestimation of the fluctuating dynamic component of the foundation force is apparent when this is derived from the fluctuating roof pressures, in comparison to the directly measured values in the field. The experimental critical local pressures (suctions) tend to be higher (lower) in comparison to the ASCE 7-05 provisions, although area-averaged pressures and suctions seem to agree much better. The results can be used to provide realistic provisions for wind design of building structural systems and foundations.

Wind Pressures on Parapets of Flat Roofs

A wind-tunnel study has been carried out to assess wind pressures acting on parapets, including their top surfaces. Local and area-averaged pressure coefficients were measured on parapets of flat-roof models with a length to height ratio (L/H) of 1:1, 2:1, and 3:1. The results were obtained for full-scale equivalent parapet heights of 1 and 2 m and for wind directions ranging from zero to 315°. The local wind load on the parapet was found to be approximately 30% larger at the windward corner of the building than at the midspan location. Maximum parapet loads for the low building model were approximately 30% larger than those for the cubical model. Parapet height did not significantly affect the peak local load on the parapet except in the corner region, where the inward load (toward the roof) for the 1 m parapet was 25% higher than that for the 2 m parapet.

Area-averaged and internal pressure coefficients measured in the field

Area-averaged pressure coefficients for a purlin and internal pressure coefficients for a building with no interior partitions are investigated using full scale data collected at the Texas Tech University Wind Engineering Research Field Laboratory (WERFL). Two methods are used to quantify the area-averaged pressure coefficients for the purlin. The first procedure is numerical averaging of pressure coefficients collected in the purlin effective area. The second procedure uses measured wind-induced purlin displacements in conjunction with a uniform pressure-purlin displacement curve to obtain area-averaged pressure coefficients. Internal pressure coefficients are investigated using data collected in the interior of the building. Results are presented for both area-averaged pressure coefficients and interior pressure coefficients before and after application of a 2 s moving average filter. Both the unfiltered and the filtered results are compared to the values given in ANSI/ASCE7-88.

Wind Loads on Buildings with Sawtooth Roofs

A windtunnel study is conducted to assess wind loads on buildings with sawtooth roofs. Local and areaaveraged pressure coefficients are measured on models with one, two, and four spans. The roof an...

Codification of Wind pressure coefficients for sawtooth roofs

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.