Uncertainty in airflow rate calculations due to the use of surface-averaged pressure coefficients

Mean wind pressure coefficients (C_p) are key input parameters for air infiltration and ventilation studies. However, building energy simulation and stand-alone airflow network programs usually only provide and/or use a limited amount of C_p data, which are based on several assumptions. An important assumption consists of using surface-averaged C_p values instead of local C_p values with a high resolution in space. This paper provides information on the uncertainty in the calculated airflow rate due to the use of surface-averaged C_p data. The study is performed using published empirical data on pressure coefficients obtained from extensive wind tunnel experiments. The uncertainty is assessed based on the comparison of the airflow rate (?) calculated using the surface-averaged C_p values (?_AV) and the airflow rate calculated using local C_p values (?_LOC). The results indicate that the uncertainty with a confidence interval of 95% is high: 0.23 ?_AV < ?_LOC < 5.07 ?_AV. In cases with the largest surface-averaged ΔC_p, the underestimation or overestimation is smaller but not negligible: 0.52 ?_AV < ?_LOC < 1.42 ?_AV. These results provide boundaries for future improvements in C_p data quality, and new developments can be evaluated by comparison with the uncertainty of the current methods.     

Improvement of natural ventilation in a large factory building using a louver ventilator

When heat generated from facilities inside a large factory building is not discharged outside the building due to a stagnant ventilation flow, the working environment of workers becomes worse, and the cooling of high-temperature products is delayed. In this study, wind tunnel tests were conducted to investigate the natural ventilation of entrained air inside a large factory building. The scale-down factory-building models were embedded in a simulated atmospheric boundary layer (ABL), and the mean and fluctuating velocity fields were measured using a two-frame particle image velocimetry (PIV) technique. For the original factory model, some of the outdoor air came in the factory building through the one-third open windward wall, while the stagnant flow region existed in the rear part of the target area. In order to improve the indoor ventilation environment of the present factory building, three different types of the louver ventilator were attached at the upper one-third open windward wall of the factory model. Among the three louver ventilators tested in this study, the ventilator model ?3 with the outer louver (θ_o=90°) and the inner louver (θ_i=−70°) was found to improve the natural ventilation inside the target factory-building model. It increased the flow rate of the entrained air by aligning the outer louver blades with the oncoming wind and guiding the entrained air down to the ground surface with the elongated inner louver blades.     

Effect of damper and heat source on wind catcher natural ventilation performance

Experimental wind tunnel and smoke visualisation testing and CFD modelling were conducted to investigate the effect of air flow control mechanism and heat source inside rooms on wind catchers/towers performance. For this purpose, a full-scale wind catcher was connected to a test room and positioned centrally in an open boundary wind tunnel. Pressure coefficients (C_p's) around the wind catcher and air flow into the test room were established. The performance of the wind catcher depends greatly on the wind speed and direction. The incorporation of dampers and egg crate grille at ceiling level reduces and regulates the air flow rate with an average pressure loss coefficient of 0.01. The operation of the wind catcher in the presence of heat sources will potentially lower the internal temperatures in line with the external temperatures.     

The effect of ceiling configurations on indoor air motion and ventilation flow rates

The purpose of this paper is to evaluate the effects of a building parameter, namely ceiling configuration, on indoor natural ventilation. The computational fluid dynamics (CFD) code Phoenics was used with the RNG k–? turbulence model to study wind motion and ventilation flow rates inside the building. All the CFD boundary conditions were described. The simulation results were first validated by wind tunnel experiment results in detail, and then used to compare rooms with various ceiling configurations in different cases. The simulation results generated matched the experimental results confirming the accuracy of the RNG k–? turbulence model to successfully predict indoor wind motion for this study. Our main results reveal that ceiling configurations have certain effects on indoor airflow and ventilation flow rates although these effects are fairly minor.     

Numerical investigation of unsteady airflow in subway influenced by piston effect based on dynamic mesh

The piston effect has a significant influence on unsteady airflows in subway stations and tunnels. This study uses in situ experimental data and a computational fluid dynamics (CFD) method to analyze the three-dimensional unsteady air flow in a subway station and tunnel. An experimental analysis of train-induced unsteady flow was measured in an actual station with platform bailout doors (PBD), and air velocity variations were recorded at regular time intervals. The unsteady numerical analysis uses a dynamic mesh method for the full-scale model. The results indicate that Standard k–ε and RNG k–ε equations are both appropriate for simulating the high Reynolds numbers in tunnel and station airflow because these equations coincide with the experimental data. Specific diversion and suction ratios exist in each channel of the airflow for piston wind. The proportions between bypass ducts and platforms are stable no matter in open or close systems. And the draught relief shaft located before station plays more important role for piston wind than the one located after the station.     

Numerical simulation of rooftop ventilator flow

Rotating rooftop turbine ventilators are cost effective environmental friendly natural ventilation devices, which are used to extract airflow from a building to improve air quality and comfort. A CFD study using the standard k-? turbulence model with multiple reference frame (MRF) meshing technique was employed to explore the suitability of numerical approach in modelling various features of a ventilator flow. The initial CFD results were validated against wind tunnel data obtained for a commercial ventilator on a simulated inclined rooftop configuration conducted at the aerodynamic laboratory of University of New South Wales. The numerical studies were then extended to examine both the internal and the external flows associated with the ventilator at different wind speeds and to quantify the performance of a rotating ventilator in terms of air extraction rate. The trend observed appeared to be in good agreement with published data suggesting that application of numerical simulation is feasible as a cost effective tool in the future design, development and performance analysis of rotating wind driven ventilation device.     

Numerical study on the mean velocity distribution law of air backflow and the effective interaction length of airflow in forced ventilated tunnels

The effective interaction length of airflow (L_e) is the key factor in the determination of the distance between an air duct nozzle and the working face in dead-end tunnels. At present, the current calculation methods of L_e consider only the tunnel cross section dimension and are therefore not suitable for large underground cavern groups. Because air backflow is the direct promoting factor in the removal of pollutants and the construction specifications in underground space require a minimum wind speed, this paper proposed that L_e is the distance between the nozzle and the critical cross section, where the mean velocity of the air backflow equals the minimum wind speed required. For this purpose, a computational study was conducted to investigate the flow pattern of air backflow in ventilated tunnels. Semi-empirical equations of the mean velocity distribution law of air backflow were derived based on the numerical results. By solving the equations, the location of the critical cross section can then be acquired. Accordingly, the value of L_e could be obtained. The proposed method is related to the tunnel width, the tunnel height, the air duct diameter and the efflux velocity. Thus, it is more comprehensive and applicable compared to the current methods.     

Computational analysis of wind driven natural ventilation in buildings

The design of natural ventilation in buildings is often performed by means of computational fluid dynamics (CFD) techniques, whose application is gaining popularity. In the present study, Reynolds averaged Navier–Stokes equation (RANS) approach is applied to wind driven natural ventilation in a cubic building. Two different models are considered, namely the two-equation k–ɛ model and the Renormalization Group (RNG) theory. The velocity and pressure distribution inside and around the building are determined, as well as the ventilation rate, for three different configurations: cross ventilation, single-sided ventilation with an opening on the windward wall and single-sided ventilation with an opening on the leeward wall. The numerical results are compared with experimental data, showing a good agreement, particularly when using RNG. The discrepancy in the determination of the ventilation rate is reasonable and the flow distribution inside the building is properly described when RNG model is used. However, the k–ɛ model fails to determine the correct velocity components near the horizontal surfaces. According to these results, the RNG model can be considered a useful tool for the study of wind driven natural ventilation, especially for the assessment of the ventilation rate and of the air distribution inside a room.     

Experimental and numerical studies of flows through and within high-rise building arrays and their link to ventilation strategy

Urban ventilation implies that wind from rural areas may supply relatively clean air into urban canopies and distribute rural air within them to help air exchange and pollutant dilution. This paper experimentally and numerically studied such flows through high-rise square building arrays as the approaching rural wind is parallel to the main streets. The street aspect ratio (building height/street width, H/W) is from 2 to 5.3 and the building area (or packing) density (λ_p) is 0.25 or 0.4. Wind speed is found to decrease quickly through high-rise building arrays. For neighbourhood-scale building arrays (1-2 km at full scale), the velocity may stop decreasing near leeward street entries due to vertical downward mixing induced by the wake. Strong shear layer exists near canopy roof levels producing three-dimensional (3D) vortexes in the secondary streets and considerable air exchanges across the boundaries with their surroundings. Building height variations may destroy or deviate 3D canyon vortexes and induced downward mean flow in front of taller buildings and upward flow behind taller buildings. With a power-law approaching wind profile, taller building arrays capture more rural air and experience a stronger wind within the urban canopy if the total street length is effectively limited. Wider streets (or smaller λ_p), and suitable arrangements of building height variations may be good choices to improve the ventilation in high-rise urban areas.     

Vibrations and aerodynamic stability of a prestressed pipeline cable bridge

A method is presented for analysing the vibrations and aerodynamic stability of a prestressed pipeline cable bridge structure. The structure of the bridge and the method for calculation of the natural vibration frequencies are described. Some of the natural vibration frequencies were tested by measuring dynamic quantities on a bridge structure in situ.A sectional model was designed for testing in a wind tunnel of diameter 1.8 m; the model was investigated by means of a drag balance, and the aerodynamic coefficients C_L, C_D and C_M were determined. The same model was suspended elastically in a wind tunnel and its response to an airstream studied. The results of the measurements for the rigidly and elastically suspended model are used in calculating the critical flutter velocity and in analysing the model's aerodynamic stability.     

Occupants’ interactions with windows in 8 residential apartments in Beijing and Nanjing,China

Occupants’ interactions with windows influence both building energy consumption and exposure to airborne pollutants indoors. Occupants’ window opening behavior varies from region to region due to physical environmental factors and social reasons. China is now confronting severe atmospheric pollution, which may affect occupants’ window opening behaviors. A field study was conducted in 8 naturally ventilated residential apartments in Beijing and Nanjing, China. This involved periodically monitoring window states of eight residential apartments within each season from October 2013 to December 2014 by magnetic induction devices (TJHY, CKJM-1). Relationships between the probability of window opening (p) and explanatory variables, including outdoor air temperature (t _o), outdoor relative humidity (RH), outdoor wind speed (V _s), and ambient PM2.5 (particles with aerodynamic diameter less than 2.5 microns) concentrations (C _p), were analyzed. Stochastic models of occupants’ interactions with windows in monitored residences were established via univariate and multivariate linear logistic regression for both cities. According to the results, t _o is the most important explanatory variable affecting occupants’ interactions with windows in monitored residences. The best multivariate linear logistic model result from the “backward selection” procedure based on “Akaike Information Criterion” (AIC) includes t _o, RH, V _s and C _p as explanatory variables, which implied that outdoor air quality, represented by C _p, has become a concern affecting Chinese residents’ interactions with windows.     

Wind response of large roofs of stadions and arena

The paper presents the experimental tests carried out in the boundary layer wind tunnel (BLWT) for the design of large roofs of the new Olympic stadium (Karaiskaki) in Pyraeus (Greece), Manfredonia (Italy) and “Delle Alpi” of Turin (Italy). In addition, a report about some results of the T.D. dynamic response analyses performed on the Karaiskaki structure and on the Olympic stadium in Rome will be given. The peculiar shape of these large structures and their particular location (two of them are in the immediate sea vicinity) let arise the question about the actual distribution of the wind loads, i.e., on the pattern of pressure coefficients (c_p) over the entire roof. For every wind direction investigated, the following quantities have been evaluated: mean values of the aerodynamic coefficients c_p, standard deviation of c_p and maximum and minimum values of c_p. Finally, the recorded data have been used for the numerical simulation of the dynamic response of the structure in Time Domain, whose aim is the definition of the design loads of the steel lattice structures. A numerical model of the “Delle Alpi” stadium is also in preparation, allowing results of dynamic response analyses, which are still in progress.     

Overview of pressure coefficient data in building energy simulation and airflow network programs

Wind pressure coefficients (C_p) are influenced by a wide range of parameters, including building geometry, facade detailing, position on the facade, the degree of exposure/sheltering, wind speed and wind direction. As it is practically impossible to take into account the full complexity of pressure coefficient variation, building energy simulation (BES) and Airflow network (AFN) programs generally incorporate it in a simplified way. This paper provides an overview of pressure coefficient data and the extent to which they are currently implemented in BES–AFN programs. A distinction is made between primary sources of C_p data, such as full-scale measurements, reduced-scale measurements in wind tunnels and computational fluid dynamics (CFD) simulations, and secondary sources, such as databases and analytical models. The comparison between data from secondary sources implemented in BES–AFN programs shows that the C_p values are quite different depending on the source adopted. The two influencing parameters for which these differences are most pronounced are the position on the facade and the degree of exposure/sheltering. The comparison of C_p data from different sources for sheltered buildings shows the largest differences, and data from different sources even present different trends. The paper concludes that quantification of the uncertainty related to such data sources is required to guide future improvements in C_p implementation in BES–AFN programs.     

Net pressure and correlation characteristics between internal and external pressures for tall building with opening

Net pressure and correlation characteristics between internal and external pressures for a typical high‐rise building with a dominant opening are detailed investigated by a series of wind tunnel experiments on a scaled rigid model. The experimental results show that internal pressures are absolutely spatially relevant. When the dominant opening faces to approaching wind, there exists strong positive correlation between internal and external pressures of each tap on the windward wall, whereas negative correlation can be observed for each tap on the other walls. At the wind directions of 15° and 75°, the highest negative correlation coefficient occurs on the right side wall and is up to ?0.6 and the mean values of area‐averaged net pressure coefficient on the right side wall also reach their maximum and minimum values, respectively; meanwhile, the values of standard deviation of area‐averaged net pressure coefficient are also much higher. Thus, it should be paid more attention to the wind‐resistant design of side wall envelopes because they are easier to be destroyed.     

Wind-induced ventilation performances and airflow characteristics in an areaway-attached basement with a single-sided opening

In the present study, we performed both wind tunnel experiments and numerical simulations on a scale model with the focus on wind-driven natural ventilation in an areaway-attached basement with a single-sided opening. In the experiments, the mean value of the effective ventilation rate, purging flow rate (PFR) was measured for nine wind incidence angels based on the homogeneous emission rate method. The experimental results were used to validate two numerical approaches: Reynolds averaged Navier–Stokes (RANS) modeling and large-eddy simulation (LES). The influences of inflow turbulent fluctuations for LES modeling were also examined. The comparisons between the experiment and the numerical simulation indicate that LES can provide more accurate results than RANS and the inflow turbulent fluctuations should be taken into account for LES modeling. Based on LES with the inflow turbulent fluctuations, the mean airflow patterns within and around the areaway-attached basement were further studied for different wind incidence angles to investigate the influence of wind direction on ventilation performance in the areaway space. Furthermore, the relationships between the effective ventilation rate and the kinetic energy in the basement space were analyzed for three wind directions: 0°, 90° and 180°. A close correlation was found between the mean values, whereas the corresponding time variations showed large discrepancies. Finally, we compared the effective ventilation rate obtained using the homogeneous emission rate method and the airflow rates through the opening using two integration procedures. The effective ventilation rates were found lower than the airflow rates through the opening.     

External flow effects on the discharge coefficients of two types of ventilation opening

Envelope flow models are commonly used in the design of naturally ventilated buildings. Such models rely on specification of the discharge coefficient, C_z, of ventilation openings, such as air vents and chimneys (stacks). Current practice is to make use of values obtained from laboratory tests under still-air conditions. These values may not be appropriate when the ventilation is due to wind, due to the external flow around the opening which is characterized by unsteadiness of both the velocity and pressure fields. Experimental tests have been carried out in a wind tunnel on two types of openings, namely a sharp-edged orifice (flush to the wall) and a long opening (chimney). Using an analytic result for the still-air case and simple dimensional analysis it is argued that the local crossflow velocity ratio, V/u, is an important parameter for the orifice and the experimental results support this (as do some simple CFD calculations). The experimental results for the sharp-edged opening are similar to earlier work on generally larger openings. For the long opening the effects of external flow are smaller. In particular, the effects are negligible when the outlet of the opening lies in the external flow. Estimates are made of the consequential uncertainties arising in envelope flow calculations for natural ventilation design.     

Airflow assessment in cross-ventilated buildings with operable façade elements

This paper presents an experimental study of basic cross-ventilation flow characteristics that are essential inputs for accurate natural ventilation modelling and design. The study focuses on a generic single-zone building model tested in a wind tunnel under isothermal flow conditions (wind-driven ventilation). An advanced experimental method based on particle image velocimetry (PIV) was developed to investigate the air velocity field in buildings with cross-ventilation. It was found that airflow patterns in rooms with cross-ventilation are complex and cannot be predicted by simplified macroscopic models such as the orifice equation. Inlet-to-outlet ratio and relative location of openings on a building façade are important parameters to be considered, in addition to the wall porosity. This study provides new insights that enable improved design and control of operable façade elements to enhance space cooling using natural ventilation.     

3D experimental and numerical analysis of wind flow around domed-roof buildings with open and closed apertures

This paper presents experimental and numerical study of airflow distribution around a reduced-scale model of a common type of domed-roof building. Measurements are performed in an open loop wind tunnel. A new modified Counihan scheme is developed for constructing a part-depth atmospheric boundary layer (ABL). Measured quantities include: wind velocity profile, turbulence intensity and airflow pattern around the building. To conduct the experiments, a 1:54 scale model of a real domed-roof building with six windows and an aperture on the roof is fabricated and placed in the test section of the wind tunnel. In addition, using a numerical modeling, turbulent airflow around such scale model in the wind tunnel is simulated and airflow field inside and outside the model as well as ventilating discharge coefficient are computed. It is illustrated that, airflow around this type of building contains complex adjacent recirculation flows. The building with open apertures has acceptable discharge coefficient for ventilation, which can be a factor to ensure comfort condition for residents as well as complying with energy-saving considerations.     

Numerical simulation of pollutant dispersion around a building complex

The dispersion of exhausted pollutants from a building roof stack situated in the wake of a neighbouring tower has been studied using computational fluid dynamics (CFD) with the realizable k–? turbulence model for closure. Two scales are considered in this work, full-scale (1:1) and wind tunnel scale (1:200).Of primary interest are the distributions of the plume and of the pollutant concentrations on the building roof as well as on the leeward wall of the tower. Two stack heights and pollutant exhaust velocities have been considered for the distribution of pollutant concentrations in the neighbourhood of the building from which the pollutant is emitted. Results are compared with measurements from field and wind tunnel experiments to estimate the accuracy of simulations.