Wind-driven Cross Ventilation Research Articles

Impacts of horizontal and vertical louvers on the natural cross-ventilation performance of a generic building

Wind-based natural cross-ventilation is defined as a process in which fresh air is injected into a building via a high-pressure windward window and exhausts into the atmosphere through a low-pressure leeward window. There are several parameters that affect the natural ventilation performance, which should be considered in detail to reach the maximum air quality inside the buildings. In the present study, the effects of horizontal and vertical louvers on the wind-driven natural cross-ventilation performance of a generic building are assessed using computational fluid dynamics. In this regard, horizontal and vertical louvers with installation louver angles of 15°, 30°, and 45° are compared with each other and corresponding non-louver cases. Computations are carried out for three different free-wind speeds of 1, 2, and 3 m/s to show the impact of the external conditions on the natural ventilation of louvered cases. The obtained results revealed that under a low free-wind speed of 1 m/s, the minimum age of air and maximum air exchange efficiency occur for vertical louvers with an installation angle of 15°, while at moderate and high free-wind speeds of 2 and 3 m/s, the horizontal louvers with an installation angle of 15° provide the best condition for a generic building. Highlights Effects of horizontal and vertical louvers on the wind-driven cross-ventilation were studied. At a free-wind speed of 1 m/s, the minimum age of air and maximum air exchange efficiency occur for vertical louver with α = 15°. At U H = 2 and 3 m/s, horizontal louver with α = 15° provides the best condition for a generic building.

Large-Eddy Simulations of Wind-Driven Cross Ventilation, Part1: Validation and Sensitivity Study

Natural ventilation is gaining popularity in response to an increasing demand for a sustainable and healthy built environment, but the design of a naturally ventilated building can be challenging due to the inherent variability in the operating conditions that determine the natural ventilation flow. Large-eddy simulations (LES) have significant potential as an analysis method for natural ventilation flow, since they can provide an accurate prediction of turbulent flow at any location in the computational domain. However, the simulations can be computationally expensive, and few validation and sensitivity studies with respect to simulation parameters such as grid resolution and boundary conditions have been reported. The objectives of this study are to validate LES of wind-driven cross-ventilation and to quantify the sensitivity of the solution to the grid resolution and the inflow boundary conditions. We perform LES for an isolated building with two openings, using three different grid resolutions and two different inflow conditions with varying turbulence intensities. Predictions of the ventilation rate are compared to a reference wind-tunnel experiment available from literature, and we also quantify the age of air and ventilation efficiency. For the cross-ventilation case modeled in this paper, the prediction of the mean ventilation flow rate is very robust, showing negligible sensitivity to the grid resolution or the inflow characteristics with the maximum error of 1.9 and 1.3% for each sensitivity study. However, a sufficiently fine grid resolution is required to obtain accurate predictions of the detailed flow pattern and the age of air as they show comparably larger errors of 10 and 20% in the grid sensitivity study, and the standard deviation of the instantaneous ventilation rate is affected by the turbulence in the inflow condition with showing 44% difference.

The effect of louver blinds on the wind-driven cross ventilation of multi-storey buildings

Louver blinds is widely used to control indoor solar heat gain at buildings' openings. However, natural ventilation can also be affected by these devices. This study has conducted a CFD investigation on the cross ventilation in a multi-storey building with louver blinds. Taking DFR and air age as the evaluation indicators, the ventilation performance of rooms with different louver angles and wind directions are surveyed. It is found that fully-opened slats may improve the indoor air quality due to its flow fairing, while the larger louver angle can result in greater blocking effect for cross ventilation. Meanwhile, it is shown that windward louvers have a greater impact on ventilation potential than leeward louvers. To provide references for ventilation analysis in louvered rooms, correlations are proposed to describe the promoting or restraining effect of louvers on indoor cross-flow. This research contributes to the integrated design of building's louver blinds.

Numerical investigation of inlet opening size on wind-driven cross ventilation

The purpose of this study is to investigate the effect of opening size on indoor airflow characteristics of a naturally ventilated building model. The numerical simulation based on 3D-steady RANS equation was performed in this study. A total of five different inlet to outlet opening ratios, namely 1:4, 1:2, 1:1, 2:1 and 9:4 were included in the study and analysed. The results of model validation and grid independence analysis were consistent with the previous study. The simulation results in this study are discussed based on the velocity vector, velocity and pressure, pressure drop and ventilation rate. Through the study, the results have clearly illustrated that the velocity vector, velocity and pressure, pressure drop and ventilation rate are highly dependent on the opening ratio. When air passes through the building model, it forms a recirculation at the top and bottom of the airflow. In addition, the contours of velocity and pressure have indicated that the lower the opening ratio, the higher the velocity and subsequently lower pressure inside the building model. For the pressure drop, the results have shown that the lower the opening ratio, the higher the pressure drop. Beyond that, when the opening ratio is less than 1, the ventilation rate was increased but when the opening ratio greater than 1, the ventilation rate begins to decrease. Therefore, change in the size of inlet opening will greatly affect the performance of ventilation rate.

Ventilation Characteristics and Performance Evaluation of Different Window-Opening Forms in a Typical Office Room

As most existing office buildings in China lack fresh air systems for ventilation, natural ventilation with windows remains the main means of improving indoor air quality and adjusting indoor thermal comfort. However, knowledge of the ventilation characteristics of various window-opening forms in actual buildings is limited and current methods for evaluating ventilation performance lack a comprehensive consideration of ventilation rate and thermal comfort. In this study, the ventilation characteristics of different window-opening forms were systematically compared by conducting computational fluid dynamics (CFD) simulations. A full-scale experiment was conducted in a typical office room in a university in Tianjin to validate the CFD simulation. Two ventilation modes (wind-driven cross-ventilation and temperature-driven single-sided ventilation), three window-opening angles, and seven window types were investigated. Additionally, the ratio of the ventilation rate to the absolute value of thermal sensation was used to quantify the indoor natural-ventilation performance. The results showed that a sliding window with a full opening has the highest discharge coefficients of 0.68 and 0.52 under wind-driven cross-ventilation and temperature-driven single-sided ventilation, respectively, and top-hung windows opening both inwards and outwards have better ventilation performance than other window types under the two ventilation modes. This study is applicable to the design and practice of natural ventilation.

Investigation on the effect of opening position across an isolated building for wind-driven cross ventilation

This study investigates the effect of opening position on the indoor airflow of a natural ventilated isolated building model. Furthermore, the opening configuration which generate the highest ventilation is to be determined. Seven different opening configurations were considered by varying the opening position on the windward and leeward walls which include three cases of aligned openings and four non-aligned openings. Models with different opening position were simulated using Computational Fluid Dynamics (CFD) with 3D RANS turbulence model Shear Stress Transport (SST) k- ω. Simulation result indicates the indoor air velocity as well as pressure distribution highly depends on both inlet and outlet opening positioning. Different opening configuration influences the recirculation zone generated within the building model. Comparison shows openings located near the roof generate 6.52% higher ventilation rate compared to openings located near the ground. The study concludes that opening position holds a crucial role in affecting internal airflow pattern, air recirculation and ventilation rate for a natural cross ventilated building.

The effect of an indoor-outdoor temperature difference on transient cross-ventilation

We examine the effect of an indoor-outdoor temperature difference on the transient wind-driven cross-ventilation of a room. Laboratory experiments are performed in a water flume using a reduced-scale model room. For solely wind-driven cross-ventilation with no initial temperature difference between the room and the external fluid, the ventilation rate is constant. In experiments, the mean dye concentration decays exponentially, which is expected when the room remains well-mixed. When there is an initial temperature difference but no wind, the buoyancy-driven exchange-ventilation results lie between a model that assumes the room is well-mixed and a new model that assumes no mixing between the incoming flow and the room. When both wind and buoyancy drive the flow, the relative importance of these two effects can be described by a Froude number, Fr. For buoyancy-dominated ventilation (Fr<1), the ventilation rate through the windows can be modelled using an exchange flow with a Fr correction to account for wind effects, which can increase the ventilation rate by up to 40%. For wind-dominated ventilation (Fr>1), a temperature difference slightly reduces the ventilation rate, but only by up to 6%, a change that can be neglected in most applications. Two processes compete to ventilate the room in combined cases: the removal of fluid from a lower layer by flow through the windows and the erosion of an upper layer by entrainment into the jet that crosses the room. The relative rates of these two processes depend on the geometry of the room.

Data-driven model for cross ventilation potential in high-density cities based on coupled CFD simulation and machine learning

Effective urban ventilation through decent urban planning and building design can alleviate the deterioration of the urban built environment. However, natural ventilation requirements and guidelines in current building codes and standards are either qualitative or quantitative but subject to an absolute indoor airspeed threshold without considering the outdoor wind environment. To fill this gap, this paper develops an urban-scale coupled indoor and outdoor computational fluid dynamics (CFD) model and defines a novel ventilation index to assess natural ventilation potential. The index considers wind environments of both indoor and outdoor spaces. First, the coupled CFD model is developed to study wind-driven cross ventilation in high-density cities. A 3D isothermal CFD simulation is solved using the RNG k-ε turbulent model. The simulation results are compared with wind tunnel experiment data from literature. Second, six key design variables are used to generate 3840 parametric design variations for natural ventilation assessment. Third, a novel integrated index CIOIv (coupled indoor and outdoor interaction) is proposed to evaluate the wind speed ratio between the indoor area and outdoor reference area. For demonstration, CIOIv,F1 is used to represent CIOIv on the ground level. Data-driven CIOIv,F1 models are developed to predict indoor building ventilation potential for quick early design support. Compared with multivariate linear regression model, the Gradient Boosting non-linear model displays much higher prediction accuracy (mean absolute percentage error = 0.16 with R2 = 0.8). In early design stage, designers and engineers can skip the computational expensive CFD simulations and use this data-drive model to quickly check the building natural ventilation potentials of different design options in an urban environment.

A holistic approach to natural ventilation studies

Natural ventilation in buildings can increase thermal comfort and reduce air-conditioning use. However, it is very challenging today to accurately determine the natural ventilation rate through a building. This paper outlines a method to calculate the wind distribution around a building site from information obtained at a meteorological station miles away. This paper also discusses the influence of surrounding buildings on the wind flow around a target building at the site, and presents various geometrical models. In addition, the use of hour-by-hour wind velocity typically available from a meteorological station may give rise to some errors because of the large time step. A correlation method can be employed to convert the hour-by-hour wind velocity to minute-by-minute velocity. One can then use CFD to calculate the airflow around the building and wind-driven cross ventilation through an apartment simultaneously. However, prediction of single-sided natural ventilation is difficult because of the bi-directional flow at the room opening and the complex flow around buildings. This paper presents an empirical model that can predict the mean and fluctuating ventilation rates due to the pulsating flow and eddy penetration of wind-driven single-sided ventilation in buildings with three types of windows. One must use the correct strategy in a building in order to achieve the maximum benefits of natural ventilation.

A holistic approach to natural ventilation studies

Natural ventilation in buildings can increase thermal comfort and reduce air-conditioning use. However, it is very challenging today to accurately determine the natural ventilation rate through a building. This paper outlines a method to calculate the wind distribution around a building site from information obtained at a meteorological station miles away. This paper also discusses the influence of surrounding buildings on the wind flow around a target building at the site, and presents various geometrical models. In addition, the use of hour-by-hour wind velocity typically available from a meteorological station may give rise to some errors because of the large time step. A correlation method can be employed to convert the hour-by-hour wind velocity to minute-by-minute velocity. One can then use CFD to calculate the airflow around the building and wind-driven cross ventilation through an apartment simultaneously. However, prediction of single-sided natural ventilation is difficult because of the bi-directional flow at the room opening and the complex flow around buildings. This paper presents an empirical model that can predict the mean and fluctuating ventilation rates due to the pulsating flow and eddy penetration of wind-driven single-sided ventilation in buildings with three types of windows. One must use the correct strategy in a building in order to achieve the maximum benefits of natural ventilation.

Comparison of analysis methods for wind-driven cross ventilation through large openings

Simplified engineering models are essential to design and control natural ventilation, yet there are not many theories that can predict cross ventilation with reasonable accuracy. Models based on the orifice equation are known to be inadequate for large openings, but there is no precise definition of the large opening. In this paper, we show that it is not just the opening size but also the wind angles that affect the prediction accuracy of the orifice based equations. The mass flow rate predicted by CFD simulations is within 10% of the flow rate predicted by the orifice equations with sealed body pressure coefficients as long as the total pressure drop in the presence of openings is nearly equal to the static pressure drop in cases with no openings. The total pressure drop in the presence of openings when the openings are aligned to the wind, i.e. when wind can pass directly through two openings, is smaller than the static pressure drop in cases with no openings. If the wind is angled, and the entering wind cannot pass undisturbed through the outlet because of the building walls, the total pressure drop will not be much smaller than the static pressure drop of the sealed building. In this condition, the orifice equation can still be used, even for porosities of up to 50%. However, when the wind can pass undisturbed between two opposite openings, the orifice based equations are not adequate, even for a porosity as small as 6%.

Wind-driven natural ventilation of greenhouses with vegetation

A large eddy simulation (LES) model was used to examine the wind-driven cross ventilation of gable-roof greenhouses containing vegetation. The obstruction of air flow by vegetation was described by a porous drag model in the numerical model, and the simulation results were validated using wind tunnel experiments. The numerical model was then utilised to inspect the influences of vegetation and greenhouse length (in the wind direction) on the ventilation rate. The results revealed that the diminishing effects of the vegetation, insect screen and internal friction on the ventilation rate can all be quantified by a physical-based resistance model. The driving force (the difference between windward and leeward pressures) of long, multi-span greenhouses was found to be less than that of a short, single-span greenhouse leading to a lower ventilation rate. The resistance factor of the vegetation and the insect screen depends on their porosity, while the resistance factor of the internal friction increased as the greenhouse length increased. In addition, the internal friction of multi-span greenhouses should be considered when the length of the greenhouse was greater than six times the greenhouse height.

CFD simulation of cross-ventilation in buildings using rooftop wind-catchers: Impact of outlet openings

Abstract Cross-ventilation using rooftop wind-catchers is very complex as it is influenced by a wide range of interrelated factors including aerodynamic characteristics of the wind catcher, approach-flow conditions and building geometry. Earlier studies on wind-driven cross-ventilation in buildings have shown the significant impact of the geometry and position of openings on the flow and ventilation performance. However, this has not yet been investigated for cross-ventilation using wind catchers. This paper, therefore, presents a detailed evaluation of the impact of the outlet openings on the ventilation performance of a single-zone isolated building with a wind catcher. The evaluation is based on three ventilation performance indicators: (i) induced airflow rate, (ii) age of air, and (iii) air change efficiency. High-resolution coupled 3D steady RANS CFD simulations of cross-ventilation are performed for different sizes and types of outlet openings. The CFD simulations are validated based on wind-tunnel measurements. The results show that using outlet openings very close to the wind catcher will not increase the induced airflow, while it leads to a considerable reduction in the indoor air quality. A combination of one-sided wind-catcher and window is superior, while the use of two-sided wind-catchers leads to the lowest indoor air quality and air change efficiency.

Experimental study for wind pressure loss rate through exterior venetian blind in cross ventilation

Exterior venetian blinds (EVBs) are widely used in commercial and residential buildings, and show a high performance level for both shading and lighting purposes. However, an EVB installed over an open window also influences natural ventilation rates. In this study, applying the pressure loss rate of an EVB to wind-driven natural ventilation rates was proposed. A mock-up building was set that the EVB was installed on the front opening. Wind generator was installed toward the front opening in order to describe wind-driven cross ventilation in a single zone. To investigate wind pressure changes through the EVB plane, 16 pressure measuring taps were installed on each of the front and back side of the EVB plane. Various wind speeds were induced toward the front opening so as to derive the pressure loss rate for the four EVB slat angle cases (0°, 45°, 90°, and no shading). The pressure loss rates for each case were derived from the field measurements. The results show that the pressure loss rates have a range of 0.22 to 0.90. In addition, the measurement results indicated that an EVB can change the air velocity by about 50% based on the slat angle. Therefore, when an EVB is installed on a window opening, the effect of the EVB on wind-driven cross ventilation rate should be taken into account.

Impact of roof geometry of an isolated leeward sawtooth roof building on cross-ventilation: Straight, concave, hybrid or convex?

The roof geometry of a leeward sawtooth roof building can have a large influence on the cross-ventilation flow. In this paper, five different leeward sawtooth roof geometries are evaluated using Computational Fluid Dynamics (CFD). The 3D CFD simulations are performed using the steady Reynolds-Averaged Navier-Stokes approach with the SST k–ω turbulence model to provide closure to the governing equations. The computational grid is based on a grid-sensitivity analysis and the computational model is successfully validated using PIV measurements for a generic isolated building from literature. The five different roof geometries that are studied include one straight and four curved roofs. The curved roofs can be subdivided in one concave, one hybrid (convex–concave) and two convex roof geometries. It is shown that a straight or convex roof geometry can maximize the underpressure in the wake of the building, where the outlet opening is located, which results in enhanced wind-driven cross-ventilation flow. Analysis of the results shows that for a normal wind incidence angle (0°) the straight and convex leeward sawtooth roof geometries can result in an increase of the volume flow rate by 13.0%, 12.5% and 12.3% respectively compared to a concave roof geometry. Furthermore, the increase of the indoor air velocity can be as high as 90% in the upper part of the occupied zone (at h=1.7m above ground level) for convex versus concave roofs.

Impact of eaves on cross-ventilation of a generic isolated leeward sawtooth roof building: Windward eaves, leeward eaves and eaves inclination

An eaves is a roof extension that can protect the indoor environment from direct solar radiation, the exterior facade from wetting of by wind-driven rain and can be useful to enhance cross-ventilation. This paper evaluates the impact of eaves configuration on wind-driven cross-ventilation of a generic leeward sawtooth roof building. Both the type of eaves (windward versus leeward) and the eaves inclination angles are investigated. Isothermal Computational Fluid Dynamics (CFD) simulations are performed using the 3D steady Reynolds-Averaged Navier–Stokes (RANS) approach. A grid-sensitivity analysis is performed and validation of the CFD results is conducted based on wind-tunnel measurements with Particle Image Velocimetry from literature. The ventilation evaluation is based on the volume flow rates and the indoor mean velocities. The eaves length is 1/4 of the building depth and the inclination is varied between 90° and −45° for both the windward and leeward eaves. The results show that windward eaves with an inclination of 27° (equal to roof inclination) result in the highest increase of the volume flow rate (15%) compared to the building without eaves. Furthermore, the flow through the occupied zone is more horizontally directed. Leeward eaves have a smaller influence on the ventilation volume flow rate than windward eaves; the maximum increase in volume flow rate is only 6% when a 90° inclination is employed. Application of both (windward and leeward eaves) results in an increase of the volume flow rate of 24%, which is 3% more than the sum of the increases by the two eaves separately.

CFD analysis of cross-ventilation of a generic isolated building with asymmetric opening positions: Impact of roof angle and opening location

The position of window openings and roof inclination are important parameters determining the effectiveness of wind-driven cross-ventilation in buildings. Many studies on natural ventilation have been performed in the past, however, a detailed review of the literature indicates that the majority of these studies focused on flat roofs with symmetric opening positions. There is a lack of research that analyzes the impact of asymmetric opening positions and roof inclination on natural ventilation potential. This paper presents Computational Fluid Dynamics (CFD) simulations to analyze the natural ventilation flow in a generic isolated building with different vertical positions of the outlet opening – yielding asymmetric opening positions – and five different roof inclination angles. The simulations are performed using the 3D steady Reynolds-Averaged Navier–Stokes (RANS) equations. They are based on a grid-sensitivity analysis and on validation with previously published wind-tunnel measurements using Particle Image Velocimetry. The results show that the shear-stress transport (SST) k-ω and the Renormalization-group (RNG) k-ε turbulence models provide the best agreement with the experimental data. It is also shown that the roof inclination angle has a significant effect on the ventilation flow; the volume flow rate increases by more than 22%. The maximum local indoor air velocity increases considerably when the inclination angle is increased, however, the differences in the average velocity in the occupied zone are only around 7%. The vertical position of the outlet opening has a relatively small impact on the volume flow rate (less than 4%), and a small influence on the average velocity in the occupied zone (<5%).

Wind-driven cross ventilation in long buildings

The rule of thumb for effective wind-driven cross ventilation suggests that the building length L should be less than five times of ceiling height H. This study uses a Large Eddy Simulation model and wind tunnel experiments to investigate the mechanism behind this rule of thumb. The numerical results reveal that the ventilation rate decreases as the building length increases. This is partly due to the pressure difference between the windward and leeward façades of long buildings (aspect ratio L/H ≥ 2.5) is smaller than that of a short building (L/H = 1.25). The other reason is owing to the internal friction, which can produce a sluggish zone with low wind speed inside the building. For buildings with aspect ratio L/H ≥ 5, the ventilation rate will be over-estimated, as much as about 20%, by ventilation models that do not consider the internal resistance. The location of the external openings can also influence the ventilation rate. When the openings are located in the opposite corners of the windward and leeward walls, also due to the internal friction, the ventilation was 15.5% less than that of openings on the centerline of the building. The mitigation effect of internal resistance on the ventilation rate can be quantified by a resistance model.

Computational fluid dynamics modelling and optimisation of an upper-room ultraviolet germicidal irradiation system in a naturally ventilated hospital ward

Ultraviolet germicidal irradiation (UVGI) has been shown to be an effective technology for reducing the airborne bioburden in indoor environments and is already advocated as a potential infection control measure for healthcare settings. However, much of the understanding of UVGI performance is based on experimental studies or numerical simulation in mechanically ventilated environments. This study considers the application of an upper-room UVGI system in a naturally ventilated multi-bed hospital ward. A computational fluid dynamics model is used to simulate a Nightingale-type hospital ward with wind-driven cross-ventilation and three wall-mounted UVGI fixtures. A parametric study considering 50 different fixture configurations and three ventilation rates was carried out using a design of experiments approach. Each configuration was assessed by calculating the UV dose distribution over the ward and at each bed. Results show that dose is influenced by the location of the fixtures and the ventilation regime. Thermal effects are likely to be important at low ventilation rates and may reduce UV effectiveness. A metamodel-based numerical optimisation was applied at a ventilation rate of 6 air changes per hour. In this case, the optimum result is achieved when UVGI fixtures are mounted on the leeward wall at their lowest mounting height.

Wind-driven cross ventilation with internal obstacles

Most ventilation models do not consider the influences of furniture and obstacles on building ventilation. This study developed a resistance model to calculate the ventilation rate of wind-driven cross ventilation in a low-rise building with vertical plates in the building. The flow resistances generated by the plates of various sizes were systemically investigated using a Large Eddy Simulation model and wind tunnel experiments. The numerical and experimental results consistently demonstrated that the resistance factor is a function of the internal blockage ratio (ratio of the plate area to the internal cross-section area) and location, but is independent of the external wind speed, building size and opening configuration. It was also found that when the wall porosity is less 3%, the resistances caused by the external openings will dominate the ventilation process and the influence of the internal obstacles on the ventilation rate can be neglected. But when the internal resistance is larger than the external resistances, the diminish effect of the obstacles on the ventilation rate should be taken into account.