The service life of building components and building materials strongly depends on their hygrothermal conditions. In this perspective, ventilated facades are employed in which a – sometimes very narrow – air gap or cavity between the rain screen and the back wall is introduced. Hygrothermal modelling of such facades requires knowledge of the air flow patterns, velocity magnitudes and ventilation rates in these cavities. These variables are determined by the complex interaction of several parameters. The following influencing parameters are distinguished: building and cavity geometry, environment topography, position of the cavity inlet and outlet openings, reference wind speed and wind direction. In the past, information on air flow in such cavities for hygrothermal modelling has been mainly obtained by simplified theoretical and/or semi-empirical expressions, which provide an estimate of the air flow rate as a function of the surfaces pressures at the inlet and outlet opening. Usually, a uniform velocity pattern is assumed in the entire cavity volume. Little is known about the actual mean air speed and turbulence intensity inside the cavity. To the knowledge of the authors, no detailed study of the relationship between these variables and the influencing parameters has yet been performed. Such detailed study would require either detailed full-scale measurements or numerical simulation with Computational Fluid Dynamics (CFD). Wind tunnel tests are considered inappropriate due to Reynolds similarity problems for the narrow cavities and the small inlet and outlet openings. This extended abstract presents preliminary results of a comprehensive CFD study on wind-induced air flow through the cavities of an exposed low-rise test house. The house exhibits different cavity assemblies for typical Scandinavian wooden claddings. A specific feature of these simulations is the full numerical simulation of both the air flow pattern around the building and the resulting flow patterns inside the cavities. The simulation results provide the relationship between cavity air change rate and cavity design, facade position, wind velocity and wind direction, and this information will be made available as input for future hygrothermal modelling efforts.
This paper discusses a procedure for the two-way run time external coupling between Building Energy Simulation (BES) and building envelope Heat, Air and Moisture (HAM) programs for enhanced wholebuilding simulation. The coupling procedure presented here involves a description of the relevant physical phenomena at the interface between the programs, domain overlaps, coupling variables, coupling strategy and types of boundary condition. The procedure is applied using the programs ESP-r and HAMFEM, where the implementation and verification issues are discussed. This work concludes that the coupling between BES and HAM programs is feasible, and it can potentially enhance the accuracy in whole-building simulation.
Vertical axis wind turbines (VAWTs) are promising for wind energy harvesting in the urban environment mainly because of their omnidirectional capability. However, currently their aerodynamic performance is not comparable with horizontal axis wind turbines (HAWTs). Therefore, to make them an ideal candidate, they need to be further improved. The aerodynamic performance of VAWTs depends on several geometrical parameters, such as solidity. However, the impact of solidity on blade aerodynamics and turbine wake has not yet been comprehensively investigated. Therefore, the current study intends to systematically study the effect of solidity on aerodynamic performance of VAWTs with different number of blades operating at various tip speed ratios to provide a deeper insight into its impact on dynamic loads on blades, turbine performance and wake. High-fidelity unsteady Reynolds-averaged Navier-Stokes (URANS) simulations extensively validated with experimental data are employed. The results show that for fixed-rotational-speed urban VAWTs, which frequently operate at high tip speed ratios, a low solidity value is more favorable. On the other hand, an optimal VAWT is a high-solidity variable-rotational-speed (fixed λ) rotor operating at a low tip speed ratio regime.
This paper provides a brief, non-exhaustive overview of the status of the application of CFD in building performance simulation for the outdoor environment. It focuses on four topics: (1) pedestrian wind environment around buildings; (2) wind-driven rain on building facades; (3) convective heat and mass transfer coefficients at building surfaces; and (4) air pollutant dispersion around buildings. For each topic, some specific difficulties, advantages and disadvantages of CFD are addressed.
The main aspect for spectator comfort in outdoor stadiums is protection from wind and rain. This paper presents an investigation of the impact of roof geometry on rain shelter. CFD simulations and Lagrangian particle tracking are performed to analyse the wind flow pattern and rainfall distribution in seven generic stadium configurations and to assess the performance of each roof type. Although most existing stadium roofs are built with a light to medium upward slope towards the field, the analysis indicates that roofs with a downward slope of 13° provide significantly better rain shelter. The reason is not only the well-known trigonometric shielding effect. In addition, this roof type – as opposed to its counterparts – restricts the extent of the primary vortex in the stadium and generates a sufficiently strong counter-rotating secondary vortex below the roof that sweeps the rain away from the stands.
This paper contains a detailed experimental analysis of an isothermal plane turbulent impinging jet (PTIJ) for two jet widths at moderate Reynolds numbers (7200–13,500) issued on a horizontal plane at fixed relative distances equal to 22.5 and 45 jet widths. The available literature on such flows is scarce. Previous studies on plane turbulent jets mainly focused on free jets, while most studies on impinging jets focused on the heat transfer between the jet and an impingement plane, disregarding jet development. The present study focuses on isothermal PTIJs at moderate Reynolds numbers characteristic of air curtains. Flow visualisations with fluorescent dye and 2D particle image velocimetry (PIV) measurements have been performed. A comparison is made with previous studies of isothermal free turbulent jets at moderate Reynolds numbers. Mean and instantaneous velocity and vorticity, turbulence intensity, and Reynolds shear stress are analysed. The jet issued from the nozzle with higher aspect ratio shows more intensive entrainment and a faster decay of the centreline velocity compared to the jet of lower aspect ratio for the same value of jet Reynolds number. The profiles of centreline and cross-jet velocity and turbulence intensity show that the PTIJs behave as a free plane turbulent jet until 70–75% of the total jet height. Alongside the information obtained on the jet dynamics, the data will be useful for the validation of numerical simulations.
Abstract: High-resolution CFD simulations and full-scale measurements have been performed to assess the dispersion of air pollutants (CO2) from the large semi-enclosed Amsterdam ArenA football stadium. The dispersion process is driven by natural ventilation by the urban wind flow and by buoyancy, and by the interaction between outdoor wind flow and indoor airflow which are only connected by the relatively small ventilation openings in the stadium facade. The CFD simulations are performed with the 3D Reynolds-averaged Navier-Stokes equations supplemented with the realizable k-e model to provide closure. The full-scale measurements include reference wind speed, wind direction, and outdoor and indoor air temperature, water vapor and indoor CO2 concentration. In particular, the focus is on CFD simulations and measurements for the few hours immediately after a concert, when the stadium roof remains closed and when indoor air temperature,water vapor and CO2 concentration have reached a maximum level due to the attendants. The removal of the sources/attendants allows an assessment of the natural ventilation rate using the concentration decay method. The CFD simulations compare favorably with the measurements in terms of mean wind velocity in the main ventilation openings and in terms of the CO2 concentration decay after the concerts. The validated CFD model will in the future be used for a detailed evaluation of indoor concentration gradients and the interaction between wind-induced and buoyancy-induced natural ventilation.
Vertical axis wind turbines (VAWTs) suffer from a poor power performance at low tip speed ratios, where their blade aerodynamics are dominated by unsteady separation and dynamic stall. Therefore, to enhance their aerodynamic performance, separation control is highly desired. The present study intends to suppress the flow separation on VAWTs using boundary layer suction through a slot located near the blade leading edge. High-fidelity computational fluid dynamics simulations extensively validated with experiments are employed. A characterization of the impact of the suction amplitude, 0.5% ≤ AS ≤ 10%, and the suction location, 8.5 ≤ XS/c ≤ 28.5, is performed. The dependency of the obtained power gain on operating conditions, i.e. tip speed ratio, 2.5 ≤ λ ≤ 3.5, Reynolds number, 0.51 × 105 ≤ Rec ≤ 2.78 × 105, and turbulence intensity, 1% ≤ TI ≤ 25%, is studied. The results show that applying suction along the chordwise extent of the laminar separation bubble (LSB) can prevent its bursting, eliminate/postpone its formation, avoid the formation of the dynamic stall vortex and trailing-edge roll-up vortex, and delay the incipient trailing-edge separation. This will significantly increase the blade lift force, decrease the drag force, delay the stall angle and suppress the aerodynamic load fluctuations. For the reference turbine and for AS = 0.5% and XS/c = 8.5%, the power coefficient at λ of 2.5, 3.0 and 3.5 is enhanced by 247%, 83% and 24%, respectively. The suction location is critical while a minimum amplitude, e.g. AS = 0.5%, suffices. The optimal suction location is insensitive to TI, weakly sensitive to λ while comparatively more sensitive to Rec.
