Accurate models for exterior convective heat transfer coefficients (CHTC) are important for predicting building energy demand. A detailed review of the literature indicates that existing CHTC models take into account the impact of building geometry either incompletely, or not at all. To the best of our knowledge, research on the impact of exterior CHTC models on the predicted energy performance of buildings with different geometry has not yet been performed. This paper, therefore, investigates the influence of CHTC models on the calculated energy demand of buildings with varying geometry. Building energy simulations are performed for three groups: buildings with Hb (building height) > Wb (building width), buildings with Hb < Wb and buildings with Hb = Wb. Six commonly used CHTC models and a new generalized CHTC model are considered. The generalized CHTC model is expressed as a function of Hb and Wb. The simulations are performed for low and high thermal resistances of the building envelope. The results show that the different CHTC models provide significantly different predictions for the building energy demand. While for annual heating demand, deviations of −14.5% are found, for the annual cooling demand a maximum deviation of +42.0% is obtained, compared to the generalized CHTC model. This study underlines the need for the CHTC models to consider building geometry in their expressions, especially for high-rise buildings. For low-rise builgings, the observed deviations between the existing and the generalized CHTC model are less pronounced.
Vertical axis wind turbines (VAWTs) are promising candidates for wind energy harvesting in the urban environment. However, their aerodynamic performance still falls behind of their horizontal axis counterparts. This could be associated to the comparatively small research they have received in the past decades as well as their complex unsteady aerodynamics. Computational Fluid Dynamics (CFD) has been widely used to evaluate and improve the aerodynamic performance of VAWTs. An extensive literature study reveals that the 2D unsteady Reynolds-Averaged Navier-Stokes (URANS) approach has been used in the majority of the CFD studies on VAWTs. The current study intends to evaluate the aerodynamic performance of a VAWT, calculated using 2D URANS, and compare it with that of 2.5D URANS and 2.5D scale-adaptive simulation (SAS). SAS is a hybrid RANS-LES model developed by Menter and Egorov [1]. The four-equation transition SST turbulence model is employed in the URANS simulations as well as in the RANS region of the hybrid RANS-LES simulation. The studied turbine is a one-bladed Darrieus H-type VAWT with a solidity of 0.125 operating at a low tip speed ratio of 2.0, which corresponds to the most complex case for VAWTs where dynamic stall is dominant. The reduced frequency is 0.125 representing the high unsteadiness in the flow. Significant benefits of the one-bladed turbine are: (i) less blade-wake interactions while the essential flow features, such as dynamic stall, are still present, (ii) reduced computational costs due to the smaller number of cells. The turbine characteristics is based on the experiment by Simao Fereira et al. [2]. Validation studies for the one-bladed turbine as well as the other turbines have been performed [3-5]. A comparative analysis of the instantaneous tangential and normal loads on the turbine (see Fig. 1), spatiotemporal distribution of pressure coefficient (see Figs. 2a-c) and skin friction coefficient (see Fig. d-f) on the blade suction side, the evolution of the shed vorticity by the blade, dynamic loads on the blade and the turbine wake are employed to evaluate the performance of URANS modeling in comparison to the SAS model. The instantaneous turbine loads calculated using the 2D and the 2.5D URANS, shown in Fig. 1, are in line with minor differences in the downwind side. Despite the 180 times higher number of cells and 10 times finer time step of the SAS modeling, an overall good agreement exists between the 2D URANS and the SAS results. The predicted thrust coefficients for 2D and 2.5D URANS and SAS are 0.422, 0.424 and 0.430, respectively. Nevertheless, there exist noticeable differences between the URANS and SAS results in the bursting location of the laminar separation bubble (LSB), the evolution of the dynamic stall vortex (DSV), the leading-edge secondary and tertiary vortices and the trailing-edge separation. The findings of the present study help to highlight the deficiencies of URANS modeling of VAWTs in dynamic stall.
Earlier research demonstrated that a car following a cyclist can provide an aerodynamic benefit to this cyclist. This effect could be large enough to potentially influence the outcome of individual time trials. This incited the International Cycling Union (UCI) in 2023 to raise the minimum distance from 10 to 25 m. However, this previous research work did not consider the bicycles typically mounted on the roof of a team car. Indeed, some teams have mounted up to ten bicycles on the roof, possibly in an attempt to optimise the aerodynamic benefit by their team car, even in individual time trials. This study employs CFD simulations validated by wind tunnel measurements to assess the benefit provided by different rooftop bicycle configurations. It is shown that the extra benefits are substantial and extend up to a distance of at least 25 m between cyclist and car, which could trigger new rules by the UCI. This study also shows that the cyclist drag reduction by a following car and for any car-cyclist separation distance d, can be estimated by the static pressure coefficient at position d in front of the isolated car, which can be obtained by a single CFD simulation.
Three different calculation models for wind-driven rain (WDR) on buildings are compared by applying them to four idealised, isolated building configurations. The three models are the semi-empirical model in the European Standard Draft for WDR assessment (ESD), the semi-empirical model by Straube and Burnett (SB) and the CFD model by Choi. The buildings are a low-rise cubic building, a low-rise wide building, a high-rise wide building and a tower building. The calculations are made for steady-state conditions of wind and rain. Validation of the CFD wind flow simulations and of the WDR simulations was performed in earlier studies. The CFD simulations of the WDR coefficient are considered as the reference case, and the performance of the two semi-empirical models is evaluated by comparison with these CFD results based on two criteria: (1) the ability to model the wind-blocking effect on the WDR coefficient; and (2) the ability to model the variation of the WDR coefficient with horizontal rainfall intensity Rh. It is shown that the ESD and SB model, as opposed to the CFD model, can not reproduce the wind-blocking effect. In addition, the ESD model provides WDR coefficients that are independent of Rh, while the SB model shows a dependency that is opposite to that by CFD. Future improvements to semi-empirical models should take into account these rather fundamental differences.
Understanding the effects of climate change on building indoor thermal conditions is of importance for providing a comfortable thermal environment for occupants. Some multi-family dwellings have already been listed as heritage in China (hereinafter referred to as heritage apartments), limiting modifications to the building envelope. However, the effect of climate change on thermal comfort in heritage apartments with a compact interior (i.e. without a living room) built before the 1980s in different Chinese climate zones has seldom been studied. This study focuses on the current and future thermal comfort in two-bedroom heritage apartments in China. The study was conducted for two different Chinese climate zones, that is, a cold climate zone (Beijing), and a hot summer and cold winter climate zone (Shanghai) and both current climate scenarios (typical meteorological years) and future climate scenarios (2050) were used. The results indicate, among other things, increases of 58%–60% and 41%–44% in the predicted average number of overheating hours in 2050 compared to the current climate for the studied bedrooms on the first floor in dwellings in Beijing and Shanghai, respectively.
The current study systematically analyzes the impact of number of blades (n) on the aerodynamic performance of 2-, 3- and 4-bladed Darrieus H-type vertical axis wind turbines (VAWTs). A large number of operational parameters, i.e., tip speed ratio (λ), Reynolds number (Re), turbulence inten-sity and reduced frequency (K) are investigated to provide a deeper insight into the impact of n on the dynamic loads on the blades, the turbine performance and the wake. High-fidelity unsteady Reynolds-averaged Navier-Stokes (URANS) simulations, extensively validated with experiments, are employed. The results show that (i) within the turbine optimal operational range, the turbine power coefficient (CP) is almost independent of n; (ii) when dynamic stall is present, CP values are dependent on n due to the impact of K; and (iii) decreasing n leads to an increase in the maximum lift coefficient, while the drag coefficient of the blade(s) reduces due to the higher K. The present findings support the optimal aerodynamic design of small- to large-scale VAWTs.
In this paper, CFD simulations of the natural ventilation of a large semi-enclosed stadium in the Netherlands during summer conditions are described. The simulations are performed to assess the air exchange rate for eight wind directions. The CFD model consists of both the complex stadium geometry and the urban environment in which the stadium is located. Validation of the CFD model is performed using full-scale 3D wind velocity measurements. The computational grid is based on a grid-sensitivity analysis. The results show that the wind direction has a significant effect on the air exchange rate; differences up to 100% are found for the air exchange rate, which can be explained by the presence and size of the buildings situated upstream of the stadium.
