Heat stress in urban areas can have detrimental effects on human health, comfort and productivity. In order to mitigate heat stress, Computational Fluid Dynamics (CFD) simulations of urban ...microclimate are increasingly used. The validation of these simulations however requires high-quality experimental data to be compared with the simulation results. Due to lack of available high-resolution high-quality experimental data, CFD validation of urban microclimate for real urban areas is normally performed based on either a limited number of parameters measured at a limited number of points in space, or on experiments for idealized generic configurations. In this study, CFD simulations of urban microclimate are performed for a dense highly heterogeneous district in Nicosia, Cyprus and validated using a high-resolution dataset of on-site measurements of air temperature, wind speed and surface temperature conducted for the same district area. The CFD simulations are performed based on the 3D Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations and the simulated period covers four consecutive days in July 2010. It is shown that the CFD simulations can predict air temperatures with an average absolute difference of 1.35 °C, wind speed with an average absolute difference of 0.57 m/s and surface temperatures with an average absolute difference of 2.31 °C. Based on the comparative results, conclusions are made regarding the performance of URANS for the selected application and possible reasons for deviations between measured and simulated results are discussed.
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•High-resolution on-site measurements of wind speed, air and surface temperature•Validation of URANS simulations of urban microclimate with high-resolution field measurements•URANS can accurately predict air temperature, surface temperature and wind speed in real urban areas.•Possible reasons for deviations between CFD and measurement are discussed.
•Framework for preliminary assessment of urban wind energy potential at large scales.•Extension of the knowledge on small-scale wind power potential assessment.•Focus on the roof-mounted wind ...turbines.•1st and 2nd step: collecting potential building data and respective wind speed characteristics.•3rd step: obtaining turbine characteristics from manufacturer, modeling or experiment.•Last step: estimating the annual urban wind energy production (AEP).
Urban wind energy can provide a decentralized local source of energy for residential areas and reduce the cost of energy by avoiding the losses/costs of long-distance energy transmission. In this perspective, a preliminary assessment of urban wind energy is highly desired by turbine developers, investors and policy makers. However, given the large number of parameters involved, predictions of the wind energy potential in urban areas are very challenging. The present paper, therefore, intends to present a straightforward framework to provide a preliminary and large-scale assessment of the urban wind energy potential, i.e. at city or country scales, for roof-mounted turbines. The framework is based on four main steps: (i) collecting the building data, i.e. the number of potential candidate high-rise buildings and their height and rooftop surface area; (ii) obtaining the annual mean wind speed statistics at the height of these buildings and sorting the building data based on these statistics; (iii) obtaining the turbine characteristics and determining the average number of turbines per building roof; (iv) calculating the annual energy production (AEP). The application of the framework is then illustrated at the country scale for the Netherlands. In this case, the urban wind energy potential is assessed by considering the installation of 18,156 small wind turbines on the roofs of 1513 existing high-rise buildings in 12 major cities in the Netherlands, yielding an annual energy production of 150.1 GWh.
Knowledge of temperature stratification in indoor environments is important for occupant thermal comfort and indoor air quality and for the design and evaluation of displacement ventilation systems. ...This paper presents a detailed and systematic evaluation of the performance of 3D steady Reynolds-Averaged Navier–Stokes (RANS) CFD simulations to predict the temperature stratification within a room with a heat source and two ventilation openings. The indoor air quality within the room is also investigated by assessing the distribution of the age of air. The evaluation is based on validation with full-scale measurements of air temperature. A sensitivity analysis is performed to investigate the impact of computational grid resolution, turbulence model, discretization schemes and iterative convergence on the predicted temperatures and age of air. The results show that steady RANS with the SST k-ω model can accurately predict the temperature stratification in this particular indoor environment. However, of the five commonly used turbulence models, only the SST k-ω model and the standard k-ω model succeed in reproducing the thermal plume structure and the associated thermal stratification, while the three k-ε models clearly fail in doing so. In addition, the iterative convergence criteria have a major impact on the predicted age of air, where it is shown that very stringent criteria in terms of scaled residuals are required to obtain accurate results. These required criteria are much more stringent than typical default settings. This paper is intended to contribute to improved accuracy and reliability of CFD simulations for displacement ventilation assessment.
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•Steady 3D RANS CFD simulations for stratification of indoor temperature and air quality.•Detailed sensitivity analysis and validation with full-scale measurements of air temperature.•The SST k-ω model shows the best agreement with the measurements: average deviation of 4.2%.•The iterative convergence criteria show the largest effect on the distribution of the age of air.
The presence of building balconies can significantly modify the near-façade wind flow pattern and surface pressures. The present study evaluates the impact of building balcony geometry on mean wind ...speed on balcony spaces and wind-induced mean surface pressure for generic high-rise buildings. The focus is on balconies that extend along the entire width of the building façade. Large-eddy simulations (LES), validated with wind tunnel experiments, are conducted to investigate the impact of (i) balconies present or not, (ii) balcony depth, (iii) balcony parapet walls, (iv) balcony partition walls, and (v) density of balconies. The results indicate that the balcony geometry can greatly affect the mean wind speed on balcony spaces and the local and façade-averaged mean pressure coefficient (Cp). The presence of balconies can increase the façade-averaged Cp over the windward and leeward façades by 5.2% and 8.9%, respectively. These numbers rise to 23.5% and 23.3% when two partition walls are added at the lateral edges of the façades. Adding five partition walls can reduce the overall area-averaged wind speed on balcony spaces by 68.0% compared to the case without partition walls. These findings can be useful in developing, designing and constructing buildings with façade geometrical details that improve building ventilation, air quality and wind comfort.
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•The geometrical characteristics of balconies significantly affect Cp and wind speed.•Deeper balconies decrease Cp on the windward façade.•Deeper balconies provide higher mean wind speed on windward balcony spaces.•Adding partition walls increases Cp and reduces mean wind speed on balcony spaces.•2 m parapet walls reduce the maximum mean wind speed on windward balcony spaces.
•The impact of relative spacing on the performance of double VAWT arrays is studied.•Double VAWT arrangements with the highest power performances are identified.•Average output over all wind ...directions is assessed for different relative distances.•Map of rotors' power performances is drafted as a function of their relative spacing.•Aerodynamic mechanisms contributing to power performance enhancement are identified.
Designing an optimal wind farm layout requires fundamental knowledge of the interaction of wind turbines in an arrangement. In this paper, extensive high-fidelity CFD simulations are performed to investigate the influence of relative spacing, i.e., distance (R) and angle (Φ), in double rotor arrangements of co-rotating Darrieus H-type vertical axis wind turbines (VAWTs) on their aerodynamic performance. The relative spacing varies within 1.25d ≤ R ≤ 10d (d: turbine diameter) and −90° ≤ Φ ≤ +90°. The turbines operate at their optimal tip speed ratio. The analysis is focused on the individual and overall power performance of the turbines and their aerodynamics. Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations, validated with experiments, are employed. It is found that an optimal region exists in which a higher overall power coefficient (CPoverall) compared to the CP of an isolated solo rotor (CPSolo) can be achieved. This region corresponds to compact rotor arrangements, i.e. R/d ≤ 3d with Φ ≥ +45° and Φ ≤ −45°, yielding a maximum 1.8% increment in CPoverall/CPSolo at R/d = 1.25 and Φ = +75°. Detailed flow analysis reveals that in the optimal spacing, a narrow passage between the two rotors is formed within which the flow accelerates, forming a high-velocity region. The downstream turbine benefits from its blade(s) passing through this region and consequently yields higher CP values. The findings highlight the high potential for compact VAWT farms with high power density and support the optimal layout design of VAWT farms.
Outdoor urban ventilation in a real complex urban area is investigated by introducing a new ventilation indicator – the “air delay”. Computational Fluid Dynamics (CFD) simulations are performed using ...the 3D steady Reynolds-Averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES) approaches. The up-to-date literature shows the lack of detailed evaluations of the two approaches for real compact urban areas. This study further presents a systematic evaluation of steady RANS and LES for the assessment of the ventilation conditions in a dense district in Nicosia, Cyprus. The ventilation conditions within the urban area are investigated by calculating the distribution of the age of air. To better assess the outdoor ventilation, a new indicator, the “air delay” is introduced as the difference between the local mean age of air at an urban area and that in an empty domain with the same computational settings, allowing the comparison of the results in different parts of the domain, without impact of the boundary conditions. CFD results are validated using wind-tunnel measurements of mean wind speed and turbulence intensity performed for the same urban area. The results show that LES can accurately predict the mean wind speed and turbulence intensity with the average deviations of about 6% and 14%, respectively, from the wind-tunnel measurements while for the steady RANS, these are 8% and 31%, respectively. The steady RANS simulations overestimate the local mean air delay. The deviation between the two approaches is 52% at pedestrian level (2 m).
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•Wind tunnel and CFD are used to evaluate ventilation conditions in real urban areas.•RANS and LES are systematically evaluated to predict ventilation conditions.•New indicator “air delay” is introduced to better assess outdoor ventilation.•Steady RANS overestimates the local mean air delay in urban areas.•Average deviation steady RANS-LES in local mean air delay is 52% at pedestrian level.
Cascaded split-source inverter (CSSI) is a new single-stage modular multilevel topology. Each cell of this converter converts dc to ac power in the buck or boost operation mode without any additional ...power switch. This article develops a design method based on a detailed model for CSSI. This model shows that energy storage elements experience double-fundamental frequency ripples besides high-frequency ones. It accurately calculates voltage gains and capacitor voltage and inductor current ripples. On the other hand, common carrier-based multilevel modulations have been modified in terms of both reference and carrier signals to control the inverter under symmetric and asymmetric conditions. With proposed simple modulations, the cell inductor is charged with the constant duty cycle. Also, the reverse recovery of diodes occurs in the minimum level of input current once per switching cycle and output power is distributed equally between cells. These characteristics reduce power losses, the input inductance of each cell, and the complexity of the system model. By phase-shifted pulsewidth modulation (PWM), the output voltage harmonics of three-cell CSSI are centralized around multiples of the triple switching frequency. In this method, high-frequency ripples on the network impedance are one-third of those in level-shifted PWM. Also, CSSI has been compared with cascaded multilevel inverters with voltage boosting ability and other SSI-based multilevel topologies. Validity of operation principles, mathematical relations, and modulation schemes are verified through simulation and experimental results.
The impact of rotor setting and relative arrangement on the individual and overall power performance and aerodynamics of double rotor vertical axis wind turbine (VAWT) arrays is investigated. Eight ...rotor settings are considered: two relative rotational directions (co-rotating, CO, and counter-rotating, CN), two relative positionings (downstream turbine positioned in the leeward, LW, and windward, WW, of the upstream rotor), and two phase lags (Δθ = 0° and 180°). For each of the eight rotor settings, 63 different relative arrangements are considered resulting in 504 unique cases. The arrangements are considered within 1.25d≤R≤ 10d (d = rotor diameter) and 0° ≤Φ≤ 90°, where R and Φ are relative distance and angle of the rotors, respectively. Unsteady Reynolds-Averaged Navier–Stokes (URANS) CFD simulations, validated with experimental data, are employed. The results show that the power performance of the array is significantly influenced by the relative rotational direction and positioning, ∼8% in power coefficient (CP), while it is marginally dependent on relative phase lag. The different performance of the studied arrays is because of different parts of the downstream turbine revolution being affected by the wake of the upstream turbine and dissimilar strength/width of the shear layer created in the two rotors’ wake overlap. The preferred rotational direction for WW arrays is co-rotating while for LW arrays counter-rotating is favored. For the same arrangement, counter-rotating turbines with LW relative positioning have the highest CP due to their downstream turbine blade moving along the flow direction in the wake overlap region resulting in little energy dissipation and weak shear layer. In contrast, counter-rotating arrays with WW relative positioning have the lowest CP, because the downstream turbine blade moves against the flow in the wake overlap region, resulting in extensive velocity deficit and a thick, strong shear layer.
•Three rotor setting and two relative arrangement parameters are studied in 504 cases.•Rotor setting can impact the double VAWT arrays' power performance by ∼8% .•The impact of phase lag on the power performance of double VAWT arrays is marginal.
Cascaded split-source inverter (CSSI) is a single-stage modular multilevel structure. This article proposes the multidimensional pulsewidth modulation (MD-PWM) for this topology. All multilevel ...modulations are a particular version of this approach. The principles of this geometrical method are based on selecting the optimal points and determining the switching sequences and time intervals. By considering the operational principles of CSSI, this method is modified to accurately control the voltage of dc links, charge inductors with constant duty cycles, control dc and ac sides of cells independently, and reduce the reverse recovery effects of input diodes. MD-PWM, with a low computational burden, can be implemented for CSSI with any number of voltage levels. The output power can be distributed equally or unequally among cells, and both buck and boost operation modes are realized for each cell. Also, this method is extended for the operation of the converter with unequal input sources. For an asymmetric topology with N cells, harmonics are located at multiples of N times switching frequency. Thus, this strategy eliminates low-frequency distortions from the output voltage of asymmetric CSSI. The simulation and experimental results verify the performance and accuracy of the proposed technique and mathematical relations.
Building-integrated wind energy harvesting offers a promising local and decentralized energy generation solution, reducing transmission losses and infrastructure expenses. However, the complex urban ...wind patterns demand accurate aerodynamic optimization for optimal performance. This study seeks to optimize the geometric design of building-integrated ducted openings to maximize wind energy harvesting. The research aims to create a data-driven model predicting the openings' performance accurately, explore the interlinked impacts of different geometrical variables, find optimal configurations across different building heights, and understand underlying aerodynamic mechanisms. The proposed design comprises a nozzle, throat, and diffuser. To increase power density, a data-driven design optimization method is employed. The optimization process involves determining the optimum opening height from the ground, along with five geometrical variables: inlet diameter, throat diameter, outlet diameter, nozzle length, and diffuser length. A comprehensive test matrix is developed using design of experiments (DoE) to build an artificial neural network (ANN) model. This neural network is developed using computational fluid dynamics (CFD) simulations validated with experimental data. Subsequently, a gradient-based global optimization method fine-tunes the neural network and identifies optimal designs for various heights. The research concludes that nozzle and diffuser opening angles within the range of 19° ≤ ΦNoz ≤ 23°and 7.5° ≤ ΦDif ≤ 8.2°, respectively, result in the optimal converging-diverging ducted opening geometry. This optimal opening can yield a power density increase of 31.5 to 35.5 times when compared to freestream flow and 17.5 to 22.4 times in comparison to cylindrical openings, depending on the opening location along the building height.
●Data-driven model is devised for optimization of building-integrated ducted openings.●Optimal ducted openings with maximum power density are found for different heights.●The optimal nozzle opening angle is found to be 19° ≤ ΦNoz ≤ 23°.●The optimal diffuser opening angle is found to be 7.5° ≤ ΦDif ≤ 8.2°.●Optimal designs can increase power density up to 35.5 times the freestream wind flow.
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