3D Reynolds-averaged Navier-Stokes Research Articles

A novel multi-fidelity optimization framework for high-altitude propellers

A novel optimization methodology is presented for optimizing high-altitude propellers operating in the lower stratosphere. The proposed methodology combines a Bayesian optimization approach for most design variables and Vortex Theory for the optimization of twist distribution and rotational velocity. For the Bayesian optimization part of the problem, a multi-fidelity approach is employed with three levels of modeling fidelity. The considered fidelity levels, from lower to higher, are: Vortex Theory, and 3D Reynolds-Averaged Navier-Stokes (RANS) with the use of γ−Reθ transition model, converged with first-order upwind, and second-order upwind for the momentum equations. Additionally, the optimization problem involves two objectives: maximizing the cruise efficiency and minimizing the total volume of the propeller. To enhance the performance of multi-fidelity, multi-objective (MFMO) acquisition functions, a novel algorithm is proposed. The proposed optimization methodology demonstrates its effectiveness in generating several high-performing propeller designs and proves to be more efficient than an equivalent problem formulation relying solely on Bayesian optimization. Post-processing of optimization data reveals new design variable bounds that correspond to more efficient blade shapes. These findings provide insights into some important geometric features of high-altitude propellers and establish guidelines for future optimization efforts within the specified thrust and power consumption ranges. Finally, simulations of the best-performing designs across a range of advance ratios and altitudes confirm their high performance in various operational conditions.

Effects of wall cavity and fuel injection pressure on the performance of a non-reacting supersonic combustor

This study numerically investigates the flow field of a non-reacting cavity-configured scramjet (Supersonic Combustion Ramjet) combustor at various fuel injection pressures by solving the 2D Reynolds-Averaged Navier-Stokes (RANS) equations, species transport equations, and Menter SST k-ω model. The aim of this research is to reveal the effects of wall cavity insertion and fuel injection pressure (FIP) on the crucial performance parameters i.e., fuel-air mixing efficiency (MxE), total pressure recovery (TPR), and mass-averaged Mach number (MAMN). Accordingly, two trapezoidal cavities of aspect ratio 7 are introduced on the opposite walls of a rectangular combustor. The combustor entrance is configured with rearward-facing steps and it intakes finite parallel air streams through finite-width inlets. Gaseous hydrogen jets are injected 30 mm downstream from the combustor entrance and 10 mm upstream from the cavity leading edge. FIP is varied according to the fuel-to-freestream pressure ratios (FFPR) of 4.5, 9.0, 13.5, and 18.0. The results of the cavity-configured combustor are then compared with the performance of the combustor in the absence of the wall cavities. The results delineate the change in flow structures with the inclusion of wall cavities and variation in FIP. Insight physics of mixing, total pressure loss, and MAMN in different regions of the combustor are studied and the results are quantified for comparison. MxE in a cavity-configured combustor does not monotonically increase with decreasing FFPR as found in the combustor without wall cavities. The shock-shear layer interactions (SSLIs) play a dominant role in mixing inside the cavity-configured combustor. The results also demonstrate that the insertion of wall cavities can increase fuel-air MxE through the formation of cavity recirculation zones. In the cavity-configured combustor, a maximum of 45% MxE is achieved for FFPR 4.5, which is 4% higher than the value obtained from the combustor without the cavities with an expense of 3% greater total pressure loss.

Impact of eave and roof pitch on cross ventilation for an isolated building with sawtooth roof

An eave refers to an extension attached to the building roof to protect the interior space from direct solar radiation and improve the performance on cross ventilation. In this study, the impact of eave inclination angle and roof pitch of an isolated sawtooth roof building on cross ventilation were investigated. The eave configurations at either windward or leeward openings were included. 3D steady Reynolds-Averaged Navier-Stokes (RANS) equation in combination with the Shear-Stress Transport model (SST k-ω model) was used for the Computational Fluid Dynamics (CFD) simulations. Grid sensitivity study was carried out and the performance of cross ventilation was evaluated based on the non-dimensional velocity magnitude, spatial distribution of pressure coefficient as well as the ventilation rate of the building. For the simulation model with 55° roof pitch, it is observed that a region with high velocity magnitude formed on top of the leeward eave due to the higher roof pitch and presence of the leeward eave. Results also indicated that the building model with 90° leeward eave and 55° roof pitch has the highest increment in ventilation rate which is 7.16%. On the other hand, the building model with 90° windward eave has the highest pressure coefficient because more blockage of airflow is caused by a steeper roof as the roof pitch of the building increases. Furthermore, the building model with 90° leeward eave shows a larger region with negative pressure at the leeward façade indicating higher airflow leaving the leeward opening. Therefore, the airflow behavior and characteristic are both dependent on the roof pitch and eave inclination angle for a naturally ventilated building.

Numerical investigation of air curtain flows in the doorway of a building using RANS and LES

Air curtains are commonly adopted in building flows to facilitate aerodynamic sealing against the exchange flow that occurs across an open doorway due to the density differences owing to buoyancy. This paper reports numerical simulations of air curtain flows using Reynolds-averaged Navier–Stokes (RANS) and large-eddy simulation (LES) methodologies. Computations are performed for various operating conditions that are described using a deflection modulus (Dm) which ranges from 0 to 1.25. Physically, Dm represents the relative strength of the jet momentum flux compared to the transverse stack effect. The value of this deflection modulus, along with qualitative distribution of buoyancy, demarcates the operating regime of an air curtain into unstable (Dm⪅ 0.1), stable (Dm⪆ 0.2), and strong installation (Dm⪆ 0.6). The quantification of aerodynamic sealing is established using effectiveness, E, which suggests that the optimal operation lies in the stable regime with the poorest performance in the unstable regime. The comparison between various simulation methodologies suggests that 2D RANS computations generally over-predict the effectiveness, whereas both 3D RANS and LES are able to estimate E reasonably well, especially at practical values of Dm. If the employed LES grid is sufficiently fine, the predicted effectiveness falls within the experimental uncertainty limits. Furthermore, we quantify the statistics of turbulence in the flow field using the transport equation of turbulent kinetic energy. This analysis reveals that shear production and turbulent dissipation are dominant processes in the flow. In comparison, the effects of transport terms and buoyancy production are rather small. These quantities are shown to scale well with w3/l where w and l are the characteristic velocity and length scales associated with an air curtain. Visualization of large-scale coherent structures using the Q-criterion depicted the presence of long spanwise vortices in the shear layer with a wide range of scales of motion at higher Dm. These vortical structures break down significantly upon impinging on the wall (floor), resulting in a complex spatiotemporal distribution of vortices.

Thermoacoustic Combustion Stability Analysis of a Bluff Body-Stabilized Burner Fueled by Methane–Air and Hydrogen–Air Mixtures

Hydrogen can play a key role in the gradual transition towards a full decarbonization of the combustion sector, e.g., in power generation. Despite the advantages related to the use of this carbon-free fuel, there are still several challenging technical issues that must be addressed such as the thermoacoustic instability triggered by hydrogen. Given that burners are usually designed to work with methane or other fossil fuels, it is important to investigate their thermoacoustic behavior when fueled by hydrogen. In this framework, the present work aims to propose a methodology which combines Computational Fluid Dynamics CFD (3D Reynolds-Averaged Navier-Stokes (RANS)) and Finite Element Method (FEM) approaches in order to investigate the fluid dynamic and the thermoacoustic behavior introduced by hydrogen in a burner (a lab-scale bluff body stabilized burner) designed to work with methane. The case of CH4-air mixture was used for the validation against experimental results and benchmark CFD data available in the literature. Numerical results obtained from CFD simulations, namely thermofluidodynamic properties and flame characteristics (i.e., time delay and heat release rate) are used to evaluate the effects of the fuel change on the Flame Response Function to the acoustic perturbation by means of a FEM approach. As results, in the H2-air mixture case, the time delay decreases and heat release rate increases with respect to the CH4-air mixture. A study on the Rayleigh index was carried out in order to analyze the influence of H2-air mixture on thermoacoustic instability of the burner. Finally, an analysis of both frequency and growth rate (GR) on the first four modes was carried out by comparing the two mixtures. In the H2-air case the modes are prone to become more unstable with respect to the same modes of the case fueled by CH4-air, due to the change in flame topology and variation of the heat release rate and time delay fields.

Determination of optimal height of barriers to reduce the amount of pollution in the viaduct settings in an idealized urban canyon: a numerical study.

In this work, we numerically investigate the process of atmospheric air pollution in idealized urban canyons along the road in the presence of a viaduct, taking into account different height of barriers. To solve this problem, the 3D Reynolds-averaged Navier-Stokes equations (RANS) were used. The closure of this system of equations was achieved by using various turbulent models. The verification of the mathematical model and the numerical algorithm was carried out using a test problem. The obtained results using various turbulent models were compared with experimental data and calculated results of other authors. The main problem considered in this work is characterized as follows: assessment of emissions of pollutants between buildings using barriers of various types in the presence of a viaduct. Computational results have shown that the barrier viaduct plays a large role in improving air quality in urban canyons. So, for example, a barrier erected on a viaduct with a height of 2m reduces the concentration value to a cross-section x = 84 by more than 2 times in comparison with the case of a complete absence of protective barriers. A similar situation was observed with barriers erected above the earth's surface: located along the road, they also significantly reduce the value of the concentration of pollutants. Thus, the presence of barriers in both cases is necessary to prevent the dispersion and deposition of pollutants.

Effect of multiple cavities and tip injection on the aerothermal characteristics of the squealer tip in turbine stage

For modern gas turbines, the blades of the first-stage high-pressure turbines are subject to significant aerodynamic and thermal loads. As the region where the aerothermal environment is most severe, the aerothermal performance of the blade tip is crucial. Either the tip injection or squealer tip with ribs can effectively improve the tip leakage flow distribution, and thus reduce the tip thermal loads, while the effect of the combination of the two remains to be explored, especially when the number of ribs increases. To further understand the combined effects of the rib number and the tip injections on the aerothermal characteristics, steady-state numerical simulations of the first stage turbine aerothermal field are carried out with varied rib numbers and tip injection configurations in the current study. The kω SST turbulence model was used to solve the 3D Reynolds-Averaged Navier-Stokes (RANS) equations. The heat transfer coefficient distributions and the leakage flow rate distributions are compared with those of the conventional squealer tip case. The results show that adding a rib inside the tip cavity would reduce the size of the reflux vortex near the leading edge of the blade (LE), and effectively reduce the averaged heat transfer coefficient on the tip. In addition, an increase in the rib number would make the coolant accumulate in the cavities, thus increasing the cooling performance. The averaged heat transfer coefficient of the coolant injected squealer tip with three ribs (Rib3C) decreases by 19.55%, compared with the performance of the conventional squealer tip, due to the combined action of adding the multiple ribs and the coolant injection. Comprehensively considering both improving the aerodynamic performance and reducing the blade tip heat load using TOPSIS analysis, the coolant injected squealer tip with two ribs (Rib2C) achieves the best overall performance. In general, ribs arranged in the upstream region of the blade would improve the coolant coverage by blocking the separation region at the cavity bottom. To improve the harsh tip thermal environment and reduce the tip aerodynamic loss, optimizing the rib layout inside the cavity and the coolant injection configurations is a practical solution.

Numerical study of high lift devices to improve airfoil aerodynamic performance

ABSTRACT High Lift Devices (HLD) are frequently used flow control devices able to increase the amount of lift generated by a wind turbine blade. The current paper presents a parametric study on the aerodynamic performance of a DU91W(2)250 airfoil with a stationary element placed at the trailing edge. The aerodynamic analysis of these devices is performed using computational fluid dynamics (CFD) based on a 2D Reynolds-averaged Navier-Stokes (RANS) K-ω Shear Stress Transport (SST) turbulence model under the effect of various high Reynolds numbers. The influence of these elements is measured for different angles of attack, lengths and tilt angles. In addition, the novel Cell-set (CS) modeling method has been compared to a fully meshed HLD model to test the reusable cell approach. Numerical results have shown that, for all 10° tilted devices at positive AoAs, high lift values are reached for HLD lengths 0.187c and 0.3c. Particularly for a 0.187c device length, lift experiences a significant enhancement at AoA 7.5°. The impact of HLDs decreases at AoAs between −2.5° and 2.5°and even underperform the baseline airfoil when studying 0.015c devices. Besides, the CS method was validated as a precise modeling tool as a maximum error of 1.93% was determined.

Development of a 3D Eulerian/Lagrangian Aircraft Icing Simulation Solver Based on OpenFOAM

A 3D icing simulation code is developed in the open-source CFD toolbox OpenFOAM. A hybrid Cartesian/body-fitted meshing method is used to generate high-quality meshes around complex ice shapes. Steady-state 3D Reynolds-averaged Navier-Stokes (RANS) equations are solved to provide the ensemble-averaged flow around the airfoil. Considering the multi-scale nature of droplet size distribution, and more importantly, to represent the less uniform nature of the Super-cooled Large Droplets (SLD), two droplet tracking methods are realized: the Eulerian method is used to track the small-size droplets (below 50 m) for the sake of efficiency; the Lagrangian method with random sampling is used to track the large droplets (above 50 m); the heat transfer of the surface overflow is solved on a virtual surface mesh; the ice accumulation is estimated via the Myers model; finally, the final ice shape is predicted by time marching. Limited by the availability of experimental data, validations are performed on 3D simulations of 2D geometries using the Eulerian and Lagrangian methods, respectively. The code proves to be feasible and accurate enough in predicting ice shapes. Finally, an icing simulation result of the M6 wing is presented to illustrate the full 3D capability.

Performance assessment of a modified wells turbine using an integrated casing groove and Gurney flap design for wave energy conversion

The present study investigates the effect of a triangular casing groove depth and width on Wells turbine performance. Furthermore, it presents an integrated method to boost the power produced by the Wells turbine and delay the stall. This integrated method is implemented by combining the effect of the triangular casing groove with a rectangular Gurney flap (GF). The Gurney flap (GF) increases the lift coefficient by modifying the Kutta condition at the trailing edge. At the same time, the triangular casing groove improves the flow near the blade tip, leading to a delay in the stall inception. The Wells turbine performance is evaluated by solving numerically 3D incompressible Reynolds-Averaged Navier-Stokes equations. To solve the flow characteristics through the turbine, an SST k-ω turbulence model is used, which is validated and verified using previous experimental and numerical works. The results proved that the Wells turbine average torque coefficient increased by 81.11% when using a triangular casing groove and a rectangular Gurney flap. Moreover, the stall inception is delayed from a flow coefficient of 0.225–0.350, which increases the operating range by 55.50% and reduces the maximum efficiency compared with the conventional Wells turbine.

CFD assessment of wind energy potential for generic high-rise buildings in close proximity: Impact of building arrangement and height

High-rise building complexes are of great importance for enabling sustainable urban development in large parts of the world. Earlier studies have indicated that high wind speed regions can be present along the passage between two high-rise buildings as well as above the roofs. At such locations, urban wind energy could be harvested by installing wind turbines between and/or above the roof of the buildings. However, the available wind energy potential around an array of generic high-rise buildings in close proximity has not yet been assessed for different building configurations. This paper conducts a detailed evaluation of the impacts of the building arrangement and height for a 2 × 2 array with a building height-to-street width ratio of 30 on the mean wind velocity and the wind energy potential along the passages between both upstream and downstream buildings as well as on their roofs. The following parameters are analyzed: (i) the passage width between the two upstream buildings (w), (ii) the streamwise distance between the upstream and downstream buildings (d), and (iii) the height difference between the upstream and downstream buildings (ΔH). The 3D steady Reynolds-averaged Navier-Stokes (RANS) equations are solved using the Reynolds stress model (RSM) turbulence model for closure. The CFD results are validated using wind-tunnel measurements of mean wind speed and turbulence intensity performed for the same building array. The results show elevated wind power density along the upstream passages for small w (=0.15B), high d (=0.6B), and equal building height (ΔH = 0). In contrast, comparatively high values of w, small d, and ΔH < 0 yield high wind power densities between the downstream buildings. Among the different wind turbine types considered, horizontally-mounted vertical axis wind turbines seem the most promising option for wind energy harvesting between the buildings.

A data-informed analytic model for turbine power prediction with anisotropic local blockage effects

This paper presents a new analytic approach for estimating the local power coefficient of a turbine experiencing anisotropic local blockage effects. Data-driven methods are employed first to approximately obtain a known analytic expression for isotropic local blockage effects, and then deployed to find candidate expressions for anisotropic local blockage effects. The dataset for the analysis of anisotropic local blockage is collected from 3D Reynolds-averaged Navier-Stokes (RANS) simulations of an infinitely wide array of actuator disks for nearly 2,000 different blockage configurations. This study builds upon previous work in array optimisation of both tidal and wind turbines, where the local power coefficient may increase substantially for optimal array configuration. A brief discussion on the relationship between the local blockage and wind farm blockage is also provided. Other theoretical approaches and possible refinements to the presented analytic model are also discussed.

2D/3D RANS and LES Calculations of Natural Convection in a Laterally-Heated Cylindrical Enclosure Using Boussinesq and Temperature-Dependent Formulations

This article presents an investigation of fluid flow and natural convection heat transfer in a cylindrical enclosure heated laterally. Two-dimensional (2D) Reynolds-averaged Navier–Stokes (RANS) equations and three-dimensional (3D) Large Eddy Simulation (LES) calculations are conducted using commercial computational fluid dynamics (CFD) software, ANSYS FLUENT. The Rayleigh number (Ra) = 2E+7 is constant in all of the simulations and is based on a length scale that is equal to the ratio of volume to the lateral area of the reactor, i.e., R/2, where R is the radius of the reactor. The validity of the Boussinesq approximation is analyzed by comparing calculations using both the Boussinesq approximation and temperature-dependent properties (non-Boussinesq approach) using 2D RANS and 3D LES (Dynamic Smagorinsky) formulations. Moreover, 2D axisymmetric k-ω SST RANS model will be implemented to investigate whether the 2D axisymmetric model can give results that are comparable to those of the 3D LES (Dynamic Smagorinsky) model when the corresponding longitudinal or azimuthal cross section are compared. In other words, the validity of using a 2D model instead of a 3D model for the current geometry, flow regime and thermal boundary conditions will be discussed. The flow and temperature contours of these two types of simulations are analyzed, compared to determine the various aspects of each case and discussed for deeper physical insight.

Impact of morphological parameters on urban ventilation in compact cities: The case of the Tuscolano-Don Bosco district in Rome

Air pollution and heat stress are major concerns associated with the liveability, resilience and sustainability of cities. They directly affect health and comfort and are associated with augmented morbidity and mortality and an increase in the energy demand for building ventilation, air cleaning and cooling. Nevertheless, the detrimental effects of poor air quality may partly be mitigated by increased urban ventilation. This strategy is closely related to the level of urbanization and the urban morphology. Therefore, detailed investigations on the impact of different morphologies on urban ventilation are of paramount importance. Computational Fluid Dynamics simulations have been widely used during the last decades to investigate the effects of the urban morphology on the urban ventilation. However, most of these studies focused on idealized building arrangements, while detailed investigations about the role of real urban morphologies are scarce. This study investigates the ventilation in a compact area in the city of Rome, Italy. 3D steady-state Reynolds-averaged Navier-Stokes simulations are performed to analyze the impact of Morphological Parameters (MP) on the urban ventilation. The results show a considerable worsening of urban ventilation with increasing building density with a reduction in the mean wind velocity up to 62% experienced at the pedestrian level (zp). Correlations between five MPs, e.g., plan area density, area-weighted mean building height, volume density, façade area density, and non-dimensional mean velocity at pedestrian level and at 10 m height are evaluated, and simple models are obtained using linear regression analysis. Among the selected MPs, the building façade area density shows a remarkable correlation with the non-dimensional mean velocity at zp (R2 = 0.82). Such correlations can be valuable tools for practitioners and urban designers, particularly during the first stage of planning, for highlighting areas potentially vulnerable to poor air conditions without running computationally expensive simulations.

An Enhanced Treatment of Boundary Conditions for 2D RANS Streamwise Velocity Models in Open Channel Flow

A 2D streamwise velocity model based on the Reynolds Averaged Navier–Stokes (RANS) is a useful approach to predict the boundary shear stress and the streamwise velocity in a free surface stream where secondary flows are not relevant. Boundary conditions treatment is a key aspect implementing these models. A low computational cost and fully predictive numerical model with a novel treatment of boundary conditions is presented. The main features of the modified model are the employment of a modified law of the wall valid for any roughness condition, the estimation of the boundary shear stress is done only focusing on the near-contour region, the use of a full-predictive physical based model for the eddy viscosity distribution and the incorporation of the free surface shear stress due to water–air interface. The validation of the proposed changes was performed with a substantial number of experimental cases available in the literature using different cross-section shapes (circular, rectangular, trapezoidal and compound section) and roughness condition with quite good agreement. Preliminary results suggest that the influence of the free surface boundary layer has a significant impact on the results for both the streamwise velocity and boundary shear stress in windy conditions. The proposed approach allows its considerations in practical applications.

Film cooling modeling of a gas turbine blade by considering different injection holes with and without opening angles through CFD

One way to achieve high performance in the gas turbine is to increase the inlet temperature of the turbine. Different cooling techniques have been carried out in order to protect the turbine blades which have been exposed to such high temperatures. Film cooling as an essential cooling method needs to be enhanced to meet the challenging demand. The purpose of the present research is to analyze the film cooling performance over a NACA 0012 gas turbine blade using six different injection holes with and without opening angles, separately through Computational Fluid Dynamics (CFD). 2D Reynolds-Averaged Navier-Stokes (RANS) equations are implemented to consider the heat transfer and flow characteristics by using CFD code Ansys Fluent v16. The flow is considered as steady, turbulent, and incompressible. The RANS equation is solved with the finite-volume method for obtaining solutions. The simulation results revealed that the k-ω SST turbulence model is suitable for simulating the flow characteristics and analyzing the performance of film cooling over the blade. Also, the opening angle has a significant effect on increasing the cooling efficiency for the upper blade surface. The highest value of cooling efficiency is obtained by the injection hole with an opening angle of 15° and height of D. In this configuration, the coolant injected from hole provides better cooling coverage for the entire blade which increases the cooling effectiveness.

Improving pedestrian micro-climate in urban canyons: City Center of Alexandria, Egypt

Due to the accelerated pace of urbanization, the study of the local urban microclimate has acquired significance. A detailed and accurate prediction of outdoor ventilation and thermal comfort is required for a healthy environment, particularly in densely built areas, due to the complex interaction of physical phenomena across different length and time scales. Engineering approaches are critical for solving the issues of wind discomfort. These approaches can however not be applied independently of the architectural context. An interdisciplinary approach can bridge the gap between architectural design and engineering wind-comfort solutions in a coastal city of developing countries with a temperate climate. The current study aims to evaluate the effect of urbanization on an asymmetrical historical canyon in the city center of Alexandria, Egypt. Challenges and limitations are imposed regarding applying retrofitting techniques to improve pedestrian comfort without changing the historical features of these areas.This paper provides a Computational Fluid Dynamics (CFD) simulation of a residential district in the modern city of Alexandria which contains the library of Alexandria and the two oldest paved roads in the world which still in use until today. The 3D steady Reynolds-averaged Navier-Stokes (RANS) equations and the standard k-ε model are used to calculate the aerodynamic flow field. These computational settings were validated with previously published wind-tunnel results. The indices of thermal comfort were calculated using a Predicted Mean Vote (PMV) model utilizing Custom Field Functions (CFF). On-site measurements of air velocity, temperature, and humidity were conducted to validate the thermo-flow characteristics with the numerical simulation and they were in a good agreement.To reach the comfort level which contributes to a pedestrian-friendly environment, it brings a range of measures that covers qualitative design guidelines on street and building level to mitigate wind discomfort and excessive solar access in urban canyons. One of these measures presents (CFD) simulations for comparing the model of the modern city with a visualized model of the ancient city of Alexandria at the end of Cleopatra's reign. This comparison aims to give a close vision on how urbanization has influenced the aerodynamics and thermal comfort in this region. The current study helps urban planners and architects to enhance the current situation for better urban microclimate at the pedestrian level, without setting up new developments during hot summer days.

The influence of terrain on the mean wind flow characteristics in a fjord

Flow conditions in complex terrains such as fjords are highly three-dimensional and thus not properly captured by the wind flow models developed for homogenous terrains. In the present study, we explore the potential of computational fluid dynamics (CFD) simulations relying on the steady 3D Reynolds-averaged Navier-Stokes equations to complement in-situ measurements from a long-span bridge in a narrow fjord. The validation is done using velocity data recorded in 2017 and 2018 by nine sonic anemometers mounted above the deck of a fjord-crossing suspension bridge. The flow characteristics studied are the along-bridge profile of the mean wind velocity, mean wind direction and mean angle of attack. The simulated flow shows that the non-uniform distributions of the mean angles of attack and wind direction along the bridge span are likely due to side-valley flows, which under certain conditions, predominate over those coming from the main valley. The measurements suggest that wind conditions corresponding to the dominating side-valley flows are associated with a high turbulence intensity at the bridge deck position. The paper highlights the complementary role of CFD studies and in-situ measurements for the design of a wind-sensitive structure, which may not be available using traditional semi-empirical modelling of topography effects.

A framework for pedestrian-level wind conditions improvement in urban areas: CFD simulation and optimization

Given the complexity of urban wind flow, acceptable wind conditions can hardly be achieved in all regions within an urban area. In some regions, high wind speeds can lead to wind discomfort and danger, while some regions suffer from poor outdoor urban ventilation. In order to evaluate and improve urban wind conditions, the combined impacts of wind flow and morphological and aerodynamic characteristics of the urban area should be taken into account. In this perspective, performing extensive and time-consuming parametric analyses is required – which is very difficult – even impossible. This paper, therefore, presents an evolutionary-based framework to optimize building heights and plan area densities to improve wind conditions in complex real urban areas. The framework consists of four steps: (i) urban area identification, (ii) optimization algorithms, (iii) CFD simulations, and (iv) evaluation. The optimization consists of Genetic Algorithm and Particle Swarm Optimization. The framework is applied to a real urban area in the city of Tehran, Iran. The focus is on minimizing low-wind-speed regions. In total, 920 possible combinations of building height variations and plan area densities are assessed. 3D steady Reynolds-averaged Navier-Stokes (RANS) simulations are performed based on a detailed validation study and grid-sensitivity analysis. The results show that the use of this framework can effectively reduce computational time and significantly improve the wind conditions in the entire urban area. Urban planners, architects, and building engineers can benefit from this framework in the early stages of design for better wind conditions in complex real urban areas.

Correlations for the forced convective heat transfer at a windward building façade with exterior louver blinds

Exterior louver blinds are increasingly utilized to protect the building envelope from solar radiation. However, with the existing correlations from studies on a smooth flat/a surface of the bluff body, the convective heat exchange at louver blinds and the façade behind them will be underestimated and overestimated respectively. In this work, Computational Fluid Dynamic (CFD) simulations have been conducted to analyze the forced convective heat transfer at the windward façade with louver blinds, which are validated with wind-tunnel experiments. The 3D steady Reynolds-averaged Navier-Stokes (RANS) function with Low-Reynolds Number Modelling (LRNM) treatment has been employed for precision improvement. The theoretical simulations are performed for 5 louver angle φ (from 0° to 60°), 3 installing distance W (from 0.025H to 0.08H), 5 slat width B (from 0.02H to 0.10H), 8 temperature difference between the façade and blinds θ (from 1 to 7) and 5 reference wind velocity UH (from 2.56 m·s−1 to 4.27 m·s−1). It was found that the convective heat loss at the louver shows a positive correlation with the growth of φ, W and the reduction of B. Meanwhile, the convective heat loss on the façade behind is positive to B, W and negative to φ and θ. Finally, with some conversions of expressions, a group of correlations are established which can predict the surface-averaged Nu of the façade and louver blinds, of which relative errors are less than 13% compared that of simulation data.