A comparative analysis of seven commonly-used eddy-viscosity turbulence models for CFD simulations of VAWTs is presented. The models include one- to four-equations, namely the Spalart-Allmaras (SA), RNG k-ε, realizable k-ε, SST k-ω, SST k-ω with an additional intermittency transition model (SSTI), k-kl-ω and transition SST (TSST) k-ω models. In addition, the inviscid modeling is included in the comparison. The evaluation is based on validation with three sets of experiments for three VAWTs with different geometrical characteristics operating in a wide range of operational conditions, from dynamic stall to optimal regime and to highly-rotational flow regime. The focus is on the turbine wake, the turbine power performance, and the blade aerodynamics. High-fidelity incompressible unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations are employed. The extensive analysis reveals high sensitivity of the simulation results to the turbulence model. This is especially the case for the turbine power coefficient CP. The results show that the inviscid, SA, RNG k-ε, realizable k-ε and k-kl-ω models clearly fail in reproducing the aerodynamic performance of VAWTs. Only the SST model variants (SST k-ω, SSTI and TSST) are capable of exhibiting reasonable agreement with all the experimental data sets, where the transitional SST k-ω versions (SSTI and TSST) are recommended as the models of choice especially in the transitional flow regime.
This article provides an overview of the application of computational fluid dynamics (CFD) in building performance simulation for the outdoor environment, focused on four topics: (1) pedestrian wind environment around buildings, (2) wind-driven rain on building facades, (3) convective heat transfer coefficients at exterior building surfaces and (4) air pollutant dispersion around buildings. For each topic, its background, the need for CFD, an overview of some past CFD studies, a discussion about accuracy and some perspectives for practical application are provided. This article indicates that for all four topics, CFD offers considerable advantages compared with wind tunnel modelling or (semi-)empirical formulae because it can provide detailed whole-flow field data under fully controlled conditions and without similarity constraints. The main limitations are the deficiencies of steady Reynolds-averaged Navier–Stokes modelling, the increased complexity and computational expense of large eddy simulation and the requirement of systematic and time-consuming CFD solution verification and validation studies.
Previous research indicated that the surface-averaged forced convective heat transfer coefficient (CHTC) at a windward building facade can vary substantially as a function of building width and height. However, existing CHTC expressions generally do not consider the building dimensions as parameters and are therefore strictly only applicable for the building geometry for which they were derived. Most CHTC expressions also categorize facades only as either windward or leeward. This indicates the need for new and more generally applicable CHTC expressions. This paper presents new generalized expressions for surface-averaged forced CHTC at building facades and roofs that contain the reference wind speed, the width and the height of the windward building facade as parameters. These expressions are derived from CFD simulations of wind flow and forced convective heat transfer for 81 different isolated buildings. The 3D Reynolds-averaged Navier-Stokes equations are solved with a combination of the high-Re number realizable k-ε model and the low-Re number Wolfshtein model. First, a validation study is performed with wind-tunnel measurements of surface temperature for a reduced-scale cubic model. Next, the actual simulations are performed on a high-resolution grid with a minimum near-wall cell size of 400 μm to resolve the entire boundary layer, including the viscous sublayer and the buffer layer, which dominate the convective surface resistance. The new CHTC expressions are analytical formulae (trivariate polynomials) that can easily be implemented in Building Energy Simulation (BES) and Building Envelope Heat-Air-Moisture (BE-HAM) transfer programs. The accuracy of the expressions is confirmed by in-sample and out-of-sample evaluations.
Air curtains are devices used to separate two different environments, in terms of air temperature and/or concentration of species. One of the main applications of air curtains consists in their placement at the open entrances of commercial buildings in order to provide an aerodynamic seal that reduces air infiltration through the entrance while allowing the unobstructed passage of people and equipment. From an energy conservation perspective, it is known that air infiltration can be responsible for a major share of the total energy losses occurring in a building. Therefore, it is relevant to understand the impact that infiltration through continuously open entrances can have on the energy demand of a building, and to comprehend how effective the placement of air curtains is as a strategy to restrict infiltration and reduce related building energy losses in such scenarios. This paper presents a study on the dynamic effect of weather conditions, during the winter season, on the total infiltration and the resulting heating energy demand of a generic retail building with an entrance door including and excluding an air curtain. For this purpose, an automated direct coupling of computational fluid dynamics (CFD) and building energy simulation (BES) is introduced to simultaneously model the complex dynamics and transport processes of the air-curtain jet flow and its contribution to the overall thermal balance of the building.
Different professional cyclists use very different hill descent positions, which indicates that prior to the present study, there was no consensus on which position is really superior, and that most cyclists did not test different positions, for example in wind tunnels, to find which position would give them the largest advantage. This paper presents an aerodynamic analysis of 15 different hill descent positions. It is assumed that the hill slope is steep enough so pedaling is not required to gain speed and that the descent does not include sharp bends necessitating changes in position. The analysis is performed by Computational Fluid Dynamics (CFD) simulations with the 3D RANS equations and the Transition SST k-ω model. The simulations are validated wind tunnel measurements. The results are analyzed in terms of frontal area, drag area and surface pressure coefficient. It is shown that the infamous “Froome” position during the Peyresourde descent of Stage 8 of the 2016 Tour de France is not aerodynamically superior to several other positions. Other positions are up to 7.2% faster and also safer because they provide more equal distribution of body weight over both wheels. Also several positions that allow larger power generation are aerodynamically superior.
Knowledge of convective heat transfer at ground surfaces around buildings is required for urban canyon models and for research on asphalt and road collectors, heat stresses in urban areas, etc. However, while many studies of convective heat transfer around buildings have been performed in the past, the focus of most of these studies was on rather simple building geometries and external surfaces of buildings. This paper, therefore, presents computational fluid dynamics (CFD) simulations and wind-tunnel measurements of convective heat transfer at ground surfaces around complex building models. The 3D Reynolds-averaged Navier Stokes equations are solved with a combination of the high-Re number realizable k- model and the low-Re number Wolfshtein model. The evaluation is based on validation with wind-tunnel measurement of ground surface temperature around a complex building model acquired through infrared thermography.
The Venturi effect refers to the increase in fluid speed due to a decrease of the flow section in confined flows. The wind speed conditions in converging and diverging passages between perpendicular buildings are studied with computational fluid dynamics to investigate the extent to which the so-called Venturi effect is present in the passages. Model validation is performed by comparing the numerical results with wind tunnel measurements. The validated model is employed for a detailed investigation of the wind speed and the flow rate in the passages for a wide range of passage widths. The simulations show an increase in wind speed near ground level, but a decrease of horizontal wind speed in the upper part of the converging passages. The reason is the wind-blocking effect, which causes a large part of the oncoming wind to flow over and around the buildings, rather than being forced through the passage. Due to this effect, the flow rates through the converging passages are consistently lower than the free-field flow rate, implying that the term Venturi effect is less applicable for such building configurations.
The accuracy of CFD simulations of atmospheric boundary layer (ABL) flow can be seriously compromised when wall-function roughness modifications based on experimental data for sand-grain roughened pipes and channels are applied at the bottom of the computational domain. This type of roughness modification is currently present in many CFD codes including Fluent 6.2 and Ansys CFX 10.0. The problems typically manifest themselves as unintended changes (streamwise gradients) in the vertical mean wind speed and turbulence profiles as they travel through the computational domain. They can-at least partly-be held responsible for the discrepancies that are sometimes found between CFD simulations and the corresponding measurements. This paper discusses this problem by focusing on a relatively simple situation: the simulation of a neutrally stratified, fully-developed, horizontally homogeneous ABL over uniformly rough, flat terrain. (author abst.)
