Application of CFD in Building Performance Simulation for the Outdoor Environment: an Overview

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

A simplified numerical model for rainwater runoff on building facades: Possibilities and limitations

A simplified numerical model for rainwater runoff on building facades is presented, evaluated and discussed. The variation of runoff film thickness is described by a first-order hyperbolic partial differential equation. This equation is derived from the continuity equation, to which the wind-driven rain (WDR) intensity and the capillary absorption flux by the wall are added as source/sink terms, and from the adoption of the parabolic velocity profile of the Nusselt solution for a simplified representation of thin film flow. Two major model simplifications are the adoption of the Nusselt solution for (1) statistically-steady, developed films, in spite of actual wave behaviour, and for (2) transient, developing films, in spite of the actual moving contact line complexity. Both simplifications are directly related to surface tension effects. Concerning the first simplification, a selective review of the literature, including experimental laboratory data, confirms the validity of the Nusselt solution for representing the time-averaged properties of thin film flow, up to film Reynolds numbers of 1000, in spite of the actual wave behaviour. Concerning the second simplification, the runoff model is evaluated by a comparison with available on-site measurements of rainwater runoff from a building facade exposed to WDR, indicating a fair qualitative and quantitative agreement. Specific attention is given to a discussion of the possibilities and limitations of the runoff model. The runoff model can easily be integrated into 2D and 3D building envelope heat-air-moisture transfer (BE-HAM) models, but further research on the simplifications and assumptions of the runoff model is required.</p

Rainwater runoff from building facades: A review

Rainwater runoff from building facades is a complex process governed by a wide range of urban, building, material and meteorological parameters. Given this complexity and the wide range of influencing parameters, it is not surprising that despite research efforts spanning over almost a century, wind-driven rain and rainwater runoff are still very active research subjects. Accurate knowledge of rainwater runoff is important for hygrothermal and durability analyses of building facades, assessment of indirect evaporative cooling by water films on facades to mitigate outdoor and indoor overheating, assessment of the self-cleaning action of facade surface coatings and leaching of particles from surface coatings that enter the water cycle as hazardous pollutants. Research on rainwater runoff is performed by field observations, field measurements, laboratory measurements and analytical and numerical modelling. While field observations are many, up to now, field experiments and modelling efforts are few and have been almost exclusively performed for plain facades without facade details. Field observations, often based on a posteriori investigation of the reasons for differential surface soiling, are important because they have provided and continue to provide very valuable qualitative information on runoff, which is very difficult to obtain in any other way. Quantitative measurements are increasing, but are still very limited in relation to the wide range of influencing parameters. To the knowledge of the authors, current state-of-the-art hygrothermal models do not yet contain runoff models. The development, validation and implementation of such models into hygrothermal models is required to supplement observational and experimental research efforts.</p

CFD simulation of the atmospheric boundary layer: wall function problems

Accurate Computational Fluid Dynamics (CFD) simulations of atmospheric boundary layer (ABL) flow are essential for a wide variety of atmospheric studies including pollutant dispersion and deposition. The accuracy of such simulations 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, previously called CFX-5. The problems typically manifest themselves as unintended streamwise gradients in the vertical mean wind speed and turbulence profiles as they travel through the computational domain. These gradients can be held responsible—at least partly—for the discrepancies that are sometimes found between seemingly identical CFD simulations performed with different CFD codes and between CFD simulations and measurements. This paper discusses the problem by focusing on the simulation of a neutrally stratified, fully developed, horizontally homogeneous ABL over uniformly rough, flat terrain. The problem and its negative consequences are discussed and suggestions to improve the CFD simulations are made

Numerical modeling of turbulent dispersion for wind-driven rain on building facades

Wind-driven rain (WDR) is one of the most important moisture sources with potential negative effects on the hygrothermal performance and durability of building facades. The impact of WDR on building facades can be understood in a better way by predicting the surface wetting distribution accurately. Computational fluid dynamics (CFD) simulations can be used to obtain accurate spatial and temporal information on WDR. In many previous numerical WDR studies, the turbulent dispersion of the raindrops has been neglected. However, it is not clear to what extent this assumption is justified, and to what extent the deviations between the experimental and the numerical results in previous studies can be attributed to the absence of turbulent dispersion in the model. In this paper, an implementation of turbulent dispersion into an Eulerian multiphase model for WDR assessment is proposed. First, CFD WDR simulations are performed for a simplified isolated high-rise building, with and without turbulent dispersion. It is shown that the turbulence intensity field in the vicinity of the building, and correspondingly the turbulence kinetic energy field, has a strong influence on the estimated catch ratio values when turbulent dispersion is taken into account. Next, CFD WDR simulations are made for a monumental tower building, for which experimental data are available. It is shown that taking turbulent dispersion into account reduces the average deviation between simulations and measurements from 24 to 15%

Numerical simulations of wind-driven rain on an array of low-rise cubic buildings and validation by field measurements

The relation between wind-driven rain (WDR) and its potential negative effects on the hygrothermal performance and durability of building facades can be better understood by the correct estimation of the spatial and temporal distribution of the WDR intensity. Computational Fluid Dynamics (CFD) simulations with Eulerian Multiphase (EM) modeling are used to obtain accurate spatial and temporal information on WDR. The EM model has the advantage of predicting the WDR intensity on all surfaces of a complex geometry within the domain at once. There is a lack of numerical studies on the WDR intensity in generic and idealized multi-building configurations. In this paper, WDR intensities on an array of 9 low-rise cubic building models for wind from three different wind directions are estimated numerically using the EM model including the turbulent dispersion of raindrops. The numerical results are validated by comparing the calculated catch ratio values with data from field measurements in Dübendorf, Switzerland after two rain events with different characteristics. The CFD simulations successfully estimate the WDR intensities at the positions of 18 WDR gauges for both rain events. The influence of turbulent dispersion is found to be lower than 3% for both rain events. It is found that, for oblique wind directions, even though the maximum WDR intensity on the facades is lower, the whole building is exposed to up to 57% more WDR.</p

Numerical modeling of turbulent dispersion for wind-driven rain on building facades

Wind-driven rain (WDR) is one of the most important moisture sources with potential negative effects on the hygrothermal performance and durability of building facades. The impact of WDR on building facades can be understood in a better way by predicting the surface wetting distribution accurately. Computational fluid dynamics (CFD) simulations can be used to obtain accurate spatial and temporal information on WDR. In many previous numerical WDR studies, the turbulent dispersion of the raindrops has been neglected. However, it is not clear to what extent this assumption is justified, and to what extent the deviations between the experimental and the numerical results in previous studies can be attributed to the absence of turbulent dispersion in the model. In this paper, an implementation of turbulent dispersion into an Eulerian multiphase model for WDR assessment is proposed. First, CFD WDR simulations are performed for a simplified isolated high-rise building, with and without turbulent dispersion. It is shown that the turbulence intensity field in the vicinity of the building, and correspondingly the turbulence kinetic energy field, has a strong influence on the estimated catch ratio values when turbulent dispersion is taken into account. Next, CFD WDR simulations are made for a monumental tower building, for which experimental data are available. It is shown that taking turbulent dispersion into account reduces the average deviation between simulations and measurements from 24 to 15 %.</p

Comparison of calculation models for wind-driven rain deposition on building facades

Wind-driven rain (WDR) is an important factor in the dry and wet deposition of atmospheric pollutants on building facades. In the past, different calculation models for WDR deposition on building facades have been developed and progressively improved. Today, the models that are most advanced and most frequently used are the semi-empirical model in the ISO Standard for WDR assessment (ISO), the semi-empirical model by Straube and Burnett (SB) and the CFD model by Choi. This paper compares the three models by applying them to four idealised buildings under steady-state conditions of wind and rain. In each case, the reference wind direction is perpendicular to the windward facade. For the CFD model, validation of wind-flow patterns and WDR deposition fluxes was performed in earlier studies. The CFD results are therefore considered as the reference case and the performance of the two semi-empirical models is evaluated by comparison with the CFD results based on two criteria: (1) ability to model the wind-blocking effect on the WDR coefficient; and (2) ability to model the variation of the WDR coefficient with horizontal rainfall intensity Rh. It is shown that both the ISO and SB model, as opposed to the CFD model, cannot reproduce the wind-blocking effect. The ISO model incorrectly provides WDR coefficients that are independent of Rh, while the SB model shows a dependency that is opposite to that by CFD. In addition, the SB model can provide very large overestimations of the WDR deposition fluxes at the top and side edges of buildings (up to more than a factor 5). The capabilities and deficiencies of the ISO and SB model, as identified in this paper, should be considered when applying these models for WDR deposition calculations. The results in this paper will be used for improvement and further development of these models.</p

Numerical simulations of wind-driven rain on an array of low-rise cubic buildings and validation by field measurements

The relation between wind-driven rain (WDR) and its potential negative effects on the hygrothermal performance and durability of building facades can be better understood by the correct estimation of the spatial and temporal distribution of the WDR intensity. Computational Fluid Dynamics (CFD) simulations with Eulerian Multiphase (EM) modeling are used to obtain accurate spatial and temporal information on WDR. The EM model has the advantage of predicting the WDR intensity on all surfaces of a complex geometry within the domain at once. There is a lack of numerical studies on the WDR intensity in generic and idealized multi-building configurations. In this paper, WDR intensities on an array of 9 low-rise cubic building models for wind from three different wind directions are estimated numerically using the EM model including the turbulent dispersion of raindrops. The numerical results are validated by comparing the calculated catch ratio values with data from field measurements in Dübendorf, Switzerland after two rain events with different characteristics. The CFD simulations successfully estimate the WDR intensities at the positions of 18 WDR gauges for both rain events. The influence of turbulent dispersion is found to be lower than 3% for both rain events. It is found that, for oblique wind directions, even though the maximum WDR intensity on the facades is lower, the whole building is exposed to up to 57% more WDR.</p

Rain on building fa\ue7ades

Rain is one of the main causes of moisture damage to the building envelope, leading to problems such as rain penetration, frost and salt damage, discoloration by leaching, soiling by differential washing, etc. The potential of deterioration due to rain depends on the fa\ue7ade material, the junction of building envelope components, the overall geometry of the building, but also the presence or absence of modulation on the fa\ue7ade. In this paper, Computational fluid dynamics (CFD) simulations are used to analyze these effects and illustrated with examples taken in Sicily. It is firstly shown that buildings protect themselves from driving rain due to the wind blocking effect, but that fa\ue7ade openings can lead to increased deposit of driving rain due to occurrence of high wind velocities originating from pressure shortcuts. The importance of fa\ue7ade modulation by cornices or roof overhangs for shelter and deflection of rain is next demonstrated. It is further highlighted that these modulations should include drips to shed rain water running down the fa\ue7ade. Fa\ue7ades without moisture buffering by capillary action of fa\ue7ade materials are found to be very sensitive to staining due to run-off of dirty water from horizontal modulations. Finally, an example illustrates the sensitivity to rain penetration and degradation of face-seal fa\ue7ades