Urban physics

Urban Physics is the multiscale and interdisciplinary research area dealing with physical processes in urban environments that influence our everyday health, comfort and productivity. It involves disciplines ranging from mesoscale meteorology to human thermophysiology. The introductory lecture addresses basic research on numerical modeling of microscale atmospheric boundary layer processes as well as practical applications such as outdoor air pollution, pedestrian wind comfort and the urban heat island effect

Impact, absorption and evaporation of raindrops on building facades

In this paper, the impact, absorption and evaporation of raindrops on building facades is investigated by experimental and numerical means. Laboratory experiments were carried out to study the impact of water drops with different diameters, impact speeds and impact angles on a porous building material surface (ceramic brick). The measurements showed that large drops with high impact speeds splash, and that drops with high impact speeds and small impact angles bounce. The measurements, furthermore, allowed measuring the maximum spreading length and width of the drops as a function of drop diameter, impact speed and impact angle. Then, a numerical analysis was performed to study the distribution of impact speed and angle for raindrops hitting the facade of a 4×4×10 m3 tower building. The results demonstrated typical and important tendencies of impact angle and speed across the facade. Finally, the experimental and numerical data were used in a more precise three-dimensional simulation of impact, absorption and evaporation of random and discrete wind-driven raindrops. This was compared with the common one-dimensional simulation of absorption and evaporation at the facade considering a continuous uniform rain load as boundary condition, and significant differences between the two approaches were observed.status: publishe

Impact, runoff and drying of wind-driven rain on a window glass surface: numerical modelling based on experimental validation

This paper presents a combination of two models to study both the impingement and the contact and surface phenomena of rainwater on a glass window surface: a Computational Fluid Dynamics (CFD) model for the calculation of the distribution of the wind-driven rain (WDR) across the building facade and a semi-empirical droplet-behaviour model. The CFD model comprises the calculation of the wind-flow pattern, the raindrop trajectories and the specific catch ratio as a measure of the WDR falling onto different parts of the facade. The droplet-behaviour model uses the output of the CFD model to simulate the behaviour of individual raindrops on the window glass surface, including runoff, coalescence and drying. The models are applied for a small window glass surface of a two-storey building. It is shown that by far not all WDR that impinges on a glass surface runs off, due to evaporation of drops adhered to the surface. The reduction of runoff by evaporation is 26% for a typical cumuliform rain event and 4% for a typical stratiform rain event. These models can be used to provide the knowledge about WDR impact, runoff and evaporation that is needed for the performance assessment of selfcleaning glass or the study of the leaching of nanoparticles from building facades

Computational Fluid Dynamics simulations of wind-driven rain on a low-rise building : new validation efforts

Despite the establishment of CFD as a tool for calculating the amount of wind-driven rain (WDR) falling onto building facades, very few efforts have been made towards the validation of CFD for this purpose. This paper presents part of a detailed CFD validation study that was conducted at the Laboratory of Building Physics, supported by a new experimental wind, rain and WDR database for a low-rise building. It will be shown that numerical simulation, if conducted with care, can provide quite accurate predictions of the amount of WDR impinging on the building facade and that the main discrepancies in this case were due to a simplification of the upstream wind conditions in the numerical model

Low-Reynolds number mixing ventilation flows:impact of physical and numerical diffusion on flow and dispersion

\u3cp\u3eQuality assurance in computational fluid dynamics (CFD) is essential for an accurate and reliable assessment of complex indoor airflow. Two important aspects are the limitation of numerical diffusion and the appropriate choice of inlet conditions to ensure the correct amount of physical diffusion. This paper presents an assessment of the impact of both numerical and physical diffusion on the predicted flow patterns and contaminant distribution in steady Reynolds-averaged Navier–Stokes (RANS) CFD simulations of mixing ventilation at a low slot Reynolds number (Re≈2,500). The simulations are performed on five different grids and with three different spatial discretization schemes; i.e. first-order upwind (FOU), second-order upwind (SOU) and QUICK. The impact of physical diffusion is assessed by varying the inlet turbulence intensity (TI) that is often less known in practice. The analysis shows that: (1) excessive numerical and physical diffusion leads to erroneous results in terms of delayed detachment of the wall jet and locally decreased velocity gradients; (2) excessive numerical diffusion by FOU schemes leads to deviations (up to 100%) in mean velocity and concentration, even on very high-resolution grids; (3) difference between SOU and FOU on the coarsest grid is larger than difference between SOU on coarsest grid and SOU on 22 times finer grid; (4) imposing TI values from 1% to 100% at the inlet results in very different flow patterns (enhanced or delayed detachment of wall jet) and different contaminant concentrations (deviations up to 40%); (5) impact of physical diffusion on contaminant transport can markedly differ from that of numerical diffusion.\u3c/p\u3

Air pollutant dispersion from a large semi-enclosed stadium in an urban area: high-resolution CFD modeling versus full-scale measurements

Abstract: High-resolution CFD simulations and full-scale measurements have been performed to assess the dispersion of air pollutants (CO2) from the large semi-enclosed Amsterdam ArenA football stadium. The dispersion process is driven by natural ventilation by the urban wind flow and by buoyancy, and by the interaction between outdoor wind flow and indoor airflow which are only connected by the relatively small ventilation openings in the stadium facade. The CFD simulations are performed with the 3D Reynolds-averaged Navier-Stokes equations supplemented with the realizable k-e model to provide closure. The full-scale measurements include reference wind speed, wind direction, and outdoor and indoor air temperature, water vapor and indoor CO2 concentration. In particular, the focus is on CFD simulations and measurements for the few hours immediately after a concert, when the stadium roof remains closed and when indoor air temperature,water vapor and CO2 concentration have reached a maximum level due to the attendants. The removal of the sources/attendants allows an assessment of the natural ventilation rate using the concentration decay method. The CFD simulations compare favorably with the measurements in terms of mean wind velocity in the main ventilation openings and in terms of the CO2 concentration decay after the concerts. The validated CFD model will in the future be used for a detailed evaluation of indoor concentration gradients and the interaction between wind-induced and buoyancy-induced natural ventilation

Validation of external BES-CFD coupling by inter-model comparison

Conflation of computational fluid dynamics (CFD) and building energy simulation (BES) has been used in recent years in order to improve the estimation of surface coefficients for studies on thermal comfort, mold growth and other performance aspects of a building. BES can provide more realistic boundary conditions for CFD, while CFD can provide higher resolution modelling of flow patterns within air volumes and convective heat transfer coefficients (CHTC) for BES. BES and CFD can be internally or externally coupled. Internal coupling is the traditional way of expanding software by which the code is expanded by adding new modules and it entails a lot of effort in terms of debugging, maintenance etc. On the other hand, by external coupling different existing numerical packages work together, using the latest advances already implemented in them.This paper focuses on the validation of a newly developed prototype performing the external coupling of BES and CFD. The validation procedure involves an inter-model comparison between a conjugate heat transfer model and the prototype

Validation of external BES-CFD coupling by inter-model comparison

Conflation of computational fluid dynamics (CFD) and building energy simulation (BES) has been used in recent years in order to improve the estimation of surface coefficients for studies on thermal comfort, mold growth and other performance aspects of a building. BES can provide more realistic boundary conditions for CFD, while CFD can provide higher resolution modelling of flow patterns within air volumes and convective heat transfer coefficients (CHTC) for BES. BES and CFD can be internally or externally coupled. Internal coupling is the traditional way of expanding software by which the code is expanded by adding new modules and it entails a lot of effort in terms of debugging, maintenance etc. On the other hand, by external coupling different existing numerical packages work together, using the latest advances already implemented in them.This paper focuses on the validation of a newly developed prototype performing the external coupling of BES and CFD. The validation procedure involves an inter-model comparison between a conjugate heat transfer model and the prototype