Computational fluid Dynamics (CFD) is increasingly used as a tool for determining the convective heat transfer coefficient (CHTC) at the surfaces of bluff bodies immersed in turbulent flows. Some previous studies on high-resolution CFD simulations of CHTC used the steady Reynolds- Averaged Navier-Stokes RANS approach. However, steady RANS is incapable of capturing the inherently transient behaviour of separation, reattachment and recirculation downstream of the windward surface and of von Karman vortex shedding in the wake. LES on the other hand can provide accurate descriptions of the mean and instantaneous flow field around bluff bodies. Therefore more accurate CHTC simulations should be pursued using LES. To gain insight into the performance of LES compared to steady RANS, this paper presents LES and RANS CFD simulations of the temperature and CHTC distributions at the surfaces of a reduced-scale wall-mounted cubic model measured in turbulent channel flow. The evaluation is based on a grid-sensitivity analysis and on validation with wind-tunnel measurements of surface temperature. The results show that LES can accurately predict the surface temperature distributions of the cube walls. Steady RANS, however, indicates a satisfactory agreement with the experiments only for the windward surface. For the windward and leeward faces, the average deviations of the obtained results by LES with the experiments are 1.4 and 1.3%, respectively. For steady RANS, these deviations are 3.3 and 5.7%. For the top and side faces, where flow separation and reattachment are very complex and intermittent, the deviations are 2.4 and 1.5% for LES, while for steady RANS they increase to 13.1 and 14.9%, respectively. This study is intended to support future CFD studies of CHTC at surfaces of buildings in urban environment.
To investigate the validity of the traditional approach to implement wind-driven rain (WDR) in numerical models of heat, air and moisture (HAM) transfer in building components, under real atmospheric conditions, a new set-up was developed at a test building. WDR intensity, other relevant environmental conditions and the resulting moisture response of the wall to these conditions (both hygroscopic loading and WDR) were simultaneously measured. The whole measurement data set was used for validation. Large differences between the measurement and simulation results were found and possible causes discussed. It is concluded that many influencing parameters interact, and that therefore precisely simulating the hygrothermal response of walls to WDR is very difficult.
The flow in a wind tunnel test section must meet high standards to obtain accurate and reliable measurement data. Good flow quality demands a certain degree of spatial uniformity and temporal steadiness of velocity and pressure. In this paper, a set of six new indices is developed and presented that relate spatial aspects of the mean velocity field to flow quality. One index quantifies the degree of uniformity of the velocity field and can be used directly as a flow quality indicator. The five other indices are related to different types of deviations from spatially uniform flow; skewed flow and angularity (up-flow and down-flow, swirl, cross-flow, diverging and converging flow). The indices can be used to evaluate the flow quality in existing tunnels and to assess the impact of design modifications. They can also be used in the CFD-based design of new wind tunnels.
Exterior convective heat and mass transfer coefficients at building surfaces are to a large extent determined by the local wind speed. Usually, empirical formulae are used to relate the reference wind speed at a meteorological station to the local wind speed near the building surface and to relate the local wind speed to surface transfer coefficients. These formulae however are based on a limited number of measurements. Little is known about the actual value and the variability of local wind speed and surface transfer coefficients across facades of different building geometries. In this paper, a validated Computational Fluid Dynamics (CFD) model is used to calculate the local wind speed near the exterior surface of a cubic building model as a function of wind speed, wind direction and the position on the facade. It is shown that the variation of the local wind speed across the facade is very pronounced and that using the available empirical formulae can yield large errors in HAM (Heat-Air-Moisture) calculations.
Pollutant dispersion in urban street canyons has been widely investigated by large-eddy simulation (LES). Many LES studies focused on generic street canyons under a wind direction perpendicular to the street axis. Accurate LES simulations require a sufficiently large domain size to minimize the effects of the artificial boundary conditions at the domain faces on the results. As opposed to RANS simulations, there is a lack of guidelines for an appropriate domain size for LES simulations of wind flow and pollutant dispersion in street canyons. The present study systematically investigates the effect of the domain width, domain height and upstream and downstream domain lengths on the wind flow and pollutant dispersion within a generic 2.5D street canyon with spanwise periodic boundary conditions. Following a validation study, 16 LES simulations are performed for different domain sizes. The results show that the minimum requirement for the domain width is 2.5H, where H is the roof height of the street canyon. For the domain height, upstream domain length and downstream domain length, 7.5H, 5H and 10H are recommended, respectively. These guidelines should help to reduce the computational costs of this type of simulation without significantly compromising the accuracy.
An experimental and numerical analysis of the flow conditions in a venturi-shaped roof has been conducted, focusing on the negative pressure in the narrowest roof section (contraction). Natural ventilation of the building zones can be driven by this negative pressure, either completely or partly. The wind tunnel experiments are performed in an atmospheric boundary layer wind tunnel at scale 1:100. The 3D CFD simulations are performed with steady RANS and the RNG k-e model. The aim of this study is to assess the magnitude of the negative pressures generated with different design configurations of the venturi-shaped roof, i.e. the presence and number of guiding vanes. The CFD simulations have been validated using the wind tunnel experiments of mean wind speed and surface pressures inside the roof. The simulated results are generally within 10% of the wind tunnel measurements, indicating a very close agreement. The study shows that the largest negative pressure coefficients are obtained for the configuration without guiding vanes, down to -1.35, with reference to the free-stream wind speed at roof height. At first sight, a counter-intuitive result is visible when comparing the design configurations with and without guiding vanes: adding guiding vanes strongly decreases the absolute value of the negative pressure. The wind tunnel measurements and the CFD simulations showed that the optimum configuration is the one without guiding vanes. An explanation can be found in the presence of the guiding vanes which increase the flow resistance inside the roof and cause more wind to flow over and around the roof, and less wind through it (wind-blocking).
This study presents results of coupled 3D steady Reynolds-averaged Navier-Stokes (RANS) Computational Fluid Dynamics (CFD) simulations of an isolated naturally-ventilated building with the application of an air curtain to prevent heat transfer across a doorway. The considered parameters include air curtain velocity, width and angle, indoor and outdoor tempera-tures, and wind flow. The results showed that higher temperatures inside the studied building were provided for: (1) higher jet velocities Vac for a jet angle a = 15deg, when jet width wjet is constant; (2) larger jet width wjet, when jet angle a and jet momentum Mac' are constant; (3) jet angle a = 0deg, when jet velocity Vac and jet width wjet are constant. The results also showed that for an air curtain with a = 0deg the air jet slightly bended to the outside that is possibly caused by stagnation-zone flow in front of the building.
Thin layer mortar brick facades are generally constructed with open vertical joints, which may increase the rain penetration risk. The study focuses on the frequency and intensity of runoff on ceramic brick facades, as a continuous water film over the open joints forms a premise for rain penetration through open joints. Combined CFD and HAM modeling reveals that large parts of the ceramic brick facades can adequately buffer all impinging driving rain: runoff will only arise occasionally at the upper edge. This low intensity runoff will however not yield a water film over the vertical joints: substantial rain penetration can therefore be considered improbable for ceramic brick facades.
Quality 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.
In team pursuit, the drag of a group of cyclists riding in a pace line is dependent on several factors, such as anthropometric characteristics (stature) and position of each cyclist as well as the sequence in which they ride. To increase insight in drag reduction mechanisms, the aerodynamic drag of four cyclists riding in a pace line was investigated, using four different cyclists, and for four different sequences. In addition, each sequence was evaluated for two arm spacings. Instead of conventional field or wind tunnel experiments, a validated numerical approach (computational fluid dynamics) was used to evaluate cyclist drag, where the bicycles were not included in the model. The cyclist drag was clearly dependent on his position in the pace line, where second and subsequent positions experienced a drag reduction up to 40%, compared to an individual cyclist. Individual differences in stature and position on the bicycle led to an intercyclist variation of this drag reduction at a specific position in the sequence, but also to a variation of the total drag of the group for different sequences. A larger drag area for the group was found when riding with wider arm spacing. Such numerical studies on cyclists in a pace line are useful for determining the optimal cyclist sequence for team pursuit.
An analysis of wind environmental conditions in the entrance channel of Ria de Ferrol in Galicia, Spain has been conducted with Computational Fluid Dynamics (CFD). The aim of the study is to provide input for real-time manoeuvring simulations to evaluate accessing the LNG terminal with larger LNG carriers. The entrance channel is enclosed by irregular hilly terrain, which is expected to yield complex wind environmental conditions in the channel and complex forces on the LNG carriers. The simulations are performed with the 3D steady Reynolds-averaged Navier-Stokes (RANS) equations and the realizable k-epsilon model. The simulation results of mean wind speed and direction are generally within 10% of the on-site measurements. Both the simulation and the measurement results illustrate the complex wind-flow patterns and the funnelling effect by the topography on the wind.
