Experimental and numerical study of a full scale ventilated enclosure: Comparison of four two equations closure turbulence models

Full-scale experimental and computational fluid dynamics (CFD) methods are used to investigate the velocity and temperature fields in a mechanically ventilated enclosure. Detailed airflow fields are measured in three cases of ventilation air temperature: an isothermal case, a hot case and a cold case. The ventilation system creates an axisymmetric jet which is developing near the ceiling. The experimental data are used to test four two equations turbulence models: a k–ε realizable model, a k–ε RNG model, a k–ω model and a k–ω SST model. It is found that, even if the models can predict reasonably the hot and isothermal cases global values of temperature and velocity, none of the models is reliable concerning the cold case. Moreover, a detailed analysis of the jet shows that none of the models is able to predict the exact experimental velocity and temperature fields.     

Improving predictions of heat transfer in indoor environments with eddy viscosity turbulence models

Heat transfer modelling in indoor environments requires an accurate prediction of the convective heat transfer phenomenon. Because of the lower computational cost and numerical stability, eddy viscosity turbulence models are often used. These models allow modification to turbulent Prandtl number, and near wall correction which influences stagnation points, entrainment, and velocity and time scales. A modified v ²–f model was made to correct the entrainment behaviour in the near wall and at the stagnation point. This new model was evaluated on six cases involving free and forced convection and room airflow scenarios and compared with the standard k–ε, and k–ω–SST models. The results showed that the modification to the v ²–f model provided better predictions of the buoyant heat transfer flows while the standard k–ε failed to reproduce and underestimate the convective heat transfer. The k–ω–SST model was able to predict the flow field well only for a 2D square cavity room, and 3D partitioned room case, while it was poor for the other four cases.     

On the numerical study of contaminant particle concentration in indoor airflow

The application of three turbulence models—standard k–ε, re-normalization group (RNG) k–ε and RNG-based large eddy simulation (LES) model—to simulate indoor contaminant particle dispersion and concentration distribution in a model room has been investigated. The measured air phase velocity data obtained by Posner et al. [Energy and Buildings 2003;35:515–26], are used to validate the simulation results. All the three turbulence model predictions have shown to be in good agreement with the experimental data. The RNG-based LES model has shown to yield the best agreement. The flow of contaminant particles (with diameters of 1 and 10 μm) is simulated within the indoor airflow environment of the model room. Comparing the three turbulence models for particle flow predictions, the RNG-based LES model through better accommodating unsteady low-Reynolds-number (LRN) turbulent flow structure has shown to provide more realistic particle dispersion and concentration distribution than the other two conventional turbulence models. As the experimental approach to access indoor contaminant particle concentration can be rather expensive and unable to provide the required detailed information, the LES prediction can be effectively employed to validate the widely used k–ε models that are commonly applied in many building simulation investigations.     

An analysis of practical RANS simulations for spinnaker aerodynamics

Reynolds-averaged Navier-Stokes (RANS) simulations were performed on 12 parametric spinnaker models which had been previously tested in a wind tunnel by the authors. Six different turbulence models were used on two of the sails: the standard k-ε, realizable k-ε, renormalization group theory (RNG) k-ε, standard k-ω, shear-stress transport (SST) k-ω and Reynolds stress (RS) models. The realizable k-ε model was the best in terms of both accuracy and robustness, and this model was used to predict the force coefficients for all 12 sails. In general, the results are good; however, the simulations fail to correctly predict the effect of camber on sail performance for beam and broad reaching conditions. Issues surrounding the convergence of the simulations are discussed, and suggestions for the practical use of RANS for spinnaker design and analysis are presented.     

CFD investigation of turbulence models for mechanical agitation of non-Newtonian fluids in anaerobic digesters

This study evaluates six turbulence models for mechanical agitation of non-Newtonian fluids in a lab-scale anaerobic digestion tank with a pitched blade turbine (PBT) impeller. The models studied are: (1) the standard k-? model, (2) the RNG k-? model, (3) the realizable k-? model, (4) the standard k-ω model, (5) the SST k-ω model, and (6) the Reynolds stress model. Through comparing power and flow numbers for the PBT impeller obtained from computational fluid dynamics (CFD) with those from the lab specifications, the realizable k-? and the standard k-ω models are found to be more appropriate than the other turbulence models. An alternative method to calculate the Reynolds number for the moving zone that characterizes the impeller rotation is proposed to judge the flow regime. To check the effect of the model setup on the predictive accuracy, both discretization scheme and numerical approach are investigated. The model validation is conducted by comparing the simulated velocities with experimental data in a lab-scale digester from literature. Moreover, CFD simulation of mixing in a full-scale digester with two side-entry impellers is performed to optimize the installation.     

A comparative study of the Mellor-Yamada and k-ε two-equation turbulence models in atmospheric application

The present study systematically compares the Mellor-Yamada (MY) model and the k−ε algebraic stress model in order to verify the possibility of using the k−ε algebraic stress model in atmospheric applications. The results of the parameterization process and atmospheric application of both models confirmed that the MY model neglected the pressure redistribution effect of buoyancy due to 〈u_iu_j〉 and 〈u_iθ〉 and that of shear due to 〈u_iθ〉. In addition, the MY model overestimated the turbulent energy dissipation. Based on the formulation of the k−ε algebraic stress model, we modified the constant value C_μ(=0.09) in the standard k−ε model to obtain the variables C_μM and C_μH to account for atmospheric stability. Finally, the results of the simulation obtained from the Wangara experiment verify the possibility of using the k−ε algebraic stress model in atmospheric application.     

Numerical predictions of indoor climate in large industrial premises. A comparison between different k–ε models supported by field measurements

This paper explores the benefits of using computational fluid dynamics (CFD) as a tool for prediction of indoor environment in large and complex industrial premises, in this case a packaging facility. This paper also presents a comparison between three eddy-viscosity turbulence models, i.e. the standard k–ε, the RNG k–ε, and the realizable k–ε, used for predictions of the flow pattern and temperature distribution in this large industrial facility. The predictions are compared with field measurements and the RNG k–ε model has been found to be the one most concurrent with the measured values.     

Numerical investigation of the effect of road structures on ambient air quality—for their better design

Two-dimensional numerical simulation for investigating wind and concentration field around a double-decked road structure was performed using a standard k-ε turbulence model. The main objective of this paper is to study how road fences installed at a double-decked road affect ambient air quality, especially, pollutant concentration at some downstream locations. For model validation, calculated results were compared with available field experiment. Performance of the standard k-ε model was also compared with that of the renormalization group k-ε model for the double-decked road. Obtained results clarified how and how much pollutant concentration distribution is influenced by road structures with and without fences: the fences on the upper deck have generally positive effect on decreasing of air pollution near ground level while those on the ground always not. The computer code we used is CFX4.     

A computational model to calculate the flow-induced pressure fluctuations on buildings

A computational model to predict the flow-induced pressure fluctuation on bluff bodies is presented. Unlike direct and large-eddy simulation, the present model employs a stochastic model to generate plausible velocity fluctuations (synthetic turbulence) that satisfy the mean turbulent quantities such as turbulent kinetic energy (k) and dissipation energy rate (ε). This model has three main components: (1) prediction of mean flow quantities by solving the 3D Navier-Stokes equations using the standard k-ε model with Kato and Launder modifications; (2) generating a synthetic turbulent velocity field using a stochastic model and finally (3) solving the Poisson equation that governs the pressure fluctuations field. Flow around the low-rise building at Texas Tech was analyzed using the developed model. Two different wind angles of attack are considered for the analysis. Results obtained using the developed model are compared with wind tunnel and field measurements. The computed rms values for pressure fluctuations show good agreement with the experimental results.     

The numerical computation of near-wall turbulent flow over a steep hill

The present work performs a detailed comparison between numerical computations for the flow over a two-dimensional steep hill and some newly obtained laboratory data. Six turbulence models were tested: four eddy-viscosity models (κ-ε, RNG-ε, κ-ω, SST) and two second-moment models (SSG-RSM-ε, BSL-RSM-ω). The experiments were conducted in a water channel and were specially planned such that the large separated flow region that is formed on the lee side of the hill could be well scrutinized. The experimental results include complete profiles of the mean velocity components and of the two-dimensional Reynolds stress tensor and were obtained through the laser Doppler anemometry. A particular concern of this work has been to achieve a detailed experimental and numerical characterization of the near-wall flow region. As such, for most of the measuring stations, at least eight points were located in the viscous sublayer. The work also shows the distribution of wall-shear stress in detail. The ω-equation-based models were observed to perform much better than the ε-equation-based models. The length of separated flow region, mean velocity profiles and wall-shear stress were all reasonably well predicted. The flow properties on the hill top were particularly difficult to describe. The turbulence properties in the reversed flow region were best simulated by the BSL-RSM model.     

The effect of ceiling configurations on indoor air motion and ventilation flow rates

The purpose of this paper is to evaluate the effects of a building parameter, namely ceiling configuration, on indoor natural ventilation. The computational fluid dynamics (CFD) code Phoenics was used with the RNG k–? turbulence model to study wind motion and ventilation flow rates inside the building. All the CFD boundary conditions were described. The simulation results were first validated by wind tunnel experiment results in detail, and then used to compare rooms with various ceiling configurations in different cases. The simulation results generated matched the experimental results confirming the accuracy of the RNG k–? turbulence model to successfully predict indoor wind motion for this study. Our main results reveal that ceiling configurations have certain effects on indoor airflow and ventilation flow rates although these effects are fairly minor.     

Numerical prediction of gas concentrations and fluctuations above a triangular hill within a turbulent boundary layer

The objective of the present work is to propose a numerical and statistical approach, using computational fluid dynamics, for the study of the atmospheric pollutant dispersion. Modifications in the standard k-ε turbulence model and additional equations for the calculation of the variance of concentration are introduced to enhance the prediction of the flow field and scalar quantities. The flow field, the mean concentration and the variance of a flow over a two-dimensional triangular hill, with a finite-size point pollutant source, are calculated by a finite volume code and compared with published experimental results. A modified low Reynolds k-ε turbulence model was employed in this work, using the constant of the k-ε model C_μ=0.03 to take into account the inactive atmospheric turbulence. The numerical results for the velocity profiles and the position of the reattachment point are in good agreement with the experimental results. The results for the mean and the variance of the concentration are also in good agreement with experimental results from the literature.     

Natural cross-ventilation in buildings: Building-scale experiments,numerical simulation and thermal comfort evaluation

The constantly increasing energy consumption due to the use of mechanical ventilation contributes to atmospheric pollution and global warming. An alternative method to overcome this problem is natural ventilation. The proper design of natural ventilation must be based on detailed understanding of airflow within enclosed spaces, governed by pressure differences due to wind and buoyancy forces. In the present study, natural cross-ventilation with openings at non-symmetrical locations is examined experimentally in a test chamber and numerically using advanced computational fluid dynamics techniques. The experimental part consisted of temperature and velocity measurements at strategically selected locations in the chamber, during noon and afternoon hours of typical summer days. External weather conditions were recorded by a weather station at the chamber's site. The computational part of the study consisted of the steady-state application of three Reynolds-Averaged Navier-Stokes (RANS) models modified to account for both wind and buoyancy effects: the standard k–?, the RNG k–? and the so-called “realizable” k–? models. Two computational domains were used, corresponding to each recorded wind incidence angle. It is concluded that all turbulence models applied agree relatively well with the experimental measurements. The indoor thermal environment was also studied using two thermal comfort models found in literature for the estimation of thermal comfort under high-temperature experimental conditions.     

Scale-adaptive simulation of unsteady flow and dispersion around a model building: spectral and POD analyses

The present paper evaluates the relative performance of scale-adaptive simulation (SAS) in modelling unsteady concentration and flow fields around a model building relative to other transient simulations such as large eddy simulation (LES) and unsteady Reynolds-averaged Navier–Stokes (URANS) models. A novel application of proper orthogonal decomposition (POD) and time–frequency analysis is carried out in order to evaluate the transient behaviour and dominant structures of the flow fields predicted by SAS and LES. Results represent the outstanding performance of SAS in comparison with the URANS computation based on the SST k–ω model. This better performance is related to the accurate reproduction of unsteady fluctuations around the model building by SAS. In addition, the quantitative and qualitative agreements for the shapes and magnitudes of POD modes between SAS and LES confirm the LES-like behaviour of SAS in the wake region. However, in terms of computational performance, SAS imposes an extra CPU cost as compared with LES for the same grid resolution.     

Experimental and numerical study of room airflow under stratum ventilation

This paper investigates the air movement, air temperature profile and gaseous contaminant transportation in an individual office with stratum ventilation. The room temperature is elevated compared with conventional standards. The experimental investigation is carried out in an environmental chamber with the presence of heat generating rectangles used to simulate an occupant and a computer. Measurements of temperature, velocity, and CO₂ concentration are carried out for nine plumb lines in the chamber. Up to sixteen points are measured along each plumb line. The experimental data of the aforesaid three parameters of the individual office in warm condition under stratum ventilation are presented. The experimental data collected are used to validate a re-normalization group (RNG) k–? turbulence model used for the warm condition. The agreements between the predicted values and experimental results are acceptable, which demonstrates the feasibility of simulating indoor airflows at elevated room temperature under stratum ventilation by the RNG k–? turbulence model.     

Evaluating the influence of the width of inlet slot on the prediction of indoor airflow: Comparison with experimental data

The aim of this work is to investigate the influence of two values of inlet slot width on the velocity characteristics and turbulent intensity of the airflow inside a rectangular room. The experimental data used to check the numerical results concerns a rectangular room where the air is supplied horizontally on the upper left and is exhausted through an opening on the lower right on the opposite side. The performance of three turbulence models, standard k-?, RNG k-?, and k-ω, in predicting the three-dimensional airflow in that room has also been investigated. The results for Reynolds number of 5000 are presented for dimensionless horizontal velocities and turbulent kinetic energy for two planes of the room and two inlet arrangements, one opening as large as the room and another with half of the width of the room. The results have indicated that the main features of the flow were captured by the three turbulence models investigated. On the whole, the performance of the standard k-? model was better than those of the other two turbulence models. In particular, the k-ω model performed better in the configuration with the largest air opening than in that with the smallest one, while the RNG k-? model presented the opposite behavior. The comparative study between both geometries demonstrated that for slots much smaller than the width of the room, three-dimensional effects become important.     

Appropriate boundary conditions for computational wind engineering models revisited

At the first Computational Wind Engineering conference in 1992 “Appropriate boundary conditions for computational wind engineering models using the k-ε turbulence model” were proposed. In this paper it is shown that these conditions can be directly derived by treating the onset flow as a horizontally homogeneous turbulent surface layer, with the flow being driven by a shear stress at the top boundary. This approach is extended to provide the inlet profiles and boundary conditions appropriate for modelling the flow using the standard k-ε, RNG k-ε, Wilcox k-ω and LRR QI turbulence models. Means for their application within the commercial CFD code CFX 12.0 are given. It is shown that within the flow the various turbulence model constants set the effective value of von Kármán's constant, which does vary slightly between models. The discrepancy between the turbulence level set by the standard turbulence model constants and that observed in the atmosphere is discussed. Problems with excessive turbulence generation near the ground and the over-prediction of stagnation pressures are discussed and possible solutions proposed.     

Numerical studies on air flow around a cube

In this paper, air approach flow moving towards a cube will be studied using computational fluid dynamics (CFD). The Reynolds Averaging of Navier-Stokes (RANS) equation types of k-ε turbulence model are used. Some RANS predicted results are compared with different upstream air speeds. Flow separation at the corner above the top of the cube, level of separation and reattachment are investigated. Reference is made to the experimental data on wind tunnels reported in the literature.A method similar to ‘recirculation bubble promoter’ is used for different approach flow speed distributions. Problems encountered in numerical simulations due to the sharp corner are discussed with a view to obtaining better prediction on recirculation flow in regions above the top of the cube. Correlations between the turbulent kinetic energy above the cube and the recirculation bubble size are derived for different distributions of approach flow speed.By limiting the longitudinal velocities in the first cell adjacent to the sharp edge of the cube or rib, and making good use of the wall functions at the intersection cells of the velocity components, positions of maximum turbulent kinetic energy and the flow separation and reattachment can be predicted by a standard k-ε model. The results agree with those obtained in the experiments.     

Parametric analysis of wind siting efficiency

The effect of wind climatic characteristics on the efficiency of the WECS is examined by means of the Weibull general model and a simple model of power curve. Two efficiency parameters—the plant utilization factor (F_u) and the site effectiveness (ε)—are considered as functions of three parameters: the Weibull shape parameter (k), a dimensionless mean wind speed (α), and the design ratio between rated and cut-in speed (φ). It is shown that only for intermediate values of α and φ the plant utilization factor can be considered independent of k, being otherwise either improved or penalized in no negligible measure by it, depending on the values of α and φ. The effect of increasing k on the site effectiveness is always beneficial. The effect of increasing α is beneficial for the plant utilization factor but largely penalizing for the effectiveness. However, no maximization is possible for neither of the efficiency parameters.It is shown that the correct use of F_u and ε for comparison is significant only for a given WECS, while the WECS data have to be used when comparing different or modified wind machines. The data of 35 commercial WECS are examined to show that the common assumption of constant rated or maximum efficiency is not acceptable.