Computational Fluid Dynamics Turbulence Models Research Articles

Adapted two-equation turbulence closures for actuator disk RANS simulations of wind & tidal turbine wakes

Reliable methods for modelling wake recovery within a farm of wind or tidal turbines are critical for obtaining accurate estimates of annual energy production, and for detailed farm layout optimization. These are important objectives for maximizing energy yield while minimizing costs. Computational fluid dynamics (CFD) simulation is rapidly being adopted as a tool for flow modelling in wind and tidal farms, gaining favour over more traditional and simpler empirically-determined wake models. The most practical methodology for CFD simulations of turbine farms uses an actuator disk (AD) representation for each rotor, which imposes the rotor forces by adding source terms to the governing equations rather than explicitly resolving the flow over the turbine blades. It is well understood that when using the AD approach, standard turbulence models tend to predict faster wake recovery than is observed in real flows. Thus, the standard CFD turbulence models must be adapted for use with the AD methodology. Additionally, because of the manner in which the AD approach distributes the rotor forces, it cannot resolve the system of discrete vortices trailed from the blade tips.This article presents two contributions to improving AD simulations of wind/tidal turbine wakes. The first is identifying that the well-established k-ω SST turbulence model is appropriate for AD simulations because it mitigates the problem of over-predicting the initial wake recovery rate. The second contribution is a method to include the typically un-modelled production of turbulent kinetic energy due to the breakdown of trailed vortices. This method was tuned to minimize the wake error for three experimental test cases with different rotors and different ambient turbulence intensities {3,10,15}%. The new model was validated and compared to existing turbulence methods for the wake of a second rotor in a tandem array configuration with different separation distances and ambient turbulence intensities. The different models were assessed using an error metric designed to estimate the error in predicting the power production of a turbine array. The reduction of this error by the new model varied from case to case, but was on the order of 3.5–10%, compared to the standard k-ε model.

An adapted steady RANS RSM wall-function for building external convection

Computational Fluid Dynamics (CFD) can improve usual estimates of building external convective heat transfer coefficients (hc,w) by accounting for the geometry of constructions, the aerodynamic field around them, the nature of convection and providing high resolution data. However, the limitations of usual mass flow descriptions and near wall treatments make the accurate prediction of hc,w challenging. Hence, this paper evaluates the ability of steady RANS Reynolds Stress (RSM) and k–ε realizable models to predict hc,w in case of isolated cubical obstacles. The accuracy of usual CFD methods and turbulence models, as well as fine grid near wall models and usual temperature wall functions (TWFs) are examined by comparison with experimental and detailed numerical data.When used with a low Reynolds number model (LRNM), both turbulence models accurately predict hc,w on the front and rear faces of the obstacle. However, they show different behaviors on the other faces and highlight issues related to the dynamic behavior of real flows. Moreover, hc,w predictions obtained using standard TWFs substantially deviate from the validated LRNM results. Therefore, a customized TWF suited for use with the RSM and forced convection problems is proposed by extending studies of Defraeye et al. (An adjusted temperature wall function for turbulent forced convective heat transfer for bluff bodies in the atmospheric boundary layer; Building and Environment, 2011, 46, 2130–2141). Customized TWFs substantially improve WF-based hc,w predictions with respect to LRNM results while keeping their cost effectiveness, and provides satisfactory results even for high z∗.

Study on Wing Aspect Ratio on the Performance of a Gliding Robotic Fish

In this paper, the performance of a gliding robotic fish with different wing aspect ratio is investigated. The gliding robotic fish, developed by Michigan State University, has the energy efficient locomotion of an underwater glider and high maneuverability of a robotic fish. ANSYS Computational Fluid Dynamics turbulence model was used to determine lift and drag coefficients for various wing aspect ratios at different angle of attack. Subsequently, the corresponding glide angle and velocity were determined analytically based on its dynamic model. The simulation results compare well with published experimental data and shows that the drag and lift coefficients are inversely proportional to the wing aspect ratio. As such, a gliding robotic fish with a low wing aspect ratio is suitable for shallow waters only, due to the high lift forces generated for a given angle of attack, requiring greater energy to sustain the glide velocity and vice versa.

Appropriate CFD Turbulence Model for Improving Indoor Air Quality of Ventilated Spaces

Abstract Accurate assessment of air-flow in ventilated spaces is of major importance for achieving healthy and comfortable indoor environment conditions. The CFD (Computational Fluid Dynamics) technique is nowadays one of the most used approaches in order to improve the indoor air quality in ventilated environments. Nevertheless, CFD has still two main challenges: turbulence modeling and experimental validation. As a result, the objective of this study is to evaluate the performance of different turbulence models potentially appropriate for the prediction of indoor airflow. Accordingly, results obtained with 6 turbulence models (standard k-ε model, RNG k-ε model, realizable k-ε model, LRN SST k-ω model, transition SST k-ω model and low Reynolds Stress-ω model) are thoroughly validated based on detailed experimental data. The configuration taken into account in this work corresponds to isothermal and anisothermal airflows produced by mixing ventilation systems in small enclosures at low room air changes per hour. In general, the transition SST k-ω model shows the better overall behavior in comparison with measurement values. Consequently, the application of this turbulence model is appropriate for air flows in ventilated spaces, being an interesting option to more sophisticated LES (Large Eddy Simulation) models as it requires less computational resources.

Multiscale image‐based modeling and simulation of gas flow and particle transport in the human lungs

Improved understanding of structure and function relationships in the human lungs in individuals and subpopulations is fundamentally important to the future of pulmonary medicine. Image-based measures of the lungs can provide sensitive indicators of localized features, however to provide a better prediction of lung response to disease, treatment, and environment, it is desirable to integrate quantifiable regional features from imaging with associated value-added high-level modeling. With this objective in mind, recent advances in computational fluid dynamics (CFD) of the bronchial airways-from a single bifurcation symmetric model to a multiscale image-based subject-specific lung model-will be reviewed. The interaction of CFD models with local parenchymal tissue expansion-assessed by image registration-allows new understanding of the interplay between environment, hot spots where inhaled aerosols could accumulate, and inflammation. To bridge ventilation function with image-derived central airway structure in CFD, an airway geometrical modeling method that spans from the model 'entrance' to the terminal bronchioles will be introduced. Finally, the effects of turbulent flows and CFD turbulence models on aerosol transport and deposition will be discussed.

Numerical aerodynamic analysis of bluff bodies at a high Reynolds number with three-dimensional CFD modeling

This paper focuses on numerical simulations of bluff body aerodynamics with three-dimensional CFD (computational fluid dynamics) modeling, where a computational scheme for fluid-structure interactions is implemented. The choice of an appropriate turbulence model for the computational modeling of bluff body aerodynamics using both two-dimensional and three-dimensional CFD numerical simulations is also considered. An efficient mesh control method which employs the mesh deformation technique is proposed to achieve better simulation results. Several long-span deck sections are chosen as examples which were stationary and pitching at a high Reynolds number. With the proposed CFD method and turbulence models, the force coefficients and flutter derivatives thus obtained are compared with the experimental measurement results and computed values completely from commercial software. Finally, a discussion on the effects of oscillation amplitude on the flutter instability of a bluff body is carried out with extended numerical simulations. These numerical analysis results demonstrate that the proposed three-dimensional CFD method, with proper turbulence modeling, has good accuracy and significant benefits for aerodynamic analysis and computational FSI studies of bluff bodies.

Analysis and Simulation of Air Drag in Vehicles Close-Following

Vehicles drag reductions arising from close spacing are studied and verified by some research institutes. But this solution was queried by an experiment carried in RMIT University in 2006. Based on two standard Ahmed Bodies, Computational Fluid Dynamics(CFD) method is used to study this problem in this paper. The calculation results by comparing four different CFD turbulence models show that the drag coefficient of the rear car is more than the lead car, and even more than the isolated car when the space of these two cars is less than one time of car length. Furthermore, the calculate results of these four turbulence models show that the standard model is the best model to simulate this problem, and different far field condition will influence the calculate results much more. Therefore, the research of this paper verifies that drag reductions from close spacing still need to be studied.

Evaluation of Turbulence Models for Airflow and Heat Transfer Prediction in BIPV/T Systems Optimization

Airflow patterns and convective heat transfer mechanisms in unglazed transpired solar collectors (UTCs) integrated with Photovoltaic-Thermal systems are crucial to their performance. Computational Fluid Dynamics (CFD) simulations can be a valuable tool for the design and analysis of these hybrid building-integrated solar energy systems while recent advances in CFD approaches and turbulence models provide a great potential for improving prediction accuracy. This study performs 3-dimensional steady Reynolds Averaged Navier-Stokes (RANS) CFD simulations and evaluates the performance of five turbulence closure models, potentially suitable for modeling UTCs in terms of accuracy and computing cost. These models include four two-equation models (Standard k-ɛ, Renormalization Group Methods k-ɛ, Realizable k-ɛ and Shear Stress Transport k-ω) and one Reynolds Stress Model. A flat UTC under free stream approaching flow conditions and a corrugated UTC subjected to a plane wall jet flow were considered. The predicted air velocity, air temperature, and turbulent kinetic energy were compared against experimental data from the literature and those obtained using a specially designed full-scale experimental set-up in a solar simulator. The results show that although the Reynolds Stress Model can provide more detailed flow features at an increased computational cost compared to other models, it does not necessarily result in better performance. Among the two- equation models, the Renormalization Group Methods k-ɛ has the best overall performance for both cases studied while the Standard k- ɛ and Shear Stress Transport k-ω models can also provide acceptable prediction accuracy. A BIPV/T system with two PV panel arrangements across a UTC under the same coverage was investigated using the CFD models developed in the present study. It was found that BIPV/T systems integrated with UTCs can result in electrical efficiency up to 1.2% higher, for the cases considered.

Evaluation of Various Turbulence Models in Predicting Airflow and Turbulence in Enclosed Environments by CFD: Part 1—Summary of Prevalent Turbulence Models

Air distribution in enclosed environments is crucial to thermal comfort and air quality. Computational fluid dynamics (CFD) has played an important role in evaluating and designing various air distribution. Many factors can influence the applications of CFD for studying air distribution. The most critical factors are the selection of an appropriate CFD approach and a turbulence model. Recent advances in CFD approaches and turbulence models provide great potential for improving prediction accuracy of air distribution in enclosed environments. This paper summarizes recent progress in CFD turbulence modeling and its application to some practical indoor environment studies. Also described are turbulence models that either are commonly used or have been proposed and used recently for indoor environment modeling. Finally, this study further identifies a few turbulence models that show great potential for modeling airflows in enclosed environments. A companion paper presents the evaluation of the selected models by using experimental data from the literature.

Guest editorial: CFD In Indoor Air

As a leading journal for members of the indoor environment community, Indoor Air strives for excellence in all published papers. For experimental findings or investigations with human subjects, measurement uncertainties and statistical significance of results are routinely reported. The editors of Indoor Air also wish to promote papers using Computational Fluid Dynamics (CFD) as a research tool. For such investigations, there are a number of corresponding measures that qualify the results and ensure quality. For that reason, we have been asked to suggest the minimum amount of information that should be included in CFD related papers submitted to the journal. Naturally, such suggestions will be biased by our personal experience, but we believe that we have covered the major topics of concern. Full assessment of the quality of a given CFD article requires 1) Adequate information pertinent to the governing equations and numerical methods, 2) justified estimates of the expected accuracy of the numerical results. If either of these is omitted, excellent studies may be considered poor and vice versa. Specifically, to ensure that all papers in Indoor Air are of high quality, our suggestion to the editors (and reviewers) is that CFD related articles contain the following information: The description of the CFD code, boundary/initial conditions, and turbulence model should be detailed enough so that the calculations can be reproduced by another investigator. Existing CFD codes should be fully cited. Topology and size of the computational grid should be described. The choice of differencing scheme should be described. Use of differencing schemes that are first order accurate (in space or time) is discouraged and solid justification should be provided if a first order scheme is used. Influence from grid-dependency should be addressed and, at a minimum, a qualitative estimate of the expected deviations from the exact solution to the governing equations must be given. The range of y+ should be stated and justified in accordance with the employed turbulence model. If possible, calculations should be validated against measurement or standard test cases of a similar problem. For researches, who routinely impose quality control in their work, it will be straightforward to comply with the above points. We do realize that, for the majority of CFD studies, significant human and computational efforts are necessary to meet the requirements. Nevertheless, we believe that the additional effort is more than compensated by the fact that the article (if accepted) is published in an archival journal that sets standards for CFD studies, therefore increasing the importance of the article. For more details on quality control for CFD calculations, see the paper by Sørensen and Nielsen (2003) in this issue of Indoor Air, as well as references herein. We strongly recommend the cited publication by the European Research Community on Flow, Turbulence and Combustion (ERCOFTAC), giving a valuable set of guidelines for CFD practitioners (Casey and Wintergerste, 2000).