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Evaluation of Various Turbulence Models in Predicting Airflow And Zhai, Z., Zhang, Z., Zhang, W., and Chen, Q. 2007. “Evaluation of various turbulence models in predicting airflow and turbulence in enclosed environments by CFD: Part-1: summary of prevent turbulence models,” HVAC&R Research, 13(6). 1 Evaluation of Various Turbulence Models in Predicting Airflow and 2 Turbulence in Enclosed Environments by CFD: Part-1: 3 Summary of Prevalent Turbulence Models 4 5 Zhiqiang Zhai Zhao Zhang Wei Zhang Qingyan Chen* 6 Member ASHRAE Student Member ASHRAE Member ASHRAE Fellow ASHRAE 7 8 Air distributions in enclosed environments are crucial to thermal comfort and air quality. 9 Computational fluid dynamics (CFD) has been playing an important role in evaluating and 10 designing various air distributions. Many factors can have influence on the applications of CFD 11 for studying air distributions. The most critical factors are the selection of an appropriate CFD 12 approach and a turbulence model. Recent advances in CFD approaches and turbulence models 13 provide a great potential of improving prediction accuracy of air distributions in enclosed 14 environments. This paper summarized recent progress in CFD turbulence modeling and 15 applications to some practical indoor environment studies. This paper also described the 16 turbulence models that either are commonly used or have been proposed and used recently for 17 indoor environment modeling. Finally, this study further identified a few turbulence models that 18 show great potential for modeling airflows in enclosed environments. A companion paper 19 presents the evaluation of the selected models by using experimental data from the literature. 20 21 INTRODUCTION 22 Enclosed environments, such as commercial, institutional, and residential buildings; 23 healthcare facilities; sport facilities; manufacturing plants; animal facilities; transportation 24 vehicles, are confined spaces with certain functionalities. It is essential to control air 25 distributions in the enclosed environments. The parameters of air distribution include, but not 26 limited to, air velocity, temperature, relative humidity, enclosure surface temperature, air 27 turbulence intensity, and concentrations of airborne gaseous, particulate, and liquid droplet 28 contaminants in the enclosed environments. The air distribution control is to create and maintain 29 a comfortable and healthy environment required by occupants and/or thermo-fluid conditions for 30 industrial processes in the enclosed environments. 31 Air distribution in an enclosed environment can be driven by different forces, for instance, 32 natural wind, mechanical fan, and/or thermal buoyancy. The combination of these flow 33 mechanisms (forced, natural, and mixed convection) creates complex indoor airflow 34 characteristics with impingement, separation, circulation, reattachment, vortices, buoyancy etc. 35 as illustrated in Figure 1. Most indoor environments have a low mean air velocity of less than 36 0.2 m/s, and the Reynolds number, Re, is generally very low (~105). The corresponding flow 37 regime may span from laminar, transitional, to turbulent flows or combination of all the flow 38 regimes under transient conditions. The complexity of indoor airflow makes experimental 39 investigation extremely difficult and expensive. * Zhiqiang Zhai is an assistant professor in the Department of Civil, Environmental & Architectural Engineering, University of Colorado, Boulder, CO. Zhao Zhang is a PhD candidate, Wei Zhang is an affiliate, and Qingyan Chen is a professor in the School of Mechanical Engineering, Purdue University, West Lafayette, IN. 1 1 With the rapid advance in computer capacity and speed, the Computational Fluid Dynamics 2 (CFD) technique has become a powerful alternative for predicting airflows in enclosed 3 environments. By solving the conservation equations of mass, momentum, energy, and species 4 concentrations, CFD can quantitatively calculate various air distribution parameters in an 5 enclosed environment. It offers richer details, a higher degree of flexibility, and lower cost than 6 experimental study. Nielsen (1974) was the first one who applied CFD for room airflow 7 prediction. Applications of CFD for airflows in enclosed spaces have been increasing since the 8 1980s. The International Energy Agency Annex 20, for instance, has sponsored a research 9 project on room airflow prediction with participants from 13 countries (Moser 1992). Jet flow Impingement Supply Separation Reattachment Exhaust Circulation Buoyancy 10 11 Figure 1. Typical flow characteristics in an enclosed environment with various flow 12 mechanisms. 13 14 CFD applications to airflow simulation for enclosed environments have achieved 15 considerable successes, as reviewed by Whittle (1986), Nielsen (1989, 1998), Liddament (1991), 16 Jones and Whittle (1992), Chen and Jiang (1992), Moser (1992), Lemaire et al. (1993), Ladeinde 17 and Nearon (1997), Emmerich (1997), Spengler and Chen (2000), Chen and Zhai (2004), and 18 Zhai (2006). These reviews concluded that CFD is a valuable tool for predicting air distributions 19 in enclosed environments. However, there are many factors influencing the results predicted. 20 Different users may obtain different results for the same problem even with the same computer 21 program. The accuracy of the simulation heavily depends on user's knowledge in fluid dynamics 22 and experience and skills in numerical techniques. Among various CFD influential factors, 23 proper selection of turbulence modeling method is a key issue that will directly affect the 24 simulation accuracy and efficiency. 25 Recent advance in CFD turbulence modeling methods may bring new potential of improving 2 1 the accuracy and efficiency of indoor airflow modeling. It is thus of great interest and value to 2 review the progress in CFD turbulence modeling and provide solid suggestions on proper 3 application of the models for indoor airflow simulation. The study consists of two parts. Part 1 4 (this paper) is to identify the most popular (and/or new) turbulence models that have been 5 (and/or have potential to be) used for modeling air distributions in enclosed environments by 6 searching the most recent literatures. Part 2 (a companion paper from Zhang et al. 2007) will 7 then evaluate systematically the identified models by comparing their prediction performance 8 against a series of benchmark experimental results so as to recommend appropriate turbulence 9 models for indoor environment modeling. 10 CFD APPROACHES 11 Generally, CFD predicts turbulent flows through three approaches: Direct Numerical 12 Simulation (DNS), Large Eddy Simulation (LES), and Reynolds-Averaged Navier-Stokes 13 (RANS) equation simulation with turbulence models. 14 DNS computes a turbulent flow by directly solving the highly reliable Navier-Stokes 15 equation without approximations. DNS resolves the whole range of spatial and temporal scales 16 of the turbulence, from the smallest dissipative scales (Kolmogorov scales) to the integral scale, 17 L (case characteristic length), which is associated with the motions containing most of the kinetic 18 energy. As a result, DNS requires a very fine grid resolution to capture the smallest eddies in the 19 turbulent flow. According to the turbulence theory (Nieuwstadt, 1990), the number of grid 20 points required to describe turbulent motions should be at least N ~ Re9/4. The computer systems 21 must become rather large (memory at least 1010 words and peak performance at least 1012 flops) 22 in order to compute the flow (Nieuwstadt et al., 1994). In other words, since the smallest eddy 23 size is about 0.01 m to 0.001 m in an enclosed environment, at least 1000 × 1000 × 1000 grids 24 are needed to solve airflow in a room. In addition, the DNS method requires very small time 25 steps, which makes the simulation extremely long. Neither existing nor near-future personal 26 computers can meet the needs so that application of DNS for indoor flows is not feasible now or 27 in the near future. 28 According to the Kolmogorov's theory of self similarity (Kolmogorov, 1941), large eddies of 29 turbulent flows depend on the geometry while the smaller scales are more universal. 30 Smagorinsky (1963) and Deardorff (1970) developed LES with the hypothesis that the turbulent 31 motion could be separated into large-eddies and small-eddies such that the separation between 32 the two does not have a significant impact on the evolution of large-eddies. The large-eddies 33 corresponding to the three-dimensional, time-dependent equations can be directly simulated on 34 existing computers. Turbulent transport approximations are made for small-eddies, which 35 eliminates the need for a very fine spatial grid and small time step. The philosophy behind this 36 approach is that the macroscopic structure is characteristic for a turbulent flow. Moreover, the 37 large scales of motion are primarily responsible for all transport processes, such as the exchange 38 of momentum and heat. The success of the method stems from the fact that the main contribution 39 to turbulent transport comes from the large-eddy motion. Thus the large-eddy simulation is 40 clearly superior to turbulent transport closure wherein the transport terms (e.g. Reynolds stresses, 41 turbulent heat fluxes, etc.) are treated with full empiricism. In the last decade, rapid advance in 42 computer capacity and speed has made it possible to use LES for some airflows related to 43 enclosed environments. LES provides detailed information of instantaneous airflow and 44 turbulence
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