Some developments in turbulence modeling for wind and environmental engineering

doi:10.1016/j.jweia.2008.02.054

Journal of Wind Engineering and Industrial Aerodynamics

Volume 96, Issues 10–11, October–November 2008, Pages 1537-1570

https://doi.org/10.1016/j.jweia.2008.02.054 Get rights and content

Abstract

We present an overview of some developments in accommodating Reynolds-averaged Navier–Stokes (RANS) models to better predict complex high Reynolds and high Rayleigh number industrial and environmental flows. Considered are several novel modifications introduced into the second-moment and the eddy-viscosity framework. The treatment of wall boundary conditions is then discussed with focus on a recently proposed robust integration up to the wall (ItW), a generalization of the standard wall functions (GWF), and a compound wall treatment (CWT) that combines both of the concepts. Arguing that large-eddy simulation (LES) will not for long be reliable and convenient for large-scale industrial and environmental computations, we consider the transient RANS approach to flows subjected to strong forcing, as well as combing RANS with LES. Several controversial issues and approaches in hybrid RANS/LES are discussed. Various concepts and improvements are illustrated by examples of wind- and buoyancy-driven environmental flows.

Introduction

Computational fluid dynamics (CFD) for wind and environmental engineering continues to rely on the conventional Reynolds-averaged Navier–Stokes (RANS) methods. Their simplicity, robustness and computational economy have not yet been seriously challenged by any other turbulence simulation methods, and it is likely that RANS methods, in the present or improved form or in combination with, e.g. large-eddy simulation (LES), will continue to dominate the industrial and environmental CFD for some time to come.

However, industrial CFD tends to use oversimplified turbulence models, which are incapable of reproducing some key phenomena encountered in wind and environmental engineering such as flow impingement, bluff body wake, unsteady and three-dimensional (3D) boundary layer separation, effects of buoyancy and others. Accurate predictions of the impingement phenomenon and of the locations at which the flow separates and possibly reattaches, are prerequisites for computing the wind load on buildings. The same can be said for effects of thermal and mass buoyancy, which often dominate flow and pollutant transport in the environment. Validation of turbulence models and various modifications in such (and a number of other) generic flows prior to their application to real-life cases have shown that most of the popular models incorporated in the industrial CFD codes perform poorly, raising concern over the models’ applicability to real-life complex flows. On the other hand, a number of relatively simple model modifications, proven to yield notable improvements in generic flows, do not seem to appeal either to CFD vendors, or to the CFD users. Instead, some CFD vendors tend to pursue their own in-house eddy-viscosity model (EVM) versions. In both communities much interest has been shown recently for LES, which have been gaining in popularity and have been regarded by many as the future industrial tool for computing building and terrain aerodynamics. LES certainly possess many desirable features and, in principle, it is superior to RANS. It requires less empiricism and provides information about (large-scale) turbulence spectrum. However, this is still a very expensive technique because the reliable computations, especially of wall-bounded flows, require very fine grid resolution, and in the near future LES does not seem feasible for computing flow over real complex objects at realistic Reynolds (Re) and Rayleigh (Ra) numbers. LES is, however, a very useful tool for studying flow physics, vortical structures and turbulence, albeit in simplified geometries and at lower Re numbers. An important application of LES is to provide the input (unsteady velocity and pressure field) for computing the aerodynamic noise. In anticipation that advancements in computer design and further developments in the LES technique will make this approach more and more attractive for industrial computations, LES deserves to be seriously considered as a complementary tool for studying wind engineering.

We begin with a brief discussion of some developments in RANS modeling for complex flows. In focus are the EVMs, but in view of some recent progress in numerical treatment of advanced RANS models, we also consider some new concepts in second-moment closures. We turn then to some new developments in the treatment of the wall boundary conditions that include the integration to the wall (ItW), generalized wall functions (GWF), and a unified, compound wall treatment (CWT) that combines both concepts. Next, we consider some improvements in modeling buoyancy effects. The last sections are focused on accommodating and sensitizing RANS to capture some parts of turbulence spectrum. Considered are the transient RANS (T-RANS), which makes possible the capture of large-scale vortical structures such as found in flows dominated by thermal convection, as well as blending of RANS and LES into a unified technique that combines the advantage of both methods. The novelties discussed here are illustrated in several generic flows, as well as in several examples of realistic environmental flows.

Section snippets

A perspective on RANS and LES

The one-point turbulence closures for RANS equations have served for over three decades as the mainstay of industrial CFD. However, the most popular and most widely used linear eddy-viscosity models (LEVM) have serious fundamental deficiencies and cannot be trusted for predicting genuinely new situations of realistic complexity. Various modifications and new modeling concepts have been proposed, ranging from ad hoc remedies, complex non-linear eddy-viscosity approaches (NLEVM) to multi-equation

The RANS models: a brief overview of the status

Most RANS turbulence models can be classified into two major classes. The most widespread are the EVM, where the turbulent stress tensor $- \bar{u_{i} u_{j}}$ (we use now the overbar to denote Reynolds-averaged quantities) is expressed in terms of the mean rate of strain S_ij=0.5(∂U_i/∂x_j+∂U_i/∂x_j) and (in some models) of the mean vorticity Ω_ij=0.5(∂U_i/∂x_j−∂U_i/∂x_j); $\bar{u_{i} u_{j}} - \frac{1}{3} \bar{u_{k} u_{k}} δ_{ij} = f (ν_{t}, S_{ij}, Ω_{ij})$ where the kinematic eddy viscosity ν_t is usually defined in term of two or more turbulence properties for which

Robust elliptic relaxation EVMs

The υ²−f model of Durbin (1991) appeared as an interesting novelty in modeling the near-wall turbulence. By introducing an additional (“wall-normal”) velocity scale υ² and an additional elliptic relaxation concept to sensitize υ² to the inviscid wall blocking effect, the model dispenses with the conventional practice of introducing empirical damping functions. Because of its physical rationale and of its simplicity, it is gaining in popularity and appeal, especially among industrial users.

Hybrid RANS/LES

It is recalled that the proper resolution of dynamically important scales with LES requires the grid density to increase with Re^0.4 in regions away from a solid wall, but this constraints becomes much more severe in near-wall regions, where the grid density should follow Re^1.8. In contrast, a RANS grid requires clustering only in the wall-normal direction, making the grid requirements proportional to ln(Re). For realistic engineering and environmental flows an attractive proposition is to

T-RANS based VLES

Many flows are dominated by large-scale coherent structures that can have a (semi) deterministic character. Common examples are vortex shedding from bluff bodies or convective rolls and cells in thermal convection. In such flows, the stochastic turbulence often behaves as a passive scalar and has little influence on large-scale structures. It may thus suffice to resolve in space and time only the very large, semi-deterministic structure (VLES), which essentially governs the momentum and heat

Conclusion

Several novel developments in RANS and combined RANS/LES methods have been presented, aimed at improving accuracy, reliability, and robustness of computations of complex flows in wind and environmental engineering. First, two new versions of the near-wall RANS models based on the elliptic relaxation concept have been presented for robust integration up to the wall (ItW), one of the eddy-viscosity type (the ζ–f model) and one at the second-moment closure level (elliptic-blending model, EBM).

Acknowledgments

The authors thank M. Hadziabdic, R. Hagenzieker, and M. Popovac for providing their research results and figures.

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Some developments in turbulence modeling for wind and environmental engineering

Abstract

Introduction

Section snippets

A perspective on RANS and LES

The RANS models: a brief overview of the status

Robust elliptic relaxation EVMs

Hybrid RANS/LES

T-RANS based VLES

Conclusion

Acknowledgments

Int. J. Heat Fluid Flow

Int. J. Heat Fluid Flow

Int. J. Heat Fluid Flow

Energy

Int. J. Heat Fluid Flow

Int. J. Heat Mass Transfer,

Int. J. Heat Fluid Flow

Int. J. Heat Fluid Flow

Int. J. Heat Fluid Flow

Atmos. Environ.

Int. J. Heat Fluid Flow

Int. J. Heat Fluid Flow

Int. J. Heat Fluid Flow

Int. J. Heat Fluid Flow

Int. J. Heat Fluid Flow

A new hybrid modelling strategy for industrial CFD

Int. J. Num. Methods Fluids

Mean flow and turbulence measurements around 2-D array of buildings in a wind tunnel

Comparison of centerline velocity measurements obtained around 2D and 3D building arrays in a wind tunnel

A new Wall Function strategy for complex turbulent flows

Num. Heat Transfer B

Hybrid LES–RANS: an approach to make LES applicable at high Reynolds number

Large eddy simulations of non-equilibrium pulsed turbulent flow using transport equations subgrid scale model

Hybrid RANS–LES modeling with the renormalization group