Turbulence Models for Sediment Transport Engineering

Abstract

In problems of civil engineering interest, sediment transport invariably occurs under turbulent-flow conditions. Traditional discussions (ASCE Manual 54; see also Chapter 2) of turbulence models have, however, been mainly restricted to the problem of determining the vertical distribution of suspended sediment in the simplest case of uniform channel flow over a plane bed. Since the appearance of ASCE Manual 54 in 1975, there have been considerable advances in our understanding and hence modeling of complex turbulent flows, and these might be expected to have a positive impact on approaches to practical problems in sediment transport. In general, the scope of the problems that can be studied has been broadened substantially, and a larger range of engineering problems can be tackled with reasonable success. The more traditional basic questions have proven more refractory and progress in answering them has been correspondingly limited. Nevertheless, with ever-growing computational capabilities and the proliferation of commercial computational fluid dynamics (CFD) software packages, numerical modeling with turbulence models will in the foreseeable future become an increasingly important engineering tool in dealing with sediment transport problems. Hence, a basic familiarity with such models, their theoretical bases, and their limitations will be useful. The present chapter describes the standard turbulence models currently being applied to problems involving sediment transport, focusing on so-called two-equation models, but also discussing more briefly simpler models that might be used judiciously for special problems, as well as more advanced models that may find more practical application in the future. In most applications thus far, the turbulence model has been taken without modification from fields where the use of these models is more solidly established, but where possibly important features unique to sediment transport are absent. The standard features, assumptions, and limitations of turbulence models are discussed in a number of monographs (Rodi 1993; Hallback et al. 1996; Chen and Jaw 1998; Wilcox 1998; Piquet 1999; Durbin and Petterson Reif 2001), as well as review articles (Launder 1984; Speziale 1991; Hanjalic 1994; Rodi 1995; Speziale 1996; Launder 1996; Spalart 2000), but, except for the review by the ASCE Task Committee on Turbulence Models in Hydraulic Computations (1988) and the works of Rodi, hydraulic or sedimentation engineering applications have not received much attention (see, however, the brief review by Lane 1998). General references on multiphase flows are also available (e.g., Crowe et al. 1998) that discuss aspects relevant to turbulent particulate flows, often, however, with gas-solid or bubbly flows in mind. Although the present chapter will necessarily rely heavily on these works, it will elaborate issues that may be of particular relevance to sediment-transport engineering applications. Turbulence modeling, especially for the very complicated problem involving sediment transport, is an extensive field undergoing continual development, and the present chapter can only serve as an introduction to the subject, targeted at a sedimentation engineering audience. For the most part, the discussion is restricted to classical problems in sediment-transport mechanics, emphasizing noncohesive uniform-sized sediment. The important case of depth-averaged models, in which complicating issues other than but closely related to turbulence modeling arise, is covered briefly in an appendix.