ABSTRACT
In large-eddy simulations the energy-containing motions are calculated directly, which allows an explicit simulation of the unsteady, large-scale turbulence structures in the flow. Large-scale turbulence structures, also called “coherent structures” or “quasi-coherent structures”, have been the subject of many experimental studies (e.g. pioneering work by Kline et al., 1967; Corino and Brodkey, 1969; Grass, 1971, Willmarth and Lu, 1972) and have been identified in these early works on boundarylayer turbulence mainly through visualization or conditional sampling techniques. Though no clear definition of “coherent structures” exists, it is commonly agreed that a coherent structure is a region of space (or time) within which the flow field has a characteristic coherent flow pattern (Pope, 2000). In his review article, Robinson (1991) provides the following definition: “a three-dimensional region of the flow over which at least one flow fundamental variable exhibits significant correlation with itself or with another variable over a range of space and/or time that is significantly larger than the smallest scale of the flow”. Nezu and Nakagawa (1993) introduce and discuss coherent structures in the context of hydraulic engineering open-channel flows and they distinguish between “bursting-structures” near the channel bed and “large-scale vortical structures” that are generated by the mean flow and/or the channel geometry away from the bed. In Table 8.1, which builds upon the categorization according to Robinson (1991), the most common coherent structures occurring in open-channel flow or in flows of hydraulic engineering interest are listed. The first five structures are considered “bursting-structures”, whereas the last seven are categorized as “large-scale vortical structures”.
