Abstract
The 3-D lattice Boltzmann method was applied to characterize roughness effects of oscillatory boundary layer flow over a rough bed. The direct numerical simulation was carried out and the flow resistance of the flat and fixed bed was investigated. The position of the theoretical bed, equivalent roughness height and the behavior of friction factor at small values of relative roughness were obtained using the log-fit method.
Highlights
In coastal area, the seabed is composed of different roughness elements in different places, such as sand, stone, coral reef and so forth
The equivalent roughness height and the friction factor could be obtained from the log-fit method, which was based on Eq 9
The term on the left-hand side was the ensemble- and space-averaged horizontal velocity. u* was the friction velocity, κ = 0.4, yt was the distance from the theoretical bed
Summary
The seabed is composed of different roughness elements in different places, such as sand, stone, coral reef and so forth. For the fixed rippled beds, it is often suggested ks is in proportion to ripple height Some investigation indicates it depends on the ripple steepness with the coefficient varying from 7.4 to 27.7 (Jonsson and Carlsen 1976; Grant and Madsen 1982; Nielsen 1992; Van Rijn 1993). Take the flat and fixed beds for instance, the coefficient 1 or 2.5 is often recommended, it still remains difficult to choose which one should be applied in practice It is often considered as the function of Reynolds number Rea and the relative roughness a / ks (Jonsson 1966), where a is the amplitude of freestream motion.
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