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Abstract

The permeability and resistance characteristics in porous structures with different grain size and shape have been studied experimentally in the paper. Two materials used in the study are crushed gravels and spherical glass balls with four sizes of 5.2 mm, 7. 4 mm, 12.2 mm, and 31.7 mm for gravels and 13.0 mm, 16.0 mm, 25. 0 mm, and 35.0 mm for balls, respectively. The permeability determined from linear Darcy Law shows a strong dependence on flow velocity in addition to grain sizes and shapes. This is not consistent to the traditional definition of permeability that is only a function of material and fluid. On the other hand, if the resistance force in porous media is quantified by the sum of linear frictional drag and nonlinear form drag suggested by Ward (1964) the turbulent frictional coefficient shows a similar trend as that of Shield Curve. The authors analyze the experimental raw data via former experienced equation (Sollitt and Cross, 1972) and compare with a new equation derived according to real physical concept considering total combined drag forces. This indicates that the intrinsic permeability change significantly as the grain size Reynolds number varies and is not constant in a non-uniform flow field such as in a wave field. Results show that porous-structure characteristics like intrinsic permeability and turbulent coefficient tend to stabilize under high Reynolds number, and description with new equation is better than former equation. The characteristics of resistance force in porous media are therefore deserved further study in order to improve related applications on coastal porous structures.

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