Experiments were conducted to systematically investigate the nearly homogeneous turbulence generated by multiple oscillating grids. Altogether 54 runs were completed by employing five different grids. For each run, four to nine layers of grids were installed, which were operated with different frequencies, strokes and grid spacings. The particle image velocimetry was used to obtain flow velocity characteristics. Instantaneous flow measurements reveal that the flow structure varies depending on the grid configuration. For regular grids, turbulent wakes of nearly uniform size were generally observed. In comparison, for double-scale grids, the size of wakes varied with the bar width. In the near-grid region, the horizontal distributions of Reynolds stresses are strongly influenced by the grid geometry, showing obvious inhomogeneity especially for wide bars. The inhomogeneity reduces as the distance from the grid increases. At the midplane between two neighboring grids, the flow characteristics appear similar for all grid configurations. The size of the region where the turbulence appears nearly homogeneous is affected by the grid spacing and stroke. Generally, the double-scale grid can produce a greater homogeneous region than the regular grid. This study shows that the decay law of turbulence intensity developed for a single oscillating grid can be successfully extended to multiple oscillating grids. In addition, a theoretical interpretation of the decay law is presented by considering the variation in the velocity and size of wakes when evolving away from the grid.

Sediment suspension is vital to the geomorphological and ecological processes in rivers. However, previous research has mainly focused on the suspension of sediments with uniform grain sizes, neglecting the complexities arising from sediment mixtures. This study investigates the suspension behaviors of both uniform and mixed-size sediment grains in an oscillating grid turbulence environment. The experimental apparatus comprised a seven-panel grid system, which oscillated at varying frequencies to generate homogeneous turbulence within a water column. The results revealed that for uniform sediments, the sediment diffusion coefficient increases nonlinearly with oscillating frequency and particle size, and for sediment mixtures, the suspension of particles is influenced not only by particle inertial effect but also by the interaction between coarse and fine particles. The presence of coarse grains leads to an enhancement in the sediment diffusion coefficient for fine grains, whereas the reverse influence is minimal. Additionally, the size fraction for the suspended sediment is influenced by various factors, including the oscillating frequency, the height of suspension, and the type of the sediment bed. This study also investigates the mechanisms of sediment-carrying capacity from an energy conservation perspective. It shows that the power required to keep sediment in suspension can be adopted as a new parameter for assessing sediment-carrying capacity. These insights are expected to have broader implications for the comprehension of suspended sediment transport in rivers.

Modeling the rheological response of coastal mud under oscillatory shear loading is essential for understanding wave-mud interactions and the dynamics of muddy seabeds. Recent studies have shown that mud exhibits a characteristic two-step yielding process. However, existing two-step yielding models have difficulty of capturing the evolution of viscoelastic behavior during the yielding process under oscillatory shear loading. Based on rheological experiments, this study developed a rheological model for the two-step yielding behavior of coastal mud using fundamental elements of viscoelastoplastic mechanics. The corresponding rheological response of the model under oscillatory shear loading is then derived analytically. In the solid state, the model reduces to a Kelvin–Voigt model; in the solid–fluid transition state, it reduces to two Kelvin–Voigt elements connected in series; and in the fluid-like state, it reduces to a Jeffreys model. Empirical relationships among the model parameters, the two yield stresses, and solid volume fraction were established for the tested mud. The model was further evaluated using published data. Results show that the proposed model captures the main features of the rheological response of coastal mud under oscillatory shear loading. This study provides scientific support for the mechanistic characterization of the oscillatory rheological behavior of coastal mud and for subsequent investigations of wave-mud interaction.

The logarithmic law of the wall is a foundational element in turbulence modeling. Its classical derivation, rooted in Prandtl’s mixing-length theory, is predicated on the existence of a constant shear stress layer. This paper demonstrates that this foundational assumption is not strictly satisfied for two-dimensional, uniform open-channel flows, where the Reynolds shear stress profile exhibits a distinct peak near the bed and never forms a true constant stress zone. We present a novel mechanistic derivation that circumvents this inconsistency. By reframing the bed shear stress as the time-averaged momentum flux from discrete, wall-coherent eddy impacts, we recover the log law through a mechanistic framework. Our model starts from the physical definition of the Reynolds stress at the bed, employs kinematic scaling for the velocity fluctuations, and incorporates the geometric constraint of eddy size. This approach does not require a constant stress layer and provides a more physically defensible explanation for the emergence and robustness of the log-law, directly linking it to the underlying structure of wall turbulence. The derivation resolves the long-standing paradox between the theory’s assumption and the empirical reality in open channel flows.