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In the present study, mesopycnal flows are investigated using direct numerical simulations. In particular, intrusive density- and particle-driven gravity currents in the lock exchange setup are simulated with the high-order finite-difference framework Xcompact3d. To account for the settling velocity of particles, a customized Fick's law for the par...
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By using direct numerical simulations (DNS) of bubbly flows with passive scalars, we show a transition in the scalar spectra from a $k^{-5/3}$ to a $k^{-3}$ scaling with the wavenumber $k$ , in contrast with those of single-phase isotropic turbulence. For cases with a mean scalar gradient in the horizontal direction, the scalar spectrum decays fast...
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... 24 A better understanding that aims at the fluid dynamics of the gravity current over the rough and permeable substrate and the analysis of its propagation evolution is needed to manage the corresponding risk posed to infrastructure on the seafloor, including gas pipelines and communication cables. [27][28][29] Previous studies have extensively investigated the influence of bed-boundary conditions on the macroscopic evolution of gravity current. Considering bed roughness factors only, physical models of different shapes were used to simulate rough elements through laboratory experiments, such as pebbles, 26,30 trigonal blocks, [31][32][33] cylinders, 16,34,35 discrete square strips, [36][37][38] and solid spheres. ...
A series of constant-flux saline and turbidity current experiments were carried out, focusing on the coupling impact of bed roughness and permeability on current propagation, mixing, and turbulence characteristics. The distinct current propagation phases on RI (rough and impermeable) and RP (rough and permeable) beds are identified respectively. Experimental results revealed that intermittently undulating bed surface breaks the strict no-slip boundary, thereby increasing local current velocity near the bed, while its roughness reduces the current peak profile velocity. Interbed pores of rough bed induce vertical fluid exchange, which synchronously decrease the current velocity, and causes the density profile no longer follow a monotonous variation trend along with water depth. The larger bed surface roughness or the interbed porosity leads to the smaller upper TKE (Turbulent Kinetic Energy) peak. On the other hand, the lower TKE peak is inversely proportional to the bed surface roughness of the RI beds but proportional to the porosity of the RP bed. Rough bed surface intensifies the asymmetry of the mean velocity distribution around peak velocity, resulting in a transfer barrier of turbulent momentum triggered by the interbed pores. For the RP bed, the cross-correlation function based on two-point statistics captures the spikes associated with pore-scale eddies locally, while on the RI condition the function only obtains the time scale characterizing the largest eddies of the current. Sediment deposition makes the turbidity current easier to separate from the RP and RI bed than the saline type, as a consequence of increasing the current height.
This study experimentally investigated the impacts of rough and porous (RP) bed and sedimentation processes on the coherent structures, turbulence intermittency, and anisotropy of saline and turbidity currents. Results reveal that the local current concentration responds immediately (saline current) or languidly (turbidity current) to turbulence bursting events. Inside the dense current, the turbulent momentum fluxes in streamwise and vertical directions transfer downstream and downward, which favor the sweep events. Inside the ambient water, the turbulent momentum fluxes in streamwise and vertical directions transfer upstream and upward, contributing to the formation of ejection events. At the current-ambient water interface, the turbulent momentum fluxes in streamwise and vertical directions do not tend to transfer in particular directions resulting in almost equal quantities of sweep and ejection events. The Gram-Charlier series expansion is strictly applicable to probability density functions (PDFs) of the sweep and ejection events but not suitable ideally for PDFs of the outward and inward interaction events. The primary anisotropy invariant map (AIM) of gravity currents starts from the two-component plain strain limit (near the bed). It is followed by the three-dimensional isotropy (inside the dense current and ambient water) and the axisymmetric contraction limit (current-ambient water interface). Finally, it ends in two-dimensional isotropy (near the free surface). This AIM is sensitive to the RP boundary and the sedimentation processes. Along the streamwise direction, the RP boundary causes alternations between the anisotropic and isotropic turbulence, but the arranged pattern of the rough units determines the period of this alternation.
