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Microfluidic droplets have emerged as novel platforms for chemical and biological applications. Manipulation of droplets has thus attracted increasing attention. Different from solid particles, deformable droplets cannot be efficiently controlled by inertia-driven approaches. Here, we report a study on the lateral migration of dual droplet trains in a double spiral microchannel at low Reynolds numbers. The dominant driving mechanism is elucidated as wall effect originated from the droplet deformation. Three types of migration modes are observed with varying Reynolds numbers and the size-dependent mode is intensively investigated. We obtain empirical formulas by relating the migration to Reynolds numbers and droplet sizes. The effect of droplet deformability on the migration and the detailed migration behavior along the double spiral channel are discussed. Numerical simulations are also performed and yielded in qualitative agreement with the experiments. could be a promising alternative to existing inertia-driven approaches bio-particles. This proposed low Re approach based on lateral migration especially concerning deformable entities and susceptible

A forward-facing step （FFS） immersed in a subsonic boundary layer is studied through a high-order flux reconstruction （FR） method to highlight the flow transition induced by the step. The step height is a third of the local boundary-layer thickness. The Reynolds number based on the step height is 720. Inlet disturbances are introduced giving rise to streamwise vortices upstream of the step. It is observed that these small-scale streamwise structures interact with the step and hairpin vortices are quickly developed after the step leading to flow transition in the boundary layer.

In recent years studies of aquatic locomotion have provided some remarkable insights into the many features of fish swimming performances. This paper derives a scaling relation of aquatic locomotion CD（Re）2 = （Sw）2 and its corresponding log law and power law. For power scaling law, （Sw）2 = βn Re 2-1/n, which is valid within the full spectrum of the Reynolds number Re = UL/v from low up to high, can simply be expressed as the power law of the Reynolds number Re and the swimming number Sw = ωAL/v as Re ∝ （Sw）σ, with σ = 2 for creeping flows, σ = 4/3 for laminar flows, σ = 10/9 and σ = 14/13 for turbulent flows. For log law this paper has derived the scaling law as Sw ∝ Re/（ln Re ＋ 1.287）, which is even valid for a much wider range of the Reynolds number Re. Both power and log scaling relationships link the locomotory input variables that describe the swimmer＇s gait A, ω via the swimming number Sw to the locomotory output velocity U via the longitudinal Reynolds number Re, and reveal the secret input-output relationship of aquatic locomotion at different scales of the Reynolds number.

A sea spray generation function （SSGF） for bubble-derived droplets that takes into account the impact of wave state on whitecap coverage was presented in this study. By combining the new SSGF with a previous wave-state-dependent SSGF for spume droplets, an SSGF applicable to both bubble-derived and spume droplets that includes the impacts of wave state was obtained. The produced SSGF varies with surface wind as well as with wave development. As sea surface wind increases, more sea spray droplets are produced, resulting in larger SSGFs and volume fluxes. Meanwhile, under the same wind conditions, the SSGF is mediated by wave state, with larger SSGFs corresponding to older waves and larger windsea Reynolds numbers. The impact of wave state on sea spray heat flux was then estimated by applying this SSGF while considering the thermodynamic feedback process. Under given atmospheric and oceanic conditions, the estimated sea spray heat flux increases with wind speed, wave age, and windsea Reynolds number.

A lattice Boltzmann (LB) model with overall second-order accuracy is applied to the 1.5-layer shallow water equation for a wind-driven double-gyre ocean circulation. By introducing the second-order integral approximation for the collision operator, the model becomes fully explicit. In this case, any iterative technique is not needed. The Coriolis force and other external forces are included in the model with second-order accuracy, which is consistent with the discretized accuracy of the LB equation. The numerical results show correct physics of the ocean circulation driven by the double-gyre wind stress with different Reynolds numbers and different spatial resolutions. An intrinsic low-frequency variability of the shallow water model is also found. The wind-driven ocean circulation exhibits subannual and interannual oscillations, which are comparable to those of models in which the conventional numerical methods are used.