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We conducted laboratory experiments and numerical simulations to investigate the formation and evolution of drops formed by partially miscible two-phase fluid, n-butanol (the continuous phase) and water (the dispersed phase), in a flow focusing microfluidic system. We carefully calibrated the numerical model to obtain good agreement with experimental data in drop velocity and mass transfer, demonstrating the model's capability to capture realistic drop dynamics. Our detailed investigation of the numerical results allowed us to determine the mechanism of drop formation and obtain a relevant criterion in terms of the disperse-to-continuous flow ratio beyond which the tubing patterns would occur. Additionally, we found that the mass transfer between the two phases, specifically at the drop interface, strongly depends on the local distribution of dissolved concentration of the dispersed phase. To enhance mass transfer, we conducted numerical simulations on alternating curved channels, which allows for the lateral advection of the dispersed phase concentration in the continuous phase at the curved section. We found that this lateral movement enhances mass transfer at the drop interface. Through detailed investigation of numerical results, we addressed mechanisms of mass transfer enhancement in the curved channel. Overall, our findings provide insight into the mechanisms of drop formation and mass transfer in partially miscible two-phase fluids in microfluidic systems, which could be useful in designing and optimizing such systems for various applications. [ABSTRACT FROM AUTHOR]

This study addresses the rolling and lifting probabilities for sediment entrainment by incorporating the probabilistic features of the turbulent fluctuation and bed grain geometry. The lognormally distributed instantaneous velocity and uniformly distributed initial grain position, along with a relation between lift coefficient and particle Reynolds number, are used to extend the theoretical formulation of the entrainment probabilities in smooth-bed, flows. The two threshold conditions identified herein enable us to precisely define the probabilities of entrainment in the rolling and lifting modes. The results obtained in this study coincide well with the published data. The lifting probability increases monotonously with the dimensionless shear stress [theta], which is consistent with the earlier results yet displays improved agreement with the experimental data. The maximum value of rolling probability, with a magnitude of 0.25, occurs at [theta] [approximately equal to] 0.15. For [theta] < 0.05 (or [theta] > 0.6), the rolling (or lifting) probability makes up more than 90% of the total entrainment probability and thus can be used as an approximation to the total probability of entrainment. The proposed rolling and lifting probabilities are further linked to the two separate criteria for incipient motion to explore the critical entrainment probabilities. The results reveal that a consistent probability corresponding to the critical state of sediment entrainment cannot be found. DOI: 10.1061/(ASCE)0733-9429(2003) 129:2(110) CE Database keywords: Entrainment; Sediment; Probability distribution; Drag coefficient; Lift coefficients.

This paper investigates the variation of the vegetative roughness coefficient with the depth of flow. A horsehair mattress is used in the experimental study to simulate the vegetation on the watercourses. Test results reveal that the roughness coefficient reduces with increasing depth under the unsubmerged condition. However, when fully submerged, the vegetative roughness coefficient tends to increase at low depths but then decrease to an asymptotic constant as the water level continues to rise. A simplified model based on force equilibrium is developed to evaluate the drag coefficient of the vegetal element; Manning's equation is then employed to convert the drag coefficient into the roughness coefficient. The data of this study are compared with those of selected previous laboratory and field tests. The results show a consistent trend of variation for the drag coefficient versus the Reynolds number. This trend can be represented by a vegetative characteristic number k. Given information such as the bed slope, the height of vegetation, and k, one can apply the proposed model to predict the roughness coefficient corresponding to different flow depths.

In this study, numerical simulations of the interaction between supersonic crossflow and a side liquid jet were performed. The processes of the liquid jet breakup, flow separation and shock waves resulting from the interaction of the gas flows and liquid droplets were predicted. In the numerical simulation, a two-way coupling Eulerian-Lagrangian method was used. The two-dimensional unsteady compressible Navier-Stokes Equations were solved based on the MUSCL type (Monotone Upstream-Centred Scheme for Conservation Laws) spatial differencing coupling with Roe-type flux-difference splitting and a third-order accurate Runge-Kutta method for time-splitting. The atomization of liquid droplets was simulated using the combined KH-RT breakup model. The modified breakup model considered both the interface instability and the sudden deformation of a liquid spray induced by the compressible flows. Shock waves, recirculation zones and breakup processes of droplet particles were captured. The predicted droplet distributions at various operating conditions were discussed. Satisfied agreement between simulations and measurements is shown in the current work. [ABSTRACT FROM AUTHOR]