The Berendsen barostat from molecular dynamics simulation is applied in both standard dissipative particle dynamics (DPD) and many-body dissipative particle dynamics (MDPD) simulations. The original Berendsen barostat works well in (M)DPD simulation of a single-component system under constant pressure condition and in nonequilibrium dynamic processes. The partial Berendsen barostat is proposed for multi-component system simulation with (M)DPD. The displacement rescaling process of the Berendsen barostat is only applied on the particles outside the center region, acting as a pressure "boundary condition." The center part forms the free zone, in which the interface shape and nonequilibrium dynamic behavior between different phases can be captured properly. An immiscible bubble in the second fluid under constant pressure condition is studied, and the oscillation of the bubble radius and fluctuation of systempressure can be obtained by the current barostat. Preliminary models for bubble growing and collapsing under square pressure wave and bubble oscillation under harmonic pressure wave are also reported in the current simulation. It shows that the partial Berendsen barostat is suitable for the modeling of nonequilibrium process of single or few droplets/bubbles in multi-component systems.

Computer modeling of complex fluid flows usually presents great challenges for conventional grid-based numerical methods. Smoothed particle hydrodynamics (SPH) is a meshfree Lagrangian particle method and has special advantages in modeling complex fluid flows, especially those with large fluid deformations, fluid-structure interactions, and multi-scale physics. In this paper, we review the recent developments of SPH in methodology and applications for modeling complex fluid flows. Specifically, in methodology, some important issues including modified SPH particle approximation schemes for improving discretization accuracy, different particle regularization techniques, and various boundary treatment algorithms for solid boundary, free surface, or multiphase interface are described. More importantly, the SPH method with ideas from the dissipative particle dynamics for complex fluids in macro- or meso-scales is discussed. In applications, different complex fluid flows, including biological flows, microfluidics and droplet dynamics, non-Newtonian fluid flows, free surface flows, multiphase flows, and flows with fluid-structure interaction, are reviewed. Some concluding remarks in SPH modeling of complex fluid flows are provided.

No abstract

Circulating tumor cells (CTCs) in blood is the direct cause of tumor metastasis. The isolation and detection of CTCs in the whole blood is very important and of clinical value in early diagnosis, postoperative review, and personalized treatment. It is difficult to separate all types of CTCs that efficiently rely on a single path due to cancer cell heterogenicity. Here, we designed a new kind of "filter chip" for the retention of CTCs with very high efficiency by integrating the effects of cell size and specific antigens on the surface of tumor cells. The filter chip consists of a semicircle arc and arrays and can separate large-scale CTC microspheres, which combined with CTCs automatically. We synthesized interfacial zinc oxide coating with nanostructure on the surface of the microsphere to increase the specific surface area to enhance the capturing efficiency of CTCs. Microspheres, trapped in the arrays, would entrap CTCs, too. The combination of the three kinds of strategies resulted in more than 90% capture efficiency of different tumor cell lines. Furthermore, it is easy to find and isolate the circulating tumor cells from the chip as tumor cells would be fixed inside the structure of a filter chip. To avoid the high background contamination when a few CTCs are surrounded by millions of nontarget cells, a digital detection method was applied to improve the detection sensitivity. The CTCs in the whole blood were specifically labeled by the antibody−DNA conjugates and detected via the DNA of the conjugates with a signal amplification. The strategy of the antibody−functional microsphere-integrated microchip for cell sorting and detection of CTCs may find broad implications that favor the fundamental cancer biology research, the precise diagnosis, and monitoring of cancer in the clinics.

We study numerically the propulsive wakes produced by a flapping foil. Both pure pitching and pure heaving motions are considered, respectively, at a fixed Reynolds number of Re = 1700. As the major innovation of this paper, we find an interesting coincidence that the efficiency maximum agrees well with the 2D-3D transition boundary, by plotting the contours of propulsive efficiency in the frequency-amplitude parametric space and comparing to the transition boundaries. Although there is a lack of direct 3D simulations, it is reasonable to conjecture that the propulsive efficiency increases with Strouhal number until the wake transits from a 2D state to a 3D state. By comparing between the pure pitching motion and the pure heaving motion, we find that the 2D-3D transition occurs earlier for the pure heaving foil than that of the pure pitching foil. Consequently, the efficiency for the pure heaving foil peaks more closely to the wake deflection boundary than that of the pure pitching foil. Furthermore, since we have drawn the maps on the same parametric space with the same Reynolds number, it is possible to make a direct comparison in the propulsive efficiency between a pure pitching foil and a pure heaving foil. We note that the maximum efficiency for a pure pitching foil is 15.6%, and that of a pure heaving foil is 17%, indicating that the pure heaving foil has a slightly better propulsive performance than that of the pure pitching foil for the currently studied Reynolds number.

Interface-resolved direct numerical simulations of downward particle-laden turbulent channel flows are performed by using a direct-forcing fictitious domain method. The effects of the particle settling coefficient, the density ratio (2, 10, and 100), and the particle size on fluid-turbulence interactions are investigated at a bulk Reynolds number of 5746 and a particle volume fraction of 2.36%. Our results indicate that the significant particle-induced reduction in the turbulence intensity does not take place for the downflow at a low density ratio of 2, and the turbulence intensity generally increases with an increasing particle Reynolds number at the same other control parameters, unlike the upflow case. The total turbulent kinetic energy (TKE) in the channel is larger for the downflow than for the upflow at the same particle Reynolds number, whereas the TKE at the channel center is roughly independent of the flow direction when the particle inertia is very large. For a density ratio of 2, the particles aggregate and are preferentially located in the low-speed streaks in the near-wall region, whereas for a density ratio of 10, the particles migrate toward the channel center, similar to the zero-gravity case. The flow friction increases with an increasing settling coefficient for the same density ratio and particle size, and the friction at the density ratio of order (10) is smallest. The pair distribution function shows the transition from the turbulence-dominated feature to the sedimentation-dominated feature, as the settling coefficient increases.

No abstract

a b s t r a c tNumerical simulations are used to study the energy harvester based on a semi-active flapping foil, in which the profile of the pitching motion is prescribed and the heaving motion is activated by the vertical hydrodynamic force. We consider a two-dimensional NACA0015 airfoil with the Reynolds number Re ¼ 1000. First, for the sinusoidal pitching, an optimal combination of the parameters of pitching amplitude q 0 ¼ 75 and reduced frequency f * ¼ 0.16 is identified, with the highest energy harvesting efficiency of 32% being recorded. Then we study non-sinusoidal pitching, with a gradual change from a sinusoid to a square wave as b is increased from one. We find that its effect of efficiency enhancement is limited for the parameters approaching their optimal values, and the upper boundary of the efficiency appears not to be increased. In detail, we report that when the pitching amplitude is small, nonsinusoidal pitching motions can indeed improve the performance of the system. However, when both the pitching amplitude and the flapping frequency are close to their optimal values, non-sinusoidal pitching motions contribute negatively to the harvesting efficiency. We suggest that a non-sinusoidal profile, at least a simple trapezoidal-like one is ineffective in the semi-active system reported by the current study.