Zhaohui Yao

Condensation and jumping relay of droplets on lotus leaf

Dynamic behavior of micro water droplet condensed on a lotus leaf with two-tier roughness is studied. Under laboratory environment, the contact angle of the micro droplet on single micro papilla increases smoothly from 80° to 160° during the growth of condensed water. The best-known “self-cleaning” phenomenon will be lost. A striking observation is the out-of-plane jumping relay of condensed droplets triggered by falling droplets, as well as its sustained speed obtained in continuous jumping relays. The underlying mechanism can be used to enhance the automatic removal of dropwise condensation without the help from any external force. The surface tension energy dissipation is the main reason controlling the critical size of jumping droplet and its onset velocity of rebounding.

A new family of high-order compact upwind difference schemes with good spectral resolution

This paper presents a new family of high-order compact upwind difference schemes. Unknowns included in the proposed schemes are not only the values of the function but also those of its first and higher derivatives. Derivative terms in the schemes appear only on the upwind side of the stencil. One can calculate all the first derivatives exactly as one solves explicit schemes when the boundary conditions of the problem are non-periodic. When the proposed schemes are applied to periodic problems, only periodic bi-diagonal matrix inversions or periodic block-bi-diagonal matrix inversions are required. Resolution optimization is used to enhance the spectral representation of the first derivative, and this produces a scheme with the highest spectral accuracy among all known compact schemes. For non-periodic boundary conditions, boundary schemes constructed in virtue of the assistant scheme make the schemes not only possess stability for any selective length scale on every point in the computational domain but also satisfy the principle of optimal resolution. Also, an improved shock-capturing method is developed. Finally, both the effectiveness of the new hybrid method and the accuracy of the proposed schemes are verified by executing four benchmark test cases.

Departure of Condensation Droplets on Superhydrophobic Surfaces

This article focuses on the departure of multidroplet coalescence on a superhydrophobic surface with nanoscale roughness. Out-of-plane jumping events triggered by multidroplet coalescence and a single fallen droplet are observed. Experimental data show that the departure of droplets due to the multidroplets coalescence and the jumping modes is dominant for the removal of condensed droplets from the substrate. The energy barrier is easier to overcome and the critical size of the self-propelled droplets could be further decreased in multidroplet coalescence jumping mode. A general theoretical model is developed which accounts quantitatively for determining the jumping velocity and the critical size of the multidroplet coalescence.

Sliding behavior of water droplet on superhydrophobic surface

We found experimentally that the advancing contact angles on micropillar-like superhydrophobic surfaces are hardly affected by the fraction of the water-solid interface area, while the receding contact angles and the sliding angles are strongly influenced by this geometrical parameter. Different from previous works, using particle image velocimetry (PIV) and particle tracking velocimetry (PTV) techniques, both rolling and slipping motions were observed for 5 μL water droplet during its sliding-down. We revealed that the rolling only occurs at the edge of the water droplet and the slip velocity is higher than the rolling velocity due to the low viscosity of water.

Experimental investigation of water flow in smooth and rough silicon microchannels

Water flow in smooth rectangular cross-section microchannels and microchannels with discrete rectangle roughness elements was experimentally investigated. The microchannels were microfabricated on silicon wafers and the hydraulic diameter ranged from 153 µm to 191 µm. The rectangle roughness elements mounted on the sidewall of the microchannel were 50 µm high and 50 µm wide. The global pressure drop and velocity field were measured for Reynolds numbers between 100 and 2300. For the smooth rectangular microchannels, the friction factor of the laminar flow agreed with the value predicted by classical theory. The transition from laminar to turbulent flow occurred at about Re = 2100. For the microchannels with roughness elements, the friction factor approaches the conventional value for Re 900. It indicates that the presence of roughness elements leads to the early transition in the microchannels. The streamwise mean velocity profiles and turbulent intensities obtained from microscopic particle image velocimetry (micro-PIV) show that the transition from laminar flow to turbulent flow occurred for Reynolds numbers ranging between 900 and 1100. This result was consistent with the experimental data of the friction factor. The experimental results of flow fields also suggest that the flow became fully developed turbulence for Re > 1400 in the rough microchannel. In the microchannel with roughness elements, large scale coherence structures are observed in the flow fields for Re < 2000.

Shape optimization of the diffuser blade of an axial blood pump by computational fluid dynamics.

Computational fluid dynamics (CFD) has been a viable and effective way to predict hydraulic performance, flow field, and shear stress distribution within a blood pump. We developed an axial blood pump with CFD and carried out a CFD-based shape optimization of the diffuser blade to enhance pressure output and diminish backflow in the impeller-diffuser connecting region at a fixed design point. Our optimization combined a computer-aided design package, a mesh generator, and a CFD solver in an automation environment with process integration and optimization software. A genetic optimization algorithm was employed to find the pareto-optimal designs from which we could make trade-off decisions. Finally, a set of representative designs was analyzed and compared on the basis of the energy equation. The role of the inlet angle of the diffuser blade was analyzed, accompanied by its relationship with pressure output and backflow in the impeller-diffuser connecting region.

Droplet Detachment by Air Flow for Microstructured Superhydrophobic Surfaces

Quantitative correlation between critical air velocity and roughness of microstructured surface has still not been established systematically until the present; the dynamics of water droplet detachment by air flow from micropillar-like superhydrophobic surfaces is investigated by combining experiments and simulation comparisons. Experimental evidence demonstrates that the onset of water droplet detachment from horizontal micropillar-like superhydrophobic surfaces under air flow always starts with detachment of the rear contact lines of the droplets from the pillar tops, which exhibits a similar dynamic mechanism for water droplet motion under a gravity field. On the basis of theoretical analysis and numerical simulation, an explicit analytical model is proposed for investigating the detaching mechanism, in which the critical air velocity can be fully determined by several intrinsic parameters: water-solid interface area fraction, droplet volume, and Youngs contact angle. This model gives predictions of the critical detachment velocity of air flow that agree well with the experimental measurements.

Low-speed gas flow subchoking phenomenon in a long-constant-area microchannel

The study on gas flow characteristics in a long-constant-area microchannel in mixed Knudsen-number-regime flows has not only theoretical meaning, but also important application in controlling outer-space aerocrafts. Flow characteristics were studied based on the experiment and approximate theoretical analysis. The inlet pressure was 130, 250, and 320 kPa, and the outlet pressure ranged from 9 to 100 kPa. Five pressure measuring points were distributed along the microchannel, and the temperature sensors were located at the inlet and outlet. The pressure distribution and the volume flow rate of air were measured experimentally. An approximate theoretical model based on Poiseuille flow was applied. Experimental investigations with the long-constant-area microchannel indicate that the mass flow rate through the microchannel changes little as the inlet-to-outlet pressure ratio reaches some critical pressure ratio. The phenomenon is defined as subchoking, and the corresponding pressure ratio is defined as the subchoking critical pressure ratio. The phenomenon of subchoking is caused by the surface effects. Moreover, the effects of the ratio of surface to volume on the critical pressure ratio are studied.

HEMOLYSIS ANALYSIS OF AXIAL BLOOD PUMPS WITH VARIOUS STRUCTURE IMPELLERS

Low hemolysis is an important factor for axial blood pumps that has been used in patients with heart failure. The structure of impellers plays a key role in the hemolytic properties of axial blood pumps. Axial blood pumps with various structure impellers exhibit different hemolytic characteristic. In the present study, we aimed to investigate the type of impellers structures in axial blood pumps that contain the best low hemolytic properties. Also, it is expensive and time-consuming to validate the axial blood pumps hemolytic property by in vivo experiments. Therefore, in the present study, the numerical method was applied to analyze the hemolytic property in a blood pump. Specifically, the hemolysis of the pump was calculated by using a forward Euler approach based on the changes in shear stress and related exposure times along the particle trace lines. The different vane structures and rotational speed that affect hemolysis were analyzed and compared. The results showed that long–short alternant vanes exhibited the best hemolytic property which could be utilized in the optimization design of axial blood pumps.

Evolutions of hairpin vortexes over a superhydrophobic surface in turbulent boundary layer flow

Turbulent flows over a superhydrophobic surface and a smooth surface have been measured and studied by particle image velocimetry technology at Reθ = 990. The Reynolds shear stress distributions over the two surfaces are significantly different. Specifically, for the superhydrophobic surface, the Reynolds shear stress is suppressed in the near-wall region (y/δ < 0.3, δ is the boundary layer thickness) and increases in the outer region (0.3 < y/δ < 0.5), which forms a second peak of the Reynolds shear stress curve. Evolutions of hairpin vortexes are analyzed to interpret differences in the Reynolds shear stress, based on some comparisons in the low-speed streaks and Q2/Q4 (ejection/sweep) events. The results show that, in the near wall region, the turbulent coherent structures (low-speed streaks and hairpin vortex) over the superhydrophobic surface are more stable and flat, due to the suppression in the strength and the lifting effect of the hairpin vortex. In the outer region, the superhydrophobic surface lifts the hairpin vortex away from the wall with a value of 0.14δ in our experiment, which makes the Q4 events occur further from the wall and contribute less to skin friction.