Experimental and computational studies on the flow fields in aortic aneurysms associated with deployment of AAA stent-grafts

Pulsatile flow fields in rigid abdominal aortic aneurysm （AAA） models were investigated numerically, and the simulation results are found in good agreement with particle image velocimetry （PIV） measurements. There are one or more vortexes in the AAA bulge, and a fairly high wall shear stress exists at the distal end, and thus the AAA is in danger of rupture. Medical treatment consists of inserting a vascular stent-graft in the AAA, which would decrease the blood impact to the inner walls and reduce wall shear stress so that the rupture could be prevented. A new computational model, based on porous medium model, was developed and results are documented. Therapeutic effect of the stent-graft was verified numerically with the new model.

Experimental investigation on drag reduction in turbulent boundary layer over superhydrophobic surface by TRPIV

This study aims at the mechanism of drag reduction in turbulent boundary layer （TBL） with superhy- drophobic surface. Comparing the time-resolved particle image velocimetry （TRPIV） measurement results with that of hydrophilic surface, the drag reduction rate over a superhydrophobic surface is approximately 10%. To investigate the characteristics of coherent structure in a drag-reduced TBL with superhydropho- bic surface, a modified multi-scale spatial locally-averaged structure function is proposed for detecting coherent structure. The conditional sampling and spatial phase-lock average methods are employed to obtain the topology of physical quantities like the velocity fluctuation, spanwise vorticity, and Reynolds stress during eject and sweep process. The results indicate that the suppression of coherent structure burst in the near-wall region is the key mechanism in reducing the skin friction drag for TBL over super- hydrophobic surface.

Slippery Properties and the Robustness of Lubricant-impregnated Surfaces

Inspired by nepenthes pitcher plants,Lubricant-Impregnated Surfaces(LISs)are surfaces with lubricant infused in the textures which form slippery interfaces.In this paper,we investigated slippery properties and the robustness of LISs with different micro-texture topologies(i.e.,no textures,micro-pillar textures and micro hole textures),including original LISs and LISs rinsed by water.We measured the static contact angle,sliding angle and droplet motions on the LISs,using a contact angle instrument and a Particle Image Velocimetry(P1V)system.Similar contact angles and small sliding angles were observed on all original LISs,which indicated that an oil-layer existed on each LIS's interface.After rinsed by water,the sliding angle increased obviously and the slip velocity decreased,which meant that the LISs slippery properties deteriorated in different degrees.Among all the LISs in our experiments,LIS with micro pillar textures has the best slippery performance and robustness before and after rinsed.We found that the LISs'slippery properties were closely related to the states of the oil-layer on the interface,which were changed after rinsed.For the LIS with micro pillar textures,a thin layer of lubricant can sustain on the interface even after rinse,which made the droplet slide smoothly on the surface,with a lower sliding angle and a larger sliding velocity.This indicates that the proper micro-texture will enhance the slippery property and make it last longer.

Numerical simulation of polygonal particles moving in incompressible viscous fluids

A two-dimensional coupled lattice Boltzmann immersed boundary discrete element method is introduced for the simulation of polygonal particles moving in incompressible viscous fluids. A collision model of polygonal particles is used in the discrete element method. Instead of a collision model of circular particles, the collision model used in our method can deal with particles of more complex shape and efficiently simulate the effects of shape on particle–particle and particle–wall interactions. For two particles falling under gravity, because of the edges and corners, different collision patterns for circular and polygonal particles are found in our simulations. The complex vortexes generated near the corners of polygonal particles affect the flow field and lead to a difference in particle motions between circular and polygonal particles. For multiple particles falling under gravity, the polygonal particles easily become stuck owing to their corners and edges, while circular particles slip along contact areas. The present method provides an efficient approach for understanding the effects of particle shape on the dynamics of non-circular particles in fluids.

Mechanical behavior of pathological and normal red blood cells in microvascular flow based on modified level-set method

The research of the motion and deformation of the RBCs is important to reveal the mechanism of blood diseases. A numerical method has been developed with level set formulation for elastic membrane immersed in incompressible fluid. The numerical model satisfies mass and energy conservation without the leaking problems in classical Immersed Boundary Method(IBM), at the same time, computing grid we used can be much smaller than the general literatures. The motion and deformation of a red blood cell(including pathological & normal status) in microvascular flow are simulated. It is found that the Reynolds number and membrane's stiffness play an important role in the transmutation and oscillation of the elastic membrane. The normal biconcave shape of the RBC is propitious to create high deformation than other pathological shapes. With reduced viscosity of the interior fluid both the velocity of the blood and the deformability of the cell reduced. With increased viscosity of the plasma both the velocity of the blood and the deformability of the cell reduced. The tank treading of the RBC membrane is observed at low enough viscosity contrast in shear flow. The tank tread fixed inclination angle of the cell depends on the shear ratio and viscosity contrast, which can be compared with the experimental observation well.