Modeling and control of MR damper considering trapped air effect

Due to the high viscosity of magnetorheological(MR)fluid,eliminating air pockets dissolved in the fluid is very difficult,which results in a force lag phenomenon.In order to evaluate the performance of a semi-active control system based on the MR damper considering the trapped air effect,a performance test on a MR damper is carried out under different loading cases,and the influence of the input current,excitation amplitude and frequency on the force lag phenomenon is analyzed.A concise and efficient parametric model,combining the simple Bouc-Wen model and a spring with small stiffness,is proposed to portray the experimental characteristics of the MR damper with force lag,and then the response analysis of the semi-active controlled single-degree-of-freedom(SDOF)structure is performed using the classic clipped-optimal control strategy based on acceleration feedback.Numerical results show that the trapped air in the MR fluid can weaken the control effect of the MR damper,and the performance of the semi-active control system will be reduced more obviously and become close to the passive-off control with the increasing content of air trapped in the MR fluid.

Development of wet adhesion of honeybee arolium incorporated polygonal structure with three-phase composite interfaces

Inspired by the dynamic wet adhesive systems in nature,various artificial adhesive surfaces have been developed but still face different challenges.Crucially,the theoretical mechanics of wet adhesives has never been sufficiently revealed.Here,we develop a novel adhesive mechanism for governing wet adhesion and investigate the biological models of honeybee arolium for reproducing the natural wet adhesive systems.Micro-nano structures of honeybee arolium and arolium-prints were observed by Cryogenic scanning electron microscopy(Cryo-SEM),and the air pockets were found in the contact interface notably.Subsequently,the adhesive models with a three-phase composite interface(including air pockets,liquid secretion,and hexagonal frames of arolium),were formed to analyze the wet adhesion of honeybee arolium.The results of theoretical calculations and experiments indicated an enhanced adhesive mechanism of the honeybee by liquid self-sucking effects and air-embolism effects.Under these effects,normal and shear adhesion can be adjusted by controlling the proportion of liquid secretion and air pockets in the contact zone.Notably,the air-embolism effects contribute to the optimal coupling of smaller normal adhesion with greater shear adhesion,which is beneficial for the high stride frequency of honeybees.These works can provide a fresh perspective on the development of bio-inspired wet adhesive surfaces.

Sessile droplet freezing and ice adhesion on aluminum with different surface wettability and surface temperature

This paper focused on the sessile droplet freezing and ice adhesion on aluminum with different wettability （hydrophilic, com- mon hydrophobic, and superhydrophobic surfaces, coded as HIS, CHS, SHS, respectively） over a surface temperature range of -9℃ to -19℃. It was found that SHS could retard the sessile droplet freezing and lower the ice adhesion probably due to the interfacial air pockets （IAPs） on water/SHS interface. However, as surface temperature decreasing, some IAPs were squeezed out and such freezing retarding and adhesion lowering effect for SHS was reduced greatly. For a surface temperature of-19℃, ice adhesion on SHS was even greater than that on CHS. To discover the reason for the squeezing out of lAPs, forces applied to the suspended water on IAPs were analyzed and it was found that the stability of IAPs was associated with surface mi- cro-structures and surface temperature. These findings might be helpful to designing of SHS with good anti-icing properties.