Applied Mechanics and Materials Vol. 461

Abstract: Nowadays falls are a serious problem for elderly people with the coming of aged society in the world. According to statistics,hip fracture accounts for the most of the deaths and costs of all the fall-induced injury. This paper presented an airbag system of hip protection, which included air source, sensors, microcontroller, gas circuit and airbags. A six-axial inertial sensor module that integrated an embedded three-axis MEMS accelerometer and three-axis MEMS gyroscope was used to collect human motion data, and a one-axis obliquity sensor was used to collect human angle data. The microcontroller was employed to recognize the activities of daily living (ADL) and falls based on fall detection algorithm and the collected data from sensors. The gas circuit was triggered once the event that the fall would be inevitable was confirmed by the microcontroller, then the compressed gas would fill into airbags through the gas circuit rapidly. Therefore, a buffer would appear between the human body and the ground before the body impacting the ground, which would reduce the impact of the human body. Compressed CO₂ was selected as the air source after we tested several kinds of gas. A 16g CO₂ pressurized cylinder could provide enough pressure and volume to inflate quickly the airbags. In order to improve the reliability of the gas circuit, a needle valve was optimized from the several designed structures by the experimental optimization methods. Finally, the airbag system was tested in various designed trials. The results indicated that the system gained the satisfaction for the design requirements and would be potential to apply to the protection of hip joint in the fall high-risk people in the future.

Abstract: Although natural shark shin surface morphology has excellent drag reduction performance, it exhibits maximum drag reduction just within swimming speed of the shark. That is, drag reduction function of shark skin is unadjustable to surrounding environment. To expand applications of bio-replicated shark skin, two novel controllable adjustments of shark skin drag reduction riblets including one-direction elongation and 3D volume swelling amplification, were explored. The validity and efficiency of the two approaches to change the drag reduction riblets were verified by comparison between microstructure of adjusted and original shark skin. And the translation of drag reduction peak of natural surface function from living environment to various application environments was proved by experimental measurement. By comparison, the elongating method is efficient but low drag reduction, while the swelling way can get the same drag reduction as shark skin but cost more time. So they should be performed according to the situation. As the optimal application velocity range of the imitative shark shin morphology can be expanded by both of the two methods, the application field of the biomimetic drag reduction surface fabricated by the bio-replicated forming technology was extended.

Abstract: Some living creatures have special structures on their body surfaces, such as smooth and elastic epidermis with subcutaneous tissue having non-smooth structures under certain conditions. The elastic epidermis coupled with non-smooth structures has a special function called bio-coupling functional surface. Imitating this functional surface and applying it in engineering has a potential to solve some engineering problems. Based on the simulation method of fluid-structure interaction (FSI), simulation calculation of the bionic functional surface coupled by the two factors, form and flexible materials was conduct using ADINA software. A viscous and weakly compressible transient flow was selected as a working medium, a discrete solver was selected in numerical calculation and the basic model was chosen as a turbulence model. It is assumed that the coupling surface of the form/flexible materials results is large deformation and large strain. The boundary condition of fluid-structure interaction was set as the calculation surface. The simulation results showed that this coupling is a dynamic process, in which the two factors (form and flexible materials) are influenced by the flow field. As the pressure and velocity of the flow field increase, the coupling process changes from partial coupling to complete coupling, the pressure drag decreased due to the maximum effective stress of bionic coupling surface is very small and the smooth and flexible materials can redistribute pressure by absorbing and releasing energy, the pressure drag thus formed is decreased. Moreover, non-smooth structures (form factor) coupled with flexible materials reduced velocity of working face and minimise energy losses effectively, enabling the bionic coupling surface to reduce drag.

Abstract: Inspired by the non-smooth structure of the leading edge of owls wing,a bionic wavy cylindrical surface is proposed in this paper to reduce the aerodynamic noise of a cylindrical rod. The effects of bionic wavy surface on the aerodynamic and aeroacoustic performance of the cylinder are investigated by wind tunnel and numerical simulation. The fluctuating pressure of the smooth cylinder and the bionic wavy surface cylinder are tested by pulsating pressure sensors in FD-09 low speed wind tunnel of China Aerospace Aerodynamics Research Institute. The fluctuating pressure of the bionic wavy surface cylinder is significantly lower than that of the smooth cylinder. We used the software ANSYS FLUENT to research the effect of the bionic wavy surface on the aerodynamic characteristics and aerodynamic noise of a cylinder by the Large Eddy Simulation (LES) and the Ffowcs Williams and Hawkings (FW-H) equation. Compared with the smooth cylinder, the aerodynamic noise of the bionic wavy cylinder is reduced by 6.7dB. A study of the relationship between the fluctuating lift and the aerodynamic noise size is conducted. We found that the sound pressure level of the wavy surface cylinder is significantly lower when the lift fluctuation amplitude decreased. Bionic wavy surface can effectively restrain the separated shear layer transition to turbulence. The frequency of vortex shedding which causes the lift fluctuations is reduced, so the aerodynamic noise of the circular cylinder is reduced. Keywords: bionics, cylindrical rod, wavy surface, aerodynamic noise, flow control

Abstract: In this paper, we have started from the point of view of bionics, doing surface bionic non-smooth design at the standard No.16 animal syringe needles. Then treating the concave as the bionic unit, we worked out the concave bionic drag reduction needles by use of the laser material remove processing means. In accordance with the national standard on the injection drag test of disposable needles, we did the puncture drag comparative test of the smooth needles and the bionic needles, getting the correspondence relationship between the drag reduction rate and the bionic unit parameters. We found that the maximum drag reduction rate up to 44.05%, and it appeared when the concave interval was 0.9mm and the concave diameter was 0.09mm. Then through discussing the drag reduction mechanism of the bionic needles, we knew that the bionic units reduced the actual contact area between the needle outer wall and the simulation skin, and it was the main reason of bionic needles puncture drag decreases.

Abstract: Mole cricket is typical insect living under the ground, which tergum is the main component that suffers the wear and tear from the soil particles. In order to adapt this environment, mole crickets have anatomically evolved well-suited tergum with wear-resistance property, which can protect the body from abrasion and damage. In this study, the surface morphology and structure of the tergum of mole cricket were studied and analyzed by stereomicroscope (SM) and Scanning Electron Microscope (SEM). By using a Universal Micro-Tribotester (UMT), the friction-wear properties and the performances of the tergum surface were tested along with the head direction, tail direction and lateral direction of body, respectively. The result shows that the setae have obvious influence on the friction characteristic of mole crickets tergum. By comparing with other direction, the tergum surface has better friction properties on the setae growth direction. This study can be a reference for the design of novel wear resistance surface.

Abstract: The pits is a typical biomimetic surface morphology, typically it is beneficial to reduce the flow drag on the wall surface. In this paper, a method combining theoretical analysis and numerical simulation is adopted, it carries out a flow and stress analysis on the wall surface distributed by 3mm-diameter pits with typical bionic surface morphology, reveals the forms down stream flow field and the pit internal as well as the distribution of stress, it also analyzes the relationship between the viscous drag of the pit wall surface, the pressure drag and the flow drag, comparing the size of the downstream shear stress of the smooth wall and wall surface distributed with pits, investigates the mechanism of the downstream shear stress decreases on the biomimetic pit wall surface.

Abstract: According to the jet hole configuration mode of bionic jet surface and its influence on the drag reduction, as the basic form of jet hole configuration is the isosceles triangle elements, so this was used to establish the computational model of jet hole configuration. In this case, the height and base of the triangles were considered as variable. The SST k-ω turbulence model was used to simulate and research the drag reduction characteristics of bionic jet surface in different configuration modes of jet holes at the main flow field velocity value of 20m/s and the jet velocity value of 0.4~2.0m/s. Also the influence of different configurations of height and base on drag reduction characteristics of bionic jet surface was studied, which got the optimum size of jet hole configuration. Results show that in triangle configuration elements, the drag reduction characteristics of bionic jet surface can be influenced by the jet hole of different configurations of height and base; the drag reduction of bionic jet surface reaches the peak of 32.74% at 8mm height, 11mm base, and the jet velocity value of 2.0m/s. At the same flow field velocity, the drag reduction rate results achieved by experimental tests and by numerical simulation were changing consistently and were found same, which verifies correctness of numerical simulation results.

Abstract: The microstructures on elytral surface of aquatic beetles belonging to Hydrophilidae and Dytiscidae were observed under an environment scanning microscope, and the wettabilities were determined with an optical contact angle meter. The results show the elytral surfaces are relatively smooth compared to the structures of other insects such as the butterfly wing scales or cicada wing protrusions. They exhibit a polygonal structuring with grooves and pores being the main constituent units. The contact angles (CAs) range from 47.1^o to 82.1^o. The advancing and receding angles were measured by injecting into and withdrawing a small amount of water on the most hydrophilic (with a contact angle of 47.1^o) and hydrophobic (with a contact angle of 82.1^o) elytral surfaces, which illustrates the vital role of three-phase contact line (TCL) in the wetting mechanism of aquatic beetle elytral surfaces.

Abstract: Oleophobic surface has broad application prospects in petroleum, petrochemical, and food industries because of its non-stick oil and self-cleaning characteristics. Oleophobic surface has mainly two features: one is fine structure with micro-nano combined scale, and another is low surface energy. An oleophobic surface was successfully prepared on X70 pipeline steel surface using shot peening, chemical etching and low surface energy modifying composite methods in this study. The surface of X70 pipeline steel sample was first treated by shot peening, forming a rough structure on the sample surface. And then the sample surface was chemically etched with hydrochloric acid (HCl) solution to further refine the surface structure, forming micro-nano combined structure. Finally the surface of the specimen was low energy modified using perfluorooctanoic acid ethanol solution (PAES). The influences of HCl concentration, chemical etching time, perfluorooctanoic acid concentration, and modification time on the contact angle were analyzed. The best process parameters were optimized. The surface morphology of X70 pipeline steel sample was observed using scanning electron microscope (SEM). It was observed that there were micron structures composed of the convexes and concaves on the specimen surface treated by shot peening and chemical etching. There were nanoscale acicular structures on the micrometer grade structure. After the micro-nano composite structure was modified with PAES, the contact angle between the sample surface and oil droplet was significantly improved. The maximum contact angle of the sample surface with oil droplet was 141°.