Wen-Li Chen

Naturally Dried Graphene Aerogels with Superelasticity and Tunable Poisson's Ratio.

A novel natural drying (ND) strategy for low-cost and simple fabrication of graphene aerogels (GAs) is highlighted. The as-formed NDGAs exhibit ultralarge reversible compressibility (99%) and tunable Poissons ratio behaviors (-0.30 < ν < 0.46), which suggests promising applications in soft actuators, soft robots, sensors, deformable electronic devices, drug release, thermal insulator, and protective materials.

Hyperbolically Patterned 3D Graphene Metamaterial with Negative Poisson's Ratio and Superelasticity.

A hyperbolically patterned 3D graphene metamaterial (GM) with negative Poissons ratio and superelasticity is highlighted. It is synthesized by a modified hydrothermal approach and subsequent oriented freeze-casting strategy. GM presents a tunable Poissons ratio by adjusting the structural porosity, macroscopic aspect ratio (L/D), and freeze-casting conditions. Such a GM suggests promising applications as soft actuators, sensors, robust shock absorbers, and environmental remediation.

Estimation and Warning of Fatigue Damage of FRP Stay Cables Based on Acoustic Emission Techniques and Fractal Theory

A damage assessment and warning method for stay cables based on the acoustic emission (AE) technique and fractal theory was developed. First, the AE signal features of Higuchis fractal dimension (FD) were analyzed at various scales. The analytical results indicated that the FD was associated with the frequency response. Meanwhile, it was found that the curve length of the original signal reflected the fluctuation of the AE signal in the time domain. Both the FD and the curve length of the original signal were related to damage evolution. Based on the above analysis, a damage index, namely, the FD-based damage assessment index, was defined by the fractal features of AE signals generated by damaged structures, including the curve length of the original signal and its FD. Fatigue tests of glass fiber-reinforced polymer (GFRP) and carbon fiber-reinforced polymer (CFRP) cables with AE sensors were performed to validate the proposed approach. The time-history responses and frequency responses of the AE signals and the corresponding damage modes were analyzed during the entire cyclic loading process. The FD-based damage indices for all the FRP cables were obtained through analysis of the AE signals. The relationships of both the time-history responses and the frequency responses with the FD-based damage index were investigated. The results indicated that the FD-based damage index increased little with the number of loading cycles at the early loading stage but increased dramatically at the final stage of the fatigue test. The results of this article demonstrate that an FD-based damage index can quantify the evolutionary process of fatigue damage in FRP stay cables.

An experimental study on the characteristics of wind-driven surface water film flows by using a multi-transducer ultrasonic pulse-echo technique

An experimental study was conducted to investigate the characteristics of surface water film flows driven by boundary layer winds over a test plate in order to elucidate the underlying physics pertinent to dynamic water runback processes over ice accreting surfaces of aircraft wings. A multi-transducer ultrasonic pulse-echo (MTUPE) technique was developed and applied to achieve non-intrusive measurements of water film thickness as a function of time and space to quantify the transient behaviors of wind-driven surface water film flows. The effects of key controlling parameters, including freestream velocity of the airflow and flow rate of the water film, on the dynamics of the surface water runback process were examined in great details based on the quantitative MTUPE measurements. While the thickness of the wind-driven surface water film was found to decrease rapidly with the increasing airflow velocity, various surface wave structures were also found to be generated at the air/water interface as the surf...

Influence of Dynamic Properties and Position of Rivulet on Rain–Wind-Induced Vibration of Stay Cables

AbstractThis paper combines an experimental study and computational fluid dynamics (CFD) simulations to investigate the influence of dynamic properties and position of upper rivulet on rain–wind-induced vibration (RWIV) of stay cables. The reproduction of the RWIV of a stay cable model is first performed based on artificial rainfall wind tunnel tests with an ultrasonic transmission thickness measurement system, which can obtain the characteristics of rivulets on the surface of the stay cable model. On the basis of the test results, CFD simulations are then used to study the aerodynamic influence of an upper rivulet using two different CFD models: a vibrating cable model with a moving upper rivulet and a vibrating cable model with a fixed upper rivulet. CFD simulations suggest that the existence of the upper rivulet do not sufficiently to excite RWIV. It is confirmed that, when an upper rivulet oscillates in a specific range at the same frequency of a cable, it can significantly vary the aerodynamic force ...

Numerical Simulation of Vortex-Induced Vibrations of Inclined Cables under Different Wind Profiles

AbstractVortex-induced vibration of an inclined cable under wind with varying velocity profiles was investigated through computational fluid dynamics numerical simulation. As a complicated fluid-structure interaction issue, the flow field around the freely oscillating cable was simulated by Ansys CFX 11.0, and the cable oscillation as calculated using the Galerkin approach (realized by the additional subroutine, which is embedded into Ansys CFX 11.0). The shear stress transport k-ω turbulent model based on the Reynolds-averaged Navier-Stokes method was employed to simulate the behavior of turbulent flow in the CFD numerical simulation. The above computational method and turbulent model were validated through comparison of the computational results with the wind tunnel test results of a rigid circular cylinder. And then, two inclination angles, 30° and 90°, were chosen for the inclined cable; wind with a uniform velocity profile (velocity profile U) and four types of velocity profiles were used as the inle...

Passive Jet Flow Control Method for Suppressing Unsteady Vortex Shedding from a Circular Cylinder

AbstractA numerical simulation is performed to investigate a passive jet flow control method for suppressing the alternating vortex shedding from the circular cylinder. A hollow pipe is tightly set on the circular cylinder, and two arrangement cases for the holes are employed: one is a five-hole case, which means that five suction holes are set near the front stagnation point and five jet holes set near the rear stagnation point of the cylinder. The other is the full-hole case, which means the holes are equidistantly arranged on the hollow pipe. The incoming flow enters the suction holes and jets into the near wake from the outlet holes. Consequently, the wake vortex shedding alternately is manipulated or destroyed. The numerical simulations of baseline cases (without control) are first conducted to verify the reliability of the numerical model. Next, the two controlled cases (five hole-case and full-hole case) are investigated at the Reynolds number R=103−105. It is found that a remarkable mitigation for...

Suppression of Vortex Shedding from a Circular Cylinder by using a Suction Flow Control Method

An experimental study was conduct to suppress the vortex shedding from a circular cylinder by using a suction flow control method. The suction flow control was accomplished using two suction holes located on the test cylinder model at an angle of 90.0 degrees in relation to the oncoming flow direction. In addition to measuring the pressure distributions around the test model, a high-resolution digital particle image Velocimetry (PIV) system was used to conduct detailed flow field measurements to quantify the transient shedding behavior of the unsteady vortex and turbulent flow structures in the wake of the test cylinder model under different suction flow control conditions. The effectiveness of the vortex shedding suppression using the suction flow control method was evaluated based on the quantitative measurement results, which include the mean and fluctuating pressure coefficient distributions around the test cylinder model, the lift and drag coefficients of the test cylinder model with and without suction flow control, the instantaneous velocity and vorticity fields, the mean velocity and turbulence kinetic energy distributions around the cylinder model, and the mean velocity profiles in the wake of the test model under different suction flow control conditions. The measurement results of the present study revealed that the suction flow control method exhibited excellent performance in suppressing the alternative vortex shedding from the circular cylinder model (i.e., change the alternative vortex shedding mode into symmetric mode). As a result, the fluctuations of the lift coefficients and drag coefficients of the cylinder model were found to be reduced dramatically by using the suction flow control method.

A FEASIBILITY STUDY TO IDENTIFY ICE TYPES BY MEASURING ATTENUATION OF ULTRASONIC WAVES FOR AIRCRAFT ICING DETECTION

Aircraft icing has been recognized as the most significant weather hazard that impacts aviation safety. A thin sheet of ice on lifting or control surfaces of an aircraft can adversely affect its flight performance by increasing drag and decreasing lift and thrust. The uncontrolled shedding of ice built up on surfaces may severely damage critical components. The occurrence of ice accretion is also a big challenge in terms of economic impact. The presence of ice can not only cause flight delays, but also reduce flight profits by increasing fuel consumptions and additional cost for de-icing operations. A better understanding of the physical mechanisms of water movement and the ice formation process on aircraft surfaces is very important and critical in ensuring safe and efficient operation of aircraft in cold weather. Generally there are two types of ice that can be deposited during flight: glaze ice and rime ice, which occur is dependent on weather conditions. Glaze ice is formed with high liquid water content (LWC) and large droplet size at temperatures just below the freezing point, and it has a smooth, clear and dense appearance. Rime ice forms with lower LWC and smaller droplet size at temperatures around or below −12 C°. It is a mixture of tiny ice particles, containing many micro bubbles and cracks, and it has a white rough appearance. These two types of ice may have significantly different effects on flight performance. However, most of the current de-icing approaches and practices do not consider this and operators potentially perform a lot of unnecessary actions. In this study, attenuation measurement of ultrasonic waves is performed to investigate the feasibility of characterization of ice types. Analysis investigates frequency dependent attenuation properties that are potentially closely related to ice acoustic properties and hence the micro-structure.Copyright

Suppression of Vortex Shedding from a Circular Cylinder by using a Traveling Wave Wall

An experimental study was conducted to suppress the unsteady vortex shedding from a circular cylinder by using a traveling wave wall (TWW). The leeward surfaces of the circular cylinder model were replaced by wave surfaces, which can move to form symmetrical TWW when driven by a motor system. The propagation speed of the wave was adjustable by controlling the rotational speed of the motor. During the experiments, while the oncoming wind speed was fixed at U ∞=9.1 m/s, the propagation speed of the TWW was adjusted to have the ratio between of the wave propagation speed and the oncoming wind speed varied from 0 (i.e., stationary case) to 0.167. It was found that, with the TWW control, the wake region behind the cylinder model was found to be shortened and the vortex shedding from the cylinder model was weakened greatly. The average drag force acting on the test model was also found to be reduced significantly. Two different mechanisms, i.e., “forced perturbation mechanism” and “resonant perturbation mechanism”, were found to play important roles for the TWW flow control.