Motion of spiral tip driven by local forcing in excitable media

We study the motion of a spiral wave controlled by a local periodic forcing imposed on a region around the spiral tip in an excitable medium. Three types of trajectories of spiral tip are observed： the epicycloid-like meandering, the resonant drift, and the hypocycloid-like meandering. The frequency of the spiral is sensitive to the local periodic forcing. The dependency of spiral frequency on the amplitude and size of local periodic forcing are presented. In addition, we show how the drift speed and direction are adjusted by the amplitude and phase of local periodic forcing, which is consistent with a theoretical analysis based on the weak deformation approximation.

QBO-like Oscillations Induced by Local Thermal Forcing

A zonalwetiCal two-dimensional equatorial model is used to study the PO￣ty that the long Period oSCillation of the zonal mean now occurring in the lower equatorial stratosphere(QBO) is cause by local thermal activihes at the tropiCal tropopause.The model sumesfully reproduces Q￣like o￣tions of the zonal mean now,suggeshng that the lOCal heating or cooling at the trOPical trOPOpose is Probably the main ￣n of QBO,s generahon. The analysis of the dependence of the oedllahon on the wave foeing iudicatw that the o￣hon is not areaible to the forCing scale.The model can reproduce QBO--like oscillahons with any forCing￣if the forcing Period and amplitude take appropriate valuex, proving that the inaneal cavity waves ge￣od by lOCal thermal sough take much important roles in QBO.

A numerical study of contact force in competitive evacuation

Crowd force by the pushing or crushing of people has resulted in a number of accidents in recent decades. The aftermath investigations have shown that the physical interaction of a highly competitive crowd could produce dangerous pressure up to 4500 N/m, which leads to compressive asphyxia or even death. In this paper, a numerical model based on discrete element method （DEM） as referenced from granular flow was proposed to model the evacuation process of a group of highly competitive people, in which the movement of people follows Newton＇s second law and the body deformation due to compression follows Hertz contact model. The study shows that the clogs occur periodically and flow rate fluctuates greatly if all people strive to pass through a narrow exit at high enough desired velocity. Two types of contact forces acting on people are studied. The first one, i.e., vector contact force, accounts for the movement of the people following Newton＇s second law. The second one, i.e., scale contact force, accounts for the physical deformation of the human body following the contact law. Simulation shows that the forces chain in crowd flow is turbulent and fragile. A few narrow zones with intense forces are observed in the force field, which is similar to the strain localization observed in granular flow. The force acting on a person could be as high as 4500 N due to force localization, which may be the root cause of compressive asphyxia of people in many crowd incidents.

Characterization of forced localization of disordered weakly coupled micromechanical resonators

The mode localization phenomenon of disordered weakly coupled resonators(WCRs)is being used as a novel transduction scheme to further enhance the sensitivity of micromechanical resonant sensors.In this paper,two novel characteristics of mode localization are described.First,we found that the anti-resonance loci behave as a linear function of the stiffness perturbation.The antiresonance behavior can be regarded as a new manifestation of mode localization in the frequency domain,and mode localization occurs at a deeper level as the anti-resonance approaches closer to the resonance.The anti-resonance loci can be used to identify the symmetry of the WCRs and the locations of the perturbation.Second,by comparing the forced localization responses of the WCRs under both the single-resonator-driven(SRD)scheme and the double-resonator-driven(DRD)scheme,we demonstrated that the DRD scheme extends the linear measurement scale while sacrificing a certain amount of sensitivity.We also demonstrated experimentally that the amplitude ratio-based sensitivity under the DRD scheme is approximately an order of magnitude lower than that under the SRD scheme,that is,the amplitude ratio-based sensitivity is−70.44%(Nm^(−1))^(−1) under the DRD scheme,while it is−785.6%(Nm^(−)1)^(−1) under the SRD scheme.These characteristics of mode localization are valuable for the design and control of WCR-based sensors.