Phase diagrams of period-4 spin chains consisting of three kinds of spins

We study a period-4 antiferromagnetic mixed quantum spin chain consisting of three kinds of spins. When the ground state is singlet, the spin magnitudes in a unit cell are arrayed as (s-t, s, s+t, s) with integer or half-odd integer s and t (0 <= t < s). The spin Hamiltonian is mapped onto a nonlinear sigma model (NLSM) in a previously developed method. The resultant NLSM includes only two independent parameters originating from four exchange constants for fixed s and t. The topological angle in the NLSM determines the gapless phase boundaries between disordered phases in the parameter space. The phase diagrams for various s and t shows rich structures. We systematically explain the phases in the singlet-cluster-solid picture.Comment: 8 pages (16 figures included

Practical Flapping Mechanisms for 20cm-span Micro Air Vehicles

[[abstract]]In the body of research relevant to high-performance flapping micro air vehicles (MAV), development of light-weight, compact and energy-efficient flapping mechanisms occupies a position of primacy due to its direct impact on the flight performance and mission capability. Realization of such versatile flapping mechanism with additional ability of producing thrust levels that fulfill requirements of cruising forward flight and vertical take-off and landing (VTOL) conditions demand extensive design validation and performance evaluation. This paper presents a concerted approach for mechanism development of a 20 cm span flapping MAV through an iterative design process and synergistic fabrication options involving electrical-discharge-wire-cutting (EDWC) and injection molding. Dynamic characterization of each mechanism is done through high speed photography, power take-off measurement, wind tunnel testing and proof-of-concept test flights. The research outcome represents best-in-class mechanism for a 20 cm span flapping MAV with desirable performance features of extra-large flapping stroke up to 100°, minimal transverse vibrations and almost no phase lag between the wings.[[notice]]補正完畢[[journaltype]]國外[[incitationindex]]SCI[[ispeerreviewed]]Y[[booktype]]紙本[[countrycodes]]US

Flapping and Flexible Wing Aerodynamics of Low Reynolds Number Flight Vehicles

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76905/1/AIAA-2006-503-331.pd

PIV on Simple Mechanical Flapping Wings for Hover-like Kinematics

A Particle Image Velocimetry (PIV) study on aspect ratio effects was performed to highlight the role of three-dimensional aerodynamic contributions relative to inertial contributions to lift and power for a hover-like passive pitch rotation flapping kinematic motion. Interest in the research and development of flapping wing micro air vehicles (MAVs) continues to grow. Despite the large body of work performed recently, an abundance of unanswered questions still exist. The approach taken in this study is to begin by reducing the complexity of the problem through parametric variation of a single geometric parameter, aspect ratio. The present PIV research has a parallel ongoing force acquisition experiment led by the US Air Force Research Labs (AFRL/RBAL). When the AFRL study has been completed, the lift and inertial force results will be used in conjunction with the PIV results to provide more insight into the physics behind the resulting forces. One important finding for the kinematics studied is that an intermediate aspect ratio (rectangular flat plate) wing will result in the greatest lift efficiency from the standpoint of both aerodynamics and power

Unsteady Flowfields Characteristics Around Two- and Three-dimensional Flapping Flight

This paper investigates aerodynamic characteristics of unsteady force generation around two- and three-dimensional flapping motions under forward flight condition. A realistic wing trajectory, called the figure-of-eight motion, is extracted from a blowflys tethered flight under freestream. In order to find out the tendency of vortex generation and flow pattern, two-dimensional simulations are firstly conducted. Computed results show vortical flow fields which exhibit very interesting and distinctive unsteady flowfields characteristics. Lift is mainly generated during downstroke motion by high effective angle of attack due to translation and lagging motion. On the other hand, a large amount of thrust is generated at the end of upstroke motion. Furthermore, analyses on three-dimensional flapping motions are performed to examine the three-dimensional counterparts of vortex formation and unsteady force generation mechanism. Three–dimensional numerical results show the existence of span-wise vortical flow that prevents the buildup of vorticity separating from the leading edge of the wing, and safely deposits it into a tip vortex. Vortical structure in the wake and the pressure field shows that the vortex pairing and vortex staying mechanism, which are firstly observed in two-dimensional flapping motion, are also observed in three-dimensional flapping motion. Consequently, they can be presented as a strong evidence for the abrupt large thrust generation.The current work is funded by the Brain Korea-21 Project for the Mechanical and Aerospace Engineering Research at Seoul National University and the Korea National e-Science project. The authors also would like to acknowledge the support from KISTI Supercomputing Center (KSC-2007-S00-1016)