A Mechanism for Ferrimagnetism and Incommensurability in One-Dimensional Systems

A mechanism for ferrimagnetism in (1+1)-dimensions is discussed. The ferrimagnetism is cased by interactions described by operators with non-zero conformal spin. Such interactions appear in such problems as the problem of tunneling between Luttinger liquids and the problem of frustrated spin ladder. I present exact solutions for a representative class of models containing such interactions together with a simple mean field analysis. It is shown that the interactions (i) dynamically generate static oscillations with a wave vector dependent on the coupling constant, (ii) give rise to a finite magnetic moment at $T = 0$ accompanied by the soft mode with a non-relativistic ({\it ferromagnetic}) dispersion $E \sim k^2$, (iii) generate massive (roton) modes.Comment: replaced by the extended version, references adde

Proper Orthogonal Decomposition of Flexible Clap and Fling Elastic Motions via High-Speed Deformation Measurements

Many complex unsteady mechanisms are thought to facilitate the high efficiency and agility commonly observed in small biological flyers. One of these, the flexible clap and fling maneuver, has not been extensively studied; an experimental characterization is the focus of this work. The clap-fling mechanism is approximated with a single flexible membrane flapping wing, replacing the symmetry plane between two wings with a splitter plate simulating the pair wing. This produces a complex vibro-impact aeroelastic problem, the deformation resulting from which is measured with a high-speed visual image correlation system. A low-dimensional representation of the ensuing large data set is obtained with proper orthogonal decomposition. The POD modes, and the relative importance of each, can help elucidate crucial mechanisms and relationships within the flapping system, and are computed for various membrane wing structures and flapping frequencies, with or without the presence of the splitter plate.Keywords: MAV, Flapping wings, VIC, Digital image correlationKeywords: MAV, Flapping wings, VIC, Digital image correlatio

Static Aeroelastic Model Validation of Membrane Micro Air Vehicle Wings

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76488/1/AIAA-2007-1067-747.pd

A Bendable Load Stiffened Wing for Small UAVs

A bendable load stiffened wing, developed at the University of Florida, has the ability to load stiffen in the positive flight load direction while remaining compliant in the opposite direction, enabling UAV storage inside smaller packing volumes. The wing employs an under-cambered airfoil with a swept planform providing dissimilar stiffness in the flight load and the folding direction. A comparative experimental study is performed using two wing geometries; straight camber and swept camber. The load stiffening ability is tested by performing three point bend tests while monitoring the wing root airfoil shape change using a visual image correlation technique. For the wing utilizing a swept camber design, increase in the root airfoil camber with increased loading resulted in a load stiffening structure. Swept camber wing showed a higher load carrying capacity (7 g's load factor) over a straight camber wing design (2 g's load factor), still maintaining the compliant nature in the folding direction. Long term storage induced creep deformations are small in both of the wing geometries. By increasing the wing stiffness, sweepback helps in reducing spanwise residual creep strain. Wind tunnel tests at Re = 7×104 of both the straight camber and the swept camber wing show similar L/D ratios. The sweepback helps in improving the static stability of the wing. Thus the bendable load stiffened wing has a clear advantage of offering stiffness improvement and reducing storage induced creep residual strains while maintaining the aerodynamic efficiency and improving the static stability of the wing

Effect of tip vortex on wing aerodynamics of micro air vehicles

Tip vortex induces downwash movement, which reduces the effective angle of attack of a wing. For a low-aspect-ratio, low-Reynolds-number wing, such as that employed by the micro air vehicle (MAV), the induced drag by the tip vortex substantially affects its aerodynamic performance. In this paper we use the endplate concept to help probe the tip-vortex effects on the MAV aerodynamic characteristics. The investigation is facilitated by solving the Navier-Stokes equations around a rigid wing with a root-chord Reynolds number of 9 × 10 4. It is confirmed that with modest angles of attack the endplate can improve the lift-to-drag ratio by reducing the drag. However, as the angle of attack becomes substantial, the wing tip vortex is stronger and the endplate loses its effectiveness. Detailed fluid flow structures are presented to offer insight into the physics responsible for the observed phenomena