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

Aerodynamics of Pitching Wings: Theory and Experiments

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/140444/1/6.2014-2881.pd

Shape Optimization of a Membrane Wing for Micro Air Vehicles

An optimization of a flexible membrane wing using a rigid wing as surrogate was performed. An analysis of the final design has verified that the flexible wing can be improved by optimization of a rigid wing with the same geometry. The optimization leads to lower camber near the root and higher camber near the tip, while still leaving the camber slightly higher at the root than the tip, when compared to the baseline. The improvement in aerodynamics of the optimized wing was largely realized via reduced-form drag and better pressure distributions

Low-reynolds number flow control using dielectric barrier discharge actuators

Dielectric Barrier Discharges (DBD), operated at KHz and KV range, can create athermal plasma via collision processes and induce near wall jet. In this paper, we investigate the potential of using DBD to conduct flow control at low Reynolds numbers, motivated by micro air vehicle (MAV) applications. A previously developed computational methodology, based on the e N transition model and the k-ω turbulence closure is adopted. The effects of actuators on the flow fields are represented with a body force model in the momentum equations. The flow control over an airfoil SD7003 at the chord Reynolds number of 6×10 4 is the focus of the present study. Specifically, we investigate both co-flow and counter-flow actuation strategies. The co-flow approach offers momentum enhancement via favorable pressure gradient in the near wall region while the counter flow approach can trigger earlier separation and transition by introducing adverse pressure gradients. The aerodynamic performance including lift and drag coefficients over a broad range of angle of attack (4 0-18 0) is studied, showing that improved lift and drag characteristics can be realized with DBD concepts for low Reynolds number aerodynamics

Membrane Wing Model for Micro Air Vehicles

A model for the interaction between a membrane wing and its surroundings for application in micro air vehicles was presented. The effect of dynamic shape change of the membrane wing on the aerodynamic characteristics was summarized. The fluid and structural solvers were synchronized through a subiteration process. Green-Lagrange strain tensor was used for the description of large strains

Membrane wing aerodynamics for micro air vehicles

The aerodynamic performance of a wing deteriorates considerably as the Reynolds number decreases from 106 to 104. In particular, flow separation can result in substantial change in effective airfoil shape and cause reduced aerodynamic performance. Lately, there has been growing interest in developing suitable techniques for sustained and robust flight of micro air vehicles (MAVs) with a wingspan of 15 cm or smaller, flight speed around 10 m/s, and a corresponding Reynolds number of 104-105. This paper reviews the aerodynamics of membrane and corresponding rigid wings under the MAV flight conditions. The membrane wing is observed to yield desirable characteristics in delaying stall as well as adapting to the unsteady flight environment, which is intrinsic to the designated flight speed. Flow structures associated with the low Reynolds number and low aspect ratio wing, such as pressure distribution, separation bubble and tip vortex are reviewed. Structural dynamics in response to the surrounding flow field is presented to highlight the multiple time-scale phenomena. Based on the computational capabilities for treating moving boundary problems, wing shape optimization can be conducted in automated manners. To enhance the lift, the effect of endplates is evaluated. The proper orthogonal decomposition method is also discussed as an economic tool to describe the flow structure around a wing and to facilitate flow and vehicle control. © 2003 Elsevier Ltd. All rights reserved

Low Reynolds number turbulent flows around a dynamically shaped airfoil

A computational investigation for flows surrounding a dynamically shaped airfoil, at a chord Reynolds number of 78,800, is conducted along with a parallel experimental effort. A piezo-actuated flap on the upper surface of a fixed airfoil is adopted for active control. The actuation frequency focused on is 500 Hz. The computational framework consists of a multi-block, moving grid technique, the e-based laminar-turbulent transition model, the two-equation turbulence closure, and a pressure-based flow solver. The moving grid technique, which handles the geometric variations in time, employs the transfinite interpolation scheme with a spring network approach. Comparing the experimental and computational results for pressure and velocity fields, implications of the detailed flap geometry, the flapping amplitude, turbulence modeling, and grid distributions on the flow structure are assessed. The effect of the flap movement on the separation location and vortex dynamics is also investigated. © 2002 Elsevier Science Ltd. All rights rerserved

Computational aeroelasticity using a pressure-based solver

The non-linear fluid-structure interaction problem is studied for two different wing configurations based on moving grid techniques. These configurations demonstrate the interaction between a rigid structure and fluid, as well as the interaction between a flexible structure and fluid. A closely-coupled approach is used to perform the combined fluid and structure interaction computations. The flow solver is an unsteady, implicit, three-dimensional, multi-block, pressure-based Navier-Stokes solver. The structure solver for the AGARD wing model is based on a linear, time-invariant model derived via classical structural finite elements whereas the flexible structural solver is based on a non-linear dynamic membrane model with the material obeying the hyperelastic Mooney's model. Suitable interfacing techniques are incorporated to couple and synchronize the flow and structure solvers. We present unsteady computations performed on both rigid and membrane wings to highlight the computational characteristics