Investigation on vortex shedding of a swept-back wing

Effects of Reynolds number and angle of attack on the vortex shedding of a finite swept-back wing are experimentally studied. The cross-sectional profile of the wing is NACA 0012, and the sweep-back angle is 15° The time series signals detected by hot-wire in the wake region shows four distinct behaviors: laminar, subcritical, transitional, and supercritical. The derived Strouhal number curves are significantly different in these four behaviors. In addition, the statistical properties of turbulence, that is, the power spectrum density function, probability density function, correlation coefficient, Lagrangian integral time scales, and length scales are also presented in this paper.     

Extracting the weak distorted fringes on the dragonfly wing by a correlation algorithm

We have proposed a simple correlation algorithm to extract weak distorted fringes buried in both background noise and random noise. The relationship among threshold value, signal to noise ratio and least frame number was discussed. The method, is efficiency to measure the shape of an object with low diffuse reflectivity. We have successfully applied the method to measure the shape of a dragonfly wing.     

Controlling the near vortex wake of a swept half-wing by means of mini-flaps

The effect of mini-flaps on the flow pattern in the near vortex wake behind a model swept half-wing is investigated. The distributions of the time-average flow velocity were measured in a subsonic wind tunnel, in a section normal to the freestream velocity vector located at a distance of 3.8 wing half-spans from its trailing edge. When mini-flaps are mounted on both upper and lower wing surfaces, two vortices (tip and auxiliary) of the same sign are observable in the above-mentioned flow section; they are separated by an extended region of vorticity of the opposite sign. The model angle-of-attack effect on the intensities of the tip and auxiliary vortices is estimated.     

大攻角运动时的机翼摇滚问题研究综述

机翼摇滚是现代战斗机、导弹设计中普遍遇到的横向不稳定现象之一,属于典型的大攻角动态特性问题. 20多年来,机翼摇滚问题在国内外航空工业界、学术界都引起了极大的重视和兴趣,并在实(试)验、计算方面发表了许多文章,对机翼摇滚问题的认识也取得了较大的进展.本文对自1981年Nguyen、Yip及Chambers最早研究机翼摇滚现象以来,在该问题研究方面的近100多篇有代表性的论文进行了综述,内容涉及实(试)验研究、计算研究的各个方面,并对今后的发展趋势提出了自己的看法.      

Spatial solitons in nonlinear liquid waveguides

Spatial solitons are studied in a planar waveguide filled with nonlinear liquids. Spectral and spatial measurements for different geometries and input power of the laser beam show the influence of different nonlinear effects as stimulated scatterings on the soliton propagation and in particular on the beam polarization. The stimulated scattering can be used advantageously to couple the two polarization components. This effect can lead to multiple applications in optical switching.     

A computational study of the wing–wing and wing–body interactions of a model insect

The aerodynamic interaction between the contralateral wings and between the body and wings of a model insect are studied, by using the method of numerically solving the Navier-Stokes equations over moving overset grids, under typical hovering and forward flight conditions. Both the interaction between the contralateral wings and the interaction between the body and wings are very weak, e.g. at hovering, changes in aerodynamic forces of a wing due to the present of the other wing are less than 3% and changes in aerodynamic forces of the wings due to presence of the body are less than 2%. The reason for this is as following. During each down- or up-stroke, a wing produces a vortex ring, which induces a relatively large jet-like flow inside the ring but very small flow outside the ring. The vortex rings of the left and right wings are on the two sides of the body. Thus one wing is outside vortex ring of the other wing and the body is outside the vortex rings of the left and right wings, resulting in the weak interactions.     

Multichanneled hierarchical porous nanocomposite CuO/carbonized butterfly wing and its excellent catalytic performance for thermal decomposition of ammonium perchlorate

To meet the requirement of generating more apparent specific heat release at lower temperatures for ammonium perchlorate (AP)-based composite solid propellants, the development of high-performance catalysts for improving the thermal decomposition properties of AP still remains essential and challenging. Herein, a novel catalyst, multichanneled hierarchical porous nanocomposite of CuO and carbonized butterfly wing (CuO/CBW), has been prepared through an in-situ reaction on original butterfly wing scales. Owing to the high active surface area and the good electrical and thermal conductivity, as well as the synergistic effect of CuO nanoparticles (20–25 nm) and CBW, CuO/CBW nanocomposite exhibits excellent catalytic activity for AP thermal decomposition in reducing the high-temperature decomposition temperature by 88.3°C, lowering the apparent activation energy from 190.0 to 103.1 kJ mol⁻¹ and increasing the heat release from 255 to 1841 J g⁻¹.     

Butterfly Methanes: Designing a Novel Class of anti-van′t Hoff Carbons

Among different possible non-classical structures, the stabilization of half-planar tetracoordinate carbon conformation is believed to be the most difficult one. Herein, we designed three types of half-planar tetracoordinate carbon compounds computationally by employing hybrid stabilization effects of substituents. The axial hydrogens of unstable half-planar methane are substituted with π-acceptor and σ-donor substituents such as −BH₂, −Li and the equatorial substituents selected are a combination of electropositive atoms (σ-donors)/electronegative atoms (σ-acceptors and π-donors). To establish the stabilization factors, we conducted a detailed study on vibrational frequency analysis, molecular orbital analysis (including Natural Bond Orbitals) and electrostatic potential (ESP) analysis of optimized molecular geometries using density functional theory.     

A mathematical model for the propulsive thrust of the thin elastic wing harmonically oscillating in a flow of non-viscous incompressible fluid

In the present paper there is proposed an analytical approach to study vibration of a rectangular elastic wing in the stationary stream of non-viscous fluid. We first develop a basic two-dimensional integral equation. Then a series expansion along the short coordinate is applied. This reduces the problem to an infinite set of one-dimensional integral equations which is studied asymptotically with respect to the large aspect ratio parameter. An example of optimization of thickness of the wing is demonstrated, to test the efficiency of the proposed method in applications.     

Propulsion velocity of a flapping wing at low Reynolds number

This paper presents a computational fluid–structure interaction analysis for free movements with a flapping wing in a quiescent fluid. We demonstrated the moving velocity of a flapping wing according to the phase difference between the angle of attack and the positional angle in the case of a fruit fly with a Reynolds number of 136. If we considered the moving velocity of the flapping wing, the physics were different from that of hovering flight of previous studies, which did not consider the propulsive velocity and presented the advanced rotation of the angle of attack as the best mechanism for propulsion force, as compared to symmetric rotation and delayed rotation. We found that symmetric rotation produced a better propulsion velocity with less fluctuation in other direction than the advanced rotation. The hairpin vortex generated at the end of a stroke did not clearly contribute to the enhancement of propulsion; the wake capture is considered to be one of the main enhancements of the advanced rotation in a previous studies. We studied the effects of the angle of attack to determine why the fruit fly uses a large angle of attack during a constant angle of attack period. Larger angles of attack produced greater propulsion velocities. Further, larger angles of attack did not generate greater peak force during the rotation of the angle of attack at the reversal of stroke, but they produced less fluctuation at the reversal of the stroke and greater force during the constant angle of attack period.