Zhichun Yang

Analysis of limit cycle flutter of an airfoil in incompressible flow

Both experimental and theoretical analyses are perfomed to investigate several types of self-excited oscillations of a two-dimensional wing model with non-linear pitching stiffiness. Free play non-linearity receives especial attention. In the low speed wind tunnel test of the wing model with free play in pitch, an interesting case of double limit cycle flutter is found. Subsequent harmonic balance analyses ascertain not only these two sustained oscillations, but also two other unstable limit cycles. A digital simulation method is also used for flutter analysis. The theoretical results agree well with the experimental data.

Chaotic motions of an airfoil with non-linear stiffness in incompressible flow

Abstract Two-dimensional airfoils with cubic pitching stiffness in incompressible flow are analyzed to investigate the chaotic behaviour of self-excited dynamical system. Airfoils with different elastic axis positions will have different aerodynamic divergence speeds and hence different post-divergence angles of attack. Also there is a chaotic region for certain elastic axis positions. Several different phase plots together with their period doubling patterns have been calculated systematically by numerical integration, and boundaries of different motion types are shown in the parameter map of airspeed and elastic axis position. A two-step approximate analysis gives some interpretation of the route to chaos for the airfoil system investigated.

Structural Damage Detection by Changes in Natural Frequencies

Based on the new conception of natural frequency vector (NFV) and natural frequency vector assurance criterion (NFVAC), a new damage detection method using the natural frequencies as damage feature is described in detail. For a specific structure, a series of damage states can be simulated based on the finite element model of the intact structure. Then, the NFVs of these damaged structures can be calculated and used to build the damage feature database of the structure. At last, the NFVAC, which is defined by the relationship of NFVs in the damage feature database and the NFV measured on the actual structure are used as damage index. An 8-story shear frame model is adopted as an example to verify the feasibility and validity of the proposed method. Both numerical simulations and model experiments for damage detection are performed, the results of damage detection for several damage cases demonstrate that using NFVAC as damage index, the damage location can be identified correctly and the damage extents can be estimated as well.Based on the new conception of natural frequency vector (NFV) and natural frequency vector assurance criterion (NFVAC), a new damage detection method using the natural frequencies as damage feature is described in detail. For a specific structure, a series of damage states can be simulated based on the finite element model of the intact structure. Then, the NFVs of these damaged structures can be calculated and used to build the damage feature database of the structure. At last, the NFVAC, which is defined by the relationship of NFVs in the damage feature database and the NFV measured on the actual structure are used as damage index. An 8-story shear frame model is adopted as an example to verify the feasibility and validity of the proposed method. Both numerical simulations and model experiments for damage detection are performed, the results of damage detection for several damage cases demonstrate that using NFVAC as damage index, the damage location can be identified correctly and the damage extents ca...

Theory of inner product vector and its application to multi-location damage detection

Structural damage detection methods using time domain vibration responses have shown appeal in recent years. In previous papers by the authors, the inner product vector (IPV) was proposed as a damage detection algorithm which uses cross correlation functions between vibration responses under white noise excitation or band pass white noise excitation. The proposed algorithm was verified by some simulative and experimental examples featuring a single damage location. However, damage at multiple locations was not considered. Therefore, this paper extends the application of IPV-based structural damage detection to the problem of multiple damage locations. Firstly, the theory of the IPV and its implementation in a damage detection context is reviewed. Then, two strategies for detecting multiple damages utilizing IPV are proposed. Finally, a damage detection experiment of a honeycomb sandwich composite beam is adopted to illustrate the feasibility and effectiveness of the IPV-based damage detection method.

Aeroelastic Tailoring of a Composite Forward-Swept Wing Using a Novel Hybrid Pattern Search Method

AbstractAeroelastic tailoring of a composite forward-swept wing model is investigated to achieve a minimum structure weight with increasing divergence speed and flutter speed. A novel hybrid pattern search method is proposed to perform the aeroelastic tailoring of the wing structure subject to multiple constraints including static deformation, strength, buckling, and aeroelastic characteristics. In the new hybrid pattern search method, the sensitivity analysis method and the genetic algorithm are combined to enhance the global convergence rate and obtain a global optimal solution, respectively. A global search is performed by using the genetic algorithm to obtain elite individuals as the initial values for further optimizations, and the sensitivity analysis method is applied to improve the optimization efficiency. The comparative study of the optimized results obtained by other existing modern heuristic optimization methods shows that the present method can achieve the lightest structure weight and cost t...

Realization of Artificial Nonlinear Spring with Controllable Stiffness

Traditionally, nonlinear stiffness is achieved using mechanical components designed for a specific structure under certain loading conditions. In the present paper, the desired stiffness nonlineari...

Steering of SH wave propagation in electrorheological elastomer with a structured meta-slab by tunable phase discontinuities

The generalized Snell’s law (GSL) with phase discontinuity proposed based on the concept of a metasurface, which can be used to control arbitrarily the reflection and refraction of waves, attracts a growing attention in these years. The concept of abnormally deflecting the incident wave has been applied to the elastic field very recently. However, most of the studies on metasurfaces are based on passive materials, which restricts the frequency or the deflected angles always working in a single state. Here, we steer elastic SH wave propagation in an electrorheological (ER) elastomer with a structured meta-slab composed of geometrically periodic wave guides by exposing the slab to the programmed electric fields. The dependence of phase velocities of SH waves on the applied electric fields can make the phase shift under the form of a special function along the slab, which will control the refraction angles of the transmitted SH waves by the GSL. Accordingly we design the meta-slab theoretically and conduct c...

Structural health monitoring of constrained tapered beamlike structures using natural frequencies and nodal points

The integrity and safety of beam-like structures are dependent in part on their boundary conditions which can vary with time due to damage or aging. Structural health monitoring of such structures should therefore include attention to boundary conditions. Where the boundary conditions can be represented by a lumped spring then the identification of associated stiffness parameter values may be a means to quantifying the integrity of the support. This paper investigates such a method for identifying the equivalent translational and rotational stiffness of a constrained tapered beam-like structure. An analytical model of a beam of tapered width and thickness is adopted as a simplified representation of a tower-like structure. The model is used to explore in what scenarios natural frequencies and/or nodal points might be sufficiently sensitive to changes in support conditions to be measurable indicators of damage. The method is evaluated by Monte Carlo simulations for a numerical example where the severity of noise can be controlled.