Stochastic collocation method for secondary bifurcation of a nonlinear aeroelastic system

A stochastic collocation method is proposed to investigate the secondary bifurcation of a two-dimensional aeroelastic system with structural nonlinearity represented by cubic restoring forces, and uncertainties expressed by random parameters in the cubic stiffness coefficient and in the initial pitch angle. The accuracy of the stochastic collocation method is improved by incorporating higher order schemes, such as piecewise cubic interpolation and piecewise cubic spline interpolation, instead of a piecewise linear interpolation formula. For an aeroelastic problem with the uncertainty expressed by a time dependent combination of five random variables, an efficient collocation method is developed using a sparse grid approach with a dimension adaptive strategy. Numerical simulations are carried out to demonstrate the effectiveness of the proposed method for long term computation and discontinuous problems.     

Hopf bifurcation analysis of an aeroelastic model using stochastic normal form

We investigate the effects of parameter uncertainties on the dynamical response of an aeroelastic model representing an oscillating airfoil in pitch and plunge with linear aerodynamics and cubic structural nonlinearities. An approach based on the stochastic normal form is proposed to determine the effects due to the variations in the flow speed and the structural stiffness terms on the stability of the aeroelastic system near the Hopf bifurcation point. This approach allows us to study analytically the bifurcation scenario and to predict the amplitude and frequency of the limit cycle oscillation (LCO). The results show that the amplitude of LCO corresponding to the supercritical Hopf bifurcation increases with the intensity of the noise perturbing the pitch and plunge cubic terms, but there is almost no effect on the LCO frequency. Uncertainties in the flow speed produce a shift in the bifurcation point, and unstable subcritical behavior may occur for values of parameters for which the corresponding deterministic model is stable. The stochastic normal form confirms and extends previously known numerical results regarding the effect of parameter variations, and offers an effective way to perform sensitivity analysis of the system's response.     

Simple method for measurement of surface roughness using spectral interferometry

The phenomenon that correlation between two beams, in a two-beam interferometer, modifies the spectral characteristics of the light field, is used to estimate the average of the roughness of the surfaces. In the case of intensity interferometer, the path difference between the two arms of the interferometer should be less than the coherence length of the light and the light should be quasi-monochromatic. However, the advantage of spectral interferometer is that the light need not be quasi-monochromatic and the path difference could be much more than the coherence length of light. It is found that more the path difference between the two arms of the interferometer, accurate are the measurements of the roughness of the surfaces. This study shows that spectral interferometry enables one to measure average roughness of the surfaces in the subscales of optical wavelengths.     

An expert system for predicting nonlinear aeroelastic behavior of an airfoil

In this paper we present an expert system to perform steady-state response predictions. We consider an aeroelastic model simulating a two degree-of-freedom airfoil oscillating in pitch and plunge with a freeplay nonlinearity in the pitch degree-of-freedom. In the proposed data-driven methodology, a freeplay is first confirmed, and then the locations of the switching points are determined. A state-space formulation is constructed to model the piece-wise linear system. The parameters of the system are estimated using the Kalman filter and the expectation maximization algorithm. The attractive feature of the present approach is its ability to accurately predict the steady-state behavior of the nonlinear aeroelastic system with freeplay, using only a limited amount of transient input data. To demonstrate the effectiveness of the proposed methodology, we present applications to freeplay aeroelastic data arising from wind tunnel experiments and numerical simulations.     

Uncertainty analysis near bifurcation of an aeroelastic system

Variations in structural and aerodynamic nonlinearities on the dynamic behavior of an aeroelastic system are investigated. The aeroelastic system consists of a rigid airfoil that is supported by nonlinear springs in the pitch and plunge directions and subjected to nonlinear aerodynamic loads. We follow two approaches to determine the effects of variations in the linear and nonlinear plunge and pitch stiffness coefficients of this aeroelastic system on its stability near the bifurcation. The first approach is based on implementation of intrusive polynomial chaos expansion (PCE) on the governing equations, yielding a set of nonlinear coupled ordinary differential equations that are numerically solved. The results show that this approach is capable of determining sensitivity of the flutter speed to variations in the linear pitch stiffness coefficient. On the other hand, it fails to predict changes in the type of the instability associated with randomness in the cubic stiffness coefficient. In the second approach, the normal form is used to investigate the flutter (Hopf bifurcation) boundary that occurs as the freestream velocity is increased and to analytically predict the amplitude and frequency of the ensuing LCO. The results show that this mathematical approach provides detailed aspects of the effects of the different system nonlinearities on its dynamic behavior. Furthermore, this approach could be effectively used to perform sensitivity analysis of the system's response to variations in its parameters.     

Efficiency of an adaptive method of unidirectional radiation transfer through a disturbed water surface

The angular structure of a light beam transferred by the previously proposed method through a disturbed water surface, modeled by a series of spherical lunes, is analyzed. The influence of aberrations of the transfer scheme and real conditions on the beam divergence is considered. Experimental data on the angular characteristics of the transmitted beam are presented, and some problems of realization of the method are discussed.     

Bifurcation analysis of a two-degree-of-freedom aeroelastic system with freeplay structural nonlinearity by a perturbation-incremental method

A perturbation-incremental (PI) method is presented for the computation, continuation and bifurcation analysis of limit cycle oscillations (LCO) of a two-degree-of-freedom aeroelastic system containing a freeplay structural nonlinearity. Both stable and unstable LCOs can be calculated to any desired degree of accuracy and their stabilities are determined by the Floquet theory. Thus, the present method is capable of detecting complex aeroelastic responses such as periodic motion with harmonics, period-doubling (PD), saddle-node bifurcation, Neimark-Sacker bifurcation and the coexistence of limit cycles. Emanating branch from a PD bifurcation can be constructed. This method can also be applied to any piecewise linear systems.     

Diffraction by a periodic surface and the adaptive collocation method

Institute of Physical Problems, Academy of Sciences of the USSR. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 33, No. 2, pp. 231–237, February, 1990.     

Determination of surface flatness by spectral interferometric method

In this study a novel method for determining surface flatness based on spectral interferometry is reported. In the traditional method a laser is used to produce intensity interference fringes and by measuring the shift in the position of a fringe when a test plate is put on a reference plate, the surface flatness of the test plate is determined. In the present study we have used a broad-band source for determining the flatness of a test plate. By studying the changes in the spectrum of the source on interference that are produced when a test plate is put on a reference plate, the surface flatness is determined. It has been found that the surface flatness determined by spectral studies comes out to be of the same order as that with a quasi-monochromatic laser light. This method might provide an edge over the present method by reducing the time taken in the study and also provide a cost-effective solution.     

Dynamic property analysis of the space-frame structure using the spectral element method

In this paper, the space-frame structure is analyzed using the spectral element method (SEM). It is periodic and composed of many geometrically uniform components. They can be considered as space beam-column elements. The spectral stiffness matrixes of the elements are established by using the variational method and the discrete Fourier transform theory. Then frequency responses of the structure are obtained by solving the spectral equations. In comparison with the results of the finite element method, those in the SEM are more accurate by taking less time, and the vibration bandgaps are observed from the responses of the structure. Moreover, the span, the substructure height, the substructure number, the material combination, the structural damping and the load location have obvious influences on the structural dynamic properties. These findings will provide the engineers with new ideas of the structural and the vibration isolation design.     

Automatic white matter lesion segmentation using an adaptive outlier detection method

White matter (WM) lesions are diffuse WM abnormalities that appear as hyperintense (bright) regions in cranial magnetic resonance imaging (MRI). WM lesions are often observed in older populations and are important indicators of stroke, multiple sclerosis, dementia and other brain-related disorders. In this paper, a new automated method for WM lesions segmentation is presented. In the proposed method, the presence of WM lesions is detected as outliers in the intensity distribution of the fluid-attenuated inversion recovery (FLAIR) MR images using an adaptive outlier detection approach. Outliers are detected using a novel adaptive trimmed mean algorithm and box-whisker plot. In addition, pre- and postprocessing steps are implemented to reduce false positives attributed to MRI artifacts commonly observed in FLAIR sequences. The approach is validated using the cranial MRI sequences of 38 subjects. A significant correlation (R=0.9641, P value=3.12×10(-3)) is observed between the automated approach and manual segmentation by radiologist. The accuracy of the proposed approach was further validated by comparing the lesion volumes computed using the automated approach and lesions manually segmented by an expert radiologist. Finally, the proposed approach is compared against leading lesion segmentation algorithms using a benchmark dataset.     

Pressure variation by a magnetohydrodynamic method at the surface of a body placed in a supersonic flow

This study is aimed at investigating the possibility of pressure variation near the surface of a body placed in a supersonic flow as a model of an aerofoil or the nose of an aircraft by organizing a surface gas discharge in a magnetic field transverse to the flow. The flow parameters and pressure are mainly affected by the ponderomotive Lorentz force acting on the gas in the direction orthogonal to the direction of the organized discharge current and leading to the removal or compression of the gas at the surface of the body and, hence, a variation of pressure. Experimental data on the visualization of the flow and on the pressure at the surface of the body are considered for various configurations of the current and intensities of the gas discharge and magnetic field; it is demonstrated that such configurations of the current and magnetic field near the surface of the body under investigation can be organized in such a way that the pressure at the front part as well as the upper and lower surfaces of the body under investigation can be increased or decreased, thus changing the aerodynamic drag and the aerofoil lift. Such a magnetohydrodynamic control over aerodynamic parameters of the aircraft can be used during takeoff and landing as well as during steady-state flight and also during the entrance into dense atmospheric layers. This will considerably reduce the thermal load on the surface of the body in the flow.     

Performance analysis of adaptive antenna array using spatial-smoothing method with optimal weighting

It is proposed that the spatial-smoothing method with optimal weighting be used to increase the efficiency of signal processing under conditions of correlated noise. It is shown that use of the method improves the characteristics of adaptive antennas. Results of a computer simulation are presented.Nizhegorod State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 34, No. 7, pp. 811–816, July, 1991.     

基于自适应核独立成分分析的活性二氧化锰助剂光谱信息的提取

利用乙醚、丙酮、氯仿、甲苯四种有机溶剂提取活性二氧化锰助剂,提取液经过滤浓缩后应用FTIR(ATR附件)测定其红外光谱.采用绝对中位差法(MAD)确定混合体系中吸光物组分数,利用核独立成分分析(KICA)提取混合光谱信号中的纯组分光谱信息,建立了自适应核独立成分分析方法(AKICA).采用AKICA提取得到的助剂光谱信...     

NDT response of spectral analysis of surface wave method to multi-layer thin high-strength concrete structures

This study presents the results of the non-destructive testing using spectral analysis of surface waves (SASW) based on high-strength concrete materials. This SASW method was used to evaluate the compressive strength of single-layer high-strength concrete slabs through a correlation with the surface wave velocities. This paper also presents the relationship between the theoretical and experimental compact dispersion curves when the SASW test is applied to multi-layer thin high-strength concrete slab systems with a finite thickness. The test results show that the surface wave velocity profile obtained from the theoretical dispersion curve has lower values than the profile obtained from the experimental compact dispersion curve under the condition of a finite thickness due to different boundary conditions and reflections from the boundaries. Based on the measured response, an experimental study was conducted to examine if the dispersive characteristics of Rayleigh wave exist in the multi-layer high-strength concrete slab systems. This study can be utilized in examining structural elements of high-strength concrete structures and can also be applied in the integrity analysis of high-strength concrete structures with a finite thickness.     

Stochastic analysis of an agent-based model

We analyze the dynamics of a forecasting game that exhibits the phenomenon of information cascades. Each agent aims at correctly predicting a binary variable and he/she can either look for independent information or herd on the choice of others. We show that dynamics can be analytically described in terms of a Langevin equation and its collective behavior is described by the solution of a Kramers’ problem. This provides very accurate results in the region where the vast majority of agents herd, that corresponds to the most interesting one from a game theoretic point of view.     

Rapid and accurate analysis of surface and pseudo-surface waves using adaptive laser ultrasound techniques

A method has been developed to measure the phase velocity of laser generated and detected surface acoustic waves. An optical grating produced by an electronically addressable spatial light modulator (SLM) was imaged onto the sample surface to generate surface acoustic waves whose frequency and wavefront was controlled by the SLM. When the grating period matched the surface acoustic wavelength, the surface wave was strongly excited, thus the wavelength and, thereby the phase velocity was determined. We present results with this method that allows the phase velocity and the angular dispersion of the generalized surface wave as well as the pseudo-surface wave on the (100) nickel to be measured. Measurements on (111) silicon single crystals are also presented. The measurement precision is approximately 0.2%. Methods to further improve the measurement precision are also discussed.