選擇卷期
- 期刊
In the optimal control theory, the Hamiltonian formulation is a famous one convenient to find an optimal designed control force. However, when the performance index is a complicated function of control force, the Hamiltonian method is not easy to find the optimal solution, because one may encounter a two-point boundary value problem of nonlinear differential algebraic equations (DAEs). In this paper we address this issue via a quite novel and effective approach, of which the optimally controlled vibration problem of Duffing oscillator is recast into a two-point nonlinear DAEs by identifying the unknown control force. We develop the corresponding SL(n, R) and GL(n, R) shooting methods, as well as a Lie-group differential algebraic equations (LGDAE) method to numerically solve the optimal control forces. Eight examples of a single Duffing oscillator and one coupled Duffing oscillators are used to test the performance of the present method.
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This study investigates three-dimensional (3D) flow fields around a flapping wing with a span of 20 cm. Numerical simulations are performed by varying the simulation mesh density for a half-flapping-wing profile with wind tunnel domain for flapping stroke angles of 53° and 90°. Wind velocities ranging from 1 to 3 m/s, variation in flapping frequency of 14-15 Hz, and corresponding lift characteristics are obtained. Comparative evaluation of numerical and experimental lift force data reveals the translational and rotational lift phenomena of Dickinson’s mechanisms. The stream line that flows over the 3D wing profile is found to be in good agreement with the existing smoke trace experimental data. The motions of a flapping wing obtained through 3D stereo photography and COMSOL Multiphysics simulation are compared and the aerodynamic characteristics are exploited. The streamwise and spanwise 3D velocity fields with intercepting planes at three chord sections are explored, and the reverse Kármán vortex sheet is observed. The comparison of the simulation and experimental results for a 3D flapping wing provides insight into the motion characteristics of a flapping wing and flapping wing aerodynamic analysis of micro air vehicles.
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An optic-based method that utilizes the particle-laden images captured during high-speed particle image velocimetry (HSPIV) measurements is presented, aiming to explore the probabilistic mean characteristics of the free surface profiles and velocity fields of a free jet with high-frequency random fluctuations over a chute. The technique based on the gray-level gradients in the smoothed gray level distribution of the contrast-enhanced images is used to determine the probabilistic mean features of the free jet, right beneath and above which the water-air interfaces have I%/(100－I)% intermittent appearance of air/water phase and (100－I)%/I% fitful show-up of water/air phase. Further, the cross-correlation calculation for HSPIV measurements is employed to obtain the instantaneous and probabilistic mean velocity fields of the free jet. A target experiment of the free jet having a mean water-depth of 2.76 cm and a Froude number of 3.92 over a 17° chute model is performed in a re-circulating water channel to demonstrate the application of this method.
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This paper proposes a filter bank multicarrier (FBMC)-based underwater transmission scheme for voice and image signals. In this scheme, FBMC transmission technology, low-density parity-check (LDPC) channel coding, adaptive binary phase shift keying (BPSK) modulation or offset quadrature amplitude modulation (OQAM), and a power assignment mechanism are integrated into an underwater voice and image transmission system. The bit error rates (BERs) of voice and image signals for underwater transmission are required to be less than 10^(-3), and 10^(-4), respectively. The BER performances of the proposed scheme in an underwater acoustic channel were demonstrated through simulations, and the power saving ratios for the underwater transmissions of voice and image signals were explored. The simulation results show that the proposed underwater transmission system can achieve a lower transmission-power consumption or a higher transmission data rates compared to a system without the power assignment mechanism. The results also indicate that the proposed system can be used for advanced underwater signal transmissions.
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In this paper, a nonlinear beam equation containing an integral term of the deformation energy, which is unknown before the solution is found, is investigated under different boundary conditions. First, we set the unknown integral term as a scalar variable and then develop a weak-form integral equation to solve the integral. By using the sinusoidal functions as test functions and bases of the numerical solution, we obtain a fast-convergence iterative scheme. Due to the orthogonality of the sinusoidal functions, the expansion coefficients of the numerical solution are in the closed form. The proposed iterative algorithms converge quickly and provide highly accurate numerical solutions of the nonlinear beam equation containing the integral term, as confirmed using five numerical examples.
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In order to accurately predict the motion performance of the ship in waves, the numerical calculation of heave, pitch and added resistance of a container ship in a regular wave is studied based on the three-dimensional fully nonlinear time-domain potential flow theory. The boundary element method is used to deal with the quadrilateral elements, and the governing equations are solved by the first-order flat-plate theory and the fourth-order Runge- Kutta time integration method. The ship hull is divided by the fixed mesh. Considering the non-linear superposition of the forward velocity, the stationary ship wave, diffracted wave, diffraction wave and incident wave field, the free surface is generated by the hybrid Euler-Lagrangian method, and the damping area is set manually at the edge. Take the KCS container ship for instance, the calculation results of heave, pitch and added resistance in waves are compared with the experimental values. The results show that the biggest advantage of this method is to get a more accurate prediction of ship motion in a short time, and this method has a broad application prospect in the analysis and motion prediction of ship hydrodynamic performance in waves.
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This study performs a series of rainfall-induced seepage and stability analyses of unsaturated slope using strength reduction method (SRM) of two-dimensional (2-D) finite element technique to inspect the efficiency of compound stabilization work adopted for the remediation of the down slope of T16-tower pier (or T16-Slope) situated at the hillside of Taipei City Mao- Kong tramway. The validities of numerical procedures and various material model parameters are verified by examining the triggering mechanism and occurrence time of landslide in T16-Slope during Jang-Mi typhoon in 2008. After the landslide in 2008, the potential sliding surfaces of T16-Slope with and without remediation are both developed at the up-middle-slope (the most critical sectional slope) where an RC-grid-beam (RC-grillage) and anchorage were constructed for remediation. The remediation enables to prevent the up-middle-slope from coming close to a critical state during torrential rainfall. The increasing FS value demonstrates the deterioration of the stability of up-middle-slope caused by groundwater rise can be mitigated by the stabilization effects of remediation. The analysis results also indicate that the stabilization effects of remediation can be greatly reduced after a rainfall with long duration and heavy intensity because of a strenuous infiltration of rainwater.
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This paper describes the fundamental characteristics of an open channel in a tidal flat. In this study, the open channel was characterized using a topographic survey. Because of the limited resources available and special measurement conditions at the study area, cranes and cables were used to transport a sounding weight to the measurement point, and pressure gauges hanging on the sounding weight were used to measure the water depth. The coordinates and altitude of the surface point above the sound weight were measured using a total station. The kriging interpolation was applied to discrete geographic data to create contour lines and a topographic map of the open channel in the tidal flat. This methodology is simple, accurate, and cost-effective. Therefore, the developed methodology and theory can be applied to bathymetry measurements of river terrains with similar conditions.
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The main purpose of this paper is to study the applications of energy storage devices based on Flexible AC Transmission System (FACTS) techniques, namely the Static Var Compensator (SVC), the Static Synchronous Compensator (STATCOM) and the Battery Energy Storage System (BESS), for the improvement of dynamic characteristics of the power systems with wind generation. The system dynamic characteristics are investigated under occurrence of a three-phase short circuit fault at the point of interconnection of the system and the wind farm, without and with the installation of SVC, STATCOM and BESS. The system dynamic behaviors are also examined when the wind power output is decreased due to the wind speed variation. The results show that with a three-phase short circuit fault occurring at the point of interconnection of the system and the wind farm, the compensation equipment will provide reactive power in order to improve the voltage profile, and when the wind power output is decreased, the BESS improves the frequency transient because it can provide real power during system transient.
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In this paper, techniques for tracking and grasping moving objects with an unknown speed on a conveyor using an eyein- hand robot arm are presented, which are useful in a production line for automatic object classification. First of all, the CAMshift (Continuously Adaptive Meanshift) algorithm is employed to continuously track a moving object in the image plane. Then, the minimum area rectangle method is integrated for correctly identifying a rectangle enclosing the target object. Object features for tracking purposes can be extracted from this rectangle. Next, through the application of an image Jacobian matrix, the tracking error in the image plane can be transformed to be the displacements of the robot's end effector. Accordingly, the robot arm can be controlled for tracking this object. However, because of sensor noise and the fact that the object is moving, tracking errors cannot be eliminated at this stage. Therefore, the Kalman filter is used to estimate the state of the moving object, especially the moving speed. Finally, on the basis of the estimated speed, the robot gripper can thus be controlled to the point on the conveyor for accurately grasping and placing the moving object to a specified location. Experimental results showed the effectiveness of the techniques for grasping different target objects with different moving speeds and at any orientations.