Journal of Aerospace Engineering

To investigate the characteristics of wind fields at the bridge site in mountain-gorge terrains more accurately and rationally, an updated curved boundary transition section (BTS) was proposed in this study. The flow transition efficiency considering the updated curved BTS was first investigated and compared with those in previous studies. Then a mountain-gorge terrain model in which a long-span bridge straddles was configured as a typical numerical example by establishing two different BTSs in the computational domain for comparison purpose. Furthermore, the effects of different BTSs on the characteristics of wind fields at the bridge site with two different ranges of terrain region were comparably investigated. The results show that the updated curved BTS is very user-friendly with a straightforward expression and has a better flow transition efficiency than those reported previously. The wind speeds at the bridge site with the updated curved BTS outperform those with the ramp BTS. An appropriate shape of BTS can probably be more important than the size factor of the terrain region in terms of predicating the characteristics of wind fields at the bridge site in such mountain-gorge terrains. It is hoped that this updated curved BTS can serve as basics for analyzing the wind fields at the bridge site in mountain-gorge terrains, which will be of benefit to investigate the structure performance under wind loads not only in the design stage but also in the service stage.

The stability of a foil with its 1/4-chord center of pressure trailing a pitch axis sprung in heave is solved algebraically to help design a fluttering windmill and perhaps watermill. Its flutter mode and frequency chord/wind speed do not depend upon its total mass or spring rate. All contours of this reduced frequency in the pitch inertia and imbalance plane pass through a nexus whose total inertia and imbalance are as if just the virtual mass were at the 3/4 chord point, with a mode of feathering in the apparent wind at this aerodynamic center. The high-frequency flutter amplitude ratio is symmetric in pitch inertia about the nexus. Similarly from the second factor in its pitch damping, each contour passes through another nearby simple point as if twice its Theodorsen factor times the virtual mass were a 1/4-chord divided by this factor behind the 1/4-chord. Therefore, twice the virtual mass at midchord gives the zero-frequency inertia and imbalance midpost furthest away from the nexus. A small trail makes the imbalance greater at the midpost than the nexus so as to slope the zero-frequency line downward. Then, the imbalance required for quasi-steady flutter decreases with pitch inertia, even below zero beyond the nexus. The trail also bends the gate of simple points to pass some low-frequency contours very slightly below the midpost to locally lower the flutter boundary. For an oscillating windmill, the net virtual mass reaction stiffens heave, opposed by the circulatory lift in flutter, because its pitch and heave are necessarily partly in phase. Such new results, and a water flutter demonstration, show a practical semirotary water blade would need a geared-up pitch flywheel for sufficient inertia to flutter well, whereas a wing is so much heavier than air that it has enough structural pitch inertia to flutter and so pump easily.

This study presents the analysis of a hybrid solar high-altitude, long-endurance unmanned aerial vehicle (HALE UAV) designed to perform meteorological observation and surveillance-related missions. For this purpose, the methodology minimizes the energy consumption of the aerial platform considering the energy balance for a climb, cruise, and descent cycle. The flight methods take into account the seasonal and diurnal variation of solar radiation in the middle latitudes. In addition, to reduce the weight of the onboard battery and regenerative fuel cell (RFC), this study analyzes the daily mission profile that consists of a climb, high-altitude cruise, descent, low-altitude cruise, and reclimb. During the daytime cruise at 18 km, extra energy is stored in the RFC. In the nighttime, the aircraft descends to 14 km using energy stored in the RFC. On the following day, the aircraft climbs again to 18 km and cruises during the daytime. The energy consumption of this one-day mission profile is compared to that of a continuous-cruise flight at 18-km altitude for a day. The effects of takeoff time on the total time and energy consumption to reach the mission altitude are investigated, and the drift due to the westerlies are calculated. The analysis of the configuration parameters and aerodynamic coefficients was conducted using software tools.

As a cost-effective high-altitude platform, stratospheric airships have attracted widespread research interest. A number of studies focused on the mechanical performance of the airship envelope, which is the key system in the construction of the whole airship structure. However, few researchers have addressed the strength correlation between the stratospheric airship envelope material and its base fabric yarns. This study uses microphotography to observe the weave geometry of a particular envelope material and conducts a decomposition process to obtain intact yarns. Tensile tests are performed on both the yarn and envelope. Stress-strain curves are drawn based on the test data, and the discreteness of the specimen is found to be noteworthy. A novel finite-element method model is proposed according to the plain weave structure and the tensile parameters of the yarn and is found to properly simulate the uniaxial tensile behavior of the envelope. Future work may focus on an extensive application of the model for fields such as biaxial tensile performance.

A structural optimization method for spoke single-layer cable-net structures based on an improved genetic algorithm (GA) is proposed. The three-cable coplanarity theory is introduced, and an exchange program between a proprietary programming language and an advanced finite-element software is presented to implement the improved GA. Then, a simplified numerical model of the Suzhou Industrial Park Stadium cable-net structure is built and optimized. The numerical example shows that the proposed structural optimization method is accurate and efficient. Because of the limited loading combinations, nodal vertical displacements, and tensile stresses of cables, the optimal $z$ coordinates of support nodes and cable nodes and subsequently, the initial pretension and cross-sectional areas of cables, are obtained. When the $z$-coordinate distribution curve of the support nodes behaves smoothly, the material property of the cables will be fully utilized with a reduction in cable weight. For the optimal mode of the simplified model, the elevations of the support nodes provide the entire cable-net structure a saddle shape, which behaves rigidly under out-of-plane loads. The proposed method can be extended to solve optimization problems of similar cable-net structures.

In order to capture noncooperative targets at close distance, vision-based target detection and tracking have been used as key technologies with great challenges. This paper proposes a novel triangular component detecting and tracking method. In the detection stage, line segments are first extracted from the regions containing triangular spans. Isolated but similar segments are merged under the distance and angle constraints. Then the endpoint distance and area ratios are used as the measurement to determine possible triangle candidates. The geometric property is introduced to discard false-positive candidates. In the tracking stage, the triangle model is characterized by the visual features and geometric relationships of its line segments. An affine transform matrix is constructed to find the correspondence between models and objects in consequent frames. Finally, a line matching–based tracking strategy is designed. Experiments on synthetic and natural images verify the effectiveness of the proposed method in detecting and tracking triangles.

Vibration-based methods are promising for damage identification; however, their capabilities for damage identification under temperature variations are usually limited. In the paper, a vibration-based nondestructive global damage identification method based on a genetic algorithm (GA) is proposed to identify structural damage location and severity under the influence of temperature variation and noise. The proposed method is verified by a number of damage scenarios of a three-span continuous beam and a two-span steel grid and shows good robustness under random noise levels. First, considering that the material properties of a structure and boundary conditions of a system are generally temperature-dependent, the relation between temperature and elastic and geometric stiffness matrices is introduced, and damage parameters along with temperature are defined as variables of the numeric model. Second, a GA is introduced where a new objective function with different weight coefficients, combined with frequencies and mode shape, is proposed and developed. Third, damage identification of a three-span continuous beam and a two-span steel grid under temperature variation is carried out numerically, considering changes of material properties and boundary conditions, and damage existence, location, and severity are accurately identified. Finally, it is shown that the proposed method is very robust even when the data are polluted with artificial noise.

Resampling of standard particle filters will cause particle depletion and require abundant particles in the state estimation, which can hardly meet the accuracy and velocity requirements of a modern radar tracking system. This paper proposes an improved multiple-maneuvering-target tracking algorithm based on a novel intelligent particle filter. The improved algorithm combines the bat algorithm and particle filters and takes particles as bats to simulate behavior of bats in pursuit of prey. By adjusting frequency, volume, and pulse rate, particle groups search for the optimal value and move to high likelihood areas intelligently under the guidance of the optimal particle. Meanwhile, it improves the optimization mechanism of the bat algorithm; dynamic control of searching velocity and perception range are proposed. It makes the algorithm seek optimization within a self-adaptive cognition range, and the optimizing rate can be adjusted dynamically to control the balance of global and local optimizing abilities. Furthermore, the improved algorithm combines interacting multiple model and joint probabilistic data association, which enables improved accuracy in target tracking and robustness in a complex environment by iterative optimization. Simulation results show that the improved algorithm enhances the performance of a multiple-maneuvering-target tracking system.

The Laplace transform-based spectral element method (LTSEM) can avoid some limitations of the conventional Fourier transform-based spectral element method (FTSEM) in handling finite-length structures, such as the wrap effect. In this paper, a damage identification technique based on the LTSEM and strain statistical moment (SSM) is proposed, which is combined with a general genetic algorithm. The dynamic responses of beam structures were first analyzed using the LTSEM and were validated by finite-element analysis results. Then, the strain statistical moment was used as the damage indicator; the damage location and severity for beam structures were numerically identified on the basis of the proposed technique. Finally, an experimental investigation for damage detection on a simply supported steel beam was also implemented. The experimental results verified that the proposed damage identification technique is feasible and has robust antinoise performance.

Aerospace and civil structures are sometimes measured under free constraints. In many substructuring methods, the substructures are independently analyzed with free constraints as well. When a structure is free because of insufficient structural constraints, the experimental flexibility is influenced by rigid body motion. This study formulates projection matrices perpendicular to rigid body motions for removing the rigid body components of flexibility of a free–free structure. Without losing generalization, a mass-normalized projection matrix is formulated for the dynamically tested free–free flexibility, and the dynamic condensation method is used to formulate the projection matrix for the partial measurement. The proposed projection matrix not only removes all rigid body components in the measured flexibility but also constructs a dual inverse between the singular free–free stiffness and flexibility, thereby making the flexibility-based model updating feasible for free–free structures. The projection matrix is applied to a laboratory subway lining placed on the ground without constraints and to the substructuring method for substructure-based model updating. The experimental examples prove that the projection matrices are effective in the identification of free–free flexibility for model updating and damage detection.

Identifying the frequencies of practical bridges can help in understanding the bridge dynamic property. However, the vibration amplitude excited by wind load and traffic load is too small, resulting in a small signal noise ratio, which affects the performance of frequency identification. This paper proposes frequency identification procedures for practical bridges to reduce the influence of noise through higher-order spectrum. First, a higher-order spectrum is introduced. Then, the frequency identification procedures are presented for practical bridges. Finally, the proposed procedures are applied to a practical bridge. A higher-order spectrum can eliminate the influence of Gaussian white noise (GWN) or reduce nonstationary random noise during frequency identification. The advantage of using the higher-order spectrum to identify frequencies is verified by simple artificial signals combined with sinusoidal signals with different frequencies and GWN. The results show that the second-order spectrum obtained by the higher-order spectrum has better frequency identification performance than the tradition spectrum and that the frequencies of the practical bridge can be identified successfully.

This paper presents a parallel mesh deformation solver using radial basis function (RBF) interpolation. The solver computes the displacement of each internal point independently without using the topological relations, and is further accelerated by an incremental approach based on the data reduction algorithm. The incremental approach makes full use of the matrix and solution of the previous step during the greedy selection procedure, and gives a better initial solution of the current RBF system of equations. To enhance the robustness and efficiency of the solver in parallel, for nonpredefined boundary movement, each CPU process computes the same interpolation function; for predefined movement, an additional process can be used to calculate the interpolation function one step earlier and broadcast it to other processes. Four typical mesh motion cases are simulated to demonstrate the deforming capability and parallel performance of the proposed method. Finally, several parametric setting rules of the deformation approach are presented for better usage.

For the computation of the dynamic properties and response of structures using the finite-element method (FEM), there are two main options for the mass matrix modeling: the lumped model and the consistent model. Both have their own advantages and drawbacks. This work focuses on the influence of mass matrix models on flutter computations of aircraft structures. Even nowadays, with high computational power resources at our disposal, a reduced and precise model for the flutter problem could be of great value given the large number of compilations required to solve it. In order to find the most suitable mass matrix to achieve this task, four different mass matrices have been considered in this work: consistent, lumped, and two optimal (proposed by the authors). Three different aircraft structures have been analyzed: a clean wing, a wing with a tip mass, and a wing with a large tip mass. For every case, the same conclusion was drawn: the result using the consistent mass matrix converges with significantly fewer elements than when using the lumped mass matrix. In addition, the proposed optimal mass matrices, which are a weighted linear combination of the lumped and consistent mass matrices, do not improve the results obtained by the consistent mass approach either computationally or in accuracy. These conclusions contradict the usual procedure followed for flutter computations within the industry, where the lumped mass model is widely spread.

This paper presents an efficient approach for dynamic stress analysis of a long-span bridge under the combined action of stochastic traffic and wind loads based on the stress influence function, which converts the stress finite-element analysis problem into a problem of stress integral or summation. The Aizhai Bridge in China is used as an example to demonstrate the numerical procedure for the stress analysis. First, the finite-element model for the Aizhai Bridge is established, and the critical locations of the bridge are determined by refined analyses. By using influence line analysis, the stress influence functions to calculate the dynamic stress response of the bridge under random traffic and wind loads are obtained through segmented polynomial fitting. Second, the random traffic flow simulation program is compiled by adopting the Monte Carlo–simulation method. The moving equivalent dynamic vehicle loading, representing different types of running vehicles on the bridge, is calculated based on the full interaction analyses of a single-vehicle–bridge–wind system, and the database of the equivalent vehicle load is established. Finally, the stress induced by the combined action of the random traffic and wind loads is obtained by summing the product of the stress influence surface function and equivalent vehicle loads and by integrating the product of the stress influence line functions and wind loads where the dynamic effect is considered by introducing the dynamic influence coefficient. The proposed method is validated by comparing the results with those from analysis with commercial software, and it is found that that the proposed method is feasible and efficient.

A new fractional-order sliding mode control (FOSMC) scheme is investigated for air-breathing hypersonic vehicles (AHVs). The control scheme proposed is designed based on linear-quadratic optimal theory. First, by using the technique of input/output feedback linearization, the longitudinal model of AHVs is linearized to be decoupled tracking-error dynamics. Second, a linear-quadratic regulator (LQR) is designed to ensure that the tracking-error dynamics converge to the equilibrium point as soon as possible. Based on LQR, a novel fractional-order sliding manifold is introduced. Subsequently, the FOSMC is designed to reject system uncertainties and reduce the magnitude of control chattering. Then, Lyapunov stability theory is used to prove the finite-time convergence of the closed-loop system. The simulations indicate that the proposed control scheme shows excellent performance and robustness in the absence of uncertainties. Compared with conventional integer-order sliding mode control (IOSMC), the high-frequency chattering of control input is drastically depressed.

This work was motivated by the need for knowledge about the natural decrease in water pressure from room temperature creep, separate from the decrease in pressure caused by leakage in tightness tests of new and/or repaired pipelines. In this context, a case study was conducted regarding the decrease in water pressure in a test pipe with dimensions of 530 mm in outside diameter and 7.8 mm in wall thickness. The system was pressurized to a pressure of 8.985 MPa, after which the water supply was stopped. The test pipe, 4 m in length, was made from linepipe steel L360NB and was closed by torispherical heads at both ends. Special attention was given to the thermal insulation of the test pipe, ensuring an average temperature of 14.5°C, with variation within 0.1°C over a period of 24 h. A nonlinear ordinary differential equation was derived to describe the time gradient of the water pressure in the test pipe in relation to time, pressure, coefficient of compressibility of water, and cross-sectional dimensions of the test pipe. The necessary creep parameters and static tensile properties were obtained from specimens manufactured from a ring 30 cm in width taken from the same pipe. The orientation of the specimens was circumferential. The creep tests were performed over a period of 24 h at ambient temperature. The creep strains were measured by the strain gauge technique, and the effect of temperature variation during the test period (24 h) was compensated by the use of a compensating strain gauge. The calculated decrease in pressure with time compared quite well with experimental results when the beginning of the pressure decrease was considered 2 h after reaching the initial pressure of water ($p_0=8.985\mathrm{MPa}$) and the supply of water was ended. The time period of 2–24 h follows the recommendations of a pipeline operating pressure standard. A family of “pressure-time” curves were constructed for a pipe from steel pressurized by water to specific initial pressure levels taken as multiples of the pressure at the yield. These curves cover a time interval of 2–24 h, and they can be used to check the total decrease in water pressure after the tightness test has been completed.