Journal of Aerospace Engineering

In this study, an adaptive critics design based on a support vector machine (SVM) is adopted to design a finite-horizon optimal feedback controller. The adaptive critics design consists of actor and critic networks. The actor (control input) and critic (cost-to-go) network are trained off-line with respect to various initial states and final times within a finite step. Using the well-trained actor-critic, the near-optimal feedback control solution can be obtained online. In the process of applying SVM to the adaptive critics, an adequate kernel function and parameters depending on the kernel function must be selected. In this study, a polynomial function and radial basis function are used for the SVM kernel function to implement the algorithm. A minimum control effort problem with final constraints for spacecraft rendezvous is considered to demonstrate the performance of the proposed the developed algorithm with respect to each kernel function and to show its potential for designing an optimal controller.

Moisture content in concrete structures is commonly associated with many durability problems (e.g., steel corrosion and freeze-thaw) and can be used in structural health monitoring and nondestructive testing of concrete structures. In this paper, the use of synthetic aperture radar (SAR) imaging for characterizing subsurface moisture content and distribution is proposed, using a concrete panel specimen as an example. A $0.3\times 0.3\times 0.05\text{-}\mathrm{m}$ concrete panel specimen cast and conditioned in laboratory was air-dried for almost 5 months and simultaneously monitored for its moisture variation by SAR imaging. From the analysis of its SAR images at different moisture contents, it was found that SAR images can not only qualitatively capture the moisture distribution inside concrete but can also quantitatively correlate with the overall moisture content inside concrete. Higher SAR amplitudes indicate stronger electromagnetic scattering response, suggesting greater values of dielectric constant. A critical contour in SAR images is proposed as an insightful measure for the subsurface sensing of concrete. An approach for determining the critical contour in each SAR image is proposed.

A strengthened concrete-filled steel tubular (CFST) arch bridge which has experienced serious damage due to long-term heavy-truck action is studied. An in-depth study of the dynamic performance of the bridge due to dynamic vehicle loading is performed by field testing and a numerical method. The dynamic performance of the reinforced bridge is more typical and representative than that of new bridges without damage experience. Dynamic field testing is conducted, and the measured data are obtained after strengthening construction for the bridge. The modal parameters and dynamic impact factors are identified, and the proposed three-dimensional vehicle and bridge coupled vibration (VBCV) analysis model is validated based on the experimental results. The influence of vehicular speed, lane position, vehicle weight, road surface condition (RSC), and local damage on the dynamic performance for the bridge due to moving vehicles is studied comprehensively. It is found that the existing design code underestimates the dynamic impact for this bridge. The obvious difference between global and local dynamic amplification effects for the bridge induced by vehicles needs to be given more attention. The dynamic performance of the bridge is not improved distinctly after conventional reinforcement, and some innovative strengthening methods which can improve structural dynamic performance when subject moving vehicles need to be further researched.

Pile barriers as an effective method for isolating ambient vibration have been used in practical engineering for many decades. However, the study of multiple rows of pile barriers in saturated soil is rare, especially of periodic pipe pile barriers in saturated soil. Based on a new method called the COMSOL partial differential equation (PDE) method, this paper investigates the vibration isolation of plane waves by periodic pipe pile barriers in saturated soil. First, through some comparisons with results from related works, the COMSOL PDE method is validated. Second, the first complete attenuation zone (CAZ) of pipe piles embedded periodically in saturated soil is studied; the effects of parameters such as the filling fraction, arrangement configuration, and filling materials on the first CAZ are discussed. Two kinds of formation mechanism of the first CAZ for periodic filled pipe pile barriers, the Bragg scattering mode and the locally resonant mode, are investigated comprehensively. Both two-dimensional (2D) and three-dimensional (3D) models with multiple rows of periodic pipe piles in saturated soil are built, and the displacement responses behind the pile barriers are analyzed in the frequency domain. This study provides a sound background for the application of periodic pipe pile barriers in saturated soil to isolate ambient vibration in engineering practice.

It is well known that concrete is the most common type of structural material, and a proper estimation of concrete strength in its early age can provide guidance for staged concrete construction and ensure the load-bearing capacity of concrete structures during construction and service periods. In this study, an active sensing approach integrated with piezoceramic-based transducers called smart aggregates (SAs) was experimentally investigated to estimate the strength development of the entire early-age (0th h to 28th day) concrete in real time. A pair of SAs was embedded in a reinforced concrete specimen prior to casting. During the 28 days of the curing period, one SA was employed as an actuator to periodically generate a designed stress wave that propagated along the specimen; meanwhile, the other SA was applied as a sensor to detect stress waves. The wave responses of the SA sensor were recorded and analyzed during the 28-day testing period. The experimental results demonstrate that the magnitude of the detected stress wave in the concrete increased with the strength of the concrete in the 28-day curing period. In addition, a hydration monitoring index was established and compared to the previous results of the 28-day concrete strength curve obtained by standard compressive tests. The proposed hydration monitoring index and the 28-day concrete strength curve show similar trends, which indicates that the proposed method has the potential to estimate early-age concrete strength in real time.

Fatigue cracks are critical structural concerns for steel highway bridges, and fatigue initiation and propagation activity continues undetected between physical bridge inspections. Monitoring fatigue crack activity between physical inspections can provide far greater reliability in structural performance and can be used to prevent excessive damage and repair costs. In this paper, a thin-film strain sensor, called a soft elastomeric capacitor (SEC) sensor, is evaluated for sensing and monitoring fatigue cracks in steel bridges. The SEC is a flexible and mechanically robust strain sensor, capable of monitoring strain over large structural surfaces. By deploying multiple SECs in the form of dense sensor arrays, it is possible to detect fatigue cracks over large regions of a structural member such as a bridge girder. Previous studies have verified the SEC’s capability to monitor fatigue cracks under idealized harmonic load cycles with a constant crack propagation rate. Here, an investigation is performed under more complex and realistic situations to translate the SEC technology from laboratory testing to field applications—specifically, as cracking propagates under (1) a decreasing crack propagation rate, and (2) random traffic load cycles with stochastic peak-to-peak amplitudes and periods. An experimental program was developed which included an efficient data collection strategy, new loading protocols, and crack-sensing algorithms. The experimental results showed an increasing trend of the fatigue damage feature, crack growth index (CGI), under crack initiation and propagation, despite decreasing crack propagation rates or random traffic load cycles. In addition, the results also showed that the SEC did not produce false-positive results when cracks stopped growing. The findings of this study significantly enhance the SEC’s fatigue sensing and monitoring capability under more realistic loading conditions, which is a critical step toward field applications of this technology.

In the postcapture phase, compliant manipulation can maintain the target undamaged and ensure the safety of the manipulation procedure. In most studies, due to the compliant equation, the compliant control scheme is not effective enough to perform all kinds of the manipulation missions in the postcapture phase. In this paper, a constrained compliant control scheme is proposed for safe manipulation of a postcapturing uncertain target with space robotic system. In order to manipulate the target safely and effectively, a compliant reference trajectory of the desired trajectory is generated considering the constraints of the contact force and torque. By constructing a Barrier Lyapunov function (BLF) based constrained tracking controller, the control performance is designed in advance, and the constraints are preserved during the whole procedure. Finally, the simulations of stabilizing an uncertain tumbling target in the postcapture phase are designed to illustrate the effectiveness of the proposed control scheme. With the comparisons to the general compliant control schemes, the safety and effectiveness of the proposed control scheme are validated via digital simulations.

This paper investigates the energy-dependent mission planning of an agile earth observation satellite. According to the similarities between energy-dependent mission planning and the dynamic traveling salesman problem (DTSP), the energy-dependent mission planning is converted into a DTSP through two mappings from observation angle and energy to city and distance. In addition, to eliminate the uncertainty of feasible attitude trajectories/energy for a given scheduling strategy, a multiple-stage optimal energy factor is developed for the model extension. The paper further uses the time-optimal $\min \mathrm{\Delta }{T}_{\mathrm{trans}i-1,i}$ in the transition time constraint as an input of the model to enlarge the optional execution time for the subsequent target in a consecutive observation. To solve this problem, a hybrid method integrating the Gauss pseudospectral method and the genetic algorithm (GPM-GA) is proposed which uses the genetic algorithm to generate the feasible solutions for scheduling process, whereas the Gauss pseudospectral method (GPM) is used to optimize the energy and the transition time constraint parameter for each solution. Extensive simulation results show that, compared with the classical genetic algorithm (CGA), the energy consumption and simulation time of the proposed algorithm are decreased effectively. In particular, the simulation time decreases more obviously with larger target sizes. Furthermore, attitude trajectory projections of satellite motion provided by GPM-GA are much smoother. These numerical and visualization results demonstrate the superiority of GPM-GA in terms of energy efficiency, computational efficiency, and attitude trajectory smoothing.

This paper presents a nondestructive rapid condition or health assessment procedure of three-dimensional civil structures following strong earthquakes. It is a nonlinear time domain system identification procedure and structures are represented by finite elements. The excitation time history may not be available soon after an earthquake and is not required to implement the procedure. The response time histories measured at a small part of a large structure, denoted as a substructure, can be noise-contaminated. The location and severity of defects in an element are evaluated by comparing its stiffness parameter with the previous value or deviation from the other similar members. The basic unscented Kalman filter concept available in the literature is significantly improved, increasing its implementation potential. The improved concept is developed by integrating the iterative least-squares, weighted global iteration, and objective function algorithms. The proposed concept is verified by assessing the health of large three-dimensional structures excited by earthquake time histories. The procedure is capable of identifying defects with different levels of severity in small or large civil structures. This study also indicates that the inspection outcomes are not sensitive to the location and configuration of the substructure. The capability and efficiency of the procedure are demonstrated with three informative examples.

Most unmanned aerial vehicles which fly with moving wings or encounter gusts at low Reynolds number conditions make pitching oscillation motions. The aerodynamic forces and flow field around the wings vary dramatically with the unsteady motion parameters (reduced frequency, mean angle of attack, and amplitude). This study conducts numerical simulations to investigate the influence of such parameters on the aerodynamic characteristics of a pitching NACA 0012 airfoil. The respective cases are researched through computational fluid dynamics (CFD) based on the finite-volume method (FVM). The governing equations are the unsteady, incompressible two-dimensional Navier–Stokes (N-S) equations. The airfoil performs sinusoidal pitching oscillations with respect to the quarter chord at the Reynolds number $2.53\times {10}^5$. A detailed analysis of the force coefficients and how their evolution is affected by the dynamics of flow structures generated during pitch oscillations is presented. The results show that these parameters change the instantaneous force coefficients quantitatively and qualitatively. The effective angle of attack is different at various locations of the airfoil chord during the oscillation motion, which determines the deviations of forces at the same angle of attack during pitch-up and pitch-down periods. It is also observed that the strength, interaction, and convection of the vortex surrounding the airfoil are significantly affected by the variations of these parameters.

The disturbance-observer-based nonlinear model predictive control (DOB-NMPC) for air-breathing hypersonic vehicles (AHVs) is proposed in this paper. The longitudinal dynamic model of generic AHVs is first redesigned and optimized for prediction, whereas the cost function of nonlinear model predictive control (NMPC) is designed with series expansions and derivative feedback. A disturbance-observer-based control (DOBC) law is then developed for enhancing robustness in the presence of external disturbances and parameter uncertainties. The relationship among tracking errors, sample time, and estimation errors is finally investigated. Compared with existing tracking control for AHVs, the proposed DOB-NMPC achieves tracking commands effectively without sacrificing the nominal tracking performance, and the convergence range of tracking errors is verified by stability analysis and simulation studies.

To simulate the conditions of the space environment at ground, the Laboratory of Application and Innovation in Aerospace Science (LAICA) of the University of Brasília (UnB) is developing a dedicated testbed to reproducing nanosatellite attitude motion. The testbed is composed of an air-bearing table and a Helmholtz cage. The air-bearing table is a spacecraft simulator that can simulate frictionless conditions with three rotational degrees of freedom. Balancing the simulator is essential in order to make the gravitational torque negligible. The testbed is also equipped with a Helmholtz cage to recreate the Earth’s magnetic field conditions that spacecrafts encounter in orbit. This paper presents the design and realization of this low-cost testbed. A simple and efficient automated balancing algorithm based on the least-squares method (LSM) is proposed and validated by experiments. The performance of the proposed simulator is evaluated and compared with previous works.

In this paper, the multiple-step loading damage evolution in two-dimensional (2D) SiC/SiC ceramic-matrix composites (CMCs) at room and elevated temperatures and subjected to different applied cycle numbers was investigated. The unloading/reloading fiber axial stress distributions were analyzed, considering different fiber–matrix interface damage regions. The interface debonding and slip lengths upon unloading/reloading were obtained, considering the coupling effects of multiple loading stress levels and interface damage. The relationships between the hysteresis loops, interface debonding and sliding, and different loading sequences were established. Comparisons of damage evolution between constant and variable fiber–matrix interface shear stress, constant peak stress with cycle loading and multiple loading, and low-high multiple loading and high-low multiple loading sequences were conducted. The experimental cyclic multiple loading/unloading stress–strain hysteresis loops and fiber–matrix interface slip lengths of 2D SiC/SiC composites were predicted for different testing temperatures and applied cycles. The effects of temperature on the fiber–matrix interactions and the evolution of interfacial shear stress and the effect of cyclic loading number on the multiple-step loading damage evolution of two different SiC/SiC composites are discussed. With increasing temperature and applied cycle number, the area of the hysteresis loops and the interface debonding/sliding lengths subjected to multiple loading stress levels increase.

Structural health monitoring (SHM) is the process of conducting structural condition diagnosis and prognosis based on appropriate analyses of in situ measurement data. Direct assessment of structural condition using time series response measurements can be classified as a type of statistical pattern recognition, in which structural condition is evaluated by comparing the statistical features of current data with those of baseline data. The philosophy behind this approach is that the time series response acquired under different structural conditions presents different statistical characteristics. As a consequence, the key step in structural condition classification is to detect the points at which the statistical properties of a time series response change; this is referred to as change-point analysis. The present study proposes the use of a computationally efficient binary segmentation (BS) approach for change-point detection in order to classify and assess structural health condition. The proposed approach, which falls into the category of data-driven diagnosis, does not require knowledge about the structure and is appealing for attaining an automated SHM system. The practicality and effectiveness are illustrated through real-world monitoring data acquired from a cable-stayed bridge and a high-speed train, both of which experienced structural damage/degradation over their service lives.

The technique of prestressing near-surface-mounted (NSM) carbon fiber–reinforced polymer (CFRP) has good potential and future prospects for structural strengthening due to its improvement of the bond between the CFRP and concrete. However, notably few investigations have been conducted to study the bond durability of the NSM CFRP–concrete interface. The present study was undertaken to increase the understanding of the durability of the bond between the NSM CFRP and concrete under freeze-thaw cycles. Pull-out tests were conducted to investigate the influence of freeze-thaw cycles on the interfacial bond with ordinary concrete and concrete with high strength or additional frost resistance. The mechanism of freeze-thaw cycles degrading the bond of the NSM CFRP is discussed. Additionally, the influence of the distance between concrete edge and groove (edge distance) and the bond length on the bond durability was studied. It was found that, first, freeze-thaw cycling results in a significant decrease in compressive strength of ordinary concrete but a unremarkable degradation of strength of high-strength concrete and concrete with water-reducing agents and admixtures of air-entraining agents. Second, freeze-thaw cycling also leads to a relatively significant decrease in the bond of the NSM CFRP systems with ordinary concrete; however, it has no substantial impact on the bond of specimens of concrete with high strength or frost resistance. This indicates that the deterioration of the bond of NSM CFRP under freeze-thaw cycles is caused by the degradation of concrete. Third, a decrease in the edge distance results in cone-shaped cracks at the loaded end. Finally, when the failure mode remains debonding at the NSM CFRP–concrete interface, the NSM CFRP system with a longer bond length exhibits more significant degradation in bond capacity compared with that with a shorter bond length. This is because the residual friction stress at the debonding interface also decreases due to freeze-thaw cycling; thus, a longer bond length corresponds to a great loss of residual friction force at the debonding zone.

A space rack suffers from shock and broadband random loads during shipment and launch, which may lead to irreversible damage until the spacecraft reaches final orbit. This paper presents a novel dual-functional electromagnetic energy harvester (DF-EEH) that is regarded as an electrically tuned mass damper (TMD) to simultaneously suppress vibration and harvest energy. The equal modal damping method is proposed to optimize parameters of the DF-EEH. Furthermore, a resonant circuit is utilized to improve the energy harvesting performance and vibration suppression efficiency of the space rack. Genetic algorithm (GA) is used to optimize tuning frequencies and damping ratios of the DF-EEH with the resonant circuit. For the DF-EEH with both the resistor and resonant circuit, the response of the space rack is reduced to 9.29, and the harvested energy is $0.41\mathrm{W}/\mathrm{N}$. Therefore, this research can provide a novel vibration control and sustainable-energy supplement concept for the low-power-consumption electronic devices of space structures.

A novel piston-type synthetic jet actuator (SJA) is developed which has an auxiliary air inlet close to bottom dead center (BDC) to alleviate the difficulty during suction duration. The flow characteristics of the novel SJA are studied numerically and experimentally. The effects of the orifice diameter, actuation frequency, and height of the auxiliary air inlet on the actuator performance are also analyzed. The results indicate that the auxiliary air inlet significantly improves the performance of the SJA, especially for small orifices and high actuation frequencies. Compared with the conventional piston-type SJA, the mass flow of the novel SJA is increased significantly, which is the major reason for the increase in cylinder pressure, maximum jet momentum, duty cycle, and efficiency of the actuator.

This paper investigates the fatigue characteristics of a new extruded aluminum 7175, with an experimental composition which uses a magnesium-calcium alloy during the alloying process instead of the standard pure magnesium. This new aluminum 7175, dubbed aluminum ECO7175v1, results in a cleaner manufacturing process and improves mechanical properties. The fatigue behavior of the new aluminum ECO7175v1 T74 temper is investigated. Experimental data show that the fatigue life of ECO7175v1-T74 aluminum can exceed ${10}^7$ cycles with a fatigue strength of approximately 207 MPa, about 36% of its tensile strength. Fractography results show that failure modes are predominately ductile near the surface and brittle toward the center. In addition, at higher stresses, crack initiation points are typically at the surface of the specimens, compared with those at lower stresses. Irrespective of the stresses to which the specimens are subjected, all crack initiation points are located at the surface and no inclusions to act as stress concentrators are seen.

Aiming at the large displacement monitoring needs of slope, road, and bridge engineering, a new method concerning the superwide-range fiber Bragg grating (FBG) displacement sensor based on eccentric gear was investigated. The structure of the sensor was designed and the formula for the wavelength-displacement relationship was established via waveform fitting from calibration tests. The lateral displacement of a first-story shear wall was successfully monitored in a hysteresis test of a steel frame–reinforced concrete infill wall. A FBG relative displacement sensor was developed refitting a superwide-range fiber Bragg grating displacement sensor based on eccentric gear. The refitted sensor was applied in a self-repairing performance test of a shape memory alloy (SMA) strengthened concrete beam. The monitoring results of the crack were compared with the results of an integrated test instrument, and the results showed that the sensor exhibited very good accuracy. The innovative FBG displacement sensor has a low influence on the structure, strong practicability, and good prospects for engineering applications.

Structural system identification has attracted much attention in the structural dynamic field over the past decades. For identifying parameters of the inelastic response of a structure under ground shaking, the Kalman filter (KF) and unscented Kalman filter (UKF) have been used extensively. In this paper, numerical and experimental investigations were carried out to test the capabilities of square-root unscented Kalman filters (SRUKF) and adaptive square-root unscented Kalman filters (ASRUKF) for identifying the parameters of the nonlinear structural system, with the Bouc-Wen model applied to describe the nonlinear hysteresis of magnetorheological elastomer materials. A new method was proposed for parameter initial values estimation, which could ensure that the parameters in the constitutive equation be identified uniquely and thus reduce the influence of the initial error on the parameter estimation. The numerical investigation showed that the ASRUKF outperformed the SRUKF in both convergence speed and estimation accuracy. Furthermore, the ASRUKF was able to track the sudden change of the parameter whereas the SRUKF was not. The experimental results indicate that the estimated Bouc-Wen model through ASRUKF not only presents a good match with the experimental data for a specific input but also keeps physical properties that are inherent to the real data, independently of the exciting input.

In this paper, solid particle erosion behaviors of titanium alloy (Ti-6Al-4V), 304 stainless steel, aluminum alloy (Al6061-T6), and pure nickel materials were experimentally and numerically investigated for different impact velocities (70, 105, 150, and $230\mathrm{m}/\mathrm{s}$) and angles (20°, 30°, 45°, 60°, and 90°). In addition, erosion performances of the same materials on erosion shields used for a helicopter rotor blade were numerically determined under conditions of different angles of attack ($-6°$, $-3°$, 0°, 3°, and 6°) and different particle impact velocities (75, 150, and $230\mathrm{m}/\mathrm{s}$). Numerical analyses were performed with a commercial software program using the finite-volumes method, discrete phase method with the Eulerian-Lagrangian approach, and an erosion model. According to the experimental results, pure nickel material exhibited the best erosion behavior for high impact velocities, whereas Al6061-T6 material exhibited the worst erosion behavior for all impact conditions. As a result of the analyses, the modeling and simulations results were in good agreement with the experimental data. Moreover, 304 stainless steel material exhibited the best erosion performance on the erosion shield surface at impact velocities of up to $150\mathrm{m}/\mathrm{s}$, whereas pure nickel exhibited the best erosion performance at impact velocities higher than $150\mathrm{m}/\mathrm{s}$.

An experimental study on a passive, self-suction, and self-jet control method was conducted to manipulate the flow around a circular cylinder at the Reynolds number $\mathsf{R}\approx 5.0\times {10}^4$. The influence of the spacing of the passive bypass jet rings on the aerodynamic forces and flow structures was investigated in detail. The results indicate that the aerodynamic forces acting on the cylinder increase with distance from the jet rings until reaching a critical distance. The critical spacing ratio, $S/D$, is approximately 2.69 for the convex surface case (Case 1) and $S/D=5.00$ for the plane surface case (Case 2). Compared to a bare cylinder, the fluctuations of the lift coefficient and mean drag coefficient are reduced by approximately 94.61% and 33.18%, respectively, when $S/D=0.38$ (the most effective case) due to the stable wake and reduction of the dissipation of turbulent kinetic energy in the near wake, as well as the extra energy input by the flow from the outlet holes.

Hypersonic vehicles have attracted lasting and worldwide attention in recent years. Considerable aerodynamic drag and severe aerothermal loads are major challenges for hypersonic vehicles. A novel combinational aerodisk and lateral jet concept is proposed for drag reduction and thermal protection in hypersonic flows. The flow field characteristics have been numerically investigated with in-house code. The Reynolds-averaged Navier-Stokes (RANS) equations were adopted to simulate the flow field, and the shear stress transport (SST) $k-\omega$ turbulence model was used to present the turbulent nature. Fluid–thermal interaction is also taken into consideration in this paper. The influences of the lateral jet pressure ratio and its location on the flow field have been thoroughly studied using numerical methods. The obtained results demonstrate that the novel concept is beneficial for drag reduction and thermal protection in hypersonic flows. Increasing the lateral jet pressure ratio can further improve drag reduction performance. In addition, the heat flux can be significantly reduced by increasing the lateral jet pressure ratio. The lateral jet location also has important effects on the flow properties. The peak values of the Stanton number and wall static pressure can even be reduced by 19.76% and 22.15%, respectively, with different lateral jet locations.

Attitude control of a conventional aircraft usually depends on its rudder surface, which reduces the aircraft’s stealth abilities and makes it heavier. To improve aerodynamic performance and optimize flight attitude control, the authors developed a set of circulation-control actuators (CCAs) based on circulation-control (CC) technology to replace the conventional rudder surface and applied these CCAs to a flight test of a small unmanned aerial vehicle (UAV). The authors measure the main characteristic parameters of the CCAs and the flow-field characteristics, which indicate that the CCAs can suppress separation and delay stall at high angles of attack. An experimental assessment of rudder efficiency shows that the CCAs can produce the rudder efficiency required for conventional flight attitude control. Specifically, the maximum roll rudder efficiency that the CCA generates is equivalent to an aileron deflection of 50°, and the maximum pitch rudder efficiency is equivalent to an elevator deflection of 10° at a speed of $10\mathrm{m}/\mathrm{s}$. Finally, a successful flight test of a rudderless (meaning no control surface) UAV based on CC technology indicates that the CCAs designed in this study could replace conventional ailerons and elevators completely to establish control of aircraft flight attitude.

In this paper, we investigate a leader-follower spacecraft formation flying (SFF) mission, which is comprised of two identical spacecraft carrying large-scale antenna arrays. The mission requires spaced antenna arrays and an ultraclose separation distance from the edge of the spaced antenna arrays in order to have an accuracy within millimeters. During maneuvers, the vibration of the antenna is excited by the translational and rotational motion of the spacecraft; in return, the coupling effect influences the accuracy of the spaced antenna arrays and leads to the risk of collision. For this reason, we present a coupled dynamics model of a flexible spacecraft utilizing dual quaternion parameterization. A novel feature of this model is that the translational motion, rotational motion, and vibration of the flex-rigid system can all be described under the same mathematical framework by representing the vibration utilizing dual quaternion parameterization in the modal coordinates. Numerical results are presented to quantify the kinematic coupling effect and to show that the effect is a key consideration for ultraclose formations.

The volute has a significant effect on the unsteady characteristics of the centrifugal compressor. The flow in the centrifugal compressor volute is unsteady and complex. In this paper, the dynamic mode decomposition (DMD) method is used to extract the most dominant flow structures in a turbocharger centrifugal compressor volute under design and mild surge conditions. Large-scale perturbations with synchronic frequencies corresponding to the shaft rotating speed are correctly captured. In addition, nonsynchronic frequency and flow mode in the volute are illustrated. The results show that the high-frequency unsteady flow mode corresponding to the main and splitter blade passing becomes weak in the volute. The development of the unsteady flow mode at the mild surge condition is vividly shown by the reconstruction. Two clear cells in the vaneless diffuser occur under the influence of the volute.

New types of hybrid cable domes combining Geiger and Levy forms are presented in this paper. Their basic configuration and geometry are proposed and discussed. The partition-assembly method for a prestress design of the hybrid-form cable dome is proposed. This method obtains the prestress of each unit by solving equilibrium equations and assembles them by the force-density method and genetic algorithm. The partition-assembly method enables prestress of hoop cables in the same ring to equal each other for an ellipsoid hybrid-form cable dome. Examples of ellipsoid hybrid-form and Geiger-form cable domes are established. Prestress of the examples are calculated by the partition-assembly method. Mechanical properties of hybrid-form and Geiger-form cable domes are analyzed and compared, and the results show that the hybrid form has better properties under nonuniform loads and larger overall stiffness than the Geiger form. Influence of prestress and proper ratio of prestress of middle to outermost hoop are discussed in the last part of the paper.

Variable-geometry structures are useful in aerospace applications because they are deployable, from a compact configuration (launch phase) to a spread geometry (operational phase). They may also benefit from the use of flexible joints, which store elastic energy for automatic deployment. Following the development of a self-deploying antenna frame structure with scissors, the geometrical configuration of a new kind of structure, simpler, lighter, and minimizing mechanical joints between elements, is proposed in this article. This topology can form a three-dimensional structure when partially opened or a planar structure when fully deployed. The applications concern autotensioning structures such as meshed space antenna, deorbiting sails, and also solar panel support structures.

This paper focuses on nonlinear oscillation of a space tether system connected to the Moon’s surface, elongating along the Earth-Moon line at $L_1$ and $L_2$ sides respectively. The full nonlinear elongation dynamics of the space tether system were established for a superlong viscoelastic massless tether with a large tip mass. The equilibria and their stabilities were investigated by removing the external force in the established dynamics. The equilibria and the stabilities depend on the elasticity and natural length of the tether, and the dependence is the key to determining the parameters for successful operation of the tether system. The equilibria were also used as reference positions to facilitate dynamic analysis. The method of multiple scales was utilized to obtain the analytical asymptotic solutions to the dynamics. The analytical results agree well with the numerical ones quantitatively and qualitatively in terms of the steady-state magnitudes of the length and the length rate of the tether, as well as the power harvested for a large range of parameters. The detailed investigation reveals that the dynamic responses were affected by three important parameters, namely, the damping, the elasticity, and the original length of the tether. In particular, the increasing damping stabilizes the motion around the expected equilibrium and makes the quasi-periodic motion periodic. It was also indicated that one should design a tether with small rigidity for efficient power generation, but the largest steady-state length should be restricted within prescribed ranges when selecting the rigidity. There is also a compromise between the power output and elastic tension when designing the tether system, i.e., the tether will experience a larger elastic tension for greater harvested power.

The present study quantifies the mixing efficiency of triangular tabs with and without corrugation, in the presence of favorable and adverse pressure gradients. Measurements were carried out to study the effects of square corrugations on triangular tabs in controlling the mixing of a supersonic elliptic jet of Mach number 1.5 of aspect ratio 3.37. The blockage caused by the simple tab is 2.5% each for two tabs. Whereas, for the corrugated tab, the corrugations over the tab side bring down the geometrical blockage to 2.2%. The experiments were carried out for nozzle pressure ratios (NPR) 3, 4, 5, 6, 7, and 8, covering both favorable and adverse pressure gradients. The pitot pressure variation along the axis (CPD) of the jets, without control, corrugated, and uncorrugated tabs, revealed that both tabs are leading to a considerable mixing enhancement of the supersonic elliptic jet at all pressure conditions studied. But, unlike the axisymmetric jet, the CPDs for both of the tabs overlap, especially in the near-field region. The core length for the uncontrolled jet is around five times the equivalent exit diameter of the nozzle exit (5D); whereas, it comes down to about one times the equivalent diameter of the nozzle exit (1D) for both the tabs at NPR 3. Therefore, both simple and corrugated triangular tabs at the minor axis could reduce the core length by about 80%. Therefore, the performance of corrugated triangular tabs to control the elliptic jet is similar to the simple triangular tabs. But, since the blockage offered by the corrugated tabs is smaller, it is advantageous from a propulsion point of view.