Journal of Aerospace Engineering

During the past few decades, various fluid–structure interaction (FSI) analysis approaches have been developed and applied to understand the physical processes related to a flexible flapping wing. In this paper, a dynamic shell analysis based on a corotational (CR) formulation is developed. Geometrically nonlinear dynamic shell formulation based on the CR framework is derived from Lagrange’s equation of motion. This brand new shell formulation is implicitly combined with a preconditioned Navier–Stokes solution for a relevant FSI analysis. Specifically, the shell analysis is extended to a multibody dynamic approach to facilitate the passive pitching motion of a flapping wing. The present analysis is validated by a comparison with the results from either previous analyses or experiments. The effect of a passive pitching motion on a flapping wing is also investigated. Finally, it is found that the presented dynamic shell analysis can enable accurate predictions, and the relevant passive pitching motion of a spanwise flexible wing may be advantageous for generating several aerodynamic loads that are applicable for the control forces of a micro aerial vehicle.

A novel optimal sensor selection approach is developed in this paper. The key innovation of this work is in formulating the stochastic optimal sensor selection problem as a second-order convex program. The approach quickly determines the required optimal sensor specifications that meet mission navigation and trajectory dispersion requirements with the lowest sensor cost. The proposed approach combines linear covariance analysis with convex optimization to describe and solve the optimal sensor selection problem. First, the trajectory dispersion of the closed-loop guidance, navigation, and control (GN&C) system based on sensor specifications is modeled using linear covariance analysis theory. Then, the linear covariance propagation and update equations are used to formulate an optimal sensor selection problem using the Kronecker product. Second-order cone programming with successive approximation techniques are used to solve the established problem. Finally, a simple nonlinear closed-loop GN&C system is investigated, and the capabilities of the proposed approach are demonstrated. The simulations show that the optimal sensor selection problem can be described and solved efficiently using the proposed approach.

Offset-feed elliptical aperture reflectors have better mechanical and electrical performance than circular aperture reflectors and are an inevitable trend for the future development of high-precision space-borne antennas. The surface mesh geometry is crucially important to ensure surface accuracy and effective aperture of mesh reflectors. This paper proposes a quadratic-curve method for mesh generation of elliptical aperture reflectors that can achieve an attractive pretension uniformity and large effective aperture. From the mechanical behavior point of view, the generated cable net is beneficial to the truss. First, the steps to generate the quadratic-curve cable net for elliptical aperture reflectors are proposed. Then, the nodal coordinates of cable net are optimized using a dynamic adjustment method of nodes. The numerical examples demonstrate the effectiveness of the proposed method.

The problem of horizontal-plane tracking control of an omnidirectional, four vectored-thruster aerostat subjected to actuator failure is considered. The actuator failures result in the aerostat’s becoming underactuated, so that it can affect only surge force and pure yaw moment about the body center. To achieve accurate position control in the horizontal plane, direct position control is used instead of heading control. This mode of control is called degraded tracking control, in contrast to full authority control of the overactuated four vectored-thruster aerostat. This degraded tracking controller uses commanded yaw rate to track lateral position, and yaw moment to eliminate lateral position error; therefore, yaw angle is not directly controlled. To guarantee the stability of the yaw motion, a virtual reference point (VRP) tracking strategy is proposed, in which the VRP is used instead of the body center (BC) in position tracking. The VRP generates a negative compensated force in the surge direction, which makes the side-force and yaw moment have the same sign and thus ensures that the aerostat is in a stable tracking configuration. In addition, the VRP decreases the transmission ratio of commanded yaw rate to commanded lateral velocity, making the aerostat’s yaw motion vary slowly during the transitional phase so that steady position tracking is obtained.

Strand corrosion can cause the flexural capacity deterioration of prestressed concrete (PC) beams. An analytical model incorporating the effects of strand cross-section reduction, material deterioration, concrete cracking, and bond degradation is proposed to predict the flexural capacity of corroded PC beams. The effect of flexural cracks is also included in the model. An equivalent bond stress concept is introduced to consider the effect of flexural cracks, which is further implemented into the flexural capacity prediction of corroded PC beams. The proposed model is validated by the experimental results collected from the previous studies. Results show that the proposed model can give an accurate prediction for the flexural capacity of corroded PC beams. Considering the effect of flexural cracks can improve the precision of the prediction model. The flexural capacity deterioration of PC beams depends on corrosion degree. Strand corrosion less than 5.5% can lead to a slight decrement of flexural capacity. As corrosion progresses the flexural capacity would exhibit a significant deterioration.

This paper presents a numerical investigation of a concept for improving film cooling performance by setting a step upstream the transverse trench. The step, which is placed upstream the transverse trench, is used to modify the approaching boundary-layer flow and its interaction with coolant to improve the lateral spreading of coolant. Five different relative distances between the steps and transverse trenches (upstream distances) are investigated, including $-20$, 0, 5, 10, and 15 mm. The film cooling performance is evaluated at a density ratio of 0.97, with the blowing ratios ranging from 0.5 to 2.0. The numerical results of film cooling with an upstream step and the numerical results of film cooling with holes embedded in transverse trenches show an agreement with the experimental data when three-dimensional average Navier–Stokes equations are solved with the standard k-$\epsilon$ model. Detailed adiabatic cooling effectiveness and total pressure loss coefficients are simulated. Results obtained indicate that film cooling performance in the region downstream from the film hole is sensitive to upstream distances. In the case with an upstream distance of 0 mm, the lateral spreading of coolant is superior to that in other cases, which leads to a higher lateral adiabatic cooling effectiveness. The case with an upstream distance of 0 mm is a better choice when the improved lateral adiabatic cooling effectiveness and the reduced total pressure loss penalty are taken into account.

Various measurements are now available for structural health monitoring (SHM) due to the fast development of sensory systems. Utilization of multitype measurements including local and global information for SHM has typically outperformed that of solo type measurements. However, the limited number of sensors for measurements hampers the effectiveness of SHM. Thus, response reconstruction at the locations of interest in which sensors are unavailable with limited measurements has drawn significant research attention. The Kalman filter (KF) is a powerful tool to estimate optimally the unknown state vector of a structure that has numerous applications in civil engineering. One main concern for KF is that it requires good estimates of the noise covariance information, which is generally difficult to determine. Therefore, this paper investigates the dual-type responses reconstruction by using the moving-window Kalman filter (MWKF) with unknown measurement noise covariance (MNC). The weighted average of the MNC was first evaluated by utilizing the moving-window estimation technique. Then the dual-type of measurements including strains and displacements were fused together to reconstruct the structural responses at unmeasured locations. Numerical and experimental investigations were conducted to verify the effectiveness and feasibility of the MWKF in dual-type response reconstruction. The results indicate that the MNC can be well estimated and the reconstructed responses agree well with the real or measured responses.

The fuel-optimal rapid cooperative rendezvous problem between two spacecraft under finite thrust, based on the indirect method, was investigated and converted into a two-point boundary value problem (TPBVP) in this study by using Pontryagin’s maximum principle. First, normalization processing of initial costate variables with unknown scopes was carried out to restrict them on a unit hypersphere. The quantum particle swarm optimization (QPSO) algorithm was used to preliminarily search for the initial costate variables of the high-dimensional energy-optimal problem, and then the results obtained were further corrected by the sequence quadratic programming (SQP) algorithm. The preceding combinatorial optimization algorithm with normalization technique considerably increases the probability of finding the approximate initial values of the globally optimal solution. Based on these modified initial costates, the smooth energy-optimal results were transitioned to the desirable nonsmooth fuel-optimal results by the homotopy method. Through the combination of the preceding effective techniques, the following difficulties were successfully overcome: (1) The optimal control was a strongly nonlinear problem under continuous high thrust; (2) in a cooperative rendezvous, the terminal rendezvous orbit was unknown and the parameter variables were doubled, leading to high-dimensional control equations; and (3) the narrow convergence domain of the shooting function made the shooting process extremely sensitive to the initial guess of the costates. The simulation results demonstrate not only the feasibility of the indirect method in solving fuel-optimal cooperative rendezvous, but also the superiority over another orbit transfer optimization method, the hybrid method.

Acoustic emission (AE) monitoring has been identified as one popular technique applicable for damage discrimination in composites. However, because the damage is not directly identified from the AE data, other nondestructive techniques are usually needed to further evaluate the damage. Moreover, there is no general guideline for AE monitoring in composites because of the unique AE signatures of each defect and the diverse properties due to varied material configurations. Therefore, this study aimed to develop a reliable method using advanced pattern-recognition techniques on AE data to identify damage modes exhibited in the glass fiber reinforced polymer (GFRP) coupons and composite systems with similar configurations. Three sets of GFRP laminate samples originated from a new fiber-reinforced polymer (FRP)–balsa wood composite bridge were tested to study critical failure modes such as fiber breakage, matrix cracking, delamination, and debonding. A methodology based on unsupervised pattern recognition was then developed. First, in order to eliminate uncorrelated AE features, the complete link hierarchical clustering algorithm and principal component analysis were used to preprocess the data and select a suitable subset of AE features for the clustering task. The unsupervised $k$-means clustering analysis was then applied to separate the date into several clusters and correlate each cluster to its corresponding failure mechanisms. After a reliable AE database was built for a typical sample of each test set, neural networks (NNs) based on multilayer perceptron (MLP) and support vector machine (SVM) algorithms were developed. Finally, the trained NNs were used for pattern recognition in samples with unknown damage modes. Most results conformed to the visual observation and thus led to the NN models with good performance. The developed network system was also applied on a full-scale GFRP bridge deck with a similar configuration, and the performance achieved further confirms the potential of the developed methodology for damage identification in full-scale structures.

Recently, a variety of nondestructive evaluation (NDE) approaches have been developed for health assessment and residual capacity estimation of timber structures. Among these methods, guided wave (GW)–based techniques are highly regarded as effective tools for potential use in real situations. Nevertheless, because it is hard to comprehensively grasp the behavior of wave propagation in a wood structure, existing NDE-based techniques mainly depend on an oversimplified hypothesis, which can result in inaccurate or even misleading results in practice. Understanding the complex behavior of GW propagation in wood structures and extracting appropriate information from captured GW signals is a key for successful assessments of in situ conditions of timber structures. This paper analyzes the existing feature extraction and damage detection algorithms, and proposes a novel approach based on an integration of wavelet packet transform (WPT) and ensemble empirical mode decomposition (EEMD) for extracting damage-sensitive patterns, and then a soft computing method like support vector machine (SVM) for pole condition identification. In the proposed method, GW signals measured from a multisensing system with pole health condition as the baseline are divided into a series of subfrequency bands based on WPT. Then EEMD is adopted to extract the intrinsic mode functions (IMFs) that possess the features extracted at corresponding subfrequency bands. Hence, the IMF component was segregated from the original signals of tested poles, and the IMF Shannon entropy was employed to build up the feature vector to effectively demonstrate the health condition. To decrease the size of the feature vector and avoid multiple collinearity among obtained patterns, principal component analysis was employed and entropy information in the feature vector was replaced with main principal components, which will be employed as input variables of the developed SVM model for identifying pole health condition. In order to reduce the assessment error of the SVM model, genetic algorithm was introduced to select optimal parameters in SVM. Finally, the performance of the proposed method was assessed using laboratory timber specimens on which the experimental tests were conducted.

The flutter mechanism for cantilevered beam in supersonic flow is studied based on Timoshenko beam model and piston theory. The partial differential equations governing transverse deflection and rotation angle of the beam are derived by analyzing the finite section of the beam. Based on the mode functions of the unperturbed system, complex natural frequencies are investigated to predict the instability of the beam by Galerkin truncation. The mechanism of both coupled-mode and single-mode flutter caused by the supersonic flow are spotted for different flow velocity with consideration of the shear deformation and rotary inertia in the beam structure by the analytical method. The instability region is discussed by Routh-Hurwitz criterion. The critical Mach number for the panel motion becoming unstable is calculated for given values of the dimensional parameters.

This paper investigates steel alloy stator blade flexural vibration characteristics and fatigue properties analytically using flexural fatigue tests. First, fatigue life data reliability is analyzed and theoretical strain endurance limit $\mathrm{\Delta }{\epsilon }_C$ deficiencies are assessed. A model from the literature is used to predict fatigue life using fatigue test data for four steel alloys to derive an extended estimation expression for the strain endurance limit. The proposed three-parameter fatigue life models can be applied to estimate steel alloy blade fatigue life in practical working environments.

This paper deals with the autonomous flight problem for a quadrotor. The quadrotor must reach its mission waypoints to complete a package delivery task autonomously while avoiding obstructing obstacles. A new linear matrix inequality–based dynamic gain adaptive robust control approach is proposed for the attitude and position controller of the quadrotor. The transient and steady-state tracking performances are guaranteed under bounded uncertain parameters of the quadrotor and external disturbances. To realize a higher level of automation for the quadrotor, an autonomous navigation strategy that consists of a trajectory generation approach and a sampling-based multicriteria local waypoint selection algorithm is presented. The trajectory generation approach is used to realize waypoint-based navigation with the proposed controller for the quadrotor. The sampling-based multicriteria local waypoint selection approach is proposed based on a deterministic sampling process and solved by a multicriteria optimization problem to balance path safety and speed in a structured environment. This autonomous navigation strategy enables the avoidance of path/trajectory planning, which could be time consuming in a cluttered environment. The simulation results demonstrate that the proposed controller and autonomous navigation strategy are useful for a variety of quadrotor tasks, including precise trajectory tracking and autonomous navigation in an unknown obstacle-laden environment.

Structural health indicators, such as modal frequencies, have been commonly utilized to interpret the health condition of monitored structures. This study modeled the relationship between structural health indicators and ambient conditions under severe typhoons. For this purpose, a two-stage Bayesian probabilistic procedure was established. In the first stage, the Bayesian spectral density approach (BSDA) is applied to identify the structural health indicators, namely the modal frequencies in this study, using the measured structural response. In the second stage, the Bayesian nonparametric general regression (BNGR) is introduced to model the relationship between the identified structural health indicators and some selected typhoon-induced ambient conditions. By using Bayesian model selection in conjunction with general regression, BNGR is able to select the most appropriate set of influencing/input variables for the prediction of the structural health indicators without prescribing any functional form. Full-scale measurements of a 22-story reinforced concrete (RC) building were used to demonstrate the efficacy of the procedure. The measurements consisted of over 280 h of structural response and the corresponding ambient conditions captured under the five most severe tropical cyclones that affected the region from 2011 to 2013. This study provides a promising framework for reliable interpretation of the variation of structural health indicators. Although the modal frequencies were considered in this study, the proposed two-stage procedure is applicable for other structural health indicators.

Defects at the interface between the slab track and the cement asphalt (CA) mortar layer can have significant effects on the dynamic response of a vehicle-track system. This paper proposes a method for analyzing the reliability of the ballastless track system affected by a CA mortar gap during service life. A vehicle-track vertical coupling dynamic model considering a CA mortar gap was proposed, and the response surface model representing the dynamic response of the vehicle-track system was obtained. Sensitivity analysis was then undertaken to evaluate the key parameters of the CA mortar gap for the long-term performance of a ballastless track system. The two most critical failure modes were identified and used to construct the corresponding limit-state equations for reliability analysis. The results from numerical studies show that the CA mortar gap has significant influence on the key dynamic responses and can significantly affect the long-term service performance of ballastless track.

Utilization of multitype measurements including local and global information for structural health monitoring (SHM) has typically outperformed that using solo-type measurements. However, in many practical situations, only partial measurements can be obtained. Therefore, multiscale response reconstruction at the key locations of interest where sensors are not available is required. The Kalman filter (KF) is a powerful tool for optimally estimating the unknown structural states. The classical KF technique is, however, not applicable when the external excitations are unknown. In this paper, a KF-based multiscale response reconstruction under unknown input (MSRR-UI) approach is proposed to circumvent the aforementioned limitations. Based on the principle of KF, an analytical recursive solution of the proposed approach is derived and given. By using a projection matrix, a revised version of the observation equation is obtained. Multitype measurements in a few locations are fused together for response reconstruction. The unknown loading is simultaneously estimated by least-squares estimation (LSE). The effectiveness of the proposed approach is demonstrated via several numerical examples.

The pounding response between a single oscillator and a rigid barrier was investigated with dimensional analysis in which the modified Kelvin pounding analytical model was used to simulate the contact force. The dimensionless equation of motion of the single pounding oscillator was deduced. In particular, the structural response parameters of the pounding oscillator were characterized by a set of dimensionless terms which were denoted by the Buckingham notation. Therefore, the key parameters governing the impact reaction were reduced so that they could effectively express the pounding response law of the structures subjected to the earthquake excitation. Numerical analysis proved the superiority of the modified Kelvin model compared with the Kelvin model. The effect of pounding on the response of the single oscillator was illustrated in three well-separated spectral regions which were described by the dimensionless system frequency parameters. Parametric analysis showed that the contact stiffness mainly affects the penetration displacements. In addition, the pounding response is insensitive to the initial spacing, but is affected obviously by the coefficient of restitution in the first spectral region.

Tornadoes are hazards with low probability of occurrence and high consequences that cost the United States billions of dollars each year. Electric power distribution systems are susceptible to damage due to tornadoes, with utility poles being the most vulnerable components. Additionally, the reliability of power distribution systems can be affected by the deterioration of the strength of utility poles with age. Many utility companies nowadays are considering the use of steel and prestressed concrete poles instead of wood poles, which are the most widely used in the United States. To date, very few studies have been performed to study the behavior of power networks when subjected to tornadoes. This paper proposes a framework to compare the reliability of wood, steel, and prestressed concrete utility poles subjected to tornadoes through fragility analysis taking into account the deterioration of the strength of poles with age. In addition, fragility analysis is used to perform life cycle cost analysis and scenario-based cost analysis on utility poles subjected to tornadoes. The results show that utility poles are vulnerable to even weaker tornadoes of EF1 and EF2 scales. Because EF1 and EF2 tornadoes have a considerable probability of occurrence, especially in tornado-active regions, this supports the direction toward designing utility poles against those tornadoes. Moreover, the age of utility poles is shown to have a large impact on their resistance to tornadoes, with aging poles vulnerable to tornadoes that are much weaker than those that new poles can resist.

Hysteresis phenomena for shock wave reflections in the supersonic jet were already studied in the literature and reported that hysteretic behaviors were affected by the transient pressure ratio. However, there is no investigation on the hysteresis phenomenon in counter-flow thrust-vector control. The objective of the present work is illustrating effective influences of hysteretic behaviors on deflection angle, static pressure distribution along the upper-suction collar, and the secondary mass-flow ratio in a counter-flow thrust-vector control system. The fully implicit finite volume method was selected to solve unsteady, compressible Reynolds-averaged Navier-Stokes equations with boundary conditions of pressure inlet and pressure outlet. Two-dimensional numerical simulations were carried out based on the standard $k\text{-}\epsilon$ turbulence model and standard wall function. Second-order accuracy was ensured to illustrate more insight into hysteresis phenomena in counter-flow thrust-vector control. Hysteretic behaviors were studied by changing pressure ratios with time linearly, which include the nozzle-pressure ratio and the secondary pressure ratio. Numerical results indicated that hysteresis loops of the deflection angle and the secondary mass-flow ratio were formed by varying the nozzle-pressure ratio or the secondary pressure ratio with time, respectively.

This paper addresses the development of control systems for the orbit control of spacecraft around irregularly shaped rotating asteroids with uncertain parameters. The objective is to steer the spacecraft along prescribed orbits. First, a nonlinear adaptive law for orbit control was designed. This was followed by the design of a supertwisting adaptive (STWA) control system. In the closed-loop system, which includes the adaptive law or the STWA law, all the signals remain bounded, and the trajectory tracking error asymptotically converges to zero for any initial condition. Finally, under the assumption of boundedness of the derivative of the uncertain functions of the model in a region of the state space, a supertwisting control (STW) law for finite-time convergence of the trajectory was obtained. Based on the Lyapunov theory, stability properties of the closed-loop systems were analyzed. Simulation results for 433 Eros and Ida asteroids were presented for illustration. The results showed that control of spacecraft along closed orbits or to a fixed point is accomplished using each of these controllers, despite uncertainties in the parameters of the asteroid models.

Modern turbomachinery is progressing with regard to its compact geometry structure, superior thermodynamic efficiency, and high power density. The rotating speed, operating temperature, and pressure ratio of the turbine are significantly increased with this development. As a result, the centrifugal load and thermal load should be treated more seriously with these latest developments. To this end, the leakage model and fluid force model of the labyrinth seal and the dynamic model of the rotor system are established to take into account the centrifugal growth and the thermal growth. The effective seal clearance, axial flow velocity, and working fluid-exciting force are updated in each calculation step with the varying rotational speed or operating temperature. Using a three-dimensional (3D) contour map, spectrum cascade, and bifurcation diagram, the influences of the radial growth on the leakage performance and the dynamic characteristics of the rotor system are analyzed at steady state. The results show that the leakage flow rate and discharge coefficient are greatly reduced by the radial growth, which implies an improved sealing effect. The dynamic responses of the rotor system remain nearly unchanged when accounting for centrifugal growth only. Excluding the impact of the thermal growth, the system stability is slightly enhanced with the operating temperature. However, the system stability appears to be much more sensitive to temperature when thermal growth is included in the dynamic model.

In this paper, a bird impact test was carried out on a static hollow blade, and a numerical model corresponding to the test was established based on the dynamic explicit finite-element software PAM-CRASH and its smoothed particle hydrodynamics (SPH) method. Good consistency between the simulating and experimental results proves the validation of the calculating method used in the model. Then, the numerical model of a rotary engine fan assembly with the same hollow blades was established, and its dynamic responses to bird impact were studied under different working conditions. Results show that the bird speed, mass, and impact location, and the rotating speed of the fan, have significant effects on the final deformation of the engine blades. The conclusions presented in this paper can provide a reference on anti-bird-strike design and airworthiness verification of the engine fan blade.

The mechanical behavior of case-carburized 18Cr2Ni4WA steel is investigated by an optimization method combined with finite-element simulation based on indentation tests. For a measured load-displacement curve, the uniqueness of the combination of the strain hardening exponent and yield stress of a material is discussed by analyzing their effects on the maximum reaction force and residual displacement. The result shows that the effects of yield stress on the load-displacement curve are more pronounced than those of the strain hardening exponent when the elastic modulus is prior known. An optimization model containing an equality constraint is constructed and carried out in every indentation location, and its results indicate that the strain hardening exponent may be constant with depth in the case-carburized steel. The conclusion is further verified through a modified optimization model proposed herein. Therefore, the gradient in the mechanical behavior for the carburized material is primarily reflected by the variation in yield stress with depth. The correlation between hardness and yield stress is established through the least squares method according to the results of the modified optimization model and should help predict the mechanical behavior of parts in the carburized material.

An enhanced approach of empirical mode decomposition based random decrement technique (EMD-RDT) is proposed for output-only modal parameter identification of structures using ambient vibration measurements. In the conventional EMD-RDT method, modal parameters are identified for each mode through Hilbert transform or least-square fitting from the free-decay modal responses, which are produced by EMD and RDT sequentially. The identification process is time consuming, and many uncertainties are involved. The novel enhancements of the proposed method lie in two aspects: computation efficiency and uncertainty treatment. On one hand, a novel decomposition method is proposed to separate the mode shape and the modal coordinates from the free-decay modal responses, thus making the identification process more efficient. On the other hand, a bootstrap approach is employed to quantify the uncertainties of modal parameters by providing surrogate estimates to generate useful statistics. Examples using both simulated data from a six degrees-of-freedom (six-DOF) system and experimental data from a three-story shear building structure are presented to demonstrate the proposed method, of which the effectiveness and the efficiency are confirmed by the identified results.

A computational fluid dynamics (CFD) investigation was aimed at accurately predicting the air flow characteristics in the vicinity of underwing-mounted instruments on the Facility for Airborne Atmospheric Measurement’s (FAAM) BAe-146-301. Perturbation of the free stream airflow as it passes through the region of detection of the underwing instruments may lead to additional uncertainties in the measurement of clouds and cloud particles. The CFD model was validated with flight data from an Aircraft-Integrated Meteorological Measurement System (AIMMS-20) in a wing-mounted instrument canister. Flow predictions showed a consistent slowing from the true air speed of the aircraft in the longitudinal direction and the introduction of horizontal and vertical flows up to 10% of the air speed. The potential impact of these flow perturbations on sizing of particles with cloud imaging probes was modeled. Sizing errors were dependent on the methodology used and the shape of the particle; those due to transverse flows remained very small but mis-sizing due to unaccounted longitudinal flow perturbations were potentially more serious.

A simplified method is developed to calculate aerodynamic heating, ablation, and structural temperature response for a body traveling at high speeds. Mach number, altitude, and angle of attack are used as a function of time. Compressibility effects are considered by using Eckert’s reference temperature approach. Convective aerodynamic heating is calculated using external flow relations. Local pressure values are found through modified Newtonian theory. An approximate recession method that uses the heat of ablation is coupled to the aerodynamic heating. An in-depth solution accounts for material decomposition; however, it does not include the energy absorption of pyrolysis gases through the material. Reduction in the heat transfer coefficient caused by the transpiring gases is estimated. An Arrhenius equation is used to model the density of the ablative material. The method is examined for the validation of six different cases, and predictions are found to be in good agreement with experimental and analytical results. The verification studies indicate that the methodology is well suited for predicting the ablation and thermal response of a thermal protection system.

Mechanical properties of composites are dependent on the constituents’ properties and microstructure. To numerically reveal the effects of fiber shape on the transverse dynamic properties of polymer composites, unit cell models considering different fiber shapes are established with the finite-element method and a polymer’s rate-dependent behavior is described using the modified Bodner-Partom model. There are some other micro characteristics including fiber distribution patterns and voids that have important influences on composites’ transverse behaviors. Models with different regular fiber distribution patterns were analyzed to evaluate the influences of fiber distribution on the response prediction. Then, to evaluate the effects of voids, results from models with different void distribution patterns, void sizes, and void shapes were compared. It shows that among all models established, the highest transverse flow stress value comes from the square-shaped-fiber model without considering the change of fiber cross-section orientation and voids that result in significant reduction in the responses. The flow stress values of most noncircular-fiber models are higher than that of the circular-shaped-fiber model. For each kind of fiber distribution pattern, the flow stress value from the model with square-shaped fibers was the highest. The location, size, modeling method, and shape of the voids affect responses of the models with different fiber shapes to different degrees.

Drive-by bridge parameter identification has been an active research area in recent years. An instrumented vehicle passing over a bridge deck captures dynamic information of the bridge structure without bridge closure and on-site instrumentation. The vehicle dynamic response includes components associated with the bridge surface roughness and the vehicle and bridge vibration. It is a challenge to separate these components and extract the bridge modal parameters from the vehicle response. A novel drive-by blind modal identification with singular spectrum analysis is proposed to extract the bridge modal frequencies from the vehicle dynamic response. The single-channel measured vehicular response is decomposed into a multichannel data set using singular spectrum analysis, and the bridge frequencies are then extracted via the blind modal identification. Numerical results showed that the proposed method is effective and robust to extract the bridge frequencies from the vehicle response measurement even with Class B road surface roughness. The effects of the moving speed and the vehicle parameters on the identification were studied. A vehicle–bridge interaction model in the laboratory was studied to further verify the proposed method using one- and two-axle vehicles.

Microwave heating has emerged as a feasible sintering and melting method for lunar regolith simulant. Unlike conventional heating methods, microwaves operating at 2.45 GHz penetrate the material and drive a dielectric heating process within the material volume itself. This property makes microwave sintering a viable and ideal candidate for localized, potentially targetable material properties modification. In this feasibility study, the impact of microwave thermal postprocessing as a fast, localized heating source to influence porosity, microstructure, crystal formation, mechanical properties, and macrocrack formation of conventionally presintered samples is investigated.

Efficient and reliable thermal insulation for cryogenic propellant storage in orbit is an essential ingredient for long-duration missions in future space exploration. A new combinational thermal insulation composed of spray-on foam insulation (SOFI) and variable density multilayer insulation (VD-MLI), is suitable for cryogenic propellant storage due to its excellent heat insulation and light quality. Based on the heat transfer calculation model of SOFI/VD-MLI for ground and space stages, the variations of total heat flux through the combinational insulation with the thickness of SOFI and VD-MLI, the warm boundary temperature, and storage medium were analyzed. Comparison between numerical results and experimental results from the literature shows that the model is feasible to be applied in engineering. In the ground stage, the heat flux through the SOFI/VD-MLI decreases by approximately 92.7% with a SOFI thickness of 10 mm than 150 mm in the condition of constant VD-MLI thickness, and declines by 8.4% with the VD-MLI thickness of 10 mm than 150 mm in the condition of constant SOFI thickness. In the space stage, however, the descent degree of the heat flux is approximately 13% and 90%, respectively, in the corresponding aforementioned conditions. Because the warm boundary temperature is constant, the total heat flux through the combinational insulation will be less sensitive to the storage medium. Furthermore, variations in VD-MLI thermal resistance and heat fluxes by solid heat conduction and heat radiation with layer position presented a jumping change at the interface of different layer density regions. The results show that there is an optimal combination thickness for SOFI and VD-MLI on the ground and in space.

The effects of Reynolds number on the wind-induced responses of a 243-m-high solar tower are modified by increasing the surface roughness of the test model. First, a series of wind tunnel tests on eight types of circular cylinder test models with different surface roughness are carried out to measure the aerodynamic forces. The results show that the most suitable surface roughness for Reynolds number modification in view of the aerodynamic forces is the height (or the width) of the rib $d/D=0.01$ and the net distance between adjacent ribs $e/D=0.02$. Second, the pressure coefficient around the circumference of the rough test model with $d=1\mathrm{mm}$ and $e=2\mathrm{mm}$ are measured to validate the results obtained from force measurement wind tunnel tests. Finally, the wind-induced responses of the 243-m-high solar tower with the selected surface roughness are measured on the basis of elastic wind tunnel tests under a structural damping ratio of 0.3%, 0.7%, 1.0%, 1.5%, and 2.0%. The results show that the RMS wind-induced responses of the solar tower are markedly reduced by taking the effects of Reynolds number into account. However, it seems that the mean wind-induced responses in the along-wind direction have nothing to do with the surface roughness. The wind-induced responses of the rough solar tower decrease with increase of the structural damping ratio. It appears that it is not suitable to totally adopt the current code to determine the wind loads on the solar tower.

Natural laminar flow (NLF) design is widely used to reduce skin friction drag to improve aircraft aerodynamic performance. In this paper, a differential evolution (DE) algorithm was applied to a NLF-designed transonic nacelle. The class shape transformation (CST) method was tested in terms of accuracy before being adopted as the geometry parameterization method that describes three longitudinal profiles constructing the nacelle surface. The purpose of this optimization is to extend the laminar length of each longitudinal profile of the nacelle while maintaining pressure drag under a preset limit. A high-fidelity computational fluid dynamics (CFD) solver was used for accurate laminar/turbulence transition prediction. It was tested in terms of pressure distribution and particularly laminar transition prediction. The whole process was executed via a Python version 3 script automatically. The laminar length was extended on longitudinal profiles after DE operation. The laminar area of the optimized nacelle surface was increased by 16.64% and total drag coefficient was decreased by 11.6 counts.