Journal of Aerospace Engineering

In the applications of composite honeycomb sandwich structures, they often are severely impacted, resulting in partial penetration or complete perforation. To study the mechanical response of composite honeycomb sandwich structures under high-velocity impact, an extended $P\text{-}\alpha$ model was developed to describe the equivalent property of honeycomb materials. Extended $P\text{-}\alpha$ model parameters were determined using analytical and experimental techniques. Based on the equivalent model, the impact response had good agreement between the experimental results and numerical simulation. By comparison with the detailed model in terms of ballistic limit, residual velocity, maximum displacement of the projectile, and contact time, the results from the equivalent model are not only reliable but also computationally efficient, because the equivalent model requires only 25% as much time as the detailed model under the same conditions.

Guided waves have been widely utilized in various structural health monitoring (SHM) techniques because of their long propagation distance and high sensitivity to small-scale defects in structural types such as plates and bars. In particular, T-bars are typical structural components commonly used in aerospace engineering, playing significant roles in load-carrying. Thus, the health states of T-bars largely affect the integrity and safety of aircraft structures. This paper presents a novel defect-detection method for T-bars relying on the examination of propagation characteristics of multimode guided waves. To select proper wave frequencies and modes that are sensitive to different locations of defects in T-bars, the dispersion curves and cross-sectional mode shapes of multimode guided waves are investigated systematically using the semianalytical finite element (SAFE) method. A defect localization strategy is then developed based on the multimode characteristics, able of identifying defects both along the longitudinal direction and the cross-sectional region. A weighted gathering method is presented to reduce the interference from the modes other than the selected ones, giving rise to increased accuracy of defect identification. Both finite element (FE) simulations and experiments are performed for an aluminum T-bar structure to validate the feasibility and precision of the proposed method.

Deployable solar arrays play an important role in the realization of various spacecraft missions. Taking the flexible spacecraft as an application background, the natural characteristics of a jointed-panel structure is investigated in this paper. Based on the energy conservation principle and Rayleigh-Ritz method, an analytical approach for obtaining the natural frequencies and extracting global mode functions of the flexible jointed-panel structure was developed. Considering the solar panel as an isotropic rectangular plate and the connection at each hinge as a joint with an additional linear rotational spring, the natural frequencies and the corresponding global mode functions were obtained theoretically for the transverse vibration of the solar array. Simulation results for a typical solar array with two panels are given and were used to validate the proposed approach by comparing the obtained natural frequencies and those calculated from the finite-element method. The discussion for the effect of the rotational spring stiffness on the accuracy of the natural frequencies obtained by the proposed approach was conducted. It is shown that the natural frequencies obtained are matched very well with those from the finite-element method for a moderate stiffness of the rotational spring. The proposed approach can provide a theoretical basis for the preliminary design, the nonlinear dynamic analysis, and the design of active vibration controller for the solar array.

Construction of habitats and infrastructure on Mars will require the extensive use of in situ materials. Regolith, which covers the planet’s surface, can be used to produce a Martian concrete but this requires the development of a binder system that is suited to unique conditions present on Mars. This paper presents the development of a magnesium silica binder system that could be produced from materials available on the surface of Mars. Magnesium oxide, amorphous silica, and water were combined with four different Mars regolith analogues from New Zealand to produce mortar cube samples. The sensitivity of the binder system to variations in a water–cement ratio was also determined for a single regolith analogue. The fresh properties of each mix were determined and the mechanical characteristics of porosity and compressive strength were assessed at 7, 28, and 90 days. The results showed that magnesium silica binder–regolith system achieved compressive strengths in excess of 35 MPa at 90 days, which would be suitable for a wide variety of Martian structural applications.

This work investigated the performance of thrust vectoring with a secondary injection in a convergent–divergent nozzle. Analytical modeling was conducted to determine the shock location and shock angle, providing a guideline for system design and performance. Thereafter, numerical simulation was performed to understand the effects of the injection pressure, injection location, injection angle, and injectant on thrust vectoring performance. Satisfactory agreement between the analytical modeling and numerical simulation suggests that shock reflection within the nozzle must be avoided to produce efficient thrust vectoring. Numerical results showed that both the injection pressure and the injection location have significant impacts on the thrust angle. The lateral thrust, and hence the thrust angle, was found to increase with the injection pressure and to be larger when the injection was situated closer to the nozzle exit. Impacts of the injection angle and the injectant on the thrust angle were found to be insignificant. However, hydrogen peroxide (${\mathrm{H}}_2{\mathrm{O}}_2$) can be more effective at producing a side thrust than can air.

Recently, studies on the control and aerodynamics of multirotor unmanned aircraft system (UAS) have been performed. However, some studies depend on methods that require significant computing time or are not applicable in the time-transient domain. Thus, this study focuses on constructing an aerodynamic analysis that is suitable for UAS flight simulation. First, unsteady dynamic inflow and blade element theory (BET) are combined in a feedback loop form. Additionally, rigid blade flapping, which is not considered for UAS analysis, is adopted. A parametric study is conducted under various forward conditions and the results of the proposed aerodynamic analysis are compared and verified with experimental results obtained in previous studies. The results indicate that the flight performance of a multirotor UAS is significantly affected when rigid blade flapping is considered. Specifically, while the drag shows discrepancy greater than 30% when the rigid blade flapping is not considered, it will decrease to a range smaller than 10% when the relevant term is considered. This matches well with the experimental results. As a result, the aerodynamic analysis presented in this study can be applied to flight simulations and nonlinear control algorithms for unsteady flight conditions.

A new set of attitude parameters that minimize the eigenangle between a body-fixed frame and a target reference frame was developed for application to rapid attitude maneuver of a spacecraft. The target reference frame is defined by using the pointing vector from the spacecraft to the target. Attitude kinematics based on the new set of attitude parameters is subsequently derived. To accomplish fast attitude maneuver, a cascade-saturation controller is then designed based on the proposed attitude parameters. The constraints of slew rate and control torque are considered in the proposed cascade-saturation controller. The stability of the resulting closed-loop system is proved by the Lyapunov theory. Simulation results verify that the proposed parameters are well-suited for spacecraft rapid target acquisition and tracking control.

Due to their construction features, many structures have confined spaces, such as the hollow tube of a wind turbine tower and the tube members of large-span space structures. To meet the requirement of aesthetics, the confined hollow space can be used to house a damper. This paper proposes new types of pounding-tuned mass damper for vibration control of a confined space (PTMD-CS). Both a double-sided PTMD-CS (DS-PTMD-CS) and a single-sided PTMD (SS-PTMD-CS) were proposed for use in a confined space. To examine the control performance and robustness of the proposed PTMDs-CS, a hollow steel tube was chosen as the control object. The proposed PTMD-CS was installed inside the confined space of the hollow steel tube. The simulation and experiments were performed to evaluate the effectiveness of the damper. To study the damper’s robustness, the natural frequency of the PTMD is tuned away from the structural resonance frequency and forced vibration tests were conducted. The simulation results indicate that the damper can reduce the vibration amplitude significantly even in the detuned cases and has a strong robustness. In addition, the experimental results agree well with the numerical results, which indicate that the pounding force model in this paper is accurate.

The filtered-x least mean square (FxLMS) algorithm has been widely used in active noise and vibration control of time-varying structural systems, where the secondary path is estimated online by injecting an auxiliary signal. However, the continuous injection of an auxiliary signal will increase the residual error. In this paper, a logic unit is proposed to remove such a drawback from the LMS-based control algorithm. The logic unit decides whether to start/stop injecting an auxiliary signal. The start procedure (SAP) and stop procedure (SOP) send out trigger signals of the logic unit. A flexible structure, which has a time-varying concentrated mass, was established to investigate the adaptive control method. Simulation and experiments were conducted to verify the feasibility and effectiveness of the logic unit. The results have proved that the adaptive control method with the logic unit can suppress vibration effectively in the presence of time-varying dynamics.

The control surfaces failures significantly reduce the maneuverability of the aircraft. The problem may appear in every flying platform, and its consequences can lead to a serious accident. To counteract the aircraft’s lower maneuverability, the adaptive system could be added. That is why, a reconfiguration of unmanned aircraft flight control system was a subject of research project. The developed system uses the other control surfaces and engine to take over scope of stuck control surface. One of the system validation test involved human operators to recognize their reactions while the system is active. The test was performed with the use of an unmanned aerial vehicle (UAV) flight simulator with implemented reconfiguration algorithm. The paper presents the results of an experiment during which the various configuration and failures were tested. A group of UAV operators repeated a simple maneuver task—slalom, under different failure settings. To assess the system and operators’ workload during the test, a subjective assessment was done. The Cooper-Harper, Bedford scales, and overall workload questionnaires were used. To get a full spectrum of the operators’ responses to the reconfiguration system, the objective evaluation was also done. The UAV flight parameters (velocities, accelerations, and position) were registered automatically during the flight tests. Based on those data, the quality index of each flight was generated, providing the objective flight performance assessment.

Modern aircraft design, given the trend to achieve smaller sizes and higher altitudes, urgently needs to introduce new configurations to achieve higher aerodynamic efficiency within a range of moderate Reynolds numbers (Re). The present study, which focuses on low-speed close-coupled biplanes, discovers a new mechanism that could increase the overall maximum lift-to-drag ratio of the configuration. This paper proposes a combination of geometrical parameters that achieves good aerodynamic performance through a set of constructive wing interferences of close-coupled NACA4412 biplanes. With this combination of parameters, the maximum lift-to-drag ratio of a biplane is improved by 3.69% relative to the maximum summative lift-to-drag ratio of two independent monoplanes at $\mathit{Re}=3\times {10}^6$. Moreover, the maximum lift-to-drag ratio of a biplane is enhanced by 3.99% relative to that of a single monoplane and is 10.26% higher than the maximum summative lift-to-drag ratio of two independent monoplanes at $\mathit{Re}=3\times {10}^5$. This paper studies three effects of the close-coupled wing interference. First, the upward wash effect on the upper wing induced by the lower wing is investigated. Second, the forward and upward pushing effects on the upper wing at the high pressure zone around the stagnation point of the lower wing is studied. Third, the pressure distribution melioration along the upper surfaces of both wings contributed by the positive pressure gradient zone near the narrow flow path between the upper and lower wings is examined. These effects collectively guarantee a high lift-to-drag ratio of a close-coupled biplane. The concept of the total pressure boundary layer is proposed to analyze the effects of the Reynolds number. The enhancement of the third effect is found to lead to a biplane’s better performance over that of a monoplane when Re decreases.

This paper presents an approach for real-time autonomous obstacle avoidance for fixed-wing unmanned aerial vehicles (UAVs) for scenarios in which a UAV is required to stay close to a reference path. A key challenge is rapid trajectory generation around obstacles while accommodating vehicle constraints. A UAV model with nonlinear dynamic constraints provides more natural accommodation of the vehicle’s constraints than a kinematic model with linear constraints. This paper presents a method for using finite horizon model predictive control with a custom solver that offers low solution time. A comparative study of a high-fidelity model and a lower-fidelity counterpart is presented. Using the proposed method, the high-fidelity model provides better trajectories than the lower-fidelity counterpart, despite both having low computational requirement for onboard trajectory generation in an embedded platform.

The dynamic sizing optimization problem of a truss structure is studied for its weight minimization with a constraint of the frequency response function (FRF) over a certain frequency bandwidth. First, a discrete representation of the FRF is proposed such that the integral FRF variation within the frequency range can be indicated effectively and efficiently by means of the dynamic responses at several specified characteristic frequency points. As a consequence of this approximation, the continuous FRF constraint is well represented through a few discrete constraints at those characteristic frequencies. Then, an evolutionary optimization procedure is developed based on the FRF sensitivity analysis. In the optimization procedure, the FRF amplitude is reduced while the resonance peaks (eigenfrequencies) within the frequency range of interest are essentially kept. The minimum weight design of a structure is obtained indirectly from a preliminary design by a lesser increase of the weight together with a significant reduction of the structural FRF. Finally, the proposed optimization method is verified through two typical examples, and the obtained results show that the present optimal design approach on an FRF constraint is superior to the natural frequency constraint for overall attenuation of the structural dynamic responses in a wide frequency range.

For precise attitude control of space robots intended to capture a target, the inertial parameters of the target need to be well estimated because the motions of arms and bases are coupled. This paper presents a momentum-based, online, one-step identification method for estimating inertial parameters of tumbling targets with linear momentum by space robots after capture. For obtaining the target’s mass and mass center, the method is the same as the traditional momentum-based method, while two contributions are made for identifying inertia tensor. The first contribution is the second-order term $m_n{\mathbf{r}}_n\times {\mathbf{v}}_n$ in the identification equation is written as the product of two functions: function of time and function of mass and mass center of the target. Secondly, the traditional recursive least squares (RLS) method is modified to obtain the corresponding inertia tensor without iteration from the beginning. To verify the validity and feasibility of the proposed identification technique, 2D and 3D models with different targets and initial velocities are both simulated and analyzed. The simulation results show that all the estimated values converge to their ideal values and the method can be easily achieved online via recursive techniques.

In the last 3 decades, sinusoidal theory has been increasingly utilized to research mechanical behaviors of layered composite and sandwich structures. Nevertheless, the existing sinusoidal model will encounter trouble in precisely yielding the buckling loads of layered structures composed of layers with different material properties. Thus, a refined sinusoidal model is offered for the buckling analysis of composite and sandwich structures that can simulate the zigzag effect of the in-plane displacement and meet the free conditions of transverse shear stresses on the surfaces. In the light of the elegant sinusoidal model, a three-node beam element has been constructed to work out the discrete eigenvalue equation coming from the stability behavior. Making use of a mixed variational theorem from the literature, the finite-element formulation can meet beforehand the continuous conditions of transverse stress at the interfaces. The three-dimensional finite element method (3D-FEM) results are utilized to evaluate the precision and efficiency of the proposed approach through a numerical example. The proposed finite-element formulation can produce satisfactory results with lower calculational cost, and some interesting conclusions are presented. However, the results for stability of layered structures made of plies with different material properties will be overestimated by utilizing the models violating the continuous prerequisite of interlaminar stresses.

A significant research effort in aviation is currently focused on the integration of electric or hybrid-electric power-trains onboard aircraft in an effort to improve efficiency and environmental friendliness. New designs incorporating these novel propulsion systems face the issue of penalizing battery characteristics, especially in terms of limited energy and power density performance, in turn imposing a toll on the inert weight of the machine. A possible solution to this issue is that of structural batteries. These are similar in structure to carbon fiber composites, where the matrix features dielectric characteristics, making the structure capable of storing electric energy while retaining the capability to withstand mechanical loads. The adoption of this technology, currently under advanced development, shall enable significant weight savings; yet it also raises relevant issues concerning aircraft sizing procedures that need to be conceived taking into account the specific characteristics of such multifunctional materials. This paper faces the new problem of aircraft initial design in presence of structural batteries. First, it presents a method for aircraft preliminary weight sizing, where the double effect of structural batteries on both structural mass and energy storage mass is considered. Subsequently, a procedure to size an airframe structure with the adoption of structural batteries in key components is shown, based on a weight-optimal approach. The complete sizing procedure is illustrated through an award-winning test case in the General Aviation category.

A novel two-scale topology optimization method is developed in this work to optimize three-dimensional (3D) plant-inspired fluidic adaptive cellular structures for morphing applications. In this method, the coupled mechanical behaviors of the 3D smart structures with fluidic cells are simulated by extended multiscale finite-element method. Multiscale base functions are constructed through the microscale computation to create the relationship between information of the single cells in the microscale and structural deformation in the macroscale. Furthermore, the 3D structural topology algorithm based on the power-low interpolation approach is combined with the multiscale method to improve the mechanical behaviors of the plant-inspired cellular structures. Consequently, the plant-inspired cellular structures can be designed by the proposed optimization method, in which the distribution of the motor cells is optimized to maximize the structural performance. Then, the smart structures based on fluid actuation of the cells can be optimized to create biomimetic compliant mechanisms, where self-actuated output displacements are set as the design objective. Moreover, the proposed two-scale optimization algorithm is investigated to optimize the number of liquid motor cells in order to minimize the weight of the cellular structure. Numerical examples including the design problems of morphing wings indicated that the two-scale topology optimization method can be effectively used to design the 3D plant-inspired cellular structures.

In this paper, a data-driven structural health monitoring (SHM) approach is proposed to conduct in situ evaluation of the structural health state, i.e., damage location and extent, in which guided Lamb wave responses at selected locations are employed. The proposed approach is composed of an offline and an online phase. The objectives of the offline phase are to carry out data dimensionality reduction and to establish the mapping relationship between sensor data and damage status. First, a comprehensive database is established via high-fidelity finite element method (FEM) simulations (ABAQUS software) to determine guided Lamb wave responses (e.g., displacement and acceleration) under various prescribed structural damage conditions. Then, the proper orthogonal decomposition (POD) method is applied to extract key features from these responses under each simulated case. Finally, a neural network-based surrogate model is developed to relate the damage status with modal coefficients of the POD. The goal of the online phase is to quantify the damage location and extent using limited sensor measurements. The gappy proper orthogonal decomposition (GPOD) is employed to reconstruct the full field information based on limited sensor data. Subsequently, the associated damage extent and location are derived by applying the surrogate model developed in the offline phase. The proposed data-driven SHM approach is comprehensively validated using simulation data harvested from both beam and plate examples. The maximum error between evaluated and actual damage values is within 10%. Parametric studies are conducted as well to investigate the effects on damage detection using different sensor placement and sensor types. In summary, the proposed approach could lead to an efficient damage detection technique for aerospace structures.

To monitor bolt connection status and detect bolt looseness in engineering structures, a bolt pretightening force sensor was designed and fabricated with a strain gauge bonded on the top surface of a bolt head. The stress–strain states of the top surface of the bolt head under different pretightening forces were analyzed using the finite element method (FEM). It was found that the radial strain on the top surface of the bolt head and the pretightening force of the bolt have a good linear relationship. A bolt sensor integrated with the designed radial strain gauge on the top surface of the bolt head was designed and fabricated, and an experimental device was also set up. The experimental results showed a good linearity between the radial strain on the top surface of the bolt head and the bolt pretightening force. The FEM and experimental results demonstrate that the proposed method and the designed sensor can effectively measure the bolt pretightening force. The proposed method and sensor are a new way to monitor bolt connection status and detect bolt looseness. The proposed method is easy to implement and has good application prospects.

Piston theory is widely used in modeling unsteady aerodynamic force caused by surface vibration. The nonlinearity of piston theory is determined by its high-order terms, which are significantly influenced by different working conditions. However, theoretical studies of the nonlinearity of high-order terms receive little attention. Based on previous works, this study discussed the physical meaning of high-order terms in piston theory and conducted a detailed theoretical analysis of the nonlinear behavior of high-order terms. Results demonstrated that the physical meaning of high-order terms denotes the nonlinearity of weak shocks (nonisentropic) and expansions, which can be approximated by isentropic theory in small vibrations. However, when the vibration amplitude and Mach number are large, shocks become stronger (the hypersonic similarity parameter exceeds 0.6) and the nonisentropic effect of shocks becomes significant, leading to the invalidation of piston theory. A spectral analysis concluded that the nonlinearity of piston theory is determined by the superposition of and counteraction between high-order terms.

Large eddy simulation (LES) of supersonic, spatially evolving mixing layers is performed. A transverse perturbation is introduced to force the development of the mixing layer. Special attention is paid to the effect of forcing frequency and amplitude on the properties of large-scale structures and growth characteristics of the mixing layer. Instantaneous plots of discrete vortices and thickness of the mixing layer are obtained to examine the variation of coherent structures and the spatial spreading rate. Results show that transverse forcing has a huge effect on the initial development of the mixing layer. The forcing frequency is able to govern the eddy roll-up and interactions, and the size of saturate eddies is determined by the convective velocity and forcing frequency and is independent on the forcing amplitude. A high forcing frequency and amplitude can enhance the initial spreading rate. In addition, two-frequency forcing with a high amplitude is suitable for increasing the spreading rate and promoting scalar mixing.

The nonlinear ultrasonic Lamb wave has attracted great attention recently due to its likelihood for microcrack detection in plate-like structures. Wave mixing technique has shown advantages over the conventional nonlinear ultrasonic technique. Most of the studies on wave mixing are based on bulk wave mixing. This paper presents a nonlinear Lamb wave mixing technique for enhanced damage detection in thin plate structures. Here, the analysis of the nonlinear interaction of the primary Lamb wave modes propagating in the opposite direction is performed using finite-element simulations and experiments. The results show that due to mutual interaction, additional combinational higher-order harmonics are generated due to nonlinearities. Experiments were conducted on a steel channel section with debond type damage. The results show that mixing of the fundamental Lamb wave modes at the defect zone increases the sensitivity of the damage detection procedure, thereby assisting in damage localization as well.

Within the scope of this study, a quantitative investigation was carried out employing the particle image velocimetry (PIV) technique to determine vortical flow characteristics of 70°/40°, 70°/50°, and 70°/55° double delta wings (DDWs). Experiments were performed in four chordwise sections ($x/c=0.5$, 0.6, 0.8, and 1) at two Reynolds numbers ($\mathsf{R}=1\times {10}^4$ and $2.5\times {10}^4$) and different angles of attack ($\alpha =5°$, 10°, 15°, and 25°). It was observed that the secondary vortex is dominant on flow characteristics of double delta wings at low Reynolds numbers. The strengths of vortices increase with increases of both the wing sweep angle and Reynolds number. Furthermore, the location of wing vortex breakdown moves farther downstream as the wing sweep angle increased. Depending on the angle of attack, Reynolds number, and the wing sweep angle, either dual (strake and wing) or triple (strake, wing, and secondary) leading-edge vortices were observed. At a Reynolds number value $\mathsf{R}=1\times {10}^4$ and angles of attack in the range of $\alpha \le 15°$, the interactions of the vortices influence the flow structure significantly. Dominant frequencies of the vortices are detected near the shear layer formed between the strake vortex and secondary vortex, generally. Moreover, the spectral analyses indicated that bursting of the vortices causes complicated flow patterns over double delta wings, including small-scale vortices. It was found that the transverse Reynolds normal stress concentrates at chordwise sections where secondary vortices are dominant, whereas the locations of peak magnitude of vertical Reynolds normal stress coincide with the strake vortex cores.

In this work, we explore the thermal properties of gallium as an effective phase change material for thermal management applications. Thermal storage and dissipation of gallium manufactured heat sinks were compared to conventional phase change heat sinks. The comparison revealed a 50-fold (80 K versus 1.5 K) potential reduction in temperature during the phase change process due to the high density, thermal conductivity, and latent heat of fusion. The gallium creates shallow thermal gradients when transiently heated, producing a nearly isothermal process. Computational estimates using lumped sum parameters were able to provide simple modeling to predict the results. Gallium based phase change devices offer a combination of low volume, small temperature drops across the device, simplicity of manufacture and design, and high energy storage applications.

Volume-constrained robotic missions seeking to obtain samples from beneath a planetary subsurface may wish to use a rigid drill string consisting of multiple, individual drill bit sections connected together, as opposed to a single, lengthy drill bit. To ensure that drill strings can be assembled and disassembled reliably, it is essential that a robust connection system be used. The authors propose a geometry that seeks to address the requirements of such a mating interface. The proposed solution is based on the bayonet interface, using L- and T-shaped so-called female grooves and male studs connected and disconnected together through a series of clockwise and counterclockwise rotations and single-point clamping events. This routine allows the transfer of both percussion through the drill string and torque in both directions of rotation, while permitting the accurate disconnection of individual drills bits at the required location. Sustained laboratory and field drilling operations suggest that bayonet-style connections offer a reliable solution to the problem of autonomous assembly and disassembly of drill strings in a planetary exploration setting. This paper discusses the development of such a connection system, based on the bayonet connection, which has been implemented in the overall architecture of the Ultrasonic Planetary Core Drill (UPCD). The design trade-off study, which sought to evaluate the use of the bayonet system in comparison with the more conventional screw thread interface, will be discussed, alongside experimental results from percussion transmission testing and drill string assembly testing.

For small fixed-wing aircraft, passive forms of extracting energy from the atmosphere have been shown to have positive improvements on their flight performance. The present work demonstrates the magnitude of performance gains possible from a gust with zero net-air motion through a detailed case study of a simplified flexible sailplane wing. Trends are established for how aeroelastic tailoring may be used as a passive means of energy extraction to further improve these performance gains. An aeroelastic solver is used that combines a higher-order potential flow method with an explicit structural-dynamics model. The aerodynamic model allows for a high force resolution, specifically in induced drag, with very few elements, while the structural model provides a low-cost means of computing elastic deformations in the time domain. By changing the location and spanwise orientation of the wing’s neutral axis, improvements in the energy extracted from a sinusoidal gust, in the range of 2%–12%, were established.

We discuss a methodology for multiobjective optimization of the U-shaped air-delivery duct, which is a part of a turboprop engine intake system. The methodology combines and extends existing techniques to optimize the real-world engineering problem efficiently. The procedure utilizes Kriging models for the approximation of objective functions. We use the Expected Hypervolume Improvement method for improving the Kriging models accuracy and creating the Pareto front. The calculations are parallel and asynchronous, allowing to reduce the time needed for finding the optimal designs. The algorithm is resistant to solver failures and mesh issues. We perform the optimization with a purpose to satisfy two objectives: reduction of a total pressure loss across the duct and improvement/not-worsening of a distortion coefficient at the duct outlet. To obtain the improvement, we modify the shape of the duct within limits to fulfill geometric constraints. We use 30 design variables to control the deformation of the duct surface based on radial basis function interpolation.

In this paper, a series of firing tests have been conducted via a connected pipe facility to investigate the influences of the paraffin blends fuel on combustion characteristics of solid-fuel ramjet engines. A three-dimensional (3D) scanner and scanning electron microscope (SEM) have been used to obtain the local regression rate and the morphology of the solid fuel after combustion, respectively. The results indicate that decreasing the inlet diameter increases the performance of a solid fuel ramjet (SFRJ), and increasing the paraffin concentration and decreasing the port diameter of the solid fuel can enhance the average regression rate but have negative effects on the performance. The SEM results indicate that increasing paraffin concentration can lead to the reduction of the size and quantity of the carbon particles attached and agglomerated at the fuel surface and also has a significant effect on the morphology of the fuel surface, such as the roughness and the separation between paraffin and high-density polyethylene for blended fuel.

This research has provided the results for a static test of a conceptual advanced jet trainer (CAJT) aircraft in a low-speed wind tunnel within take-off and landing angle of attack limits. Test data includes longitudinal aerodynamic properties in take-off and landing phases. The analysis includes characteristics for lift and longitudinal stability, horizontal tail effectiveness, and studying the ground effect on the longitudinal aerodynamic properties. The tests have a Reynolds number of more than 1 million. Experiments were conducted in a low-speed, closed-circuit wind tunnel with a test section of 2.2 m width and 2.8 m height. The tests consist of measuring aerodynamic forces and moments using the six-component balance for strain metering. The slope of lift coefficient curve, lift coefficient value, and maximum aerodynamic efficiency value increase due to the presence of the ground. Also, the total aerodynamic center location moves backward as a result of the presence of the ground. It was observed by comparing the data for ground effect with those for out-of-ground effect that ground proximity significantly increases horizontal tail effectiveness as a component of pitching moment control.

This paper presents exceedance probability assessment, which is a statistical evaluation method for pedestrian wind environment based on numerical simulations with multiscale coupling techniques. The Meixi Lake community in Changsha, China, was selected as a case study to demonstrate the assessment procedure. First, two numerical models, the large domain model (LDM) and small domain model (SDM), were established for large eddy simulations (LES). Second, the multiscale coupling technique, which combined weather research forecasting (WRF) and computation fluid dynamics (CFD), was used to obtain the wind inlet boundary for LDM. Further coupling between LDM and SDM was conducted to obtain the detailed wind field distribution at the inlet boundary for SDM by using the polynomial fitting method. Finally, the exceedance probability assessment of the pedestrian wind environment in the sample community was demonstrated on the basis of joint probability density distribution function of the wind speed and wind direction obtained from historical meteorological data. Results show that multiscale coupling and high-precision interpolation techniques can provide reasonable inlet velocity boundary conditions, and the rationality was successfully verified by the measured data. Furthermore, reasonable inlet boundaries can be obtained to evaluate the pedestrian wind environment using the multiscale coupling technique. The uncomfortable wind environment usually occurs outside groupings of buildings and near high-rise buildings. This assessment framework can serve as a practical tool to evaluate pedestrian safety and comfort among grouped buildings in communities.

The evaluation of stationarity, such as the widely used steady-state response assessment in stepped-sine testing, is a critical issue in structural dynamics. Stepped-sine excitation is a crucial testing method for nonlinear structures. This method can provide high signal-to-noise measurement data but requires a long measurement time. The disadvantage of lengthy measurements stems mainly from the prolonged time for a structure to reach a steady-state response in each frequency step. Although stepped-sine testing has been applied in numerous studies, there is still no uniform criterion to exactly indicate the moment when the response of a testing structure begins to achieve a steady state. In this study, a precise and novel criterion for evaluating stationarity in nonlinear structural dynamics based on the trend of the squared 2-norm of sinusoidal fitting residual vectors is proposed. Three numerical examples show that the steady-state response determined periodically by the criterion is in good agreement with the results of the theoretical analysis, which demonstrates that the presented criterion not only can assist engineers and researchers in accurately checking steady-state response but also can reduce the unnecessary and redundant waiting time during stepped-sine testing; therefore, the comprehensive performance of stepped-sine testing applied in nonlinear structures can be improved.

The capacity to manufacture complex structures directly without the need for a mold gives additive manufacturing (AM) technologies a major advantage compared with conventional manufacturing. An interesting and challenging field is to manufacture unmanned aerial vehicle (UAV) components and models using three-dimensional (3D) printing. This study focuses on the design, preliminary aerodynamics analysis, manufacture, and assembly of a small UAV using fused deposition modeling (FDM) technology. From the preliminary aerodynamic analysis of the UAV model it was found that the highest $C_l/C_d$ of the UAV model is 14.67 at a 4° angle of attack, which corresponds with cruise airspeed of $20\mathrm{m}/\mathrm{s}$. The UAV model developed in this paper presents dense ribs for wings and formers for fuselage, all of them manufactured by FDM technology. The study also includes a description of the 3D printing procedure of the components of the UAV model and the results of the flight tests. Following the flight tests, it can be said that the UAV model made by FDM technology is stable and has a wide airspeed range available, stall characteristics are good, and it operates with good aerodynamic characteristics and high maneuverability. Thus, the manufacturing technique presented in this paper can be used in the fast and efficient manufacture of scale aircraft prototypes with the purpose of determining the flight performance by conducting flight tests.