Journal of Aerospace Engineering

The present study investigated the time-dependent deformation, damage, and fracture tendencies of fiber-reinforced ceramic-matrix composites (CMCs) that were exposed to stress-rupture loading at average environmental temperatures. The composite micro stress field and tensile constitutive relationship of the damaged CMCs were examined to characterize their time-dependent damage mechanisms. The relationships between stress-rupture lifetime, constant peak stress level, time-dependent composite deformation, and evolution of internal damages are established. The effects of composite material properties, composite damage state, and environmental temperature on stress-rupture lifetime, time-dependent composite deformation, and evolution of the internal damages of $\mathrm{SiC}/\mathrm{SiC}$ are analyzed. $\mathrm{SiC}/\mathrm{SiC}$ was monitored at various constant peak stress levels to predict their experimental stress-rupture lifetime, time-dependent composite deformation, and internal damage evolution. When the fiber volume increases, the stress-rupture lifetime increases, the time-dependent composite strain and the debonding fraction at the interface decrease, and the oxidation fraction at the interface increases. When the saturation space between the matrix cracking increases, the time-dependent composite deformation and the debonding fraction at the interface decrease, and the time for complete debonding and oxidation at the interface increases. When the constant peak stress level and environmental temperature increase, the stress-rupture lifetime decreases, the time-dependent composite deformation and the debonding fraction at the interface increase, and the oxidation fraction at the interface decreases.

A wireless sensor network (WSN) eliminates physical cables and reduces the difficulty of structural information collection in a structural health monitoring (SHM) system. This paper presents a methodology for placing wireless sensors in SHM systems to obtain the most useful information and the highest WSN performance. The problem of optimal wireless sensor placement (OWSP) first was formulated as a multiobjective optimization in which the information effectiveness is specified as the modal assurance criterion and the network performance is measured by the energy efficiency combined with the network connectivity. Then a multiobjective discrete firefly algorithm based on neighboring searching (MDFA/NS) is proposed to solve the rather difficult combinatorial optimization problem presented by the OWSP. The neighboring information instead of global knowledge is utilized to drive fireflies to move toward the Pareto front at high speed. Elite preservation was adopted to enhance algorithm robustness. Numerical experiments demonstrated that the optimal criterion maintains a trade-off between information effectiveness and network performance, and the MDFA/NS outperforms the popularly used nondominated sorting genetic algorithm II.

Civil structures are affected by some environmental factors, such as traffic, ambient temperature, and noises. The change of structural dynamic characteristics arising from these factors may cover up those coming from structural damage, which makes the evaluation of structural health conditions based on vibration data more difficult. To overcome this difficulty, a novel method based on the support vector machine (SVM) and moth-flame optimization (MFO) is proposed to identify the damage of structures considering temperature variations. First of all, SVM is adopted to determine temperature variations and possible damage locations using the first six natural frequencies, and MFO is exploited to locate and quantify the damage accurately through the objective function constructed with a frequency-based multiple damage location assurance criterion (FMDLAC) and modal strain energy-based index (MSEBI). The combination of MFO and SVM can promote the efficiency of damage identification and accurately analyze environmental effects, which is a creative method with good robustness to solve the issue of damage identification considering environmental factors. To verify the effectiveness of the proposed method, a numerical simply-supported beam example considering temperature variations, as well as random noise, is investigated, and the optimal parameters for the method are acquired. Finally, a practical engineering example, the I-40 Bridge, is adopted to confirm the feasibility of the method further. The results demonstrate that the proposed approach is of a good optimization performance and can identify the damage of large complex structures considering temperature variations, which is of great practical application value.

Transmission lines (TLs) are vulnerable to wind actions. Notwithstanding the fact that the behavior of transmission lines under strong winds has been investigated in numerous studies, wind-induced failures of TLs are still common worldwide, which are mainly due to the lack of accurate understanding of the tremendous variability in wind speed. The main goal of this paper is to extend the concept of nonlinear dynamic analysis to identify and characterize the dynamic structures hidden in wind speed fluctuation, which are responsible for generating the variability in wind speed. Two well-established nonlinear dynamic analysis techniques, namely, recurrence plot and recurrence quantification analysis, are applied to diagnose the underlying dynamic characteristics. The results indicate that wind speed time series, regardless of the type of wind event, are likely to exhibit chaotic behavior. Often, the dynamics associated with monsoons are more complex than those for tropical cyclone and thunderstorm winds. Moreover, site-to-site variability in wind speed dynamics is also apparent, which is due mainly to the terrain-enhanced effect. The outcomes of this study are expected to provide a new avenue to aid the wind-resistant design of TL systems, and to accelerate the advancement in wind load design codes and climate-adapted decision-making procedures.

For small satellite electrothermal or chemical thrusters, nozzle throat diameters may be less than 1 mm. At these sizes, the effects of flow surface roughness on nozzle performance must be considered. In this research, nozzles were processed using a nickel alloy by laser powder bed fusion (LPBF), a form of additive manufacturing. Experimentally, the thrust coefficient was measured over a range of targeted throat Reynolds numbers and nozzle expansion ratios. Additional nozzles were manufactured using traditional machining practices to compare performance at similar flow conditions. An analytic model was then developed to determine that performance may not be predicted by traditional viscous loss theory. Surface features of the LPBF manufactured nozzles appear to have caused bow shock systems that overwhelm all other impacts, limiting performance of the as-printed nozzles. This imperfection was then implemented into a computational fluid dynamics (CFD) flow study, showing shock wave reflections and similar thrust losses.

This paper aims to investigate the interlaminar stresses of laminated composite doubly-curved shells on a rectangular planform subjected to concentrated force using various equivalent single layer shell theories recovered via generalized mathematical formulation. A generalized higher-order shell theory presented herein uses various polynomial and nonpolynomial type shear strain functions available in the literature to account for the effect of transverse shear deformation. Closed-formed analytical solutions are obtained using the Navier’s solution technique. Interlaminar stresses are recovered based on the direct integration of three-dimensional equilibrium equations of the theory of elasticity to capture the effect of stress concentration due to concentrated forces. The numerical results for transverse deflection are compared with previously published results wherever possible. Through-the-thickness distributions of in-plane normal and transverse shear stresses are presented to clearly understand the stress concentration effects due to concentrated force.

Aeroelastic instabilities are some of the critical issues affecting the reliability and safety of military and commercial aircraft. In this study, a coupled computational fluid dynamics and computational structural dynamics (CFD-CSD) capability was developed for transonic aeroelasticity analysis in the time domain. To expedite application of the CFD solver for aeroelastic simulations, a morphing technique was developed for mesh deformation in CFD, eliminating successive calling for a grid generator. The CFD solution was then tightly coupled with CSD by a fully implicit method. The three-dimensional (3D) CSD solver is a finite element model solving 3D elasticity equations using second-order tetrahedron elements. The coupling between aerodynamic loads and elastic deflections is developed based on spline matrices using radial basis functions. The results from the coupled CFD-CSD simulations for a two-dimensional (2D) rigid airfoil and the 3D elastic AGARD 445.6 wing are in reasonable agreement with the experimental and computational data available in the public domain. Also, the results reported in this paper highlight the importance of viscous effects for Mach numbers at and above the transonic dip, thereby highlighting the necessity to use a Navier-Stokes-based CFD solution as opposed to Euler. It is shown that the use of 3D elasticity enables the consideration of complex aircraft configurations such as with underwing stores, which can then be coupled in aircraft flutter simulations and sensitivity studies.

With the rapid urbanization and unprecedented development of urban traffic, there is an increasing need for river crossings with large-diameter tunnels in short spans, such as in the Yangtze River Delta, China. In order to satisfy the slope requirement of the road during the river crossing, the thickness of the overburden beneath rivers is inevitably very shallow. Meanwhile, the slurry pressure of a shield tunnel during construction is difficult to control when crossing under the river embankment, usually built with roller-compacted soil rock, where the thickness of the overburden varies widely. The structure stiffness of such an embankment is low and the integrality is poor. In this scenario, there will, without doubt, be an increase in risks and difficulties for river-crossing shield tunnel construction. In this paper, key construction technologies adopted to overcome the difficulties in large river-crossing slurry shield tunnel construction are put forward based on the Jiangyin West Chengjiang Road Tunnel project. A test of advance was conducted for the length of 100 m (67 rings) to optimize the shield excavation parameters and make shield operators skilled through continuous tests and explorations. Then supplementary countermeasures were carried out to ensure the safety of the shield tunneling. Multiple countermeasures provided a powerful guarantee for successfully tunneling under the rivers.

The aerodynamics of the airfoil National Advisory Committee for Aeronautics (NACA) 4412 moving in proximity to a wavy water surface are investigated using numerical methods, and the conditions under different values of the Reynolds numbers and Froude numbers are investigated. The aerodynamics of the airfoil over a wavy water surface are compared with those pertaining to rigid wavy ground. The results show that the aerodynamic coefficients for the wavy water surface are significantly different than those for rigid wavy ground. The changes in the Reynolds number and Froude number considerably influence the aerodynamics. The analysis of the flow field shows that the flow mechanisms of the airfoil moving over the wavy water surface and rigid wavy ground are significantly different. The particles on the wavy water surface and wavy ground surface exhibit a vertical and horizontal velocity, respectively. The analysis of the flow mechanism clarifies the reasons for the different aerodynamics under the two boundary conditions. This study has a certain guiding significance for the design and realization of safe flight of wing-in-ground crafts.

The occurrence of shock and boundary layer interactions (SBLIs) in aircraft-engine intakes often results in poor total pressure recovery, a degraded boundary layer, and, in worst cases, flow separation. Hence, the SBLIs must be controlled to reduce/eliminate the losses. In this study, the efficacy of conventional microvortex generators (MVGs), deployed before and at the interaction region of a Mach 5.7 mixed-compression intake, has been investigated experimentally. Specifically, an array of MVGs, placed in the spanwise direction, was found to be useful in reducing the extent of the interactions. The height of the MVGs was varied as 0.5, 0.7, and 1 mm. Both quantitative and qualitative investigations have been carried out by measuring the static pressure distribution over the surface of the ramp and visualizing the SBLI phenomena at various time instants during the test run using the time-resolved Schlieren technique. The MVGs with a height of 1.0 mm, deployed at the interaction region, appeared to be quite effective in reducing the wall static pressure, with a maximum of a 13.57% reduction at the downstream proximal location. In addition, the Schlieren images show a decrease in the separation bubble size in all the intakes controlled with MVGs; a maximum reduction of 31.25% in the bubble length is achieved with 0.7 mm MVGs, which is deployed upstream of the interaction point.

In this study, the fatigue characteristics are evaluated for a new extruded aluminum 7175 with an experimental composition that uses a magnesium-calcium alloy during the alloying process instead of the standard pure magnesium. Specimens of ECO-7175-v3 were fabricated and subjected to fatigue life testing, fatigue life data analysis, and observation of their fracture characteristics through optical microscopy and scanning electron microscopy (SEM), and metallography to study their grains and surface characteristics. The $S\text{-}N$ curve shows that the fatigue life for the new fabricated ECO-7175-v3 aluminum can exceed $5\times {10}^7$ cycles, with a fatigue strength of approximately 220 MPa or less, about 40% of its tensile strength. The fatigue strength of ECO-7175-v3 is an improvement over ECO-7175-v1, which is also shown by its higher fatigue strength coefficient, ${\sigma }_F^{\prime }$, of 1,589.7 MPa. When ECO-7175-v3 specimens were subjected to stresses of up to 67% of the ultimate tensile strength (UTS), the fracture surfaces are shown generally to have softly defined fracture features and propagation bands. These fracture surface characteristics are unique to ECO-7175-v3 and are attributed to its larger average grain size. Irrespective of the stress amplitude, like ECO-7175-v1, all crack initiation points of all specimens are seen at the surface and no inclusions to act as stress concentrators are seen. Reliability analysis shows the hazard rates for ECO-7175-v3 remain relatively constant most of the time before increasing towards the end. The trend of the hazard rates indicates failures are due to wear-outs and not due to defects. Reliability analysis also shows that at any given fatigue life cycle and stress level, ECO-7175-v3 has a lower probability of failure compared to ECO-7175-v1.

The annular lift fan aircraft is a new conceptual vertical takeoff and landing (VTOL) vehicle for which the tremendous momentum drag and pitching moment in transition is a big problem to overcome. In this study, different strut configurations and transitional strategies for the annular lift fan aircraft are numerically investigated using computational fluid dynamics (CFD) software. The results show that the longitudinal struts can significantly reduce the oscillations of lift and drag caused by the interactions between the radial struts and the incoming flow and that the gliding transition strategy reduces the total required thrust by 47% without the need for a skirt. This gliding transition is also faster, taking only about 20 seconds to complete because part of the thrust can be used for the forward acceleration.

Spinning electrodynamic tether systems are considered ideal platforms for payload transportation, removal of space debris, artificial gravity, and so on, for they provide a propellantless solution to orbital maneuver and good centrifugal stability. However, all spinning electrodynamic tether systems have to transition into a spinning state from the equilibrium state, during which tethers are likely to become deformed because of Lorentz forces. This paper studies tether deformation during such a transition process. Two open-loop programs are proposed in the Lagrangian model as the reference trajectories of acceleration under different mission backgrounds. The dynamic characteristic of tether is studied in a more accurate model with distributed parameters (bead model). Considering the significant tether deformation in the case of high electrical current, an optimal controller is proposed based on Bellman dynamic programming. Numerical results indicate that the proposed control laws can ensure a safe transition of the proposed tether system into a spin and limit tether deformation to a reasonable level.

This paper presents fluid transients in a pipeline with the sudden opening of a valve using a network flow solver based on the finite-volume method. There is entrapped air downstream of the valve. The mathematical model is formulated by integrating the flow equations in the liquid (water) zone and compressibility of the entrapped air and the structural deformation of the pipe. An earlier study by the authors shows a good agreement of computed results using the rigid pipe assumption with the experimental data when the percentage of entrapped air is reasonably high (about 50%). However, as the pressure amplitudes go higher when there is a relatively low volume of entrapped air present, the computed results differ from experimental data both in the amplitude of the pressure oscillations and corresponding frequencies. The use of structural deformation modeling coupled with the fluid dynamics problem reduces the difference for much better comparisons with the experimental data.