Low-thrust Lambert transfer based on two-stage constant-vector thrust control method

The low-thrust Lambert transfer refers to that the spacecraft achieves the orbital transfer whose boundary conditions are represented by two sets of orbital elements at initial and final time by the low-thrust propulsion system. The modulus and direction of the low-thrust solutions in previous methods change with time, which leads to high control requirements for the engine. In this paper, to reduce the requirements of the engine, a practical two-stage constant-vector thrust control method is proposed, in which the magnitude and direction of the thrust are deemed as segmental constant value in TNH frame, where three components of the thrust are f_t, f_n, and f_h. First, the mathematical model of the two-stage constant-vector thrust is formulated, and a rapid algorithm is presented to obtain the solution based on the linearized sensitivity matrix, which describes the relationship between the constant-vector thrust and the change of the orbital elements approximately. Furthermore, two low-thrust Lambert strategies based on the two-stage constant-vector thrust are presented for cases of short-time transfer and long-time transfer. A sequence of numerical simulations demonstrated the efficiency of the proposed approaches. The proposed control strategies are solved rapidly, and they are also suitable for different types of orbits with J₂ perturbation, which are practical options for engineering applications.

     

Nonlinear vibration and instability of functionally graded nanopipes with initial imperfection conveying fluid

In this paper, the nonlinear vibration and instability of a fluid-conveying nanopipe made of functionally graded (FG) materials with consideration of the initial geometric imperfection are investigated. The material properties are assumed to vary smoothly along the radial direction according to a power-law exponent form. The fluid-conveying FG nanopipe is modeled as a Euler-Bernoulli beam, and the governing equation is derived based on the nonlocal strain gradient theory incorporating the effects of Von-Karman geometrical nonlinearity and initial imperfection. The nonlinear frequency and critical fluid velocity are achieved via He's Hamiltonian approach. After verifying the present model with comparison of several previous studies, the effect of several different system parameters including the amplitude of the nonlinear oscillator, the initial geometric imperfection, size-dependent parameters, and the power-law index on the frequency response of the fluid-conveying FG nanopipe are explored. Moreover, the critical velocity of the conveying fluid under different system parameters is also investigated and discussed in detail. The developed size-dependent nonlinear model is expected to provide a possible theoretical way to guide the application of FG nanopipe as micro/nanofluidic devices.     

Frequency analysis of carbon and silicon nanosheet with surface effects

Silicon-based microelectromechanical system (MEMS) and nanoelectromechanical systems (NEMS) have been used to design and fabricate sensitive sensors and actuators. Recent research trends show that graphene and carbon nanotubes (CNTs) have been used to change the surface properties of silicon-based MEMS and NEMS to improve different mechanical, optical and electrical properties of silicon-based composites. In this paper, we focus on analyzing the vibrational characteristics of silicon-based devices when the surface of silicon is coated with single-layer graphene and horizontally aligned carbon nanotubes (HACNTs). To perform the analysis, we use multi-scale finite element approach for developing graphene–silicon nanocomposites (GSNCs) and carbon nanotube-silicon nanocomposites (CSNC) composites in which interface layer of silicon with graphene or CNT is modeled using bonded contact element. Subsequently, we performed modal analysis to find the first transverse mode frequency of GSNC and CSNC composites for beam with smaller as well as longer lengths. The numerical model is compared with classical beam theory with and without surface effect. For GSNCs composites, we take a fixed-free case with lengths in the range of (20 Å–120 Å) and (400 Å–2000 Å), respectively. For CSNC composites, CNT diameter is varied from (5 Å–30 Å) for single walled nanotube. Subsequently, we analyze the influence of HACNTs-on-silicon on its vibrational characteristics. The analysis presented in the paper demonstrate that GSNCs offer a higher bending stiffness compared to single layer graphene (SLGs) and isolated silicon nanosheet which lead to higher natural frequency. A similar trend is found in the case of HACNTs on silicon NS when the number of tubes increases.     

Magic Labeling of Disjoint Union Graphs

Let G be a graph with vertex set V (G), edge set E(G) and maximum degree Δ respectively. G is called degree-magic if it admits a labelling of the edges by integers {1, 2, …,|E(G)|} such that for any vertex v the sum of the labels of the edges incident with v is equal to (1+|E(G)|)/2·d(v), where d(v) is the degree of v. Let f be a proper edge coloring of G such that for each vertex v ∈ V (G),|{e:e ∈ E_v, f(e) ≤ Δ/2}|=|{e:e ∈ E_v, f(e) > Δ/2}|, and such an f is called a balanced edge coloring of G. In this paper, we show that if G is a supermagic even graph with a balanced edge coloring and m ≥ 1, then (2m + 1)G is a supermagic graph. If G is a d-magic even graph with a balanced edge coloring and n ≥ 2, then nG is a d-magic graph. Results in this paper generalise some known results.     

Practical limitations to the influence of through-thickness normal stress on sheet metal formability

According to a recent (original) model, when hardening properties and the ratio of through-thickness normal stress to the first principal stress (γ≡σ₃/σ₁) are held constant, sheet metal formability can be increased dramatically through the introduction of a compressive through-thickness normal stress, σ₃. In practice, however, both the hardening properties and γ evolve with the progression of deformation. To manage most efficiently the evolution of the hardening properties and γ, the original model is cast into a more compact form and presented as a proposed alternative form (proposed model). When the evolution of the hardening properties and γ is considered, the proposed model is shown to be in very good agreement with observed data; the influence of through-thickness normal stress on sheet metal formability is quite small for all practical purposes. Because the structure of the original model is similar to that of the proposed model, the original model is also validated. Ultimately, it is verified that although the theory of the original and proposed model may be acceptable, the implications of such theories are less profound than those first proposed when practical limitations are considered. This work serves as a useful basis for: (1) further understanding the limitations of the influence of compressive through-thickness normal stress on sheet metal formability and (2) exploring opportunities for improving sheet metal formability.     

Investigating ego modules involved in TGFβ3-induced chondrogenesis in mesenchymal stem cells based on ego network

ObjectiveThis paper aimed to investigate ego modules for TGFβ3-induced chondrogenesis in mesenchymal stem cells (MSCs) using ego network algorithm.MethodsThe ego network algorithm comprised three parts, extracting differential expression network (DEN) based on gene expression data and protein-protein interaction (PPI) data; exploring ego genes by reweighting DEN; and searching ego modules by ego gene expansions. Subsequently, permutation test was carried out to evaluate the statistical significance of the ego modules. Finally, pathway enrichment analysis was conducted to investigate ego pathways enriched by the ego modules.ResultsA total of 15 ego genes were obtained from the DEN, such as PSMA4, HNRNPM and WDR77. Starting with each ego genes, 15 candidate modules were gained. When setting the thresholds of the area under the receiver operating characteristics curve (AUC) ≥0.9 and gene size ≥4, three ego modules (Module 3, Module 8 and Module 14) were identified, and all of them had statistical significances between normal and TGFβ3-induced chondrogenesis in MSCs. By mapping module genes to confirmed pathway database, their ego pathways were detected, Cdc20:Phospho-APC/C mediated degradation of Cyclin A for Module 3, Mitotic G1-G1/S phases for Module 8, and mRNA Splicing for Module 14.ConclusionsWe have successfully identified three ego modules, evaluated their statistical significances and investigated their functional enriched ego pathways. The findings might provide potential biomarkers and give great insights to reveal molecular mechanism underlying this process.     

Structural evolution of turbostratic carbon: Implications in H2 storage

Structural evolution of turbostratic carbon samples as a function of annealing temperature has been investigated in detail using small angle X-ray scattering (SAXS), solid state nuclear magnetic resonance (NMR) and Raman spectroscopic techniques. From these studies, it is established that, samples heated at lower temperatures (700 °C and 800 °C) consist carbon particles with rough surfaces forming structure of surface fractal in nature. Whereas the sample heated at higher temperature (900 °C) consists of larger clusters with nearly smooth surface as well as smaller size particles forming dense mass fractal structure. For this sample, solid state NMR and Raman Spectroscopic studies indicate an increased extent of overlapping of 2p_z orbital of carbon atoms due to improved long range ordering and clustering. Hydrogen adsorption studies further substantiated that energetically more homogeneous surface exists for particles of 900 °C heated sample as compared to those of 700 °C and 800 °C heated samples. A highest hydrogen storage capacity of 0.152 H/M has been observed at 123 K and 45 bar pressure for the sample heated at 900 °C.     

Accurate kinematic measurement at interfaces between dissimilar materials using conforming finite-element-based digital image correlation

Digital image correlation (DIC) is now an extensively applied full-field measurement technique with subpixel accuracy. A systematic drawback of this technique, however, is the smoothening of the kinematic field (e.g., displacement and strains) across interfaces between dissimilar materials, where the deformation gradient is known to be large. This can become an issue when a high level of accuracy is needed, for example, in the interfacial region of composites or joints. In this work, we described the application of global conforming finite-element-based DIC technique to obtain precise kinematic fields at interfaces between dissimilar materials. Speckle images from both numerical and actual experiments processed by the described global DIC technique better captured sharp strain gradient at the interface than local subset-based DIC.     

Hybrid uncertainty propagation of coupled structural–acoustic system with large fuzzy and interval parameters

Based on the finite element framework and uncertainty analysis theory, this paper proposes a first-order subinterval perturbation finite element method (FSPFEM) and a modified subinterval perturbation finite element method (MSPFEM) to solve the uncertain structural–acoustic problem with large fuzzy and interval parameters. Fuzzy variables are used to represent the subjective uncertainties associated with the expert opinions; whereas, interval variables are adopted to quantify the objective uncertainties with limited information. By using the level-cut strategy and subinterval methodology, the original large fuzzy and interval parameters are decomposed into several subintervals with small uncertainty level. In both FSPFEM and MSPFEM, the subinterval matrix and vector are expanded by the Taylor series. The inversion of subinterval matrix in FSPFEM is approximated by the first-order Neumann series, while the modified Neumann series with higher order terms is adopted in MSPFEM. The eventual fuzzy interval frequency responses are reconstructed by the interval union operation and fuzzy decomposition theorem. A numerical example evidences the remarkable accuracy and effectiveness of the proposed methods to solve engineering structural–acoustic problems with hybrid uncertain parameters.     

Study on ethyl groups with two different orientations in [N(C2H5)4]2CuBr4

The crystal structure and phase transition temperature of [N(C₂H₅)₄]₂CuBr₄ are studied using X-ray diffraction and differential scanning calorimetry (DSC); measurements revealed a tetragonal structure and the two phase transition temperatures T_C of 204 K and 255.5 K. The structural geometry near T_C is discussed in terms of the chemical shifts for ¹H magic angle spinning (MAS) nuclear magnetic resonance (NMR) and ¹³C cross-polarization (CP)/MAS NMR. The two inequivalent ethyl groups are distinguishable by the ¹³C NMR spectrum. The molecular motions are discussed in terms of the spin–lattice relaxation times T_1ρ in the rotating frame for ¹H MAS NMR and ¹³C CP/MAS NMR. The T_1ρ results reveal that the ethyl groups undergo tumbling motion, and furthermore that the ethyl groups are highly mobile.