In this paper, a novel family of fourth-order accurate explicit time integration schemes is developed by combining the new time approximations and the explicit fourth-order Runge–Kutta (RK4) method. Inaccurate predictions due to the presence of excessive numerical dissipation are often observed in practical analyses of structural dynamics when the RK4 is employed for both displacement and velocity approximations. To remedy this, novel time approximations with adjustable algorithmic parameters are employed for the displacement vectors while the velocity vectors are approximated by using the RK4. For the complete elimination of numerical dissipation, algorithmic parameters are unconventionally determined by taking the determinant of the amplification matrix as unity. A set of algorithmic parameters obtained from this process makes the new schemes completely non-dissipative while keeping the computational cost the same as the RK4. Due to this improvement, the new schemes have enhanced total energy-conserving capabilities for nonlinear systems and give noticeably more accurate predictions in practical analyses. Until now, controllable numerical dissipation and fourth-order accuracy are not attained simultaneously in a unified set of time schemes. In the new schemes, however, a systematic way to adjust the level of numerical dissipation is also presented while attaining fourth-order accuracy. The numerical results of various test problems show that the new time schemes can provide more accurate predictions for nonlinear conservative dynamic problems than the existing time schemes.
The objective of this study was to determine the effect of a synthesis procedure of Sr hexaaluminate on catalytic performance during the decomposition of ionic liquid monopropellants based on ammonium dinitramide (ADN) and hydroxyl ammonium nitrate (HAN). Sr hexaaluminates were prepared via both coprecipitation and a sol–gel process. The surface area of hexaaluminate synthesized via the coprecipitation method was higher than that of hexaaluminate synthesized by the sol–gel process, and calcined at the same temperature of 1200 °C or more. This is because of the sintering of α-Al2O3 on the hexaaluminate synthesized via the sol–gel process, which could not be observed on the catalysts synthesized via the coprecipitation method. The hexaaluminate synthesized via coprecipitation showed a lower decomposition onset temperature during the decomposition of ADN- and HAN-based liquid monopropellants in comparison with the catalysts synthesized via the sol–gel process, and calcined at the same temperature of 1200 °C or more. This is attributed to the differences in the Mn3+ concentration and the surface area between the two hexaaluminates. Consequently, the hexaaluminate synthesized via coprecipitation which calcined above 1200 °C showed high activity during the decomposition of energetic ionic liquid monopropellants compared with the hexaaluminate synthesized via the sol–gel process.
In this article, third- and fourth-order accurate explicit time integration methods are developed for effective analyses of various linear and nonlinear dynamic problems stated by second-order ordinary differential equations in time. Two sets of the new methods are developed by employing the collocation approach in the time domain. To remedy some shortcomings of using the explicit Runge-Kutta methods for second-order ordinary differential equations in time, the new methods are designed to introduce small period and damping errors in the important low-frequency range. For linear cases, the explicitness of the new methods is not affected by the presence of non-diagonal damping matrix. For nonlinear cases, the new methods can handle velocity dependent problems explicitly without decreasing order of accuracy. The new methods do not have any undetermined algorithmic parameters. Improved numerical solutions are obtained when they are applied to various linear and nonlinear problems.
In this study, mixed finite element models of plate bending are developed to include other variables (e.g., the membrane forces and shear forces) in addition to the generalized displacements to investigate their effect on nonlinear response. Various finite element models are developed using the weighted-residual statements of suitable equations. The classical plate theory and the first-order shear deformation plate theory are used in this study and the von Karman nonlinear strains are accounted for. Each newly developed model is examined and compared with displacement finite element models to evaluate their performance. Numerical results show that the new mixed models developed herein show better accuracy than existing displacement based models.
새로운 고체산화제 화합물인 pyridinium dinitramide (Py-DN)는 환경 및 인체에 독성이 적은 비염소계의 에너지물질로서 고체 추진제 뿐만 아니라 단일계 추진제로 활용이 가능한 high performance green propellant (HPGP) 물질이다. 합성반응은 술팜산칼륨(potassium sulfamate, $NH_2SO_3K$)을 출발물질로 시작하였으며, 합성된 Py-DN의 화학적인 구조특성을 적외선분광법과 가시광선-자외선분광법으로 관찰하였다. 또한, 유사한 물성의 친환경 고체산화제인 ammonium dinitramide[ADN, $NH_4N(NO_2)_2$]와 guanidine dinitramide[GDN, $NH_2C(NH_2)NH_2N(NO_2)_2$]의 열특성을 TG/DSC로 분석하여 상대 비교하였다. 본 연구에서 합성한 Py-DN염의 흡열온도는 $77.4^{\circ}C$, 분해온도는 $144.7^{\circ}C$, 발열에너지는 1739 J/g으로 기존의 DN계열 물질보다 열적 반응이 빠르므로 분해온도가 상대적으로 낮아 단일계 추진제의 촉매 분해 시 촉매의 예열온도를 낮출 수 있어 로켓추력기의 연료로 활용할 경우, 낮은 분해온도 적용성에 장점이 있다. A new solid oxidizer, pyridinium dinitramide (Py-DN) is a low toxic energetic material which can be utilized as a HPGP (high performance green propellant). In this work, Py-DN was synthesized using various starting materials including potassium sulfamate, pyridine hydrochloride, strong nitric acid and sulfuric acid. Physical and chemical properties of the Py-DN were characterized using UV-Vis, FT-IR and a thermal analyzer and their properties were compared to those of previously prepared salts including ammonium dinitramide[ADN, $NH_4N(NO_2)_2$] and guanidine dinitramide[GDN, $NH_2C(NH_2)NH_2N(NO_2)_2$] in our lab. Endothermic and exothermic decomposition temperatures of Py-DN were $77.4^{\circ}C$ and $144.7^{\circ}C$, respectively. The combustion caloric value was 1739 J/g, which is thermally more sensitive than that of conventional dinitramides. It may enable to lower the decomposition temperature, which can reduce preheating temperature required for satellite thruster applications.
Ultrashort pulse lasers are emerging as an advanced tool of distance measurement, with their unique temporal and spectral characteristics being extended to diverse principles of absolute ranging and instrumentation. Here, a systematic methodology is presented for absolute ranging by means of the time-of-flight measurement of ultrashort light pulses using dual-comb asynchronous optical sampling. Based on an elaborate uncertainty analysis, influencing system parameters such as the pulse duration, repetition rate, and averaging time are optimized to achieve a sub-µm measurement accuracy. The absolute ranging system developed in this study demonstrates a combined standard uncertainty of 0.986 µm for a 0.5 ms averaging over a distance range of 3.0 m, with a further reduction to 0.056 µm when the averaging time is increased to 0.5 s. The outstanding performance leads to unprecedented multitarget applications: machine feed control with thermal error compensation in real time as well as the nondestructive inspection of multilens assembly in a production line.
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