Recent progress in modeling of cryogenic cavitation for liquid rocket propulsion

Thermal effects substantially impact the cavitation dynamics of cryogenic fluids. The present article reviews recent progress made toward developing modeling and computational strategies to simulate cryogenic cavitation relevant to liquid rocket propulsion applications. We re-examine previously developed cavitation models, including thermal effect and turbulence closures. The experimentally observed “frosty” appearance within the cavity is modeled as a mushy phase boundary. The impact of model parameters and material properties on the prediction is probed by global sensitivity techniques. Performance of the reported cavitation models is compared against the existing cavitation models and experimental data, under both non-cryogenic and cryogenic conditions. Time-dependent computations for various cases of cryogenic cavitation are further reviewed. Impact of the cryogenic environment and inflow perturbations on the flow structure and instabilities is explained via the simulated flow fields and the reduced order strategy of proper orthogonal decomposition (POD).     

Optimal satellite formation reconfiguration actuated by inter-satellite electromagnetic forces

The inter-satellite electromagnetic forces generated by the magnetic dipoles on neighboring satellites provide an attractive control actuation alternative for satellite formation flight due to the prominent advantages of no propellant consumption or plume contamination. However, the internal force nature as well as the inherent high nonlinearity and coupling of electromagnetic forces bring unique dynamic characteristics and challenges. This paper investigates the nonlinear translational dynamics, trajectory planning and control of formation reconfiguration actuated by inter-satellite electromagnetic forces. The nonlinear translational dynamic model is derived by utilizing analytical mechanics theory; and analysis on the dynamic characteristics is put forward. Optimal reconfiguration trajectories of electromagnetic force actuated formation are studied by applying optimal control theory and the Gauss pseudospectral method. Considering the high nonlinearity and uncertainty in the dynamic model, an inner-and-outer loop combined control strategy based on feedback linearization theory and adaptive terminal sliding mode control is proposed with finite-time convergence capability and good robust performance. Theoretical analysis and numerical simulation results are presented to validate the feasibility of the proposed translational model, reconfiguration trajectory optimization approach and control strategy.     

Tracking characteristics of tracer particles for PIV measurements in supersonic flows

The tracking characteristics of tracer particles for particle image velocimetry (PIV) mea-surements in supersonic flows were investigated. The experimental tests were conducted at Mach number 4 in Multi-Mach Wind Tunnel (MMWT) of Shanghai Jiao Tong University. The motion of tracer particles carried by the supersonic flow across shockwaves was theoretically modelled, and then their aerodynamic characteristics with compressibility and rarefaction effects were evalu-ated. According to the proposed selection criterion of tracer particles, the PIV measured results clearly identified that the shockwave amplitude is in good agreement with theory and Schlieren visu-alizations. For the tracer particles in nanoscales, their effective aerodynamic sizes in the diagnostic zone can be faithfully estimated to characterize the tracking capability and dispersity performance based on their relaxation motion across oblique shockwaves. On the other hand, the seeding system enabled the tracer particles well-controlled and repeatable dispersity against the storage and humidity.     

学生成绩排名信息管理系统的设计与实现

成绩排名是一个评价和测量每个学生相对于其他学生在校表现的很有效的技术;排名结果可以根据数据库中每个学生各个学期的各科成绩数据计算出来,并且根据该排名可以对学生进行奖学金的评奖以及做为其他评优活动的依据。     

An autonomous celestial navigation method for LEO satellite based on unscented Kalman filter and information fusion

A reliable and secure navigation system and assured autonomous capability of satellite are in high demand in case of emergencies in space. Celestial navigation is a fully autonomous navigation method for satellite. To near earth satellite, the earth direction is the most important measurement and the horizon sensing accuracy is the most important factor which effects celestial navigation accuracy. According to the mode of acquiring horizon measurement, satellite celestial navigation methods can be broadly classified into two approaches: directly sensing horizon using earth sensor and indirectly sensing horizon by observation of starlight atmospheric refraction. For these two methods are complementary to each other, a new Unscented Kalman Filter (UKF) based information fusion method is proposed here for hybriding them. Compared to the traditional celestial navigation method, this method can provide better navigation performance and higher reliability. The hardware-in-loop test results demonstrate the feasibility and effectiveness of this method, in most cases the accuracy is sufficient for near earth civilian satellite and moreover it can be used as a backup system to provide redundancy.     

Command shaping for a flexible satellite platform controlled by advanced fly-wheels systems

In this paper we consider the coupling between the flexibility of a spacecraft equipped with variable speed control moment Gyros and its attitude control system, in the framework of command shaping techniques. The analysis is performed on the set of equation of motion written in an explicit form describing a flexible platform and N wheels gimballed to it. The attitude control system is designed upon a Lyapunov based feedback relying upon a rigid body model. Whenever the structure is not stiff enough, flexibility degrades the performances of the controller that fails to track the desired history. We therefore study the possibility of altering the tracking signal fed to the controller trying to get rid of the relevant frequencies notch filtering them in order to reduce vibrations. The results obtained for a test case attitude acquisition maneuver are presented and the benefit deriving from the command shaping technique evaluated and discussed.     

Design and implementation of the GPS subsystem for the Radio Aurora eXplorer

This paper presents the design and implementation of the Global Positioning System (GPS) subsystem for the Radio Aurora eXplorer (RAX) CubeSat. The GPS subsystem provides accurate temporal and spatial information necessary to satisfy the science objectives of the RAX mission. There are many challenges in the successful design and implementation of a GPS subsystem for a CubeSat-based mission, including power, size, mass, and financial constraints. This paper presents an approach for selecting and testing the individual and integrated GPS subsystem components, including the receiver, antenna, low noise amplifier, and supporting circuitry. The procedures to numerically evaluate the GPS link budget and test the subsystem components at various stages of system integration are described. Performance results for simulated tests in the terrestrial and orbital environments are provided, including start-up times, carrier-to-noise ratios, and orbital position accuracy. Preliminary on-orbit GPS results from the RAX-1 and RAX-2 spacecraft are presented to validate the design process and pre-flight simulations. Overall, this paper provides a systematic approach to aid future satellite designers in implementing and verifying GPS subsystems for resource-constrained small satellites.     

The ASTROD I charging simulation and disturbances due to solar energetic particles at 0.5 AU

The electrostatic charging of ASTROD I test mass (the Astrodynamical Space Test of Relativity using Optical Devices I) will disturb its pure geodesic motion due to spurious Coulomb and Lorentz forces. The three main disturbances associated with the charging are the acceleration noise of the test mass, the coupling between the test mass and the spacecraft, the appearance of unwanted coherent Fourier components in the measurement bandwidth. By implementing the same geometry and physics models in our previous work, a positive charging rate of 9369 e⁺/s for the solar energetic particles (SEPs) event on October 19th 1989 is predicted. Based on these charging rates, we calculated the acceleration noise and stiffness associated with charging. We also compared the acceleration noises arising from Coulomb and Lorentz effects using different discharge characteristics. Although the noise due to charging exceeds the ASTROD I budget for the two events at 0.5 AU, it can be suppressed through continuous discharging. In addition to Monte Carlo uncertainty, an error of ±30% in the net charging rates was added to account for uncertainties in the primaries spectra, physics and geometry models.     

廊坊市构建文化强市战略的实施路径研究

近年来，廊坊市高度重视文化产业发展，把文化产业确立为影响廊坊未来发展的战略产业，作为构建现代产业体系的重要内容，先后出台了一系列政策措施，廊坊取得了前所未有的成绩。虽然廊坊市文化的发展优势得天独厚且已初具规模，但文化缺乏战略性管理、资金短缺、竞争力不强等问题制约着廊坊市文化的进一步跨越发展，如何构建和完善文化强市战略目标，如何发挥文化对经济的强劲作用，这对廊坊经济稳步、可持续发展具有十分重要的现实意义。     

YES2 optimal trajectories in presence of eccentricity and aerodynamic drag

YES2 (launching 2007) aims to demonstrate a tether-assisted re-entry concept, whereby payload will be returned to Earth using momentum provided from a swinging tether. Deployment takes place in two phases: (1) deployment of 3.5 km of tether to the local vertical and hold, and (2) deployment to 30 km for a swinging cut. Optimal trajectories are determined for both phases after comparing the effect of different cost functions on the deployment dynamics. Closed-loop control is provided by linearizing the dynamics around the optimal trajectories and solving a receding horizon control problem for a set of linear feedback gains. The controllers are tested in a flexible tether model with large disturbances to the hardware model and environmental variables. Closed-loop simulations show that the system can be controlled quite well using only feedback of length and length rate.