A deformation model of flexible, high area-to-mass ratio debris under perturbations and validation method

A new type of space debris was recently discovered by Schildknecht in near -geosynchronous orbit (GEO). These objects were later identified as exhibiting properties associated with High Area-to-Mass ratio (HAMR) objects. According to their brightness magnitudes (light curve), high rotation rates and composition properties (albedo, amount of specular and diffuse reflection, colour, etc), it is thought that these objects are multilayer insulation (MLI). Observations have shown that this debris type is very sensitive to environmental disturbances, particularly solar radiation pressure, due to the fact that their shapes are easily deformed leading to changes in the Area-to-Mass ratio (AMR) over time. This thesis proposes a simple effective flexible model of the thin, deformable membrane with two different methods. Firstly, this debris is modelled with Finite Element Analysis (FEA) by using Bernoulli-Euler theory called “Bernoulli model”. The Bernoulli model is constructed with beam elements consisting 2 nodes and each node has six degrees of freedom (DoF). The mass of membrane is distributed in beam elements. Secondly, the debris based on multibody dynamics theory call “Multibody model” is modelled as a series of lump masses, connected through flexible joints, representing the flexibility of the membrane itself. The mass of the membrane, albeit low, is taken into account with lump masses in the joints. The dynamic equations for the masses, including the constraints defined by the connecting rigid rod, are derived using fundamental Newtonian mechanics. The physical properties of both flexible models required by the models (membrane density, reflectivity, composition, etc.), are assumed to be those of multilayer insulation. Both flexible membrane models are then propagated together with classical orbital and attitude equations of motion near GEO region to predict the orbital evolution under the perturbations of solar radiation pressure, Earth’s gravity field, luni-solar gravitational fields and self-shadowing effect. These results are then compared to two rigid body models (cannonball and flat rigid plate). In this investigation, when comparing with a rigid model, the evolutions of orbital elements of the flexible models indicate the difference of inclination and secular eccentricity evolutions, rapid irregular attitude motion and unstable cross-section area due to a deformation over time. Then, the Monte Carlo simulations by varying initial attitude dynamics and deformed angle are investigated and compared with rigid models over 100 days. As the results of the simulations, the different initial conditions provide unique orbital motions, which is significantly different in term of orbital motions of both rigid models. Furthermore, this thesis presents a methodology to determine the material dynamic properties of thin membranes and validates the deformation of the multibody model with real MLI materials. Experiments are performed in a high vacuum chamber (10-4 mbar) replicating space environment. A thin membrane is hinged at one end but free at the other. The free motion experiment, the first experiment, is a free vibration test to determine the damping coefficient and natural frequency of the thin membrane. In this test, the membrane is allowed to fall freely in the chamber with the motion tracked and captured through high velocity video frames. A Kalman filter technique is implemented in the tracking algorithm to reduce noise and increase the tracking accuracy of the oscillating motion. The forced motion experiment, the last test, is performed to determine the deformation characteristics of the object. A high power spotlight (500-2000W) is used to illuminate the MLI and the displacements are measured by means of a high resolution laser sensor. Finite Element Analysis (FEA) and multibody dynamics of the experimental setups are used for the validation of the flexible model by comparing with the experimental results of displacements and natural frequencies.

Cultural citizenship and its implications for citizenship education: Chinese university students’ civic experience in relation to mass media and the university citizenship curriculum

A growing body of research has argued that university citizenship curricula are inefficient in promoting civic participation, while there is a tendency towards a broader citizenship understanding and new forms of civic engagements and citizenship learning in everyday life. The notion of cultural citizenship in this thesis concentrates on media practices’ relation to civic expression and civic engagement. This research thus argues that not enough attention has been paid to the effects of citizenship education policy on students and students’ active citizenship learning in China. This thesis examines the civic experience of university students in China in the parallel contexts of widespread adoption of mass media and of university citizenship education courses, which have been explicitly mandatory for promoting civic morality education in Chinese universities since 2007. This research project raises significant questions about the meditating influences of these two contexts on students’ perceptions of civic knowledge and civic participation, with particular interest to examine whether and how the notion of cultural citizenship could be applied in the Chinese context and whether it could provide certain implications for citizenship education in China. University students in one university in Beijing contributed to this research by providing both quantitative and qualitative data collected from mixed-methods research. 212 participants contributed to the questionnaire data collection and 12 students took part in interviews. Guided by the theoretical framework of cultural citizenship, a central focus of this study is to explore whether new forms of civic engagement and civic learning and a new direction of citizenship understanding can be identified among university students’ mass media use. The study examines the patterns of students’ mass media use and its relationship to civic participation, and also explores the ways in which mass media shape students and how they interact and perform through the media use. In addition, this study discusses questions about how national context, citizenship tradition and civic education curricula relate to students’ civic perceptions, civic participation and civic motivation in their enactment of cultural citizenship. It thus tries to provide insights and identify problems associated with citizenship courses in Chinese universities. The research finds that Chinese university students can also identify civic issues and engage in civic participation through the influence of mass media, thus indicating the application of cultural citizenship in the wider higher education arena in China. In particular, the findings demonstrate that students’ citizenship knowledge has been influenced by their entertainment experiences with TV programs, social networks and movies. However, the study argues that the full enactment of cultural citizenship in China is conditional with regards to characteristics related to two prerequisites: the quality of participation and the influence of the public sphere in the Chinese context. Most students in the study are found to be inactive civic participants in their everyday lives, especially in political participation. Students express their willingness to take part in civic activities, but they feel constrained by both the current citizenship education curriculum in universities and the strict national policy framework. They mainly choose to accept ideological and political education for the sake of personal development rather than to actively resist it, however, they employ creative ways online to express civic opinions and conduct civic discussion. This can be conceptualised as the cultural dimension of citizenship observed from students who are not passively prescribed by traditional citizenship but who have opportunities to build their own civic understanding in everyday life. These findings lead to the conclusion that the notion of cultural citizenship not only provides a new mode of civic learning for Chinese students but also offers a new direction for configuring citizenship in China. This study enriches the existing global literature on cultural citizenship by providing contemporary evidence from China which is a developing democratic country, as well as offering useful information for Chinese university practitioners, policy makers and citizenship researchers on possible directions for citizenship understanding and citizenship education. In particular, it indicates that it is important for efforts to be made to generate a culture of authentic civic participation for students in the university as well as to promote the development of the public sphere in the community and the country generally.

Eulerian-Lagrangian definition of coarse bed-load transport: Theory and verification with low-cost inertial measurement units

Fluvial sediment transport is controlled by hydraulics, sediment properties and arrangement, and flow history across a range of time scales. This physical complexity has led to ambiguous definition of the reference frame (Lagrangian or Eulerian) in which sediment transport is analysed. A general Eulerian-Lagrangian approach accounts for inertial characteristics of particles in a Lagrangian (particle fixed) frame, and for the hydrodynamics in an independent Eulerian frame. The necessary Eulerian-Lagrangian transformations are simplified under the assumption of an ideal Inertial Measurement Unit (IMU), rigidly attached at the centre of the mass of a sediment particle. Real, commercially available IMU sensors can provide high frequency data on accelerations and angular velocities (hence forces and energy) experienced by grains during entrainment and motion, if adequately customized. IMUs are subjected to significant error accu- mulation but they can be used for statistical parametrisation of an Eulerian-Lagrangian model, for coarse sediment particles and over the temporal scale of individual entrainment events. In this thesis an Eulerian-Lagrangian model is introduced and evaluated experimentally. Absolute inertial accelerations were recorded at a 4 Hz frequency from a spherical instrumented particle (111 mm diameter and 2383 kg/m3 density) in a series of entrainment threshold experiments on a fixed idealised bed. The grain-top inertial acceleration entrainment threshold was approximated at 44 and 51 mg for slopes 0.026 and 0.037 respectively. The saddle inertial acceleration entrainment threshold was at 32 and 25 mg for slopes 0.044 and 0.057 respectively. For the evaluation of the complete Eulerian-Lagrangian model two prototype sensors are presented: an idealised (spherical) with a diameter of 90 mm and an ellipsoidal with axes 100, 70 and 30 mm. Both are instrumented with a complete IMU, capable of sampling 3D inertial accelerations and 3D angular velocities at 50 Hz. After signal analysis, the results can be used to parametrize sediment movement but they do not contain positional information. The two sensors (spherical and ellipsoidal) were tested in a series of entrainment experiments, similar to the evaluation of the 111 mm prototype, for a slope of 0.02. The spherical sensor entrained at discharges of 24.8 ± 1.8 l/s while the same threshold for the ellipsoidal sensor was 45.2 ± 2.2 l/s. Kinetic energy calculations were used to quantify the particle-bed energy exchange under fluvial (discharge at 30 l/s) and non-fluvial conditions. All the experiments suggest that the effect of the inertial characteristics of coarse sediments on their motion is comparable to the effect hydrodynamic forces. The coupling of IMU sensors with advanced telemetric systems can lead to the tracking of Lagrangian particle trajectories, at a frequency and accuracy that will permit the testing of diffusion/dispersion models across the range of particle diameters.