1,134 research outputs found
The implementation of cost effective debris protection in unmanned spacecraft
No full textProper characterisation of the survivability of an unmanned spacecraft to debris impact must go beyond just a simple assessment of the probability of penetration. Some penetrative damage may be survivable, particularly if critical internal equipment is arranged judiciously. Consideration of the satellite architecture can be seen as a potentially cost-effective and complementary approach to the more traditional method of adding shielding mass. To quantify the benefits of both strategies, and identify candidate protection solutions for a typical satellite design, a new model called SHIELD has been developed. Competing protection options are evaluated using a survivability metric. Rapid convergence on one or more ‘good' designs can also be achieved with a built-in genetic algorithm search method. SHIELD's potential as a project support tool is illustrated by applying it to the survivability evaluation of a satellite currently under design. The effectiveness of the genetic algorithm is also demonstrated, but on a more idealised spacecraft design
Measuring scapular kinematics during arm lowering using the acromion marker cluster
No full textThe aim of the present study was to examine the acromion marker cluster (AMC) method of measuring scapular kinematics during the arm lowering, eccentric, phase. Twenty six participants completed arm elevation and lowering in the sagittal, frontal and scapular plane. The participants held their arm at 30° increments while the orientation of the scapula was recorded using an AMC and a scapular locator (SL). There were no significant differences between the AMC and SL during the lowering phase for sagittal and scapular plane arm movements. The AMC significantly underestimated upward rotation (max RMSE = 6.0°), and significantly overestimated posterior tilt (max RMSE = 7.2°) during arm lowering in the frontal plane. The reported root mean square errors, however, were within the ranges observed during the elevation phase and reported in previous literature. The AMC therefore provides a reasonable description of scapular kinematics during the arm lowering phase
Historical evolution of the low Earth orbit debris environment
No full textThe importance of modelling the historical evolution of the orbital debris environment is discussed. The DERA IDES debris model is introduced and the elements of the model that are used to simulate historical environment evolution are briefly described. High resolution IDES model simulations of the historical debris environment evolution in Low Earth Orbit (LEO) are then presented for centimetre and decimetre object size thresholds. The simulated environment evolution dynamics are fully explained. Discrete increases in the LEO object density can be correlated to specific debris source events. Examples of the IDES model validation process are given, consisting of comparisons between the model simulation and debris measurement data. It was found that the model exhibited reasonable accuracy for the centimetre and decimetre object size thresholds, leading to improved confidence when performing long-term environment projections
Enhancement and validation of the IDES orbital debris environment model
No full textOrbital debris environment models are essential in predicting the characteristics of the entire debris environment, especially for altitude and size regimes where measurement data is sparse. Most models are also used to assess mission collision risk. The IDES (Integrated Debris Evolution Suite) simulation model has recently been upgraded by including a new sodium–potassium liquid coolant droplet source model and a new historical launch database. These and other features of IDES are described in detail. The accuracy of the IDES model is evaluated over a wide range of debris sizes by comparing model predictions to three major types of debris measurement data in low Earth orbit. For the large-size debris population, the model is compared with the spatial density distribution of the United States (US) Space Command Catalog. A radar simulation model is employed to predict the detection rates of mid-size debris in the field of view of the US Haystack radar. Finally, the small-size impact flux relative to a surface of the retrieved Long Duration Exposure Facility (LDEF) spacecraft is predicted. At sub-millimetre sizes, the model currently under-predicts the debris environment encountered at low altitudes by approximately an order of magnitude. This is because other small-size debris sources, such as paint flakes have not yet been characterised. Due to the model enhancements, IDES exhibits good accuracy when predicting the debris environment at decimetre and centimetre sizes. Therefore, the validated initial conditions and the high fidelity future traffic model enables IDES to make long-term debris environment projections with more confidenc
The induced mean flow of surface, internal and interfacial gravity wave groups
No full textAlthough the leading-order motion of waves is periodic - in other words backwards and forwards - many types of waves including those driven by gravity induce a mean flow as a higher-order effect. It is the induced mean flow of three types of gravity waves that this thesis examines: surface (part I), internal (part II) and interfacial gravity waves (part III). In particular, this thesis examines wave groups. Because they transport energy, momentum and other tracers, wave-induced mean flows have important consequences for climate, environment, air traffic, fisheries, offshore oil and other industries. In this thesis perturbation methods are used to develop a simplified understanding of the physics of the induced mean flow for each of these three types of gravity wave groups. Leading-order estimates of different transport quantities are developed. For surface gravity wave groups (part I), the induced mean flow consists of two compo- nents: the Stokes drift dominant near the surface and the Eulerian return flow acting in the opposite direction and dominant at depth. By considering subsequent orders in a separation of scales expansion and by comparing to the Fourier-space solutions of Longuet-Higgins and Stewart (1962), this thesis shows that the effects of frequency dis- persion can be ignored for deep-water waves with realistic bandwidths. An approximate depth scale is developed and validated above which the Stokes drift is dominant and below which the return flow wins: the transition depth. Results are extended to include the effects of finite depth and directional spreading. Internal gravity wave groups (part II) do not display Stokes drift, but a quantity analogous to Stokes transport for surface gravity waves can still be developed, termed the “divergent- flux induced flow” herein. The divergent-flux induced flow it itself a divergent flow and induces a response. In a three-dimensional geometry, the divergent-flux induced flow and the return flow form a balanced circulation in the horizontal plane with the former transporting fluid through the centre of the group and the latter acting in the opposite direction around the group. In a two-dimensional geometry, stratification inhibits a balanced circulation and a second type of waves are generated that travel far ahead and in the lee of the wave group. The results in the seminal work of Bretherton (1969b) are thus validated, explicit expressions for the response and return flow are developed and compared to numerical simulations in the two-dimensional case. Finally, for interfacial wave groups (part III) the induced mean flow is shown to behave analogously to the surface wave problem of part I. Exploring both pure interfacial waves in a channel with a closed lid and interacting surface and interfacial waves, expressions for the Stokes drift and return flow are found for different configurations with the mean set-up or set-down of the interface playing an important role
Stereoscopic PIV measurement in laminar rotating plane Couette flow
No full textPlane Couette flow with spanwise system rotation shows structures of streamwise-oriented roll cells that arise due to the instability by the Coriolis force when the system rotation is in the opposite direction to the mean flow vorticity. The momentum transport caused by such roll cell structures makes the flow tend to exhibit zero absolute vorticity. In the present study, stereoscopic PIV measurements in the rotating plane Couette flow were carried out in order to further illuminate the vortex structures and transport phenomena in this flow. The Reynolds stresses and some terms of its transport equation were evaluated to discuss the transport phenomena caused by the coherent structure. Furthermore, the wall shear stress was evaluated based on the measurement results of the Reynolds and viscous shear stresses and its variation with the system rotation rate is also presented
Generalized diagnostic scaling for high-order moments in turbulent boundary layers
No full textThe present work builds upon the diagnostic plot for the streamwise turbulence intensity [Alfredsson & Örlü, 2010] and generalises it for higher-order (even and odd) moments providing a general description of the probability density distribution of streamwise velocity fluctuations. Turbulent boundary layers (up to a friction Reynolds number of 20'000) are employed and demonstrate its feasibility to scale data throughout the overlap and outer region
PV architectures for DC microgrids using buck or boost exclusive microconverters
No full textDC microgrids can connect directly dc renewable energy sources with increasing amount of dc loads. In this paper it is looked for possible architectures for integrating PV panels into dc microgrids, by means of microconverter strings. Three topologies are considered, featuring only buck microconverters and only boost microconverters, since they promise higher efficiency due to fewer semiconductors in the current path. The topologies under exam are tested with a perturb and observe MPPT, in the cases of abrupt local shading and uneven shading over a solar panels' array. Among the three, the best topology by response time and control ease is found.Accepted Author ManuscriptOld - EWI-ESE-DC&S DC systems & StorageElectrical Power Processin
A differentially interpolated direct forcing immersed boundary method for predicting incompressible Navier-Stokes equations in time-varying complex geometries
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