14532 research outputs found
Sort by
Direct detection of neutral species from ionic liquid electrospray thrusters in the purely ionic regime
Get PDFRecent experiments on ionic liquid electrospray thrusters revealed lower efficiency than expected from time-of-flight (ToF) measurements, despite operating in the purely ionic regime (PIR). This discrepancy was attributed to excessive propellant consumption caused by neutral particles generated during ion emissions. In this study, ToF spectrometry was performed using a channel electron multiplier (CEM) with an externally wetted emitter and two propellants: 1-ethyl-3-methylimidazolium tetrafluoroborate and 1-ethyl-3-methylimidazolium dicyanamide. ToF measurements confirmed that the thruster operated in the PIR. While the ion species composition remained unchanged with increasing CEM gain, the zero levels of ToF signals differed between conditions with and without ion emissions. Additionally, the CEM signal increased with rising emitter current, even when the ion beam was completely blocked by the deflector gate. These findings indicate that neutrals generated in both acceleration and field-free regions can be directly detected by the CEM. Considering the lower detection efficiency of neutrals compared to that of ions, the number of generated neutral particles was higher than expected, which could explain the excessive propellant mass loss. Such neutral species are overlooked when a metal plate or a Faraday cup is employed as a collector for ToF measurements
Enhancing computational efficiency of iLQR and DDP via the parametric representation of control inputs
Get PDFEfficiently solving nonlinear optimal control problems is crucial in trajectory planning and model predictive control. This can be achieved by utilizing differential dynamic programming (DDP) and iterative linear quadratic regulator (iLQR), which have recently gained attention. As these algorithms partition the problem into subproblems at each time step, they exhibit linear complexity of one iteration in the length of the prediction horizon. While these methodologies are computationally efficient, industrial applications demand further improvements in computational efficiency, primarily due to the limitations of embedded CPUs. The parametric representation of control inputs has been widely adopted to reduce the dimensionality of decision variables in optimal control problems. However, the subproblem partitioning inherent in DDP and iLQR presents challenges for directly incorporating this representation. In this study, we present a computationally efficient algorithm that integrates a parametric representation of control inputs into DDP- or iLQR-like algorithms. We exemplified a scenario in which parametric representation was introduced by considering interior-point DDP and iLQR, which could handle nonlinear inequality constraints. The effectiveness of this approach for practical applications was demonstrated through a series of numerical experiments. In particular, these numerical experiments mainly focused on key real-world problems, such as trajectory planning for forklifts and optimal excavation trajectory planning for an excavator. Regarding trajectory planning for forklifts and excavators, we achieved a maximum reduction of about 70% in the total computation time
Spatial dispersal of environmental DNA particles in lentic and marine ecosystems: An overview and synthesis
Get PDFKnowledge of the transport of environmental DNA (eDNA) particles, including its downstream transport, dispersal, and deposition, is crucial for properly assessing the spatial scale of its signals in the field. Despite the relative progress in research on riverine eDNA transport, the mechanism of eDNA dispersal in lentic (ponds and lakes) and marine (oceans and coasts) ecosystems remains understudied. Here, we reviewed and integrated the previous findings of horizontal and vertical eDNA dispersal in lentic and marine ecosystems. We showed the lack of studies in these ecosystems and substantial variabilities among their results, which could be induced by a variety of ecological, hydrological, and geographical factors. We also re-analyzed the previous studies to estimate the rates of horizontal eDNA dispersal in lentic and marine ecosystems, indicating that eDNA particles can reach substantially farther in the oceans and coasts than in the ponds and lakes. The literature review and analysis suggested that the level of eDNA dispersal influenced the resolution of eDNA signals to reflect the spatial community structures, which could further pose positive and negative impacts on the eDNA sampling strategy. Future efforts to understand such technical background and relate eDNA dispersal dynamics to environmental parameters will help properly interpret the eDNA detection results and make eDNA analysis more effective for conservation and environmental management
A study on the effects of small-scale vacant spaces in densely urban areas
Get PDF横浜国立大学Yokohama National University博士（工学）Doctor of Engineerin
Variability trend and geometry effect of a single emitter on ion emission in passively fed porous ionic liquid electrospray thrusters
Get PDFIonic liquid (IL) electrospray thrusters offer a promising space propulsion solution for small spacecraft; however, their performance is limited by variability in ion emission current. Understanding these variations is critical for the reliability of the thrusters. This study focuses on passively fed porous emitters, which are commonly used for their effective IL transport via capillary action without active flow control. Nonetheless, such configurations often suffer from poor repeatability in ion emission current versus applied voltage (I–V) characteristics, known as I–V variability. We systematically investigated the trends and contributing factors to I–V variability by identifying the emitter filling state—linked to the surface pressure on the porous emitter—as a key driver. We also examined the impact of emitter geometry and found that increased emitter tip curvature reduces the threshold electric field, thereby affecting the I–V characteristics. These findings provide valuable insights into the mechanisms driving emission variability and offer guidance for the design optimisation of passively fed IL electrospray thrusters
A study on damage mechanisms and retrofit strategies for RC structures with axially degraded columns
Get PDF横浜国立大学博士（工学）Doctor of Engineerin
Proposal of analysis method for evaluation of soundness of shield tunnel considering joint corrosion
Get PDF横浜国立大学博士（工学）Doctor of Engineerin
