Fixation and a 180 Degree View Simplify Ego Motion Estimation

Although moving human observers actively fixate their eyes on points in the world, computer vision algorithms designed for the estimation of structure-from-motion or egomotion typically do not make use of this constraint. The main contribution of this work is to precisely specify the form of the optical flow field for a fixating observer. In particular, we show theoretically that the use of a hemispherical (retinal) imaging surface generates an optical flow field of a particularly simple form. The predictions of this theory are tested using the first actual hemispherical lens-camera system in computer vision, involving a 180 degree field of view lens. A further contribution is the finding that the sign of flow at the retinal periphery can be used to predict collisions

Velocity fields in and around sunspots at the highest resolution

The flows in and around sunspots are rich in detail. Starting with the Evershed flow along low-lying flow channels, which are cospatial with the horizontal penumbral magnetic fields, Evershed clouds may continue this motion at the periphery of the sunspot as moving magnetic features in the sunspot moat. Besides these well-ordered flows, peculiar motions are found in complex sunspots, where they contribute to the build-up or relaxation of magnetic shear. In principle, the three-dimensional structure of these velocity fields can be captured. The line-of-sight component of the velocity vector is accessible with spectroscopic measurements, whereas local correlation or feature tracking techniques provide the means to assess horizontal proper motions. The next generation of ground-based solar telescopes will provide spectropolarimetric data resolving solar fine structure with sizes below 50 km. Thus, these new telescopes with advanced post-focus instruments act as a "zoom lens" to study the intricate surface flows associated with sunspots. Accompanied by "wide-angle" observations from space, we have now the opportunity to describe sunspots as a system. This review reports recent findings related to flows in and around sunspots and highlights the role of advanced instrumentation in the discovery process.Comment: 6 pages, 1 figure, to be published in "Physics of Sun and star spots", Proc. IAU Symp. 273, D.P. Choudhary and K.G. Strassmeier (eds.

Computational Modelling of Aqueous Humor Dynamics And Drug Delivery For Intraocular Pressure Control In Glaucoma

We present a computational model of sustained delivery of ocular pressure-lowering drugs (Timolol Maleate) to the anterior segment of the eye using drug infused contact lens. Ocular structures, aqueous humor flow and their interaction were modeled as linearly elastic solids, viscous fluid flow and fluid-structure interaction (FSI) respectively in COMSOL 5.3 as axisymmetric models. Timolol distribution from contact lens was modeled as transport of diluted species coupled with the flow dynamics (FSI velocity coupled). Aqueous humor production in the ciliary body was simulated as an inlet with a mass flow rate of 5e-8 kg/s. Over a 10-hour duration, drug transport was simulated using the diffusion-FSI coupled model and a model without FSI coupling. The coupled model provided a more accurate characterization of the drug transport and distribution. The model may be useful to study ocular drugs and their delivery to the anterior and posterior segment of the eye more accurately

Numerical Study to Investigate the Effect of Lens and Nozzle Geometry on Aerodynamic Focusing Lens Performance

During the sampling of atmospheric aerosol particles from ambient environments, particles must be separated from bulk fluid flow into a narrow flight path in order to perform accurate mass spectrometry under vacuum. To establish and maintain a low-divergence aerosol flight path, an aerodynamic focusing lens system (AFL) is used. A series of aerodynamic lenses are employed to moderate the pressure drop along the length of the system and separate nano- to supermicron-particles from the bulk fluid. AFLs require a steady laminar flow to enable particle convergence along the center axis, and thus the generation of turbulent eddies is to be altogether avoided. The objective of this thesis is to enhance the performance of AFL inlets over a wide range of particle sizes (from 10 nm to 10 µm) by (a) investigating the effect of aerodynamic lens and nozzle geometry on the particle flow and (b) investigating the early signs of laminar-to-turbulence transition in the AFL inlets. Three geometric modifications within the selected AFL inlet were analyzed: varying half-angles of a divergent lens, varying the length and diameter of the capillary step of the accelerating nozzle, and varying the nozzle length. The Reynolds number threshold that leads to turbulent structure generation in the selected AFL inlets is also studied. The simulation results can be used to enhance the design of AFL inlets for a particle size range of approximately 10 nm to 2.5 μm, with the possibility of extension to 10 μm

An X-Ray Microlensing Test of AU-Scale Accretion Disk Structure in Q2237+0305

The innermost regions of quasars can be resolved by a gravitational-lens {\lq}telescope{\rq} on scales down to a few AU. For the purpose, X-ray observations are most preferable, because X-rays originating from the innermost regions, can be selectively amplified by microlensing due to the so-called `caustic crossing'. If detected, X-ray variations will constrain the size of the X-ray emitting region down to a few AU. The maximum attainable resolution depends mainly on the monitoring intervals of lens events, which should be much shorter than the crossing time. On the basis of this idea, we performe numerical simulations of microlensing of an optically-thick, standard-type disk as well as an optically-thin, advection-dominated accretion flow (ADAF). Calculated spectral variations and light curves show distinct behaviors, depending on the photon energy. X-ray radiation which is produced in optically thin region, exhibits intensity variation over a few tens of days. In contrast, optical-UV fluxes, which are likely to come from optically thick region, exhibit more gradual light changes, which is consistent with the microlensing events so far observed in Q2237+0305. Currently, Q2237+0305 is being monitored in the optical range at Apache Point Observatory. Simultaneous multi-wavelength observations by X-ray sattelites (e.g., ASCA, AXAF, XMM) as well as HST at the moment of a microlens event enable us to reveal an AU scale structure of the central accretion disk around black hole.Comment: 10 pages LaTeX, 3 figures, accepted to ApJ Letter. e-mail: [email protected]

Experimental and computational analysis of purge systems for radiation pyrometers

Maximizing the turbine entry temperature (TET) is fundamental to increase engine efficiency and reducing fuel consumption. Nonetheless, safety and reliability requirements have to be fulfilled. The life of gas turbine blades is strictly connected to their temperature through the creep deformation process. For this reason temperature monitoring is an essential requirement. Commonly this is achieved by means of devices such as thermocouples which are placed in the bulk flow. The usefulness of these devices as the means of supplying turbine blade temperature information is limited given their slow response time and the fact that the blade temperature is inferred from that of the surrounding gas rather than measured directly. This in turn means that critical blades parts (e.g., trailing edge) or the presence of hot spots are not identified in a discrete manner. These drawbacks can be addressed by using instead a radiation pyrometer, which is characterized by a fast response time, high accuracy, and by being contactless. The pyrometer optical front-end is a lens which collects the radiation emitted by a spot on the turbine blades. However, since the lens is exposed to the harsh engine environment, contaminants entrapped by the turbine flow can therefore be easily deposited on the lens thus filtering the radiation and resulting in an under-estimation of the actual blade temperature. The fouling of the lens is generally tackled by using a purge air system that employs air bled from the compressor to divert those particles whose trajectory is directed towards the lens. Currently the employment of optical pyrometry is often confined to military applications due to the fact that their turbine entry temperatures are higher than in civil applications. Besides, the maintenance schedule established for military engines is far more frequent than what is practiced in airline engines. Therefore, the design of current purge air systems reflects these facts. Before optical pyrometers can be commonly used for civil applications more research is required since some of the fundamentals of the fouling mechanisms remain to be clarified. This is then the knowledge gap the present research sought to fill. Its aim was to conduct a comprehensive investigation of the phenomena that underpins the lens fouling process in order to provide a set of guidelines for optimising the design of purge air systems. The initial part of the research was dedicated to the study of the purge flow inside a given pyrometer configuration. The scope was to identify the main flow structure that determines the fouling process and at the same time to validate the results obtained via computational fluid dynamics (CFD) analyses conducted in a second phase of the research. Given the reduced dimensions of the pyrometer purge system, it was not possible to gain the appropriate optical access to take flow measurements. Consequently, a large scaled experiment was performed, employing the Particle Image Velocimetry (PIV) technique for the acquisition of experimental data of the flow field. The distortion of the image and light reflection introduced by the presence of curved glass surfaces was investigated by means of a feasibility experiment. The experimental study highlighted the presence of a large recirculation zone that can trap contaminants and direct them towards the pyrometer lens. The experimental data were in agreement with computational fluid dynamics results obtained by using two different turbulence models. In a second instance, attention was focused on the particle deposition as seen from a fluid dynamics perspective. A computational fluid dynamics analysis aimed at reproducing the flow field of an existing pyrometer purge system enabled the identification of those features that can significantly impact on the lens fouling process. It was found that the geometry of the air curtain configuration plays a fundamental role. However, given the high speeds involved, the main force governing the contaminants deposition is the drag. Additionally, particles with high inertia hit the purge tube wall and then bounce towards the pyrometer lens, while contaminants with low inertia can be trapped by a large recirculation zone and subsequently directed towards the lens. In a third phase of the research, the impacts between the contaminant particles and the lens were investigated through a finite element analysis (FEA) aimed at identifying the most important factors that contribute to the lens fouling process. Particles moving at low speed can be deposited on the lens by means of electrostatic and Van der Waals forces. Conversely, particles with very high velocity can be deposited on the lens through the same mechanisms involved in the cold spraying process, which is a technique commonly used for coating deposition. A local melting can occur at the interface between the lens and the contaminants due to the high stresses created by the asperities and high sliding velocity of the particles. As a result, while large particles bounce back, debris remains bonded to the lens surface. Last but not least, the findings of the several steps of the present research have been brought together in order to produce guidelines to be followed by engineers engaged in the redesign of more efficient pyrometer purge systems

A Detailed Thermal Analysis of the Binospec Spectrograph

Refractive optics in astronomical instruments are potentially sensitive to temperature gradients and temperature transients. This sensitivity arises from thermally dependent refractive indices, lens spacings, and lens dimensions. We have therefore undertaken a detailed thermal analysis of Binospec, a wide-field optical spectrograph under development for the converted MMT. Our goals are to predict the temperature gradients that will be present in the Binospec optics and structure under realistic operating conditions and to determine how design choices affect these gradients. We begin our analysis by deriving thermal time constants for instrument subassemblies. We then generate a low-resolution finite difference model of the entire instrument and high-resolution models of sensitive subassemblies. This approach to thermal analysis is applicable to a variety of other instruments. We use measurements of the ambient temperature in the converted MMT's dome to model Binospec's thermal environment. During moderate conditions we find that the Binospec lens groups develop 0.14 C axial and radial temperature gradients and that lens groups of different mass develop 0.5 C temperature differences; these numbers are doubled for the extreme conditions. Internal heat sources do not significantly affect these results; heat flow from the environment dominates. The instrument must be periodically opened to insert new aperture masks, but we find that the resulting temperature gradients and thermal stresses in the optics are small. Image shifts at the detector caused by thermal deflections of the Binospec optical bench structure are approx 0.1 pixel/hr. We conclude that the proposed Binospec design has acceptable thermal properties, and briefly discuss design changes to further reduce temperature gradients.Comment: 11 pages, to appear in PASP v114 Dec 200