A new theory aiming to define the mechanisms of material removal in electroerosion

Formulation of an electrohydrodynamic hypothesis to explain the mechanism of material removal in electroerosion, and results of experiments designed to verify the validity of the proposed hypothesis is discussed. A hypothesis is proposed according to which material is removed by a high-power liquid microjet which forms as a result of implosion of a vapor cavity between the two electrodes. Using a model of the implosion of a vapor cavity, a study is made of the energy dissipated in the arc zone as a function of the expended electrical energy

Outsourcing Creative Work: a Study of Mobile Application Development

As the mobile phone industry converges with the IT/software industry, the emerging internetenabled multifunctional mobile devices are bringing together many diverse stakeholders. This paper examines the under-researched area of mobile application developers. We frame our study within the wider context of the mobile application industry in order to illustrate how the business models of large IT firms shape the everyday practices of systems developers, specifically those working on iPhone and Android platforms. By broadening out the study in this way we are able to situate seemingly new and distinctive technological artefacts and practices and show how they build on a number of continuing trends within the field. Drawing on qualitative data with developers working in this emerging market, we analyse their practices in order to (a) provide detail on this under-studied area and (b) show the ways in which developers adapt to the turbulent environment of the IT sector

Geographic information retrieval in a mobile environment: evaluating the needs of mobile individuals

This paper describes research that aims to define the information needs of mobile individuals, to implement a mobile information system that can satisfy those needs, and finally to evaluate the performance of that system with end-users. First a review of the emerging discipline of geographic information retrieval (GIR) is presented as background to the more specific issue of mobile information retrieval. Following this, a user needs study is described evaluating the requirements of potential users of a mobile information system; the study finds that there is a strong geographic component to users' information needs. Next, four geographic post-query filters are described which attempt to represent the region of space associated with an individual's query made at some specific spatial location. These filters are spatial proximity (distance in space), temporal proximity (travel time), speed-heading prediction surfaces (likelihood of visiting locations) and visibility (locations that can be seen). Two of these filters — spatial proximity and speed-heading prediction surfaces — are implemented in a mobile information system and subsequently evaluated with users in an outdoor setting. The results of evaluation suggest that retrieved information to which post-query geographic filters have been applied is considered more relevant than unfiltered information, and that users find information sorted by spatial proximity to be more relevant than that sorted by a prediction surface of likely future locations. The paper closes with a discussion of the wider implications of these results for developers of mobile information systems and location-based services

A dialogue based mobile virtual assistant for tourists: The SpaceBook Project

Ubiquitous mobile computing offers innovative approaches in the delivery of information that can facilitate free roaming of the city, informing and guiding the tourist as the city unfolds before them. However making frequent visual reference to mobile devices can be distracting, the user having to interact via a small screen thus disrupting the explorative experience. This research reports on an EU funded project, SpaceBook, that explored the utility of a hands-free, eyes-free virtual tour guide, that could answer questions through a spoken dialogue user interface and notify the user of interesting features in view while guiding the tourist to various destinations. Visibility modelling was carried out in real-time based on a LiDAR sourced digital surface model, fused with a variety of map and crowd sourced datasets (e.g. Ordnance Survey, OpenStreetMap, Flickr, Foursquare) to establish the most interesting landmarks visible from the user's location at any given moment. A number of variations of the SpaceBook system were trialled in Edinburgh (Scotland). The research highlighted the pleasure derived from this novel form of interaction and revealed the complexity of prioritising route guidance instruction alongside identification, description and embellishment of landmark information – there being a delicate balance between the level of information ‘pushed’ to the user, and the user's requests for further information. Among a number of challenges, were issues regarding the fidelity of spatial data and positioning information required for pedestrian based systems – the pedestrian having much greater freedom of movement than vehicles

Étude de l'écoulement d'une émulsion deuxième partie application de l'effet fusée mécanisme de l'érosion de cavitation

A gas or vapor bubble moving in translation in a liquid and varying its volume propels itself by a rocket effect, which occurs in foams moving in pressure or temperature fields with a non-zero gradient. It is notably observed in boiling and centrifugal pump operation, which latter features the following two characteristic bubble types : __ 1. Gas bubbles, which are apt to emerge faster from the pump than the liquid; 2. Vapor bubbles (cavitalion). When a bubble in translatory motion implodes, energy is transferred: potential pressure energy is converted info kinetic implosion energy, which in turn becomes kinetic translation energy. The instrument for this energy transfer process is a "micro-jet" following the bubble. It is shown that a considerable increase in kinetic translation energy density occurs, which is converted into potential pressure energy on impact against a solid obstacle. The resulting pressures (10,000 kg/sq.cm) explain the mechanical aspect of cavitation erosion. Several experimental results have confirmed these theoretical considerations. The development of a rotoscope is also described, this being a device enabling a centrifugal pump impeller (for example) to be "stopped" so that only the relative motion of the flow particles remains. Unlike with a stroboscope, time exposures can he taken with this device. With more thorough knowledge of foam mechanisms, and especially of cavitalion bubble behavior, it should be possible to design cavitation erosion-proof impeller blades

Visualisation des écoulements relatifs dans les machines tournantes rotoscope

It is often useful to be able to establish the fluid stream lines or the paths of given particles in suspension in a rotary machine. The various methods whereby this can he achieved are reviewed in this article, with special emphasis on the use of the rotoscope, an instrument designed to 'freeze' the rotation al motion and only leave relative motion visible. Where stream lines are made visible by the addition of different substances to the fluid under investigation__especially gas bubbles__very careful Interpretation is necessary in analysing any results. Stroboscope technique can be improved with special lighting, for example where threads are used as an aid to visual observation, but such methods are unsuitable where there are gas bubbles in the flow. The rotoscope has been designed in an attempt to overcome these limitations. It is similar to a Poggendorf mirror and comprises an electrically-driven prism rotating at half the speed of the impeller under investigation (see Fig. 1). A pump impeller viewed through a rotoscope appears to be stationary. Relative particle paths and directions of motion can be shown up by taking a time exposure. By using stroboscopic lighting, they can be related to a time base and their velocity and acceleration defined. This is a difficult instrument to set up. The optical axis of the prism must coincide with the axis of rotation. Several adjustment methods are available. Image stability depends on adjustment accuracy. The speed of the pump tested varied from rest up to several thousand r. p. m. The rotoscope faithfully followed the impeller motion and constantly gave a fixed image providing a fresh view of the various effects investigated and making them easier to comprehend. The equipment can also be used to study leakage along blades or at blade tips, or cavitation. A large number of photographs of various techniques show such familiar effects as stream lines in various impeller types, swirl, bubble motion, and the flow pattern in a bladeless impeller. Used in conjonction with such visual observation aids as threads or corpuscules, the rotoscope becomes an essential instrument for researchers and test bed operators interested in relative flows in hydraulic and gas flow rotary machines

Étude de l'écoulement d'une émulsion : Première partie, étude théorique du mouvement d'une bulle, effet fusée. Pébullon

A gas or vapour bubble moving at a high velocity of translation within a liquid sometimes enjoys a high degree of independence. By varying its volume, it may propel itself in the same manner as a rocket : this is the rocket effect. An increase in volume accelerates it and a decrease in volume slows it down. This result is obtained from the generalised Bernoulli equation (1) from which identical relationships (9), (10) and (11) are established. With a few simplifying assumptions, we have defined a particle that is equivalent to the bubble in translatory motion : the "pebullon". This "pebullon" conserves constant momentum (14) and is able to propel itself by change of mass. If this particle is assumed to be in a vacuum, a relationship has to be found between the mass variation and the energy involved: m'↔E = m' W2 The rocket effect occurs in a pressure field (centrifugal pump) or a temperature field (boiling). The acceleration to which the "pebullon" is subjected may become quite considerable, up to several thousand m/sec2. An air or vapour bubble inside a centrifugal pump is quasi-covered independent. Its relative velocity is proportional to distance run. Although it is subjected to a considerable buoyancy force it can still escape from the pump, often faster than the liquid. Noteworthy in the study of this problem are considerable kinetic energy variations involved. A subsequent article will show how this directed mechanical energy flux provides an explanation for erosion by mechanical cavitation process. A bubble should be considered not as a physical ball but as a cavity, or a deformation of the medium within which it is moving. This deformation propagates itself in the same manner as an energy wave. The "pebullon" concept is a practical, fertile model for the analysis of complex bubble motion. Whenever a problem involves a foam, it may fruitfully be considered from this angle. This will be done in a subsequent paper referring to an experimental study of the rocket effect

Appareil de mesure des tensions superficielles

A new apparatus for measuring the surface tension of liquids is described. It can be used to study the pressure required to create a gas bubble of given radius inside the liquid. Liquids either wetting or not wetting glass can be investigated, over a wide range of tempe-rature and pressure conditions.Nous présentons un nouvel appareil de mesure des tensions superficielles. Il permet d'étudier la pression nécessaire pour former, dans un liquide, une bulle de gaz d'un diamètre donné. Cet appareil peut être utilisé avec des liquides mouillant ou ne mouillant pas le verre dans une large gamme de température et de pression

Mouvement d'une bulle dans un liquide. Applications de l'effet fusée (fonctionnement des pompes érosion de cavitation, modèles réduits, ébullition usinage par électroérosion et par ultrasons, etc.)

Une bulle de gaz ou de vapeur de volume variable et en mouvement de translation dans un liquide est soumise à des accélérations importantes. Celles-ci entraînent des variations considérables de densité d'énergie cinétique de translation. Ce phénomène que nous appelons « l'effet fusée » trouve des applications dans les domaines les plus divers. Nous l'avons considéré successivement dans l'écoulement d'une émulsion à l'intérieur d'une pompe centrifuge, dans l'érosion de cavitation, dans quelques problèmes d'hydraulique, dans l'ébullition, dans l'usinage par ultrasons ou par électroérosion, dans la formation des aérosols et dans quelques autres cas possibles

Mouvement d'une bulle dans un liquide. Applications de l'effet fusée (fonctionnement des pompes érosion de cavitation, modèles réduits, ébullition usinage par électroérosion et par ultrasons, etc.)

A gas or vapour bubble, of variable volume, having translation motion in a liquid, is submitted to considerable acceleration. This phenomenon, called the "rocket effect", finds applications in various fields. We study those relative to the flow of emulsions in centrifugal pumps, to cavitation erosion, to some problems of hydraulics, to ebullition, to machining by ultrasonics or electroerosion, to formation of aerosols, and to some other possible cases.Une bulle de gaz ou de vapeur de volume variable et en mouvement de translation dans un liquide est soumise à des accélérations importantes. Celles-ci entraînent des variations considérables de densité d'énergie cinétique de translation. Ce phénomène que nous appelons « l'effet fusée » trouve des applications dans les domaines les plus divers. Nous l'avons considéré successivement dans l'écoulement d'une émulsion à l'intérieur d'une pompe centrifuge, dans l'érosion de cavitation, dans quelques problèmes d'hydraulique, dans l'ébullition, dans l'usinage par ultrasons ou par électroérosion, dans la formation des aérosols et dans quelques autres cas possibles