322,833 research outputs found
Assessing the observability of demand pattern multipliers in water distribution systems using algebraic and numerical derivatives
No full textAbstract not availableAngela Marchi, Graeme C. Dandy, Dominic L. Boccelli and S. M. Masud Ran
14-moment maximum-entropy modelling of collisionless ions for Hall thruster discharges
No full textIons in Hall effect thrusters are often characterized by a low
collisionality. In the presence of acceleration fields and azimuthal electric
field waves, this results in strong deviations from thermodynamic equilibrium,
introducing kinetic effects. This work investigates the application of the
14-moment maximum-entropy model to this problem. This method consists in a set
of 14 PDEs for the density, momentum, pressure tensor components, heat flux
vector and fourth-order moment associated to the particle velocity distribution
function. The model is applied to the study of collisionless ion dynamics in a
Hall thruster-like configuration, and its accuracy is assessed against
different models, including the Vlasov kinetic equation. Three test cases are
considered: a purely axial acceleration problem, the problem of ion-wave
trapping and finally the evolution of ions in the axial-azimuthal plane.
Most of this work considers ions only, and the coupling with electrons is
removed by prescribing reasonable values of the electric field. This allows us
to obtain a direct comparison among different ion models. However, the
possibility to run self-consistent plasma simulations is also briefly
discussed, considering quasi-neutral or multi-fluid models. The maximum-entropy
system appears to be a robust and accurate option for the considered test
cases. The accuracy is improved over the simpler pressureless gas model (cold
ions) and the Euler equations for gas dynamics, while the computational cost
shows to remain much lower than direct kinetic simulations
Fracture-directed Waterjet Needle Steering: Design, Modeling, and Path Planning
No full textSteerable needle technology has the promise of improving outcomes by enhancing the accuracy of different therapies and biopsies, as they can be steered to a target location around obstacles. Achieving small radius of curvature and being able to control both radius of curvature and tip travel are of paramount importance in steerable needles to accomplish the increase in efficacy of the medical procedures. In this paper, we present a new class of the steerable needles, which we call waterjet-directed steerable needles, where the underlying principle is to first control the direction of tissue fracture with waterjet, after which the needle will follow during subsequent insertion. In this paper, the direction of the tissue fracture is controlled by an angled waterjet nozzle and control of the water velocity, and then the flexible Nitinol needle follows. It is shown that by changing the velocity of waterjet and thus depth of cut, radius of curvature can be controlled. A discrete-step kinematic model is used to model the motion of the waterjet steerable needle. This model consist of two parts: (1) the mechanics based model predicts the cut-depth of waterjet in soft tissue based on soft tissue properties, waterjet diameter, and water exit velocity, and (2) a discrete-step kinematic unicycle model of the steerable needle travel. Path planning is accomplished through a genetic algorithm, and the efficacy of waterjet steerable needle is tested for different paths. The key finding of the paper is that the radius of curvature of the waterjet steerable needle can be controlled by a fixed waterjet tip angle and varying water exit velocity to control the depth of cut
Analysis of the void coefficient in Pavia TRIGA Mark-II reactor: Monte Carlo numerical evaluation and comparison with experimental data
No full textLagrangian diffusive reactor for detailed thermochemical computations of plasma flows
No full textThe simulation of a thermochemical nonequilibrium for atomic and molecular energy level populations in plasma flows requires a comprehensive modeling of all the elementary collisional and radiative processes involved. Coupling detailed chemical mechanisms to flow solvers is computationally expensive and often limits their application to 1D simulations. We develop an efficient Lagrangian diffusive reactor moving along the streamlines of a steady baseline flow simulation to compute detailed thermochemical effects. In addition to its efficiency, the method allows us to model both continuum and rarefied flows, while including mass and energy diffusion. The Lagrangian solver is assessed for several testcases including strong normal shockwaves, as well as 2D and axisymmetric blunt-body hypersonic rarefied flows. In all the testcases performed, the Lagrangian reactor improves drastically the baseline simulations. The computational cost of a Lagrangian recomputation is typically orders of magnitude smaller with respect to a full solution of the problem. The solver has the additional benefit of being immune from statistical noise, which strongly affects the accuracy of calculations obtained by means of the Direct Simulation Monte Carlo method, especially considering minor species in the mixture. The results demonstrate that the method enables applying detailed mechanisms to multidimensional solvers to study thermochemical nonequilibrium flows
A self-consistent method for the simulation of meteor trails with an application to radio observations
No full textContext. Radio-based techniques allow for a meteor detection of 24 h. Electromagnetic waves are scattered by the electrons produced by the ablated species colliding with the incoming air. As the electrons dissipate in the trail, the received signal decays. The interpretation of these measurements entails complex physical modelling of the flow. Aims. In this work, we present a procedure to compute extensive meteor trails in the rarefied segment of the trajectory. This procedure is a general and standalone methodology, which provides meteor physical parameters at given trajectory conditions, without the need to rely on phenomenological lumped models. Methods. We started from fully kinetic simulations of the evaporated gas that describe the nonequilibrium in the flow and the ionisation collisions experienced by metals in their encounter with air molecules. These simulations were employed as initial conditions for performing detailed chemical and multicomponent diffusion calculations of the extended trail, in order to study the processes which lead to the extinction of the plasma. In particular, we focused on the evolution of the trail generated by a 1 mm meteoroid flying at 32 km s-1, above 80 km. We retrieved the ambipolar diffusion coefficient and the electron line density and compared the outcome of our computations with classical results and observational fittings. Finally, the electron field was employed to estimate the resulting reflected signal, using classical radio-echo theory for underdense meteors. Results. A global and constant diffusion coefficient is sufficient to reproduce numerical profiles. A good agreement is found when we compare the extracted diffusion coefficients with theory and observations
Predictive mechanics-based model for depth of cut (DOC) of waterjet in soft tissue for waterjet-assisted medical applications
No full textThe use of waterjet technology is now prevalent in medical applications including surgery, soft tissue resection, bone cutting, waterjet steerable needles, and wound debridement. The depth of the cut (DOC) of a waterjet in soft tissue is an important parameter that should be predicted in these applications. For instance, for waterjet-assisted surgery, selective cutting of tissue layers is a must to avoid damage to deeper tissue layers. For our proposed fracture-directed waterjet steerable needles, predicting the cut depth of the waterjet in soft tissue is important to develop an accurate motion model, as well as control algorithms for this class of steerable needles. To date, most of the proposed models are only valid in the conditions of the experiments and if the soft tissue or the system properties change, the models will become invalid. The model proposed in this paper is formulated to allow for variation in parameters related to both the waterjet geometry and the tissue. In this paper, first the cut depths of waterjet in soft tissue simulants are measured experimentally, and the effect of tissue stiffness, waterjet velocity, and nozzle diameter are studied on DOC. Then, a model based on the properties of the tissue and the waterjet is proposed to predict the DOC of waterjet in soft tissue. In order to verify the model, soft tissue properties (constitutive response and fracture toughness) are measured using low strain rate compression tests, Split-Hopkinson-Pressure-Bar (SHPB) tests, and fracture toughness tests. The results show that the proposed model can predict the DOC of waterjet in soft tissue with acceptable accuracy if the tissue and waterjet properties are known. [Figure not available: see fulltext.]
Diffusive author(s), cohesive author: Analysis of S/N (1994)
No full textThis study indicates the ways in which various aspects of the author(s) are brought forth in Dumb type’s performance art, the S/N production. Previous research has suggested a non-hierarchical organization of Dumb type and the absence of a “privileged author” in Dumb type’s collaborative work, S/N. However, the results that I have investigated from member’s interviews on the creative process of S/N along with my analysis of the recorded images of S/N, indicate a different aspect of the author(s). First, S/N was created through, so to speak, the collective ideas of the members of Dumb type. Further, S/N has at least nine quotations from previous performances, installations, and printed writings, besides the work-in-progress technique. Explicating one of the “author functions” as given by Michel Foucault, each text has plural subjects of the author. However, it has been revealed from members’ interviews that Teiji Furuhashi had a decision-making role in selecting the members’ ideas within the performance. Since then, S/N has had plural subjects of creation; however, Furuhashi is one of the subjects of creation along with the “privileged author.” S/N has plural authors (diffusive authors) yet at the same time, it has a “privileged author,” Teiji Furuhashi (cohesive author)
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
A 14-moment maximum-entropy description of electrons in crossed electric and magnetic fields
No full textA 14-moment maximum-entropy system of equations is applied to the description of non-equilibrium electrons in crossed electric and magnetic fields and in the presence of low collisionality, a characteristic of low-temperature plasma devices. The flexibility of this formulation is analyzed through comparison with analytical results for steady-state non-equilibrium velocity distribution functions and against particle-based solutions of the time-dependent kinetic equation. Electric and magnetic source terms are derived for the 14-moment equations, starting from kinetic theory. A simplified collision term based on the Bhatnagar-Gross-Krook operator is formulated to describe the collision of electrons with background neutrals, accounting for the large mass disparity and energy exchange. An approximated expression is proposed for the collision frequency, to include the effect of the electrons' drift velocity, showing good accuracy in the considered conditions. The capabilities of the proposed 14-moment closure to accurately capture the non-equilibrium behavior of electrons for space homogeneous problems under conditions representative of those found in Hall thrusters are demonstrated
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