3,021 research outputs found
Migration arising from gradients in shear stress: Particle distributions in Poiseuille flow
Experimental evidence for the existence of shear induced migration processes is reviewed and the mechanism by Leighton and Acrivos (1987b) is described in detail. The proposed mechanism is shown to lead to the existence of an additional shear induced migration in the presence of gradients in shear stress such as would be found in Poiseuille flow, and which may be used to predict the amplitude of the observed short-term viscosity increase. The concentration and velocity profiles which result from such a migration are discussed in detail and are compared to the experimental observations of Karnis, Goldsmith and Mason (1966)
Accurate measurement of the piezoelectric coefficient of thin films by eliminating the substrate bending effect using spatial scanning laser vibrometry
One of the major difficulties in measuring the piezoelectric coefficient d(33,f)
for thin films is the elimination of the contribution from substrate bending. We
show by theoretical analysis and experimental measurements that by bonding thin
film piezoelectric samples to a substantial holder, the substrate bending can be
minimized to a negligible level. Once the substrate bending can be effectively
eliminated, single-beam laser scanning vibrometry can be used to measure the
precise strain distribution of a piezoelectric thin film under converse
actuation. A significant strain increase toward the inside edge of the top
electrode (assuming a fully covered bottom electrode) and a corresponding strain
peak in the opposite direction just outside the electrode edge were observed.
These peaks were found to increase with the increasing Poisson's ratio and
transverse piezoelectric coefficient of the piezoelectric thin film. This is due
to the non-continuity of the electric field at the edge of the top electrode,
which leads to the concentration of shear stress and electric field in the
vicinity of the electrode edge. The measured d(33,f) was found to depend not
only on the material properties such as the electromechanical coefficients of
the piezoelectric thin films and elastic coefficients of the thin film and the
substrate, but also on the geometry factors such as the thickness of the
piezoelectric films, the dimensions of the electrode, and also the thickness of
the substrate
Prediction of far-field acoustic emissions from cavitation clouds during shock wave lithotripsy for development of a clinical device
This study presents the key simulation and decision stage of a multi-disciplinary project to develop a hospital device for monitoring the effectiveness of kidney stone fragmentation by shock wave lithotripsy (SWL). The device analyses, in real time, the pressure fields detected by sensors placed on the patient's torso, fields generated by the interaction of the incident shock wave, cavitation, kidney stone and soft tissue. Earlier free-Lagrange simulations of those interactions were restricted (by limited computational resources) to computational domains within a few centimetres of the stone. Later studies estimated the far-field pressures generated when those interactions involved only single bubbles. This study extends the free-Lagrange method to quantify the bubble–bubble interaction as a function of their separation. This, in turn, allowed identification of the validity of using a model of non-interacting bubbles to obtain estimations of the far-field pressures from 1000 bubbles distributed within the focus of the SWL field. Up to this point in the multi-disciplinary project, the design of the clinical device had been led by the simulations. This study records the decision point when the project's direction had to be led by far more costly clinical trials instead of the relatively inexpensive simulations. <br/
Wall influence on dynamics of a microbubble
The nonlinear dynamic behaviour of microscopic bubbles near a wall is
investigated. The Keller-Miksis-Parlitz equation is adopted, but modified to
account for the presence of the wall. This base model describes the time
evolution of the bubble surface, which is assumed to remain spherical, and
accounts for the effect of acoustic radiation losses owing to liquid
compressibility in the momentum conservation. Two situations are considered:
the base case of an isolated bubble in an unbounded medium; and a bubble near a
solid wall. In the latter case, the wall influence is modeled by including a
symmetrically oscillating image bubble. The bubble dynamics is traced using a
numerical solution of the model equation. Subsequently, Floquet theory is used
to accurately detect the bifurcation point where bubble oscillations stop
following the driving ultrasound frequency and undergo period-changing
bifurcations. Of particular interest is the detection of the subcritical period
tripling and quadrupling transition. The parametric bifurcation maps are
obtained as functions of non-dimensional parameters representing the bubble
radius, the frequency and pressure amplitude of the driving ultrasound field
and the distance from the wall. It is shown that the presence of the wall
generally stabilises the bubble dynamics, so that much larger values of the
pressure amplitude are needed to generate nonlinear responses.Comment: 25 pages, 14 figure
Recognition of micro-scale deformation structures in glacial sediments - pattern perception, observer bias and the influence of experience
It is a scientist's mission to try to remain unbiased. However, certain factors play a role in scientific analyses that are not controlled by conscious thought. These factors are potentially very important in areas of science where interpretations are based on a scientist's ability to identify patterns or structures. One such area is the micromorphology of glacial sediments. In this paper we investigate the role of an analyst's experience in relation to pattern perception with specific reference to turbate microstructures in glacial diamictons. An experiment was conducted on 52 participants, which demonstrated that, as may be expected, more experienced (glacial) micromorphologists tend to exhibit a higher sensitivity-to-signal, but that complete novices, if given clear instructions, can reach levels of sensitivity similar to those of experts. It also showed, perhaps more surprisingly, that response bias does not decrease with experience. We discuss psychological factors, such as the drive for success and consistency, that may have contributed to these results and investigate their possible implications in the micromorphological analysis and interpretation of glacial sediments
Parallel Exhaustive Search without Coordination
We analyze parallel algorithms in the context of exhaustive search over
totally ordered sets. Imagine an infinite list of "boxes", with a "treasure"
hidden in one of them, where the boxes' order reflects the importance of
finding the treasure in a given box. At each time step, a search protocol
executed by a searcher has the ability to peek into one box, and see whether
the treasure is present or not. By equally dividing the workload between them,
searchers can find the treasure times faster than one searcher.
However, this straightforward strategy is very sensitive to failures (e.g.,
crashes of processors), and overcoming this issue seems to require a large
amount of communication. We therefore address the question of designing
parallel search algorithms maximizing their speed-up and maintaining high
levels of robustness, while minimizing the amount of resources for
coordination. Based on the observation that algorithms that avoid communication
are inherently robust, we analyze the best running time performance of
non-coordinating algorithms. Specifically, we devise non-coordinating
algorithms that achieve a speed-up of for two searchers, a speed-up of
for three searchers, and in general, a speed-up of
for any searchers. Thus, asymptotically, the speed-up is only four
times worse compared to the case of full-coordination, and our algorithms are
surprisingly simple and hence applicable. Moreover, these bounds are tight in a
strong sense as no non-coordinating search algorithm can achieve better
speed-ups. Overall, we highlight that, in faulty contexts in which coordination
between the searchers is technically difficult to implement, intrusive with
respect to privacy, and/or costly in term of resources, it might well be worth
giving up on coordination, and simply run our non-coordinating exhaustive
search algorithms
Low-profile and wearable energy harvester based on plucked piezoelectric cantilevers
The Pizzicato Energy Harvester (EH) introduced the technique of frequency up-conversion to piezoelectric EHs wearable on the lateral side of the knee-joint. The operation principle is to pluck the piezoelectric bimorphs with plectra so that they produce electrical energy during the ensuing mechanical vibrations. The device presented in this work is, in some ways, an evolution of the earlier Pizzicato: it is a significantly more compact and lighter device; the central hub holds 16 piezoelectric bimorphs shaped as trapezoids, which permits a sleek design and potentially increased energy output for the same bimorph area. Plectra were formed by Photochemical Machining of a 100-μm-thick steel sheet. To avoid the risk of short-circuiting, the plectra were electrically passivated by sputtering a 100 nm layer of ZrO2. Bench tests with the steel plectra showed a very large energy generation. Polyimide plectra were also manufactured with a cutting plotter from a 125μm-thick film. Besides bench tests, a volunteer wore the device while walking on flat ground or climbing stairs, with a measured energy output of approximately 0.8 mJ per step. Whereas most of the tests were performed by the traditional method of discharging the rectified output from the EH onto a resistive load, tests were performed also with a circuit offering a stabilised 3.3 V supply. The circuit produced a stable 0.1 mA supply during running gait with kapton plectra
Surface Roughness Dominated Pinning Mechanism of Magnetic Vortices in Soft Ferromagnetic Films
Although pinning of domain walls in ferromagnets is ubiquitous, the absence
of an appropriate characterization tool has limited the ability to correlate
the physical and magnetic microstructures of ferromagnetic films with specific
pinning mechanisms. Here, we show that the pinning of a magnetic vortex, the
simplest possible domain structure in soft ferromagnets, is strongly correlated
with surface roughness, and we make a quantitative comparison of the pinning
energy and spatial range in films of various thickness. The results demonstrate
that thickness fluctuations on the lateral length scale of the vortex core
diameter, i.e. an effective roughness at a specific length scale, provides the
dominant pinning mechanism. We argue that this mechanism will be important in
virtually any soft ferromagnetic film.Comment: 4 figure
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