3 results
A model for confined vortex rings with elliptical-core vorticity distribution
- Ionut Danaila, Felix Kaplanski, Sergei S. Sazhin
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- Journal:
- Journal of Fluid Mechanics / Volume 811 / 25 January 2017
- Published online by Cambridge University Press:
- 07 December 2016, pp. 67-94
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We present a new model for an axisymmetric vortex ring confined in a tube. The model takes into account the elliptical (elongated) shape of the vortex ring core and thus extends our previous model (Danaila et al. J. Fluid Mech., vol. 774, 2015, pp. 267–297) derived for vortex rings with quasi-circular cores. The new model offers a more accurate description of the deformation of the vortex ring core, induced by the lateral wall, and a better approximation of the translational velocity of the vortex ring, compared with the previous model. The main ingredients of the model are the following: the description of the vorticity distribution in the vortex ring is based on the previous model of unconfined elliptical-core vortex rings (Kaplanski et al. Phys. Fluids, vol. 24, 2012, 033101); Brasseur’s approach (Brasseur, NASA Tech. Rep. JIAA TR-26, 1979) is then applied to derive a wall-induced correction for the Stokes streamfunction of the confined vortex ring flow. We derive closed formulae for the flow streamfunction and vorticity distributions. An asymptotic expression for the long-time evolution of the drift velocity of the vortex ring as a function of the ellipticity parameter is also derived. The predictions of the model are shown to be in agreement with direct numerical simulations of confined vortex rings generated by a piston–cylinder mechanism. The predictions of the model support the recently suggested heuristic relation (Krieg & Mohseni Trans. ASME J. Fluids Engng, vol. 135, 2013, 124501) between the energy and circulation of vortex rings with converging radial velocity. A new procedure for fitting experimental and numerical data with the predictions of the model is described. This opens the way for applying the model to realistic confined vortex rings in various applications including those in internal combustion engines.
Nanoscale Ruthenium Coatings of MEMS Switches Contacts
- Sergey Karabanov, Andrey S. Karabanov, Dmitriy V. Suvorov, Benoit Grappe, Caroline Coutier, Henri Sibuet, Boris N. Sazhin, Anatoliy A. Krutilin
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- Journal:
- MRS Online Proceedings Library Archive / Volume 1249 / 2010
- Published online by Cambridge University Press:
- 01 February 2011, 1249-F08-09
- Print publication:
- 2010
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The paper reports the tests results of ruthenium contacts coatings of magnetically controlled MEMS switches. During the tests the contact resistance was measured and the lifetime of MEMS switches was evaluated. After testing the analysis of the form and contact surface structure using SEM-method was carried out. The experiment results showed that the application of ruthenium nanolayers as the contact coating at slight increase of the contact resistance improves the lifetime of MEMS switches considerably.
Nowadays the increase of the lifetime of all MEMS switch types (RF MEMS relays, magnetically controlled on-off MEMS switches) is the most actual problem. The work resource and operating characteristics, first of all, contact resistance, of any switching unit, the basis of the construction of which is a dry contact, are determined by the contact coating properties. This is true for MEMS switches too.
The paper presents the results of the study of the operating characteristics of magnetically controlled MEMS switches with ruthenium and gold contact coatings of up to 100 nm thickness. During tests the following switching mode was used: switching voltage – 3 V, current - 10 μA. Each MEMS switch was subjected to 100 million cycles switching at the frequency of 100 Hz. After testing the contact surface investigation by SEM-method and electrical characteristics measurement was carried out. The paper presents SEM-images of the contacts surface and statistical date of electrical characteristics.
After 100 million switching cycles MEMS switches with nanoscale ruthenium coating have shown 100% operating capacity; 16% of switches with gold contacts turned out to be inoperative due to electrical contacts sticking.
The results of researches by SEM-method show that the contacts without nanoscale coating have the traces of strong erosion and melting; the contacts with nanoscale ruthenium coating practically did not change the form and flatness.
So, the test results indicate that nanoscale ruthenium coatings of the electrical contacts provide excellent resource for the switches operation of hundred millions and more cycles.
A generalized vortex ring model
- FELIX KAPLANSKI, SERGEI S SAZHIN, YASUHIDE FUKUMOTO, STEVEN BEGG, MORGAN HEIKAL
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- Journal:
- Journal of Fluid Mechanics / Volume 622 / 10 March 2009
- Published online by Cambridge University Press:
- 10 March 2009, pp. 233-258
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A conventional laminar vortex ring model is generalized by assuming that the time dependence of the vortex ring thickness ℓ is given by the relation ℓ = atb, where a is a positive number and 1/4 ≤ b ≤ 1/2. In the case in which , where ν is the laminar kinematic viscosity, and b = 1/2, the predictions of the generalized model are identical with the predictions of the conventional laminar model. In the case of b = 1/4 some of its predictions are similar to the turbulent vortex ring models, assuming that the time-dependent effective turbulent viscosity ν∗ is equal to ℓℓ′. This generalization is performed both in the case of a fixed vortex ring radius R0 and increasing vortex ring radius. In the latter case, the so-called second Saffman's formula is modified. In the case of fixed R0, the predicted vorticity distribution for short times shows a close agreement with a Gaussian form for all b and compares favourably with available experimental data. The time evolution of the location of the region of maximal vorticity and the region in which the velocity of the fluid in the frame of reference moving with the vortex ring centroid is equal to zero is analysed. It is noted that the locations of both regions depend upon b, the latter region being always further away from the vortex axis than the first one. It is shown that the axial velocities of the fluid in the first region are always greater than the axial velocities in the second region. Both velocities depend strongly upon b. Although the radial component of velocity in both of these regions is equal to zero, the location of both of these regions changes with time. This leads to the introduction of an effective radial velocity component; the latter case depends upon b. The predictions of the model are compared with the results of experimental measurements of vortex ring parameters reported in the literature.