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Air Layer

The Experts below are selected from a list of 7788 Experts worldwide ranked by ideXlab platform

Y. Leviatan – 1st expert on this subject based on the ideXlab platform

  • Analysis of transient interaction of electromagnetic pulse with an Air Layer in a dielectric medium using wavelet-based implicit TDIE formulation
    IEEE Transactions on Microwave Theory and Techniques, 2002
    Co-Authors: Y. Shifman, Y. Leviatan

    Abstract:

    The interaction of transient electromagnetic pulse with an Air Layer in a dielectric medium is formulated in terms of a time-domain integral equation and solved numerically via the method of moments. Previous related works pointed to the inherent inadequacy of the marching-on-in-time method in this case, but suggested no remedy. This paper explains why an implicit modeling scheme would work effectively in this case. It is also noted that the use of an implicit scheme would normally involve a solution of a very large and dense matrix equation. To alleviate this drawback of the implicit scheme, the use of a wavelet-based impedance-matrix-compression technique, which has facilitated in the very recent past solutions of time-domain problems with greater efficiency, is described.

  • Transient analysis of Air Layer in a dielectric medium using wavelet-based implicit TDIE formulation
    IEEE Antennas and Propagation Society International Symposium. 2001 Digest. Held in conjunction with: USNC URSI National Radio Science Meeting (Cat. N, 2001
    Co-Authors: Y. Shifman, Y. Leviatan

    Abstract:

    The interaction of a transient electromagnetic pulse with an Air Layer in a dielectric medium is formulated in terms of a time-domain integral equation and solved numerically via the method of moments. Application of the standard marching-on-in-time approach in this case can not yield a solution. Hence, we utilize an implicit modeling scheme, and, to reduce the computational complexity, resort to a previously proposed time-domain impedance matrix compression method. This method uses spatio-temporal wavelet basis functions to construct and solve a reduced-rank matrix equation. Furthermore, by modeling the problem simultaneously at all the time steps, we can obtain a solution which roughly has the same level of accuracy for all these times.

Marc Perlin – 2nd expert on this subject based on the ideXlab platform

  • on the scaling of Air Layer drag reduction
    Journal of Fluid Mechanics, 2013
    Co-Authors: Brian R Elbing, Marc Perlin, David R. Dowling, Simo A Makiharju, Andrew Wiggins, Steven L. Ceccio

    Abstract:

    Air-induced drag reduction was investigated on a 12.9 m long flat plate test model at a free stream speed of $6. 3~\mathrm{m} ~{\mathrm{s} }^{- 1} $
    . Measurements of the local skin friction, phase velocity profiles (liquid and gas) and void fraction profiles were acquired at downstream distances to 11.5 m, which yielded downstream-distance-based Reynolds numbers above 80 million. Air was injected within the boundary Layer behind a 13 mm backward facing step (BFS) while the incoming boundary Layer was perturbed with vortex generators in various configurations immediately upstream of the BFS. Measurements confirmed that Air Layer drag reduction (ALDR) is sensitive to upstream disturbances, but a clean boundary Layer separation line (i.e. the BFS) reduces such sensitivity. Empirical scaling of the experimental data was investigated for: (a) the critical Air flux required to establish ALDR; (b) void fraction profiles; and (c) the interfacial velocity profiles. A scaling of the critical Air flux for ALDR was developed from balancing shear-induced lift forces and buoyancy forces on a single bubble within a shear flow. The resulting scaling successfully collapses ALDR results from the current and past studies over a range of flow conditions and test model configurations. The interfacial velocity and void fraction profiles were acquired and scaled within the bubble drag reduction (BDR), ALDR and transitional ALDR regimes. The BDR interfacial velocity profile revealed that there was slip between phases. The ALDR results showed that the Air Layer thickness was nominally three-quarters of the total volumetric flux (per unit span) of Air injected divided by the free stream speed. Furthermore, the Air Layer had an average void fraction of 0.75 and a velocity of approximately 0.2 times the free stream speed. Beyond the Air Layer was a bubbly mixture that scaled in a similar fashion to the BDR results. Transitional ALDR results indicate that this regime was comprised of intermittent generation and subsequent fragmentation of an Air Layer, with the resulting drag reduction determined by the fraction of time that an Air Layer was present.

  • bubble induced skin friction drag reduction and the abrupt transition to Air Layer drag reduction
    Journal of Fluid Mechanics, 2008
    Co-Authors: Brian R Elbing, Eric S Winkel, Steven L. Ceccio, David R. Dowling, Marc Perlin

    Abstract:

    To investigate the phenomena of skin-friction drag reduction in a turbulent boundary Layer (TBL) at large scales and high Reynolds numbers, a set of experiments has been conducted at the US Navy’s William B. Morgan Large Cavitation Channel (LCC). Drag reduction was achieved by injecting gas (Air) from a line source through the wall of a nearly zero-pressure-gradient TBL that formed on a flat-plate test model that was either hydraulically smooth or fully rough. Two distinct drag-reduction phenomena were investigated; bubble drag reduction (BDR) and AirLayer drag reduction (ALDR). The streamwise distribution of skin-friction drag reduction was monitored with six skin-friction balances at downstream-distance-based Reynolds numbers to 220 million and at test speeds to 20.0ms −1 . Near-wall bulk void fraction was measured at twelve streamwise locations with impedance probes, and near-wall (0 Y Results from the BDR experiments indicate that: significant drag reduction (>25%) is limited to the first few metres downstream of injection; marginal improvement was possible with a porous-plate versus an open-slot injector design; BDR has negligible sensitivity to surface tension; bubble size is independent of surface tension downstream of injection; BDR is insensitive to boundary-Layer thickness at the injection location; and no synergetic effect is observed with compound injection. Using these data, previous BDR scaling methods are investigated, but data collapse is observed only with the ‘initial zone’ scaling, which provides little information on downstream persistence of BDR. ALDR was investigated with a series of experiments that included a slow increase in the volumetric flux of Air injected at free-stream speeds to 15.3ms −1 . These results indicated that there are three distinct regions associated with drag reduction with Air injection: Region I, BDR; Region II, transition between BDR and ALDR; and Region III, ALDR. In addition, once ALDR was established: friction drag reduction in excess of 80% was observed over the entire smooth model for speeds to 15.3ms −1 ; the critical volumetric flux of Air required to achieve ALDR was observed to be approximately proportional to the square of the free-stream speed; slightly higher injection rates were required for ALDR if the surface tension was decreased; stable Air Layers were formed at free-stream speeds to 12.5ms −1 with the surface fully roughened (though approximately 50% greater volumetric Air flux was required); and ALDR was sensitive to the inflow conditions. The sensitivity to the inflow conditions can be mitigated by employing a small fAired step (10mm height in the experiment) that helps to create a fixed separation line.

Y. Shifman – 3rd expert on this subject based on the ideXlab platform

  • Analysis of transient interaction of electromagnetic pulse with an Air Layer in a dielectric medium using wavelet-based implicit TDIE formulation
    IEEE Transactions on Microwave Theory and Techniques, 2002
    Co-Authors: Y. Shifman, Y. Leviatan

    Abstract:

    The interaction of transient electromagnetic pulse with an Air Layer in a dielectric medium is formulated in terms of a time-domain integral equation and solved numerically via the method of moments. Previous related works pointed to the inherent inadequacy of the marching-on-in-time method in this case, but suggested no remedy. This paper explains why an implicit modeling scheme would work effectively in this case. It is also noted that the use of an implicit scheme would normally involve a solution of a very large and dense matrix equation. To alleviate this drawback of the implicit scheme, the use of a wavelet-based impedance-matrix-compression technique, which has facilitated in the very recent past solutions of time-domain problems with greater efficiency, is described.

  • Transient analysis of Air Layer in a dielectric medium using wavelet-based implicit TDIE formulation
    IEEE Antennas and Propagation Society International Symposium. 2001 Digest. Held in conjunction with: USNC URSI National Radio Science Meeting (Cat. N, 2001
    Co-Authors: Y. Shifman, Y. Leviatan

    Abstract:

    The interaction of a transient electromagnetic pulse with an Air Layer in a dielectric medium is formulated in terms of a time-domain integral equation and solved numerically via the method of moments. Application of the standard marching-on-in-time approach in this case can not yield a solution. Hence, we utilize an implicit modeling scheme, and, to reduce the computational complexity, resort to a previously proposed time-domain impedance matrix compression method. This method uses spatio-temporal wavelet basis functions to construct and solve a reduced-rank matrix equation. Furthermore, by modeling the problem simultaneously at all the time steps, we can obtain a solution which roughly has the same level of accuracy for all these times.