scholarly journals Experimental studies of vortex disconnection and connection at a free surface

1996 ◽  
Vol 321 ◽  
pp. 59-86 ◽  
Author(s):  
Morteza Gharib ◽  
Alexander Weigand
Keyword(s):  
Free Surface ◽  
Vortex Ring ◽  
Experimental Studies ◽  
Local Strain ◽  
Surface Region ◽  
Vorticity Field ◽  
Core Diameter ◽  
Near Surface ◽  
Spatial And Temporal Scales ◽  
Rate Analysis

An experimental study is presented that examines the interaction of a vortex ring with a free surface. The main objective of this study is to identify the physical mechanisms that are responsible for the self-disconnection of vortex filaments in the near-surface region and the subsequent connection of disconnected vortex elements to the free surface. The understanding of those mechanisms is essential for the identification and estimation of the appropriate spatial and temporal scales of the disconnection and connection process. In this regard, the velocity and vorticity fields of an obliquely approaching laminar vortex ring with a Reynolds number of 1150 were mapped by using Digital Particle Image Velocimetry (DPIV). The evolution of the near-surface vorticity field indicates that the connection process starts in the side regions of the approaching vortex ring where surface-normal vorticity already exists in the bulk. A local strain rate analysis was conducted to support this conclusion. Disconnection in the near-surface tip region of the vortex ring occurs because of the removal of surfaceparallel vorticity by the viscous flux of vorticity through the surface. Temporal and spatial mapping of the vorticity field at the surface and in the perpendicular plane of symmetry shows that the viscous flux is balanced by a local deceleration of the flow at the surface. It is found that the observed timescales of the disconnection and connection process scale with the near-surface vorticity gradient rather than with the core diameter of the vortex ring.

Head-on collision of two coaxial vortex rings: experiment and computation

1995 ◽  
Vol 296 ◽  
pp. 39-71 ◽  
Author(s):  
Chin-Chou Chu ◽  
Chi-Tzung Wang ◽  
Chien-Cheng Chang ◽  
Ray-Yu Chang ◽  
Wen-Tyzh Chang
Keyword(s):  
Vortex Ring ◽  
Viscous Dissipation ◽  
Numerical Data ◽  
Vortex Rings ◽  
Visualization Technique ◽  
Vorticity Field ◽  
Core Diameter ◽  
Flow Development ◽  
Three Stages ◽  
Two Stages

Head-on collision of two coaxial vortex rings has been studied by joint experimental and numerical investigation. The Reynolds number, ReΓ, based on the initial circulation of the vortex rings, ranged from 400 to 2700. Besides numerical data, the vorticity field was also resolved by a non-intrusive visualization technique, LIPA, which enabled simultaneous measurement of velocities at multiple locations on a plane area. It was found that the enstrophy, rather than circulation, revealed three stages of evolution of the vortex rings prior to their breakdown. These include the free-travelling stage, stage of vortex stretching and the stage of viscous dissipation dominance. The results indicate that it would be incorrect to neglect the viscous effect, in particular, for the latter two stages of flow development. In fact, the rebound behaviour of the vortex rings for lower ReΓ is essentially a viscous phenomenon and is found to be closely related to the dissipation of enstrophy when the vortex rings are brought to interact actively with each other and is also related to the increase of the vorticity core diameter in the stage of dominance of viscous dissipation. Furthermore, an instant dimensionless group, Nt/ReΓ, based on the local vorticity distribution and the radius of a vortex ring, is found to be appropriate to characterize the onset of instability. Our investigation indicates that, in the range of observation, bulging instability will be observed during collision when Nt/ReΓ exceeds a critical value, (Nt/ReΓ)cr, which is a function of the initial core-size of the vortex ring. Comparisons showed that the numerical, measured, and visualization results were in consistent agreement; this not only enables us to assess the range of validity of the axisymmetry assumed for the numerical simulation, but also provides us with a rational basis for further analysis of azimuthal instability.

Effect of Ge Nanoislands on Lateral Photoconductivity of Ge-SiOX-Si Structures

2011 ◽  
Vol 276 ◽  
pp. 179-186 ◽  
Author(s):  
V.S. Lysenko ◽  
Yu.V. Gomeniuk ◽  
Yu.N. Kozyrev ◽  
M.Yu. Rubezhanska ◽  
V.K. Skylar ◽  
...  
Keyword(s):  
Density Of States ◽  
Experimental Studies ◽  
Surface Recombination ◽  
Fundamental Absorption Edge ◽  
Surface Region ◽  
Near Surface ◽  
Surface Recombination Rate ◽  
Equilibrium Charge ◽  
Effects Of Disorder ◽  
Gap Width

The results of the experimental studies of the effect of nanoislands on the lateral photoconductivity in structures with Ge nanoislands formed on the SiOx layer using molecular beam epitaxy are reported. It is shown that nanoislands increase the surface recombination rate and affect the fundamental absorption edge of c-Si. The generation of lateral photocurrent in the range 0.8 – 1.0 eV was observed due to transitions between tails in the density of states of the near-surface c-Si, which is described by Urbach dependence. It was shown that the absorption spectrum of nanoislands is typical for the disordered Ge and is due to transitions between density-of-states tails of the valence and conductance bands. The mechanism is proposed of lateral photoconductivity involving the non-equilibrium charge carriers, generated in Ge nanoislands. It is suggested that the optical absorption and lateral photocurrent in Ge-SiOx-Si structures are affected by fluctuations of the surface potential in the near-surface region of c-Si, fluctuations of the Si band gap width and by effects of disorder in Ge nanoislands.

scholarly journals Some Aspects of Near-Surface Vortices

1994 ◽  
Vol 47 (6S) ◽  
pp. S157-S162 ◽  
Author(s):  
M. Gharib
Keyword(s):  
Experimental Investigation ◽  
Particle Image Velocimetry ◽  
Free Surface ◽  
Vortex Ring ◽  
Particle Image ◽  
Digital Particle Image Velocimetry ◽  
Near Surface ◽  
Surface Circulation ◽  
Image Velocimetry ◽  
Vorticity Flux

An experimental investigation of the dynamics of near surface vortices is presented. Using digital particle image velocimetry, the velocity and vorticity fields of a vortex ring and a pair of undulated vortices were mapped as they rise toward a free surface. Circulation measurements indicate that vortex disconnection and appearance of the surface-normal vortices are simultaneous. Vortex disconnection due to the vorticity flux through the surface is shown to be related to the free-surface deceleration.

Preparation of cross-sectional samples for near-surface TEM studies

1987 ◽  
Vol 45 ◽  
pp. 380-381
Author(s):  
R.C. Dickenson ◽  
K.R. Lawless
Keyword(s):  
Standard Sample ◽  
Surface Region ◽  
Specimen Preparation ◽  
Thick Sheet ◽  
Drill Bit ◽  
Sample Holder ◽  
Metal Interface ◽  
Cross Sectional ◽  
Near Surface ◽  
Cold Worked

In thermal oxidation studies, the structure of the oxide-metal interface and the near-surface region is of great importance. A technique has been developed for constructing cross-sectional samples of oxidized aluminum alloys, which reveal these regions. The specimen preparation procedure is as follows: An ultra-sonic drill is used to cut a 3mm diameter disc from a 1.0mm thick sheet of the material. The disc is mounted on a brass block with low-melting wax, and a 1.0mm hole is drilled in the disc using a #60 drill bit. The drill is positioned so that the edge of the hole is tangent to the center of the disc (Fig. 1) . The disc is removed from the mount and cleaned with acetone to remove any traces of wax. To remove the cold-worked layer from the surface of the hole, the disc is placed in a standard sample holder for a Tenupol electropolisher so that the hole is in the center of the area to be polished.

The Effects of Ion Implantation on the Surface Features of Inconel 600

1985 ◽  
Vol 43 ◽  
pp. 288-289
Author(s):  
John D. Rubio
Keyword(s):  
Grain Boundary ◽  
Ion Implantation ◽  
Nuclear Power ◽  
Power Plants ◽  
Surface Region ◽  
Selective Dissolution ◽  
Boundary Region ◽  
Near Surface ◽  
Inconel 600 ◽  
Steam Generator Tubing

The degradation of steam generator tubing at nuclear power plants has become an important problem for the electric utilities generating nuclear power. The material used for the tubing, Inconel 600, has been found to be succeptible to intergranular attack (IGA). IGA is the selective dissolution of material along its grain boundaries. The author believes that the sensitivity of Inconel 600 to IGA can be minimized by homogenizing the near-surface region using ion implantation. The collisions between the implanted ions and the atoms in the grain boundary region would displace the atoms and thus effectively smear the grain boundary.To determine the validity of this hypothesis, an Inconel 600 sample was implanted with 100kV N2+ ions to a dose of 1x1016 ions/cm2 and electrolytically etched in a 5% Nital solution at 5V for 20 seconds. The etched sample was then examined using a JEOL JSM25S scanning electron microscope.

Microstructural changes of Aluminum Nitride after laser irradiation and electroless copper deposition

1994 ◽  
Vol 52 ◽  
pp. 636-637
Author(s):  
S. Cao ◽  
A. J. Pedraza ◽  
L. F. Allard
Keyword(s):  
Aluminum Nitride ◽  
Laser Irradiation ◽  
Electroless Deposition ◽  
Thermal Evaporation ◽  
Surface Region ◽  
Near Surface ◽  
Laser Patterning ◽  
Electroless Copper ◽  
Excimer Laser Irradiation ◽  
Laser Effect

Excimer-laser irradiation strongly modifies the near-surface region of aluminum nitride (AIN) substrates. The surface acquires a distinctive metallic appearance and the electrical resistivity of the near-surface region drastically decreases after laser irradiation. These results indicate that Al forms at the surface as a result of the decomposition of the Al (which has been confirmed by XPS). A computer model that incorporates two opposing phenomena, decomposition of the AIN that leaves a metallic Al film on the surface, and thermal evaporation of the Al, demonstrated that saturation of film thickness and, hence, of electrical resistance is reached when the rate of Al evaporation equals the rate of AIN decomposition. In an electroless copper bath, Cu is only deposited in laser-irradiated areas. This laser effect has been designated laser activation for electroless deposition. Laser activation eliminates the need of seeding for nucleating the initial layer of electroless Cu. Thus, AIN metallization can be achieved by laser patterning followed by electroless deposition.

Annealing Effects at the Near-Surface Region of Gallium Arsenide

1992 ◽  
Vol 105-110 ◽  
pp. 1383-1386 ◽  
Author(s):  
Hugh E. Evans ◽  
D.L. Smith ◽  
P.C. Rice-Evans ◽  
G.A. Gledhill ◽  
A.M. Moore
Keyword(s):  
Gallium Arsenide ◽  
Surface Region ◽  
Near Surface ◽  
Annealing Effects

scholarly journals Interactions of Ruddlesden-Popper Phases and Migration-Induced Field-Stabilized Polar Phase in Strontium Titanate

Crystals ◽  
2021 ◽  
Vol 11 (6) ◽  
pp. 693
Author(s):  
Christian Ludt ◽  
Elena Ovchinnikova ◽  
Anton Kulikov ◽  
Dmitri Novikov ◽  
Sibylle Gemming ◽  
...  
Keyword(s):  
Strontium Titanate ◽  
Density Functional ◽  
Surface Region ◽  
Point Of View ◽  
Polar Phase ◽  
Near Surface ◽  
Atomic Displacements ◽  
Induced Field ◽  
And Migration ◽  
Novel Concept

This work focuses on the validation of a possible connection of the known Ruddlesden-Popper (RP) phases and the novel concept of the migration-induced field-stabilized polar (MFP) phase. To study this subject, model structures of RP phases in bulk strontium titanate are analyzed by means of density functional theory (DFT). The obtained geometries are compared to experimental MFP data. Good agreement can be found concerning atomic displacements in the pm range and lattice strain inferred by the RP phases. Looking at the energy point of view, the defect structures are on the convex hull of the Gibb’s free energy. Although the dynamics to form the discussed defect models are not addressed in detail, the interplay and stability of the described defect model will add to the possible structure scenarios within the near-surface region of strontium titanate. As a result, it can be suggested that RP phases generally favor the MFP formation.

Defect Structure of MEV Si Implantation in GaAs

1988 ◽  
Vol 126 ◽  
Author(s):  
S.-Tong Lee ◽  
G. Braunstein ◽  
Samuel Chen
Keyword(s):  
Free Surface ◽  
Defect Structure ◽  
Room Temperature ◽  
Surface Region ◽  
Peak Position ◽  
Lattice Disorder ◽  
Depth Profiles ◽  
Trim Calculation ◽  
Good Agreement

ABSTRACTThe defect and atomic profiles for MeV implantation of Si in GaAs were investigated using He++ channeling, TEM, and SIMS. Doses of 1–10 × 1015Si/cm2 at 1–3 MeV were used. MeV implantation at room temperature rendered only a small amount of lattice disorder in GaAs. Upon annealing at 400°C for 1 h or 800°C for 30 a, we observed a ‘defect-free’ surface region (- 1 μ for 3 MeV implant). Below this region, extensive secondary defects were formed in a band which was 0.7 μ wide and centered at 2 μ for 3 MeV implant. These defects were mostly dislocations lying in the [111] plane. SIMS depth profiles of Si implants showed the Si peak to be very close to the peak position of the defects. The experimental profiles of Si were compared to the TRIM calculation; generally good agreement existed among the peak positions.

Electronic Impact of Inclusions in Diamond

2009 ◽  
Vol 1203 ◽  
Author(s):  
Erik M. Muller ◽  
John Smedley ◽  
Balaji Raghothamachar ◽  
Mengjia Gaowei ◽  
Jeffrey W. Keister ◽  
...  
Keyword(s):  
Charge Trapping ◽  
Secondary Phase ◽  
Surface Region ◽  
Phase Region ◽  
Electron Trapping ◽  
Near Surface ◽  
X Ray ◽  
Photoconductive Gain ◽  
Single Crystal Diamond ◽  
Topography Data

AbstractX-ray topography data are compared with photodiode responsivity maps to identify potential candidates for electron trapping in high purity, single crystal diamond. X-ray topography data reveal the defects that exist in the diamond material, which are dominated by non-electrically active linear dislocations. However, many diamonds also contain defects configurations (groups of threading dislocations originating from a secondary phase region or inclusion) in the bulk of the wafer which map well to regions of photoconductive gain, indicating that these inclusions are a source of electron trapping which affect the performance of diamond X-ray detectors. It was determined that photoconductive gain is only possible with the combination of an injecting contact and charge trapping in the near surface region. Typical photoconductive gain regions are 0.2 mm across; away from these near-surface inclusions the device yields the expected diode responsivity.

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