Buoyancy transfer across a diffusive interface

1989 ◽  
Vol 209 ◽  
pp. 1-34 ◽  
Author(s):  
Harindra J. S. Fernando
Keyword(s):  
Interfacial Layer ◽  
Cold Water ◽  
Lower Layer ◽  
Layer Structure ◽  
Constant Heat Flux ◽  
Time Interval ◽  
Interfacial Layers ◽  
Density Interface ◽  
Turbulent Eddies ◽  
Opposing Effects

An experimental investigation of various aspects of buoyancy transfer across a diffusive density interface that separates stably stratified, turbulently convecting layers of relatively fresh cold water overlying hot salty water is described. It is argued that the interfacial layer should possess a double boundary-layer structure, in which the thicknesses of the salt and heat interfacial layers are determined by a balance between the opposing effects of diffusion and entrainment. Based on this argument, a simple theory, that predicts the interfacial-layer thickness, the diffusive heat and salt fluxes across the density interface, and the time variation of the temperature and salt concentrations in the convecting layers, is proposed for the case in which the convection is driven by a constant heat flux supplied to the lower layer. During a certain time interval, the theory and experiment agree well, but thereafter distinct differences can be seen. Measurements suggest that these differences may be due to the distortion of the density interface at low interfacial stabilities by turbulent eddies, which leads to a change in the buoyancy transfer mechanism. When the Richardson number falls below a critical value Riv, the interface was found to migrate slowly upwards and the mechanism of entrainment was the detachment of thin sheets of fluid by eddies scouring the interface.

The role of dissipation and mixing in exchange flow through a contracting channel

2000 ◽  
Vol 407 ◽  
pp. 265-290 ◽  
Author(s):  
KRAIG B. WINTERS ◽  
HARVEY E. SEIM
Keyword(s):  
Numerical Model ◽  
Interfacial Layer ◽  
Large Scale ◽  
Lower Layer ◽  
Layer Structure ◽  
Finite Thickness ◽  
Three Dimensional ◽  
Exchange Flow ◽  
Flow Through ◽  
Scale Turbulence

We investigate the transport of mass and momentum between layers in idealized exchange flow through a contracting channel. Lock-exchange initial value problems are run to approximately steady state using a three-dimensional, non-hydrostatic numerical model. The numerical model resolves the large-scale exchange flow and shear instabilities that form at the interface, parameterizing the effects of subgrid-scale turbulence. The closure scheme is based on an assumed steady, local balance of turbulent production and dissipation in a density-stratified fluid.The simulated flows are analysed using a two-layer decomposition and compared with predictions from two-layer hydraulic theory. Inter-layer transport leads to a systematic deviation of the simulated maximal exchange flows from predictions. Relative to predictions, the observed flows exhibit lower Froude numbers, larger transports and wider regions of subcritical flow in the contraction. To describe entrainment and mixing between layers, the computed solutions are decomposed into a three-layer structure, with two bounding layers separated by an interfacial layer of finite thickness and variable properties. Both bounding layers lose fluid to the interfacial layer which carries a significant fraction of the horizontal transport. Entrainment is greatest from the faster moving layer, occurring preferentially downstream of the contraction.Bottom friction exerts a drag on the lower layer, fundamentally altering the overall dynamics of the exchange. An example where bed friction leads to a submaximal exchange is discussed. The external forcing required to sustain a net transport is significantly less than predicted in the absence of bottom stresses.

Entrainment and mixing in stratified shear flows

2001 ◽  
Vol 428 ◽  
pp. 349-386 ◽  
Author(s):  
E. J. STRANG ◽  
H. J. S. FERNANDO
Keyword(s):  
Internal Waves ◽  
Richardson Number ◽  
Transition Period ◽  
Lower Layer ◽  
Buoyancy Flux ◽  
Shear Instability ◽  
Mixing Efficiency ◽  
Buoyancy Frequency ◽  
Entrainment Rate ◽  
Turbulent Eddies

The results of a laboratory experiment designed to study turbulent entrainment at sheared density interfaces are described. A stratified shear layer, across which a velocity difference ΔU and buoyancy difference Δb is imposed, separates a lighter upper turbulent layer of depth D from a quiescent, deep lower layer which is either homogeneous (two-layer case) or linearly stratified with a buoyancy frequency N (linearly stratified case). In the parameter ranges investigated the flow is mainly determined by two parameters: the bulk Richardson number RiB = ΔbD/ΔU2 and the frequency ratio fN = ND=ΔU.When RiB > 1.5, there is a growing significance of buoyancy effects upon the entrainment process; it is observed that interfacial instabilities locally mix heavy and light fluid layers, and thus facilitate the less energetic mixed-layer turbulent eddies in scouring the interface and lifting partially mixed fluid. The nature of the instability is dependent on RiB, or a related parameter, the local gradient Richardson number Rig = N2L/ (∂u/∂z)2, where NL is the local buoyancy frequency, u is the local streamwise velocity and z is the vertical coordinate. The transition from the Kelvin–Helmholtz (K-H) instability dominated regime to a second shear instability, namely growing Hölmböe waves, occurs through a transitional regime 3.2 < RiB < 5.8. The K-H activity completely subsided beyond RiB ∼ 5 or Rig ∼ 1. The transition period 3.2 < RiB < 5 was characterized by the presence of both K-H billows and wave-like features, interacting with each other while breaking and causing intense mixing. The flux Richardson number Rif or the mixing efficiency peaked during this transition period, with a maximum of Rif ∼ 0.4 at RiB ∼ 5 or Rig ∼ 1. The interface at 5 < RiB < 5.8 was dominated by ‘asymmetric’ interfacial waves, which gradually transitioned to (symmetric) Hölmböe waves at RiB > 5:8.Laser-induced fluorescence measurements of both the interfacial buoyancy flux and the entrainment rate showed a large disparity (as large as 50%) between the two-layer and the linearly stratified cases in the range 1.5 < RiB < 5. In particular, the buoyancy flux (and the entrainment rate) was higher when internal waves were not permitted to propagate into the deep layer, in which case more energy was available for interfacial mixing. When the lower layer was linearly stratified, the internal waves appeared to be excited by an ‘interfacial swelling’ phenomenon, characterized by the recurrence of groups or packets of K-H billows, their degeneration into turbulence and subsequent mixing, interfacial thickening and scouring of the thickened interface by turbulent eddies.Estimation of the turbulent kinetic energy (TKE) budget in the interfacial zone for the two-layer case based on the parameter α, where α = (−B + ε)/P, indicated an approximate balance (α ∼ 1) between the shear production P, buoyancy flux B and the dissipation rate ε, except in the range RiB < 5 where K-H driven mixing was active.

scholarly journals Effect of organic cathode interfacial layers on efficiency and stability improvement of polymer solar cells

RSC Advances ◽  
2017 ◽  
Vol 7 (50) ◽  
pp. 31158-31163 ◽  
Author(s):  
Mingguang Li ◽  
Wen Zhang ◽  
Honglei Wang ◽  
Lingfeng Chen ◽  
Chao Zheng ◽  
...  
Keyword(s):  
Solar Cells ◽  
Interfacial Layer ◽  
Polymer Solar Cells ◽  
High Mobility ◽  
Interfacial Layers

Simultaneously enhanced efficiency and stability can be achieved by using an organic cathode interfacial layer with high mobility and coarse morphology.

Fluorescent dye measurements of the mixing and transport of wastewater discharge in the bosphorus

1995 ◽  
Vol 32 (2) ◽  
pp. 61-68 ◽  
Author(s):  
Emin Özsoy ◽  
Mohammed Abdul Latif ◽  
Sükrü T. Besiktepe ◽  
Alexander Gaines
Keyword(s):  
Extreme Events ◽  
Interfacial Layer ◽  
Lower Layer ◽  
Fluorescent Dye ◽  
Longitudinal Dispersion ◽  
Wastewater Discharge ◽  
Exchange Flows ◽  
Background Levels ◽  
Good Agreement ◽  
Mixing And Transport

The performance of the wastewater discharge of Istanbul is evaluated based on fluorescent dye technique, supported by hydrographic and current measurements. Under the normal exchange flows, as well as during extreme events of blocking, the dye essentially remained confined in the lower layer. The maximum upper layer average dye concentration in the Bosphorus was close to the background levels, while the interfacial layer values were larger. Good agreement is found between the observations and simple models of longitudinal dispersion in the lower layer of the Bosphorus.

scholarly journals Double-layer structure in polar mesospheric clouds observed from SOFIE/AIM

2017 ◽  
Vol 35 (2) ◽  
pp. 295-309 ◽  
Author(s):  
Haiyang Gao ◽  
Gordon G. Shepherd ◽  
Yuanhe Tang ◽  
Lingbing Bu ◽  
Zhen Wang
Keyword(s):  
Double Layer ◽  
Lower Layer ◽  
Layer Structure ◽  
Polar Mesospheric Clouds ◽  
Double Peaks ◽  
Double Layer Structure ◽  
Ice Water Content ◽  
Background Temperature ◽  
Mesospheric Clouds ◽  
Ice Water

Abstract. Double-layer structures in polar mesospheric clouds (PMCs) are observed by using Solar Occultation for Ice Experiment (SOFIE) data between 2007 and 2014. We find 816 and 301 events of double-layer structure with percentages of 10.32 and 7.25 % compared to total PMC events, and the mean distances between two peaks are 3.06 and 2.73 km for the Northern Hemisphere (NH) and Southern Hemisphere (SH) respectively. Double-layer PMCs almost always have less mean ice water content (IWC) than daily IWC during the core of the season, but they are close to each other at the beginning and the end. The result by averaging over all events shows that the particle concentration has obvious double peaks, while the particle radius exhibits an unexpected monotonic increase with decreasing altitude. By further analysis of the background temperature and water vapour residual profiles, we conclude that the lower layer is a reproduced one formed at the bottom of the upper layer. 56.00 and 47.51 % of all double-layer events for the NH and SH respectively have temperature enhancements larger than 2 K locating between their double peaks. The longitudinal anti-correlation between the gravity waves' (GWs') potential energies and occurrence frequencies of double-layer PMCs suggests that the double-layer PMCs tend to form in an environment where the GWs have weaker intensities.

Low-temperature Crystallization of SrBi2Ta2O9 Thin Filmswith Bi2O3 Interfacial Layers by Liquid-delivery Metalorganicchemical Vapor Deposition

2002 ◽  
Vol 17 (1) ◽  
pp. 26-30 ◽  
Author(s):  
Woong-Chul Shin ◽  
Kyu-Jeong Choi ◽  
Soon-Gil Yoon
Keyword(s):  
Interfacial Layer ◽  
Deposition Temperature ◽  
Remanent Polarization ◽  
Annealing Temperature ◽  
Axis Orientation ◽  
Interfacial Layers ◽  
Si Substrates ◽  
Low Temperature Crystallization ◽  
Temperature Crystallization ◽  
Liquid Delivery

Ferroelectric SrBi2Ta2O9 (SBT) thin films and Bi2O3 interfacial layers were depositedonto the Pt/Ti/SiO2/Si substrates via liquid-delivery metalorganic chemical vapordeposition. The SBT films with a 5-nm-thick Bi2O3 interfacial layer were well crystallized without c-axis orientation, even at deposition temperature of 540 °C and showed a stronger (115) orientation than those without a Bi2O3 layer with increasing annealing temperature. The remanent polarizations of SBT films with Bi2O3 interfacial layer were significantly improved in comparison with those without Bi2O3 layer. The remanent polarization (2Pr) and coercive field (Ec) of SBT films without and with aBi2O3 interfacial layer annealed at 750 °C were 12 and 21 μC/cm2 and 60 and38 kV/cm, respectively, at an applied voltage of 5 V.

Numerical simulations of the role of a ferroelectric polymer interfacial layer in organic solar cells

2016 ◽  
Vol 18 (7) ◽  
pp. 5412-5418
Author(s):  
Bo Liu ◽  
Feng Xu ◽  
Xinghua Zhang ◽  
Dadong Yan ◽  
Dan Lu
Keyword(s):  
Solar Cells ◽  
Master Equation ◽  
Organic Solar Cells ◽  
Interfacial Layer ◽  
Vinylidene Fluoride ◽  
Bulk Heterojunction ◽  
Interfacial Layers ◽  
Polymer Bulk ◽  
Pauli Master Equation

A Pauli master equation method is adopted for the simulation of polymer bulk heterojunction (BHJ) solar cells with vinylidene fluoride–trifluoroethylene copolymer (P(VDF–TrFE)) films as interfacial layers.

scholarly journals Stress-strain state of a three-layer cylindrical shell under internal axisymmetric pulse load

2020 ◽  
pp. 145-151
Author(s):  
Viktor Gaidaichuk ◽  
Kostiantyn Kotenko
Keyword(s):  
Finite Element ◽  
Cylindrical Shell ◽  
Strain State ◽  
Layer Structure ◽  
Three Dimensional ◽  
Pulse Load ◽  
Geometrical Parameters ◽  
Time Interval ◽  
Stress Strain ◽  
Stress Strain State

The problem of dynamic deformation of a three-layer cylindrical shell under non-stationary loads in the case of rigid clamping of the shell ends is considered. The article presents the results of assessing the stress-strain state of a three-layer cylindrical shell, taking into account its structural feature, the ratio of the sheathing thickness and the physical and mechanical characteristics of a one-piece polymer filler. Calculations were performed by software complex Nastran. The values of displacements and stresses were calculated by the algorithm of direct transient dynamic process. The step duration of the time interval was 0.0000025 s, and the total number of steps was 200. The choice of the type of three-dimensional finite element was due to obtaining more detailed and accurate calculation results. The finite element model included 19000 three-dimensional finite elements and numbered 20800 nodes. The influence of geometrical parameters of shell layers with different physical and mechanical properties of one-piece filler on the stress-strain state under axisymmetric internal impulse load is investigated. Numerical results on the dynamics of the three-layer structure, obtained by the finite element method, allow to characterize the stress-strain state of the three-layer elastic structure of the cylindrical type at any time in the studied time interval. Optimization of the shell design is recommended. Changing the ratio of the thickness of the internal and external shells of the shell significantly affects the stress-strain state of the shell and its performance. Increasing the thickness of the internal layer of the shell significantly contributes to the increase of the latter. Comparison of the given results with materials of other similar researches and positions, testify to objectivity of the made approach.

Testing of Rectangular Channels With Perturbed Entrances for Temperature Difference Under Constant Heat Flux

2011 ◽  
Author(s):  
Jacek Marek Matyja ◽  
Tunde Bello-Ochende
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Transfer Performance ◽  
Cold Water ◽  
Constant Heat Flux ◽  
Entrance Region ◽  
Average Error ◽  
Transfer Performance ◽  
Rectangular Duct ◽  
Constant Heat

In this paper convective heat transfer performance of various duct geometries are compared using theoretical and experimental analyses. The experiments stretch further by perturbing the entrance region of the 2:1 rectangular duct (both inwards and outwards) and to obtain the effect on heat transfer performance. The cross-sectional area and length of the ducts are fixed and constant heat flux is applied to the ducts while cold water is used as the flow stream. The laminar flow regime is analysed. The theoretical and experimental cases are in agreement, with slight deviances attributed to certain assumptions made during the theoretical analysis and non-ideal testing conditions. The analyses concludes that perturbing the entrance region of a standard rectangular duct, both inwards and outwards, has a visible increase in heat transfer performance. The inward perturbed duct shows the highest increase in performance. The average variation between the theoretical and experimental case is about 18% for constant heat flux. The average error imposed on the results due experimental equipment is about 3% for constant heat flux experiments.

Experimental Study of Mixing of Two-Layer Density-Stratified Fluid by a Vortex Ring

2019 ◽  
Author(s):  
Lile Cao ◽  
Ryo Ito ◽  
Tomohiro Degawa ◽  
Yu Matsuda ◽  
Kotaro Takamure ◽  
...  
Keyword(s):  
Vortex Ring ◽  
Lower Layer ◽  
Stratified Fluid ◽  
Nacl Solution ◽  
Density Jump ◽  
Density Interface ◽  
Layer Density ◽  
Visualization Experiment ◽  
Planar Laser ◽  
Lower Fluid

Abstract This study experimentally investigates the mixing of a two-layer density-stratified fluid of water (upper layer) and aqueous sodium chloride (NaCl) solution (lower layer) induced by the interaction between a vortex ring and the density interface. The vortex ring, which consists of water, is launched from an orifice in the upper layer toward the density interface, after which its motion, along with the behavior of the lower fluid, is visualized through a planar laser-induced fluorescence method. The Atwood number that expresses the nondimensional density jump across the density interface is set at 0.0055, and the Reynolds number Re of the vortex ring is varied from 2050 to 3070. The visualization experiment clarifies that the vortex ring penetrating the density interface is bounced while collapsing in the lower fluid. Furthermore, it elucidates that the bounced upper fluid entrains the lower fluid into the upper layer by inducing a second vortex ring consisting of the lower fluid. Thus, this study reveals the effect of Re on the mixing of the upper and lower fluid induced by the launched vortex ring.

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