scholarly journals Continental Shelf Baroclinic Instability. Part II: Oscillating Wind Forcing

2016 ◽  
Vol 46 (2) ◽  
pp. 569-582 ◽  
Author(s):  
K. H. Brink ◽  
H. Seo
Keyword(s):  
Flow Field ◽  
Continental Shelf ◽  
Large Scale ◽  
Baroclinic Instability ◽  
Wind Forcing ◽  
Develop Model ◽  
Bottom Slope ◽  
Coriolis Parameter ◽  
Wide Range ◽  
Eddy Field

AbstractContinental shelf baroclinic instability energized by fluctuating alongshore winds is treated using idealized primitive equation numerical model experiments. A spatially uniform alongshore wind, sinusoidal in time, alternately drives upwelling and downwelling and so creates highly variable, but slowly increasing, available potential energy. For all of the 30 model runs, conducted with a wide range of parameters (varying Coriolis parameter, initial stratification, bottom friction, forcing period, wind strength, and bottom slope), a baroclinic instability and subsequent eddy field develop. Model results and scalings show that the eddy kinetic energy increases with wind amplitude, forcing period, stratification, and bottom slope. The dominant alongshore length scale of the eddy field is essentially an internal Rossby radius of deformation. The resulting depth-averaged alongshore flow field is dominated by the large-scale, periodic wind forcing, while the cross-shelf flow field is dominated by the eddy variability. The result is that correlation length scales for alongshore flow are far greater than those for cross-shelf velocity. This scale discrepancy is qualitatively consistent with midshelf observations by Kundu and Allen, among others.

On the generation of large-scale eddy-driven patterns: the average eddy model

2016 ◽  
Vol 809 ◽  
pp. 316-344 ◽  
Author(s):  
Timour Radko
Keyword(s):  
Multiscale Analysis ◽  
Large Scale ◽  
Baroclinic Instability ◽  
Spontaneous Generation ◽  
Physical Processes ◽  
Layer System ◽  
Large Scale Structures ◽  
Wide Range ◽  
Scale Structures ◽  
Eddy Field

A theoretical model is developed which illustrates the dynamics of the spontaneous generation of large-scale structures in baroclinically unstable eddying flows. Techniques of asymptotic multiscale analysis are used to identify instabilities resulting from the positive feedback of the background eddies on large-scale perturbations. The novelty of the proposed approach lies in the choice of a dynamically consistent time-dependent background eddy field, which is taken from simulations of baroclinic instability in the Phillips two-layer system. The resulting solutions differ considerably from those of traditional multiscale models, in which the background eddy field is represented by steady analytical patterns. The present formulation makes it possible to (i) test the multiscale theory against the corresponding numerical simulations, (ii) unambiguously interpret the key physical processes at play and (iii) rationalize the emergence of large-scale patterns for certain background parameters. While the proposed approach to multiscale modelling is illustrated on a particular example of the Phillips baroclinic instability model, it is our belief that the presented technique is readily adaptable to a wide range of applications.

Spatially amplifying modes of the Charney baroclinic-instability problem

1986 ◽  
Vol 170 ◽  
pp. 293-317 ◽  
Author(s):  
R. T. Pierrehumbert
Keyword(s):  
Broad Class ◽  
Baroclinic Instability ◽  
Surface Wind ◽  
Stationary Wave ◽  
Vertical Shear ◽  
Coriolis Parameter ◽  
Spatial Instability ◽  
Wide Range ◽  
Amplification Rate ◽  
Zero Frequency

We determine the circumstances under which baroclinic instability in the Charney model subjected to localized time-periodic forcing manifests itself as a wavetrain that oscillates at the source frequency and amplifies in space with distance from the source; analytical and numerical results describing the salient characteristics of such waves are presented. The spatially amplifying disturbance is a hitherto unsuspected part of the response to a pulsating source, and coexists with the more familiar neutral Rossby wavetrains; it is likely to play a role in a wide range of atmospheric and oceanic phenomena.The central results rely on a careful application of a causality criterion due to Briggs. These results illustrate a practical means of attacking spatial instability problems, which can be applied to a broad class of systems besides the one at hand. We have found that the Charney problem with positive vertical shear is not absolutely unstable, so long as the wind at the ground is non-negative. This implies that spatial instability and forced stationary-wave problems are well posed in an open domain under typical atmospheric circumstances.The amplifying waves appear on the downstream side of the source, have eastward (downstream) phase propagation and have wavelengths that increase monotonically with decreasing frequency, becoming infinite at zero frequency. When the surface wind is not too large, the spatial amplification rate has a single maximum near the frequency ωm= (f/N)Uz, wherefis the Coriolis parameter,Nis the stability frequency andUzis the vertical shear; the rate approaches zero at zero frequency and asymptotes algebraically to zero at large frequency for any positive surface wind. Distinct Charney and Green modes do not appear until the surface wind is made very large. The amplification rate at ωmbecomes infinite as surface wind approaches zero, suggesting a mechanism for the concentration of eddy activity.We also discuss the relationship of these results to the structure of low- and high-frequency atmospheric variability.

scholarly journals Shear Rheology of Unentangled and Marginally Entangled Ring Polymer Melts from Large-Scale Nonequilibrium Molecular Dynamics Simulations

Polymers ◽  
2019 ◽  
Vol 11 (7) ◽  
pp. 1194 ◽  
Author(s):  
Alexandros J. Tsamopoulos ◽  
Anna F. Katsarou ◽  
Dimitrios G. Tsalikis ◽  
Vlasis G. Mavrantzas
Keyword(s):  
Molecular Dynamics ◽  
Flow Field ◽  
Large Scale ◽  
Nonequilibrium Molecular Dynamics ◽  
Survival Times ◽  
Dynamic Heterogeneity ◽  
Shear Rheology ◽  
Time Dynamics ◽  
Shear Rates ◽  
Wide Range

We present results for the steady state shear rheology of non-concatenated, unentangled and marginally entangled ring poly(ethylene oxide) (PEO) melts from detailed, atomistic nonequilibrium molecular dynamics (NEMD) simulations, and compare them to the behavior of the corresponding linear melts. The applied flow field spans a wide range of shear rates, from the linear (Newtonian) to the highly non-linear (described by a power law) regime. For all melts studied, rings are found to exhibit shear thinning but to a lesser degree compared to linear counterparts, mostly due to their reduced deformability and stronger resistance to alignment in the direction of flow. These features are attributed to the more compact structure of ring molecules compared to linear chains; the latter are capable of adopting wider and more open conformations even under shear due to the freedom provided by the free ends. Similar to linear melts, rings also exhibit a first and a second normal stress coefficient; the latter is negative. The ratio of the magnitude of the two coefficients remains practically constant with shear rate and is systematically higher than the corresponding one for linear melts. Emphasis was also given to the statistics of terminal (re-orientational) relaxation times which we computed by analyzing all chains in the simulated systems one by one; it was demonstrated that long time dynamics are strongly heterogeneous both for rings and (especially) linears. Repeating the analysis under flow conditions, and as expected, we found that the applied flow field significantly suppresses dynamic heterogeneity, especially for high shear rates well beyond the Newtonian plateau. Finally, a detailed geometrical analysis revealed that the average population of ring–ring threading events in the longest melt studied here (the PEO-5k ring) remains practically unaffected by the imposed flow rate even at strong shear rates, except for multi-threadings which disappear. To further analyze this peculiar and rather unexpected effect, we computed the corresponding survival times and penetration lengths, and found that the overwhelming majority of threadings under shear are extremely weak constraints, as they are characterized by very small penetration lengths, thus also by short survival times. They are expected therefore to play only a minor (if any) role on chain dynamics.

Ignition and Flame Stabilization of a Premixed Jet in Hot Cross Flow

2013 ◽  
Author(s):  
Denise Schmitt ◽  
Michael Kolb ◽  
Johannes Weinzierl ◽  
Christoph Hirsch ◽  
Thomas Sattelmayer
Keyword(s):  
Flow Field ◽  
High Speed ◽  
Equivalence Ratio ◽  
Large Scale ◽  
Large Diameter ◽  
Flame Temperature ◽  
Cross Flow ◽  
Flame Stabilization ◽  
Wide Range ◽  
Cold Case

At the Institute of Thermodynamics, Technical University of Munich a large scale atmospheric combustion test rig has been designed and set up. The experimental setup is comprised of two burning zones: A first zone consists of 16 burners providing vitiated air at 1776K, into which a secondary fuel-air mixture jet is injected and ignited by the hot cross flow. The phenomenon is known in the literature as a reacting jet in hot cross flow. The hot data is compared to the cold case in order to show differences in the flow field due to flame propagation. For evaluating the flow field several experimental analyses have been applied so far (OH*, High-Speed PIV, Mixture Analysis). The focus of this paper is on the momentum ratios J = 4–10 with Jet Reynolds Numbers between 20,000 and 80,000. For the cold case the flow field is measured and compared with the reacting jet. In the injector the air and the natural gas are perfectly premixed. The equivalence ratio of the jet is varied over a wide range of mixtures (ϕ = 0.05–0.77) resulting in an adiabatic flame temperature of the jet between 800 and 2200K. As the pictures of the chemiluminescence analysis show the jet gas ignites immediately upon entering the hot cross flow. The distinct influence of the equivalence ratio on the flame length and shape can be seen in the data. The trajectory of the flame penetrates further into the channel compared to the trajectory of the cold case caused by the reaction in the flame and its resulting gas expansion. Due to the large diameter of the jet in the experiment the origins of the dominant flow patterns are obtained with high spatial resolution. Following this, flame anchoring mechanisms at different operation points are derived.

scholarly journals Planetary–Geometric Constraints on Isopycnal Slope in the Southern Ocean

2015 ◽  
Vol 45 (12) ◽  
pp. 2991-3004 ◽  
Author(s):  
Daniel C. Jones ◽  
Takamitsu Ito ◽  
Thomas Birner ◽  
Andreas Klocker ◽  
David Munday
Keyword(s):  
Southern Ocean ◽  
Large Scale ◽  
Baroclinic Instability ◽  
Surface Wind ◽  
Relative Vorticity ◽  
Geometric Constraints ◽  
Coriolis Parameter ◽  
Sector Model ◽  
Surface Wind Stress ◽  
Flux Parameterization

AbstractOn planetary scales, surface wind stress and differential buoyancy forcing act together to produce isopycnal surfaces that are relatively flat in the tropics/subtropics and steep near the poles, where they tend to outcrop. Tilted isopycnals in a rapidly rotating fluid are subject to baroclinic instability. The turbulent, mesoscale eddies generated by this instability have a tendency to homogenize potential vorticity (PV) along density surfaces. In the Southern Ocean (SO), the tilt of isopycnals is largely maintained by competition between the steepening effect of surface forcing and the flattening effect of turbulent, spatially inhomogeneous eddy fluxes of PV. Here quasigeostrophic theory is used to investigate the influence of a planetary–geometric constraint on the equilibrium slope of tilted density/buoyancy surfaces in the SO. If the meridional gradients of relative vorticity and PV are small relative to β, then quasigeostrophic theory predicts ds/dz = β/f0 = cot(ϕ0)/a, or equivalently r ≡ |∂zs/(β/f0)| = 1, where f is the Coriolis parameter, β is the meridional gradient of f, s is the isopycnal slope, ϕ0 is a reference latitude, a is the planetary radius, and r is the depth-averaged criticality parameter. It is found that the strict r = 1 condition holds over specific averaging volumes in a large-scale climatology. A weaker r = O(1) condition for depth-averaged quantities is generally satisfied away from large bathymetric features. The r = O(1) constraint is employed to derive a depth scale to characterize large-scale interior stratification, and an idealized sector model is used to test the sensitivity of this relationship to surface wind forcing. Finally, the possible implications for eddy flux parameterization and for the sensitivity of SO circulation/stratification to changes in forcing are discussed.

LES Simulation of Backflow Vortex Structure at the Inlet of an Inducer

2006 ◽  
Vol 129 (5) ◽  
pp. 587-594 ◽  
Author(s):  
Nobuhiro Yamanishi ◽  
Shinji Fukao ◽  
Xiangyu Qiao ◽  
Chisachi Kato ◽  
Yoshinobu Tsujimoto
Keyword(s):  
Flow Field ◽  
Vortex Structure ◽  
Large Scale ◽  
Experimental Studies ◽  
Flow Coefficient ◽  
Dimensional Structure ◽  
Limited Information ◽  
Inlet Flow ◽  
New Findings ◽  
Wide Range

Turbopump inducers often have swirling backflow under a wide range of flow rates because they are designed with a certain angle of attack even at the design point in order to attain high cavitation performance. When the flow rate is decreased, the backflow region extends upstream and may cause various problems by interacting with upstream elements. It is also known that the backflow vortex structure occurs in the shear layer between the main flow and the swirling backflow. Experimental studies on the backflow from an inducer have given us insight into the characteristics of backflow vortex structure, but the limited information has not lead to the complete understanding of the phenomena. Numerical studies based on Reynolds-averaged Navier-Stokes (RANS) computations usually deteriorate when the flow field of interest involves large-scale separations, as shown by a previous study by Tsujimoto et al. (2005). On the other hand, the numerical approach using the Large Eddy Simulation (LES) technique has the potential to predict unsteady flows and/or flow fields that include regions of large-scale separation much more accurately than RANS computations does in general. The present paper describes the application of the LES code developed by one of the authors (Kato) to further understand the backflow vortex structure at the inlet of an inducer. First, the internal flow of the inducer was simulated, as a way to evaluate the validity of the proposed method, under a wide range of inlet flow coefficients. The static pressure peformance and the length of the backflow region was compared with measured values, and good agreement was obtained. Second, using the validated LES code, the fundamental characteristics of the backflow vortex was investigated in detail. It was found that the backflow vortices are formed in a circumferentially twisted manner at the boundary between the swirling backflow and the straight inlet flow. Also, the backflow vortices rotate in the same direction as the inducer, but with half of the circumferential flow velocity in the backflow region. Another finding was that the backflow region expands toward the center of the flow field and the number of vortices decrease, as the flow coefficient decreases. To the best of our knowledge, this is the first computation of the backflow at the inducer inlet to achieve quantitative agreement with measured results, and give new findings to the complicated three-dimensional structure of the backflow, which was very limited under experimental studies.

Validation of process-based sand wave models: applying a linear and nonlinear sand wave model to the Netherlands Continental Shelf

2020 ◽  
Author(s):  
Geert Campmans ◽  
Pieter Roos ◽  
Thaiënne Van Dijk ◽  
Suzanne Hulscher
Keyword(s):  
Continental Shelf ◽  
Linear Model ◽  
Large Scale ◽  
Model Performance ◽  
Sand Wave ◽  
Earth Surface ◽  
Geophysical Research ◽  
Wave Dynamics ◽  
Wide Range ◽  
Linear And Nonlinear

<p>Tidal sand waves are dynamic large-scale bed forms occurring in tide-dominated, sandy shelf seas such as the North Sea. Since they may interfere with various activities, understanding sand wave dynamics is important from a practical point of view. Recently, two process-based model studies were carried out to investigate the influence of storm processes on sand wave dynamics (Campmans et al., CSR2017; JGR2018). While this type of model gives insight in the morphodynamic mechanisms, quantitative comparison with field observations remains a challenge.</p><p> </p><p>Here we present a systematic validation of the afore mentioned linear and nonlinear models, against a wide range of sand wave observations from the entire Netherlands Continental Shelf (Damen et al., JGR2018). Specifically, from the available locations with sand wave observations and environmental characteristics, we have chosen a grid for calibration and, staggered to that, a grid for validation. For the so-called calibration locations, we tuned the linear model (using local environmental conditions) in order to minimize the difference between observed and modelled wavelengths. Next, on the validation locations, we used the thus obtained parameter settings (location-independent values of slip parameter and effective wave period) to test our model performance, both in the linear and nonlinear regime. First results demonstrate fair agreement for the wavelengths from the linear model and indicate a systematic overestimation of sand wave heights by the nonlinear model.</p><p> </p><p><strong>References</strong></p><p>Campmans, G.H.P., Roos, P.C., De Vriend, H.J., Hulscher, S.J.M.H., 2017.  Modeling the influence of storms on sand wave formation: A linear stability approach. Continental Shelf Research 137, 103–116.</p><p>Campmans, G.H.P., Roos, P.C., De Vriend, H.J., Hulscher, S.J.M.H., 2018. The influence of storms on sand wave evolution: a nonlinear idealized modeling approach. Journal of Geophysical Research: Earth Surface 123, 2070-2086.</p><p>Damen, J.M., Van Dijk, T.A.G.P., Hulscher, S.J.M.H., 2018. Spatially varying environmental properties controlling observed sand wave morphology. Journal of Geophysical Research: Earth Surface 123, 262-280.</p>

scholarly journals Ergodicity of stochastically forced large scale geophysical flows

2001 ◽  
Vol 28 (6) ◽  
pp. 313-320 ◽  
Author(s):  
Jinqiao Duan ◽  
Beniamin Goldys
Keyword(s):  
Large Scale ◽  
Geophysical Flows ◽  
Ensemble Average ◽  
Wind Forcing ◽  
Statistical Ensemble ◽  
Time Interval ◽  
Coriolis Parameter ◽  
Random Forcing ◽  
Time Average

We investigate the ergodicity of 2D large scale quasigeostrophic flows under random wind forcing. We show that the quasigeostrophic flows are ergodic under suitable conditions on the random forcing and on the fluid domain, and under no restrictions on viscosity, Ekman constant or Coriolis parameter. When these conditions are satisfied, then for any observable of the quasigeostrophic flows, its time average approximates the statistical ensemble average, as long as the time interval is sufficiently long.

Large-Eddy Simulation Study of Log Laws in a Neutral Ekman Boundary Layer

2018 ◽  
Vol 75 (6) ◽  
pp. 1873-1889 ◽  
Author(s):  
Qingfang Jiang ◽  
Shouping Wang ◽  
Peter Sullivan
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Large Scale ◽  
Geostrophic Wind ◽  
Coriolis Parameter ◽  
Ekman Boundary Layer ◽  
Wide Range ◽  
Large Eddy ◽  
Log Law ◽  
Flux Layer

Abstract The characteristics of wind profiles in a neutral atmospheric boundary layer and their dependence on the geostrophic wind speed Ug, Coriolis parameter f, and surface roughness length z0 are examined utilizing large-eddy simulations. These simulations produce a constant momentum flux layer and a log-law layer above the surface characterized by a logarithmic increase of wind speed with height. The von Kármán constant derived from the mean wind profile is around 0.4 over a wide range of control parameters. The depths of the simulated boundary layer, constant-flux layer, and surface log-law layer tend to increase with the wind speed and decrease with an increasing Coriolis parameter. Immediately above the surface log-law layer, a second log-law layer has been identified from these simulations. The depth of this upper log-law layer is comparable to its counterpart in the surface layer, and the wind speed can be scaled as , as opposed to just in the surface log-law layer, implying that in addition to surface processes, the upper log-law layer is also influenced by Earth’s rotation and large-scale conditions. Here is the friction velocity at the surface, and h is the boundary layer depth. An analytical model is proposed to assist in the interpretation of the log laws in a typical Ekman boundary layer. The physics and implications of the upper log-law layer are discussed.

Electron microscopy study of reactively sputter-deposited Ti/N films on silicon

1992 ◽  
Vol 50 (2) ◽  
pp. 1362-1363
Author(s):  
V. C. Kannan ◽  
A. K. Singh ◽  
R. B. Irwin ◽  
S. Chittipeddi ◽  
F. D. Nkansah ◽  
...  
Keyword(s):  
Integrated Circuits ◽  
Large Scale ◽  
Hexagonal Phase ◽  
Microscopy Study ◽  
Electron Microscopy Study ◽  
Process Conditions ◽  
Tin Films ◽  
Wide Range ◽  
Fcc Phase ◽  
Circuits Vlsi

Titanium nitride (TiN) films have historically been used as diffusion barrier between silicon and aluminum, as an adhesion layer for tungsten deposition and as an interconnect material etc. Recently, the role of TiN films as contact barriers in very large scale silicon integrated circuits (VLSI) has been extensively studied. TiN films have resistivities on the order of 20μ Ω-cm which is much lower than that of titanium (nearly 66μ Ω-cm). Deposited TiN films show resistivities which vary from 20 to 100μ Ω-cm depending upon the type of deposition and process conditions. TiNx is known to have a NaCl type crystal structure for a wide range of compositions. Change in color from metallic luster to gold reflects the stabilization of the TiNx (FCC) phase over the close packed Ti(N) hexagonal phase. It was found that TiN (1:1) ideal composition with the FCC (NaCl-type) structure gives the best electrical property.

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