scholarly journals Permanent bedforms in a theoretical model of wave-sea-bed interactions

2000 ◽  
Vol 7 (1/2) ◽  
pp. 31-35 ◽  
Author(s):  
J. M. Becker ◽  
D. Bercovici
Keyword(s):  
Theoretical Model ◽  
Nonlinear Response ◽  
External Surface ◽  
Lower Layer ◽  
Sea Waves ◽  
Layer Model ◽  
Periodic Forcing ◽  
Sea Bed ◽  
Two Layer Model ◽  
Large Wavelength

Abstract. The interaction between sea waves and a deformable sea-bed is studied with a simple two-layer model in which the upper-layer fluid is inviscid and the lower-layer fluid is bi-viscous to account for non-Newtonian behaviour of sand and sediments. The nonlinear response of the system to periodic forcing by an external surface pressure is determined. It is shown that a simple bi-viscous rheology allows small wavelength morphology in the lower layer to be generated from large wavelength surface waves in the upper inviscid layer, although the morphology is not permanent. For a bi-viscous rheology with a pressure-dependent yield stress (which accounts for the fact that sand yields less readily under loading than unloading), however, small wavelength and permanent features are formed in the seabed.

On the Thickness Ratio in the Quasigeostrophic Two-Layer Model of Baroclinic Instability

2013 ◽  
Vol 70 (5) ◽  
pp. 1505-1511 ◽  
Author(s):  
Noboru Nakamura ◽  
Lei Wang
Keyword(s):  
Growth Rate ◽  
Vertical Structure ◽  
Lower Layer ◽  
Baroclinic Instability ◽  
Thickness Ratio ◽  
Layer Model ◽  
Optimal Thickness ◽  
Zonal Wavenumber ◽  
The Mean ◽  
Two Layer Model

Abstract It is shown that the classical quasigeostrophic two-layer model of baroclinic instability possesses an optimal ratio of layer thicknesses that maximizes the growth rate, given the basic-state shear (thermal wind), beta, and the mean Rossby radius. This ratio is interpreted as the vertical structure of the most unstable mode. For positive shear and beta, the optimal thickness of the lower layer approaches the midheight of the model in the limit of strong criticality (shear/beta) but it is proportional to criticality in the opposite limit. For a set of parameters typical of the earth’s midlatitudes, the growth rate maximizes at a lower-layer thickness substantially less than the midheight and at a correspondingly larger zonal wavenumber. It is demonstrated that a turbulent baroclinic jet whose statistical steady state is marginally critical when run with equal layer thicknesses can remain highly supercritical when run with a nearly optimal thickness ratio.

scholarly journals Examining cerebral hemodynamics in a two-layer model of the human head using broadband near-infrared spectroscopy

2021 ◽  
Author(s):  
Olivia Pucci
Keyword(s):  
Optical Properties ◽  
Near Infrared ◽  
Continuous Wave ◽  
Lower Layer ◽  
Homogeneous Medium ◽  
Human Head ◽  
Cerebral Hemodynamics ◽  
Layer Model ◽  
Adult Human ◽  
Two Layer Model

The development of a continuous-wave method is presented, to quantify accurately the optical properties of a two-layer model of the human head using a broadband spectral approach. In particular, focus is put on the reconstruction of the absolute absorption and scattering properties of a two-layered phantom model of the human head with steady-state multi-distance measurements by performing differential fit analysis of the near-infrared (NIR) reflectance spectrum between 700 nm and 1000 nm. The two-layer model approximation was fitted to experimental broadband absorbance measurements obtained from two-layered phantoms with known optical properties. Results demonstrated that the suggested method was able to determine the optical properties of the lower layer with minimal error at specific source-detector distances. Preliminary results on the non-invasive measurement of the optical properties of the adult human brain in a two-layer approximation are presented. Finally, a mobile wireless NIR device is used to measure changes in the temporal characteristics of cerebral hemodynamic responses to functional brain activity, in particulaur the effect of smoking. Results suggest that assuming homogeneous medium for the adult human head severely underestimates the changes in cerebral hemodynamics. Hence, it is important to take surrounding layers into consideration when performing cerebral measurements using NIR spectroscopy.

ELECTROMAGNETIC COUPLING BETWEEN GROUNDED WIRES AT THE SURFACE OF A TWO‐LAYER EARTH

Geophysics ◽  
1973 ◽  
Vol 38 (5) ◽  
pp. 854-863 ◽  
Author(s):  
Gerald W. Hohmann
Keyword(s):  
Numerical Solution ◽  
Lower Layer ◽  
Electromagnetic Coupling ◽  
Induced Polarization ◽  
Electrode Configuration ◽  
Smooth Transition ◽  
Layer Model ◽  
The Earth ◽  
Two Layer Model

A technique has been developed for computing the effects of electromagnetic coupling in induced polarization surveys, when the earth can be approximated by a two‐layer model. Results are given for the dipole‐dipole array, but the numerical solution described can be applied to any electrode configuration. No unusual effects were observed for the models computed. As the thickness of the upper layer is increased, there is a smooth transition between the coupling response of a homogeneous earth having the resistivity of the upper layer and that of a homogeneous earth having a resistivity equal to that of the lower layer.

A two-layer model of Gulf Stream separation

1969 ◽  
Vol 39 (3) ◽  
pp. 511-528 ◽  
Author(s):  
A. T. Parsons
Keyword(s):  
Wind Stress ◽  
Gulf Stream ◽  
Stream Flow ◽  
Lower Layer ◽  
Western Boundary ◽  
Boundary Current ◽  
Layer Model ◽  
Layer Depth ◽  
Boundary Currents ◽  
Two Layer Model

A study is made of the wind-driven circulation of a two-layer ocean within a square basin, with a view to describing the observed separation of western boundary currents. The lower layer is allowed to surface and the line along which the upper-layer depth vanishes is interpreted as the region of the surfacing thermocline. For a representative wind stress the theory predicts the gross features of the Gulf Stream flow, the region adjacent to the surfacing line containing the separated boundary current. By assuming that the effects of friction and inertia are confined to regions of a boundary-layer character, the position of a separated current is shown to depend only on the degree of stratification and certain integral properties of the applied wind stress.

scholarly journals Coronal influence on dynamos

2013 ◽  
Vol 9 (S302) ◽  
pp. 134-137 ◽  
Author(s):  
Jörn Warnecke ◽  
Axel Brandenburg
Keyword(s):  
Reynolds Number ◽  
Convection Zone ◽  
Magnetic Energy ◽  
Lower Layer ◽  
Solar Radius ◽  
Magnetic Reynolds Number ◽  
Layer Model ◽  
Dynamo Action ◽  
Turbulent Dynamo ◽  
Two Layer Model

AbstractWe report on turbulent dynamo simulations in a spherical wedge with an outer coronal layer. We apply a two-layer model where the lower layer represents the convection zone and the upper layer the solar corona. This setup is used to study the coronal influence on the dynamo action beneath the surface. Increasing the radial coronal extent gradually to three times the solar radius and changing the magnetic Reynolds number, we find that dynamo action benefits from the additional coronal extent in terms of higher magnetic energy in the saturated stage. The flux of magnetic helicity can play an important role in this context.

scholarly journals Examining cerebral hemodynamics in a two-layer model of the human head using broadband near-infrared spectroscopy

2021 ◽  
Author(s):  
Olivia Pucci
Keyword(s):  
Optical Properties ◽  
Near Infrared ◽  
Continuous Wave ◽  
Lower Layer ◽  
Homogeneous Medium ◽  
Human Head ◽  
Cerebral Hemodynamics ◽  
Layer Model ◽  
Adult Human ◽  
Two Layer Model

The development of a continuous-wave method is presented, to quantify accurately the optical properties of a two-layer model of the human head using a broadband spectral approach. In particular, focus is put on the reconstruction of the absolute absorption and scattering properties of a two-layered phantom model of the human head with steady-state multi-distance measurements by performing differential fit analysis of the near-infrared (NIR) reflectance spectrum between 700 nm and 1000 nm. The two-layer model approximation was fitted to experimental broadband absorbance measurements obtained from two-layered phantoms with known optical properties. Results demonstrated that the suggested method was able to determine the optical properties of the lower layer with minimal error at specific source-detector distances. Preliminary results on the non-invasive measurement of the optical properties of the adult human brain in a two-layer approximation are presented. Finally, a mobile wireless NIR device is used to measure changes in the temporal characteristics of cerebral hemodynamic responses to functional brain activity, in particulaur the effect of smoking. Results suggest that assuming homogeneous medium for the adult human head severely underestimates the changes in cerebral hemodynamics. Hence, it is important to take surrounding layers into consideration when performing cerebral measurements using NIR spectroscopy.

scholarly journals Time-Dependent Response to Cooling in a Beta-Plane Basin

2006 ◽  
Vol 36 (11) ◽  
pp. 2185-2198 ◽  
Author(s):  
Joseph Pedlosky
Keyword(s):  
Steady State ◽  
Vertical Motion ◽  
Ocean Basin ◽  
Time Dependent ◽  
Western Boundary ◽  
Boundary Current ◽  
Layer Model ◽  
Periodic Forcing ◽  
Beta Plane ◽  
Two Layer Model

Abstract The time-dependent response of an ocean basin to the imposition of cooling (or heating) is examined in the context of a quasigeostrophic, two-layer model on the beta plane. The focus is on the structure and magnitude of the vertical motion and its response to both a switch-on forcing and a periodic forcing. The model employed is a time-dependent version of an earlier model used to discuss the intensification of sinking in the region of the western boundary current. The height of the interface of the two-layer model serves as an analog of temperature, and the vertical velocity at the interface consists of a cross-isopycnal velocity modeled in terms of a relaxation to a prescribed interface height, an adiabatic representation of eddy thickness fluxes parameterized as lateral diffusion of thickness, and the local vertical motion of the interface itself. The presence of time dependence adds additional dynamical features to the problem, in particular the emergence of low-frequency, weakly damped Rossby basin modes. If the buoyancy forcing is zonally uniform the basin responds to a switch-on of the forcing by coming into steady-state equilibrium after the passage of a single baroclinic Rossby wave. If the forcing is nonuniform in the zonal direction, a sequence of Rossby basin modes is excited and their decay is required before the basin achieves a steady state. For reasonable parameter values the boundary layers, in which both horizontal and vertical circulations are closed, are quasi-steady and respond to the instantaneous state of the interior. As in the steady problem the flow is sensitive to small nonquasigeostrophic mass fluxes across the perimeter of the basin. These fluxes generally excite basin modes as well. The basin modes will also be weakly excited if the beta-plane approximation is relaxed. The response to periodic forcing is also examined, and the sensitivity of the response to the structure of the forcing is similar to the switch-on problem.

Coherent structures of nonlinear barotropic-baroclinic interaction in unequal depth two-layer model

2021 ◽  
Vol 408 ◽  
pp. 126347
Author(s):  
Jiaqi Zhang ◽  
Ruigang Zhang ◽  
Liangui Yang ◽  
Quansheng Liu ◽  
Liguo Chen
Keyword(s):  
Coherent Structures ◽  
Layer Model ◽  
Two Layer Model
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