Vertical viscosity coefficient increased for high-resolution modeling of the Tsushima/Korea Strait

This study clarifies the necessity of an extraordinary large coefficient of vertical viscosity for dynamical ocean modeling in a shallow and narrow strait with complex bathymetry. Sensitivity experiments and objective analyses imply that background momentum viscosity is at the order of 100 cm2/s, while tracer diffusivity estimates are on the order of 0.1 cm2/s. The physical interpretation of these estimates is also discussed in the last part of this paper. To obtain reliable solutions, this study introduces cyclic application of the dynamical response to each parameter to minimize the number of long-term sensitivity experiments. The recycling Green’s function method yields weaker bottom friction and enhanced latent heat flux simultaneously with the increased viscosity in high-resolution modeling of the Tsushima/Korea Strait.

Lagrangian diffusivity estimated from coherent mesoscale eddies in an idealized basin circulation model

<div> <div> <div> <p>Lagrangian methods have been used to estimate the lateral eddy diffusivity in the ocean using surface drifter and subsurface float tracks and using the numerical particles advected by satellite-derived velocity fields. The diffusivity is estimated from the rate of dispersion of these particles. Accurate point-wise estimates of diffusivity generally require averages over a large number of drifters or floats, but the distribution of drifters and floats is generally sparse and many tracks of drifters are contaminated by winds. On the other hand, the convergence time for the particle-based diffusivity is on the order of a month for both in situ and numerical particles, which makes the estimates inefficient and allows for the accumulation of measurement error. Studies of vortex-dominated 2D turbulence have found that particle dispersion is dominated by the movement of coherent eddies, and that the dispersion rate of coherent eddies themselves can provide accurate estimates of the Lagrangian diffusivity. We found that the potential vorticity diffusivity in two-layer quasigeostrophic turbulence can also be accurately estimated by the rate of dispersion of coherent eddies, and this estimate converges more than four times faster than the diffusivity estimated from particles inserted uniformly in the flow. If this result also holds for oceanic mesoscale turbulence, it can form the basis for a potentially useful technique for diagnosing mesoscale diffusivity based on the tracks of coherent mesoscale eddies.</p> <p>This presentation examines the relation between the dispersion of coherent eddies and tracer diffusivity in an idealized configuration of Massachusetts Institute of Technology general circulation model which contains multiple gyres, boundary currents, and a zonally reentrant channel flow analogous to the Antarctic Circumpolar Current. The coherent eddies are identified and tracked from the sea surface height snapshots, and the diffusivity estimated from coherent eddies is compared to the tracer diffusivity diagnosed by a tracer inversion method. The diffusivity inferred from dispersion of coherent eddies generally converges within 15 days. Direct comparison of two diffusivity estimates is not straightforward, since the tracer-based diffusivity varies vertically. Approaches for reconciling the two estimates are discussed. This study shows the possibility of relating the Lagrangian movement of coherent eddies to the Eulerian tracer diffusivity.</p> </div> </div> </div>

Theory of tracer diffusion in concentrated hard-sphere suspensions

A phenomenological theory of diffusion and cross-diffusion of tracer particles in concentrated hard-sphere suspensions is developed. Expressions for the diffusion coefficients as functions of the host particle volume fraction are obtained up to the close-packing limit. In concentrated systems the tracer diffusivity decreases because of the reduced pore space available for diffusion. The tracer diffusivity can be modelled by a Stokes–Einstein equation with an effective viscosity that depends on the pore size. Tracer diffusion and segregation during sedimentation cease at a critical trapping volume fraction corresponding to a tracer glass transition. The tracer cross-diffusion coefficient, however, increases near the glass transition and diverges in the close-packed limit.

Monte Carlo Simulation of Correlation Effects in a Random SC Alloy via Interstitialcy Mechanisms

In this paper some recent progress in the area of Monte Carlo simulation of diffusion via the interstitialcy mechanism in a randomly ordered binary alloy is reviewed. Topics discussed include the calculation of tracer correlation factorsfA and fBas a function of composition and jump frequency ratiowA/wBand interstitialcy correlation factors fI; which play a crucial role in the interpretation of ion-conductivity data. The percolation behavior of fI when wA ≪ wB is analysed in detail and limits of the tracer diffusivity ratios bD A/bD B for alloy compositions below thepercolation threshold are presented. Allowance for non-collinear jumps (partly) replacing concurrent collinear site exchanges leads to a reduction of diffusion correlation effects.This goes along with a shift of the diffusion percolation threshold to lower concentrations of the (more) mobile component B. Even stronger changes of mass and charge transport compared to an exclusively collinear interstitialcy scheme are observed for additional contributions of direct interstitial jumps. It is remarkable that for both extensions of interstitialcy-mediated diffusion the Haven ratio appears to be greater than unity in certain compositionranges poor in B.

Collective motion in a suspension of micro-swimmers that run-and-tumble and rotary diffuse

Recent experiments have shown that suspensions of swimming micro-organisms are characterized by complex dynamics involving enhanced swimming speeds, large-scale correlated motions and enhanced diffusivities of embedded tracer particles. Understanding this dynamics is of fundamental interest and also has relevance to biological systems. The observed collective dynamics has been interpreted as the onset of a hydrodynamic instability, of the quiescent isotropic state of pushers, swimmers with extensile force dipoles, above a critical threshold proportional to the swimmer concentration. In this work, we develop a particle-based model to simulate a suspension of hydrodynamically interacting rod-like swimmers to estimate this threshold. Unlike earlier simulations, the velocity disturbance field due to each swimmer is specified in terms of the intrinsic swimmer stress alone, as per viscous slender-body theory. This allows for a computationally efficient kinematic simulation where the interaction law between swimmers is knowna priori. The neglect of induced stresses is of secondary importance since the aforementioned instability arises solely due to the intrinsic swimmer force dipoles.Our kinematic simulations include, for the first time, intrinsic decorrelation mechanisms found in bacteria, such as tumbling and rotary diffusion. To begin with, we simulate so-called straight swimmers that lack intrinsic orientation decorrelation mechanisms, and a comparison with earlier results serves as a proof of principle. Next, we simulate suspensions of swimmers that tumble and undergo rotary diffusion, as a function of the swimmer number density$(n)$, and the intrinsic decorrelation time (the average duration between tumbles,${\it\tau}$, for tumblers, and the inverse of the rotary diffusivity,$D_{r}^{-1}$, for rotary diffusers). The simulations, as a function of the decorrelation time, are carried out with hydrodynamic interactions (between swimmers) turned off and on, and for both pushers and pullers (swimmers with contractile force dipoles). The ‘interactions-off’ simulations allow for a validation based on analytical expressions for the tracer diffusivity in the stable regime, and reveal a non-trivial box size dependence that arises with varying strength of the hydrodynamic interactions. The ‘interactions-on’ simulations lead us to our main finding: the existence of a box-size-independent parameter that characterizes the onset of instability in a pusher suspension, and is given by$nUL^{2}{\it\tau}$for tumblers and$nUL^{2}/D_{r}$for rotary diffusers; here,$U$and$L$are the swimming speed and swimmer length, respectively. The instability manifests as a bifurcation of the tracer diffusivity curves, in pusher and puller suspensions, for values of the above dimensionless parameters exceeding a critical threshold.

Determining a Tracer Diffusivity by way of the Darken-Manning Equation for Interdiffusion in Binary Alloy Systems

The self-or tracer diffusivity of one component in a binary alloy is often required when there is knowledge of the other component’s self-or tracer diffusivity and the interdiffusivity (and the thermodynamic factor). In the present paper, this problem is addressed for the random alloy model by applying three possible approximations having different levels of accuracy: Darken (low level of accuracy), Manning (medium level of accuracy) and Moleko, Allnatt and Allnatt (MAA) (high level of accuracy). There are unexpectedly large differences between the results of these approximations that sometimes are reflected in the high sensitivity of the vacancy-wind factor to the level of approximation. Generally, for the application of Manning and the MAA approximations, it is found that there is a difference in the number of self-diffusivity roots depending on whether the tracer diffusivity is available for the faster diffuser or for the slower diffuser and depending on how close the composition is to the forbidden (according to Manning’s description) region. Provided that the interdiffusion coefficient (divided by the thermodynamic factor) is greater than the available self-diffusion coefficient multiplied by its complementary composition, the application of the Darken approximation always results in one self-diffusivity root.