794 research outputs found
Two-dimensional Moist Stratified Turbulence and the Emergence of Vertically Sheared Horizontal Flows
Moist stratified turbulence is studied in a two-dimensional Boussinesq system
influenced by condensation and evaporation. The problem is set in a periodic
domain and employs simple evaporation and condensation schemes, wherein both
the processes push parcels towards saturation. Numerical simulations
demonstrate the emergence of a moist turbulent state consisting of ordered
structures with a clear power-law type spectral scaling from initially
spatially uncorrelated conditions. An asymptotic analysis in the limit of rapid
condensation and strong stratification shows that, for initial conditions with
enough water substance to saturate the domain, the equations support a
straightforward state of moist balance characterized by a hydrostatic,
saturated, vertically sheared horizontal flow (VSHF). For such initial
conditions, by means of long time numerical simulations, the emergence of moist
balance is verified. Specifically, starting from uncorrelated data, subsequent
to the development of a moist turbulent state, the system experiences a rather
abrupt transition to a regime which is close to saturation and dominated by a
strong VSHF. On the other hand, initial conditions which do not have enough
water substance to saturate the domain, do not attain moist balance. Rather,
the system remains in a turbulent state and oscillates about moist balance.
Even though balance is not achieved with these general initial conditions, the
time scale of oscillation about moist balance is much larger than the imposed
time scale of condensation and evaporation, thus indicating a distinct dominant
slow component in the moist stratified two-dimensional turbulent system.Comment: 23 pages. 9 figure
Hybrid deterministic stochastic systems with microscopic look-ahead dynamics
We study the impact of stochastic mechanisms on a coupled hybrid system consisting of a general advection-diffusion-reaction partial differential equation and a spatially distributed stochastic lattice noise model. The stochastic dynamics include both spin-flip and spin-exchange type interparticle interactions. Furthermore, we consider a new, asymmetric, single exclusion pro- cess, studied elsewhere in the context of traffic flow modeling, with an one-sided interaction potential which imposes advective trends on the stochastic dynamics. This look-ahead stochastic mechanism is responsible for rich nonlinear behavior in solutions. Our approach relies heavily on first deriving approximate differential mesoscopic equations. These approximations become exact either in the long range, Kac interaction partial differential equation case, or, given sufficient time separation con- ditions, between the partial differential equation and the stochastic model giving rise to a stochastic averaging partial differential equation. Although these approximations can in some cases be crude, they can still give a first indication, via linearized stability analysis, of the interesting regimes for the stochastic model. Motivated by this linearized stability analysis we choose particular regimes where interacting nonlinear stochastic waves are responsible for phenomena such as random switching, convective instability, and metastability, all driven by stochasticity. Numerical kinetic Monte Carlo simulations of the coarse grained hybrid system are implemented to assist in producing solutions and understanding their behavior
Structural, magnetic, dielectric and mechanical properties of (Ba,Sr)MnO ceramics
Ceramic samples, produced by conventional sintering method in ambient air,
6H-SrMnO(6H-SMO), 15R-BaMnO(15R-BMO),
4H-BaSrMnO(4H-BSMO) were studied. In the XRD measurements
for SMO the new anomalies of the lattice parameters at 600-800 K range and the
increasing of thermal expansion coefficients with a clear maximum in a vicinity
at 670 K were detected. The Nel phase transition for BSMO was
observed at =250 K in magnetic measurements and its trace was detected in
dielectric, FTIR, DSC, and DMA experiments. The enthalpy and entropy changes of
the phase transition for BSMO at were determined as 17.5 J/mol and 70
mJ/K mol, respectively. The activation energy values and relaxation times
characteristic for relaxation processes were determined from the Arrhenius law.
Results of ab initio simulations showed that the contribution of the exchange
correlation energy to the total energy is about 30%.Comment: 12 pages, 12 figure
A dissipative random velocity field for fully developed fluid turbulence
We investigate the statistical properties, based on numerical simulations and
analytical calculations, of a recently proposed stochastic model for the
velocity field of an incompressible, homogeneous, isotropic and fully developed
turbulent flow. A key step in the construction of this model is the
introduction of some aspects of the vorticity stretching mechanism that governs
the dynamics of fluid particles along their trajectory. An additional further
phenomenological step aimed at including the long range correlated nature of
turbulence makes this model depending on a single free parameter that
can be estimated from experimental measurements. We confirm the realism of the
model regarding the geometry of the velocity gradient tensor, the power-law
behaviour of the moments of velocity increments (i.e. the structure functions),
including the intermittent corrections, and the existence of energy transfers
across scales. We quantify the dependence of these basic properties of
turbulent flows on the free parameter and derive analytically the
spectrum of exponents of the structure functions in a simplified non
dissipative case. A perturbative expansion in power of shows that
energy transfers, at leading order, indeed take place, justifying the
dissipative nature of this random field.Comment: 38 pages, 5 figure
How Cell Geometry and Cellular Patterning Influence Tissue Stiffness
Cell growth in plants occurs due to relaxation of the cell wall in response to mechanical forces generated by turgor pressure. Growth can be anisotropic, with the principal direction of growth often correlating with the direction of lower stiffness of the cell wall. However, extensometer experiments on onion epidermal peels have shown that the tissue is stiffer in the principal direction of growth. Here, we used a combination of microextensometer experiments on epidermal onion peels and finite element method (FEM) modeling to investigate how cell geometry and cellular patterning affects mechanical measurements made at the tissue level. Simulations with isotropic cell-wall material parameters showed that the orientation of elongated cells influences tissue apparent stiffness, with the tissue appearing much softer in the transverse versus the longitudinal directions. Our simulations suggest that although extensometer experiments show that the onion tissue is stiffer when stretched in the longitudinal direction, the effect of cellular geometry means that the wall is in fact softer in this direction, matching the primary growth direction of the cells
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