Wave-in-ice: theoretical bases and field observations.

There has been a significant increase of studies on wave-ice interactions in the past decades. Through a close look at a representative set of theories, this paper investigates different physical processes that have produced different wave dispersion and attenuation. The existing theories have considered four major processes: scattering, flexural damping, viscoelastic damping and basal friction. Each theory looked into one of these processes and used a different mathematical formulation to model these processes. The low-frequency behaviours of the resulting spectral attenuation in these theories are fundamentally different from each other. Recent field observations have produced a large amount of data to calibrate and validate these theories. The uncertainties in using field measurements to determine attenuation due to ice covers are discussed. Both observational data and applications of these theories in field conditions suggest a multi-physics approach. A number of studies to further the theoretical development are recommended. It will take time for wave-in-ice models to reach the same level of performance as wave models for the open ocean, relying on the combined effort of theoretical, modelling and observational studies. This article is part of the theme issue 'Theory, modelling and observations of marginal ice zone dynamics: multidisciplinary perspectives and outlooks'.

A major effect of simulated microgravity on several stages of preimplantation mouse development is lethality associated with elevated phosphorylated SAPK/JNK.

We tested whether microgravity affects mouse development during a period when gravity cues chick and frog embryo development. A rotating vessel developed approximately 0.1% simulated microgravity (MGS) for embryos. Microgravity simulation resulted in blocked cell accumulation in E2.5 embryos. E1.5 and E3.5 embryos showed lesser effects. For E1.5/2.5 embryos, cell accumulation block was followed by lethality at 48 hours after MGS. For E3.5 embryos, MGS blocked development without lethality but with apoptosis. E1.5-3.5 embryos from the rotational control developed lesser effects than MGS embryos. Embryonic stress-activated protein kinase (SAPK) was phosphorylated during MGS and mediated apoptosis. Increased pSAPK suggested that lethality is due to cellular stress induced by MGS, unlike the dysfunctional development after gravitational disorientation in frog and chick embryos. Thus, MGS causes lethality, a novel phenotype not often observed in microgravity or MGS. Embryonic lethality at E2.5 and apoptosis at E3.5 are associated with SAPK function, suggesting that MGS causes a general stress response that immediately affects many aspects of development. In addition, MGS and many aspects of In vitro fertilization/assisted reproductive technologies (IVF/ART) produce nonphysiological, nonevolutionary stresses that are mediated by SAPK, suggesting the primacy of this protein kinase in a wide range of mechanisms mediating negative reproductive outcomes in IVF/ART and potentially in spaceflight.

Internal length and time scales in a simple shear granular flow.

Kinetic theory has been successfully applied to mathematically model the constitutive relations for flowing granular materials. However, the basis for kinetic theory is the assumption of binary collisions between particles. Both physical and numerical experiments of granular flows have questioned the validity of this assumption. It is known that when solid concentration or shear-rate increase, collision contact time becomes long relative to free-flight duration. Multiple collisions begin to prevail. Interactions between groups of particles may dominate the dynamics of the flow. This paper addresses both the size and lifetime of multiple collision groups in a granular flow. Computer simulations of a simple shear two-dimensional assembly of visco-elastic particles with or without friction are performed. It is found that as the shear-rate or solid concentration increase the shearing particles begin to form distinct groups within each group particles collide simultaneously. The group size grows with further increase of shear-rate or solid concentration to the extent that force chains spanning the whole shearing assembly may form. Concomitantly the collision duration also increases far beyond that of a binary collision. The evolution of the group size and the collision duration are associated with the change of the constitutive behavior of the granular materials. The existence of additional length and time scales demands new formulation for granular flows.

Stability and bifurcations of a stationary state for an impact oscillator.

The motion of a vibroimpacting one-degree-of-freedom model is analyzed. This model is motivated by the behavior of a shearing granular material, in which a transitional phenomenon is observed as the concentration of the grains decreases. This transition changes the motion of a granular assembly from an orderly shearing between two blocks sandwiching a single layer of grains to a chaotic shear flow of the whole granular mass. The model consists of a mass-spring-dashpot assembly that bounces between two rigid walls. The walls are prescribed to move harmonically in opposite phases. For low wall frequencies or small amplitudes, the motion of the mass is damped out, and it approaches a stationary state with zero velocity and displacement. In this paper, the stability of such a state and the transition into chaos are analyzed. It is shown that the state is always changed into a saddle point after a bifurcation. For some parameter combinations, horseshoe-like structures can be observed in the Poincare sections. Analyzing the stable and unstable manifolds of the saddle point, transversal homoclinic points are found to exist for some of these parameter combinations. (c) 1994 American Institute of Physics.