Splash wave and crown breakup after disc impact on a liquid surface

In this paper we analyze the impact of a circular disc on a free surface using experiments, potential flow numerical simulations and theory. We focus our attention both on the study of the generation and possible breakup of the splash wave created after the impact and on the calculation of the force on the disc. We have experimentally found that drops are only ejected from the rim located at the top part of the splash --giving rise to what is known as the crown splash-- if the impact Weber number exceeds a threshold value \Weber_{crit}\simeq 140. We explain this threshold by defining a local Bond number $Bo_{tip}$ based on the rim deceleration and its radius of curvature, with which we show using both numerical simulations and experiments that a crown splash only occurs when $Bo_{tip}\gtrsim 1$, revealing that the rim disrupts due to a Rayleigh-Taylor instability. Neglecting the effect of air, we show that the flow in the region close to the disc edge possesses a Weber-number-dependent self-similar structure for every Weber number. From this we demonstrate that \Bond_{tip}\propto\Weber, explaining both why the transition to crown splash can be characterized in terms of the impact Weber number and why this transition occurs for $We_{crit}\simeq 140$. Next, including the effect of air, we have developed a theory which predicts the time-varying thickness of the very thin air cushion that is entrapped between the impacting solid and the liquid. Our analysis reveals that gas critically affect the velocity of propagation of the splash wave as well as the time-varying force on the disc, $F_D$. The existence of the air layer also limits the range of times in which the self-similar solution is valid and, accordingly, the maximum deceleration experienced by the liquid rim, what sets the length scale of the splash drops ejected when We>\Weber_{crit}

Micromorphological and preliminary X-ray observations on a basal till from Lunteren, The Netherlands

Undisturbed box samples have been taken from a Saalian till complex and associated shear zone in the Central Netherlands. Microscopic studies of vertical thin sections of so-called mammoth-size (7 x 14 cm) and microdensitometer readings taken from X-ray images reveal amongst others (1) a closely spaced textural banding, (2) the presence of two types of unconsolidated pebbles, i.e. mud pebbles and till pebbles, (3) different types of plasmic fabric, i.e. skel-lattisepic fabric for most of the till matrix and till pebbles and a strong unistrial fabnc associated with shear zones. The present observations are in accordance with earlier macroscopic field and laboratory studies, indicating till formation under conditions of continuous subglacial shearing, by which much local material is reworked as well

Micromorphological and preliminary X-ray observations on a basal till from Lunteren, The Netherlands

Undisturbed box samples have been taken from a Saalian till complex and associated shear zone in the Central Netherlands. Microscopic studies of vertical thin sections of so-called mammoth-size (7 x 14 cm) and microdensitometer readings taken from X-ray images reveal amongst others (1) a closely spaced textural banding, (2) the presence of two types of unconsolidated pebbles, i.e. mud pebbles and till pebbles, (3) different types of plasmic fabric, i.e. skel-lattisepic fabric for most of the till matrix and till pebbles and a strong unistrial fabnc associated with shear zones. The present observations are in accordance with earlier macroscopic field and laboratory studies, indicating till formation under conditions of continuous subglacial shearing, by which much local material is reworked as well

Hysteretic clustering in granular gas

Granular material is vibro-fluidized in N=2 and N=3 connected compartments, respectively. For sufficiently strong shaking the granular gas is equi-partitioned, but if the shaking intensity is lowered, the gas clusters in one compartment. The phase transition towards the clustered state is of 2nd order for N=2 and of 1st order for N=3. In particular, the latter is hysteretic. The experimental findings are accounted for within a dynamical model that exactly has the above properties

Estimation of Density-Dependent Mortality of Juvenile Bivalves in the Wadden Sea

We investigated density-dependent mortality within the early months of life of the bivalves Macoma balthica (Baltic tellin) and Cerastoderma edule (common cockle) in the Wadden Sea. Mortality is thought to be density-dependent in juvenile bivalves, because there is no proportional relationship between the size of the reproductive adult stocks and the numbers of recruits for both species. It is not known however, when exactly density dependence in the pre-recruitment phase occurs and how prevalent it is. The magnitude of recruitment determines year class strength in bivalves. Thus, understanding pre-recruit mortality will improve the understanding of population dynamics. We analyzed count data from three years of temporal sampling during the first months after bivalve settlement at ten transects in the Sylt-Rømø-Bay in the northern German Wadden Sea. Analyses of density dependence are sensitive to bias through measurement error. Measurement error was estimated by bootstrapping, and residual deviances were adjusted by adding process error. With simulations the effect of these two types of error on the estimate of the density-dependent mortality coefficient was investigated. In three out of eight time intervals density dependence was detected for M. balthica, and in zero out of six time intervals for C. edule. Biological or environmental stochastic processes dominated over density dependence at the investigated scale

First Report of a Preserved Weichselian Periglacial Surface in NW Europe—the "P. van der Lijn": Geological Reserve in The Netherlands

Durch Frost gespaltene Steine und Blöcke und durch Windschliff polierte und auf der Oberfläche sortierte Steine in einem vegetationsfreien geologischen Reservat im Zentrum der Niederlande werden beschrieben. Sie werden als Anzeichen für eine weichselzeitliche periglaziale Oberfläche gedeutet. Bisher hat man sie als eine holozäne Abrasionsfläche aufgefaßt.researc