Impact of mega-scale sand extraction on tidal dynamics in semi-enclosed basins: An idealized model study with application to the Southern North Sea

Long-term considerations of repeated and increasing sand extraction on the Netherlands Continental Shelf (North Sea) may lead to the creation of a mega-scale extraction trench in front of the Dutch coast (length hundreds of km, width over 10 km, depth several m). We investigate the impact of such a huge topographic intervention on tidal dynamics, which is a key aspect in hydrodynamics, and indirectly also affecting morphodynamics and ecology.     

Prediction of hydrodynamic loading on a mini TLP with free surface effects

This paper describes the extension of a fluid-flow simulations method to capture the free surface evolution around a full-scale Tension Leg Platform (TLP). The focus is on the prediction of the resulting hydrodynamic loading on the various elements of the TLP in turbulent flow conditions and, in particular, on quantifying the effects of the free surface distortion on this loading. The basic method uses finite-volume techniques to discretize the differential equations governing conservation of mass and momentum in three dimensions. The time-averaged forms of the equations are used, and the effects of turbulence are accounted for by using a two-equation, eddy-viscosity closure. The method is extended here via the incorporation of surface-tracking algorithm on a moving grid to predict the free-surface shape. The algorithm was checked against experimental measurements from two benchmark flows: the flow over a submerged semi-circular cylinder and the flow around a floating parabolic hull. Predictions of forces on a model TLP were then obtained both with and without allowing for the deformation of the free surface. The results suggest that the free surface effects on the hydrodynamic loads are small for the values of Froude number typically encountered in offshore engineering practice.     

Stability of a Riemann S Ellipsoid with a Spheroidal Halo

The stability of Riemann S ellipsoids inside an oblate halo with respect to the second form of oscillations is investigated. It is shown that some ellipsoids with reverse internal circulation of matter, which are stable inside a spherical halo or in its absence, become unstable with respect to second odd forms of oscillation inside an oblate halo. Here there is asymmetry between conjugate ellipsoids from the standpoint of their stability. Only those conjugate ellipsoids that correspond to higher frequencies of reverse circulation of matter than their corresponding angular rotation rates are unstable. The domains of instability of light and heavy conjugate embedded ellipsoids are obtained as a function of the oblateness measure and relative density of the halo.     

Morphology, photometry and kinematics of N-body bars – I. Three models with different halo central concentrations

We discuss the morphology, photometry and kinematics of the bars which have formed in three N -body simulations. These have initially the same disc and the same halo-to-disc mass ratio, but their haloes have very different central concentrations. The third model includes a bulge. The bar in the model with the centrally concentrated halo (model MH) is much stronger, longer and thinner than the bar in the model with the less centrally concentrated halo (model MD). Its shape, when viewed side-on, evolves from boxy to peanut and then to 'X'-shaped, as opposed to that of model MD, which stays boxy. The projected density profiles obtained from cuts along the bar major axis, for both the face-on and the edge-on views, show a flat part, as opposed to those of model MD which are falling rapidly. A Fourier analysis of the face-on density distribution of model MH shows very large  m=2  , 4, 6 and 8 components. Contrary to this, for model MD the components  m=6  and 8 are negligible. The velocity field of model MH shows strong deviations from axial symmetry, and in particular has wavy isovelocities near the end of the bar when viewed along the bar minor axis. When viewed edge-on, it shows cylindrical rotation, which the MD model does not. The properties of the bar of the model with a bulge and a non-centrally concentrated halo (MDB) are intermediate between those of the bars of the other two models. All three models exhibit a lot of inflow of the disc material during their evolution, so that by the end of the simulations the disc dominates over the halo in the inner parts, even for model MH, for which the halo and disc contributions were initially comparable in that region.     

High-velocity stars from decay of small stellar systems

In this paper we present numerical results on the decay of small stellar systems under different initial conditions (multiplicity 3 ≤  N  ≤ 10, and various mass spectra, initial velocities and initial configurations). The numerical treatment uses the CHAIN1 code (Mikkola &38; Aarseth). Particular attention is paid to the distribution of high-velocity escapers: we define these as stars with velocity above 30 km s⁻¹. These numerical experiments confirm that small N -body systems are dynamically unstable and produce cascades of escapers in the process of their decay. It is shown that the fraction of stars that escape from small dense stellar systems with an escape velocity greater than 30 km s⁻¹ is ∼1 per cent for all systems treated here. This relatively small fraction must be considered in relation to the rate of star formation in the Galaxy in small groups: this could explain some moderately high-velocity stars observed in the Galactic disc and possibly some young stars with relatively high metallicity in the thick disc.     

Dynamical stability and evolution of the discs of Sc galaxies

We examine the local stability of galactic discs against axisymmetric density perturbations with special attention to the different dynamics of the stellar and gaseous components. In particular, the discs of the Milky Way and of NGC 6946 are studied. The Milky Way is shown to be stable, whereas the inner parts of NGC 6946, a typical Sc galaxy from the Kennicutt sample, are dynamically unstable. The ensuing dynamical evolution of the composite disc is studied by numerical simulations. The evolution is so fierce that the stellar disc heats up dynamically on a short time-scale to such a high degree, which seems to contradict the morphological appearance of the galaxy. The star formation rate required to cool the disc dynamically is estimated. Even if the star formation rate in NGC 6946 is at present high enough to meet this requirement, it is argued that the discs of Sc galaxies cannot sustain such a high star formation rate for extended periods.