Unsteady hydromagnetic thermal boundary layer flow

With viscous dissipation and Joule heating taken into account, solution of the energy equation is obtained for unsteady hydromagnetic thermal boundary layer flow past a porous wall (e.g., surface of a star) in presence of a transverse magnetic field, under the condition of zero heat transfer between the fluid and the boundary — the so-called plate thermometer problem in MHD. Solution of the problem, in the form of power series, is obtained under certain valid simplifying assumptions, when (i) the wall is subjected to a normal velocity of suction/injection which is proportional tot ^–1/2, and (ii) the wall has a velocity given by t ⁿ . The variation of temperature is shown graphically and is followed by a quantitative discussion therein also signifying the importance of retaining the Joule heating term in the energy equation.Part I is the article inAstrophysics and Space Science, Vol.45, No. 2, 1976, pp. 397–410.On study-leave from Defence Science Laboratory, Delhi, India.     

Hydromagnetic flow near an oscillating wall

The flow of a viscous incompressible and electrically conducting fluid produced by harmonically oscillating wall of infinite extent in presence of a transverse magnetic field is considered. Exact solutions for velocity, induced magnetic field, electrical current density and skin-friction are obtained when the magnetic Prandtl number is unity. It is shown that the velocity has a phase lag with respect to the oscillations of the wall. This phase lag is found to be significantly affected by the applied magnetic field.On study-leave from Defence Science Laboratory, Delhi, India.     

A grid search for three-dimensional motions and three new types of such motions

A grid search method aimed at locating ‘all’ doubly symmetric orbits of the three-dimensional restricted problems of one, two, etc. revolutions is developed and applied numerically on the CDC-3300 computer. Three new types of orbits have thus been located and a second order ‘predictorcorrector’ method is applied in order to determine a certain number of members of the families of which the ‘located’ orbits are members. The stability of these members is also discussed.     

Application of higher order variations of motion

The theoretical considerations developed in a previous paper (Kazantzis, 1973) are applied with the aid of the CDC-3300 computer and examples of new families of doubly symmetric solutions of the restricted three-dimensional problem obtained thereof are presented. Rough estimates of initial conditions of these families have been obtained by applying an extension of the scanning technique developed by Hénon (1966).     

Restricted problem stability and the solar system

Reference periodic orbits are determined accurately for the planets of the solar system, using a restricted problem model with the Sun and Jupiter as the two primaries. The prediction is verified that stability of the planetary orbits should imply stability of those reference orbits that are simple-periodic.     

Effects of mass transfer on the free convective flow in the Stokes' problem for an infinite vertical limiting surface

An exact analysis of the mass transfer effects on the free convection flow of an incompressible viscous fluid past an impulsively started infinite vertical (wall) limiting surface (Stokes's or Rayleigh's problem) has been carried out. Expressions for the velocity, temperature, species concentration and skin friction are obtained by using the Laplace transform technique. The velocity field and the skin friction are shown graphically for air (P=0.71) and mercury (P=0.025). The effects ofG (Grashof number),G _c (the modified Grashof number) andS _c (Schmidt number) are considered qualitatively during the course of discussion.     

The third integral and the Hamiltonian

It is reiterated that any suggestion of the existence of a third integral is at variance with Poincaré's theorem on the non-existence of such integrals. Even in a purely numerical approach no form of a new integral can be constructed that is valid in every domain of the phase space; and it is devoid of meaning to use as a third integral different forms of functions in various cases.     

Holocene Treeline History and Climate Change Across Northern Eurasia

Radiocarbon-dated macrofossils are used to document Holocene treeline history across northern Russia (including Siberia). Boreal forest development in this region commenced by 10,000 yr B.P. Over most of Russia, forest advanced to or near the current arctic coastline between 9000 and 7000 yr B.P. and retreated to its present position by between 4000 and 3000 yr B.P. Forest establishment and retreat was roughly synchronous across most of northern Russia. Treeline advance on the Kola Peninsula, however, appears to have occurred later than in other regions. During the period of maximum forest extension, the mean July temperatures along the northern coastline of Russia may have been 2.5° to 7.0°C warmer than modern. The development of forest and expansion of treeline likely reflects a number of complimentary environmental conditions, including heightened summer insolation, the demise of Eurasian ice sheets, reduced sea-ice cover, greater continentality with eustatically lower sea level, and extreme Arctic penetration of warm North Atlantic waters. The late Holocene retreat of Eurasian treeline coincides with declining summer insolation, cooling arctic waters, and neoglaciation.     

Directional‐oriented wavefield imaging: a new wave‐based subsurface illumination imaging condition for reverse time migration

The key objective of an imaging algorithm is to produce accurate and high‐resolution images of the subsurface geology. However, significant wavefield distortions occur due to wave propagation through complex structures and irregular acquisition geometries causing uneven wavefield illumination at the target. Therefore, conventional imaging conditions are unable to correctly compensate for variable illumination effects. We propose a generalised wave‐based imaging condition, which incorporates a weighting function based on energy illumination at each subsurface reflection and azimuth angles. Our proposed imaging kernel, named as the directional‐oriented wavefield imaging, compensates for illumination effects produced by possible surface obstructions during acquisition, sparse geometries employed in the field, and complex velocity models. An integral part of the directional‐oriented wavefield imaging condition is a methodology for applying down‐going/up‐going wavefield decomposition to both source and receiver extrapolated wavefields. This type of wavefield decomposition eliminates low‐frequency artefacts and scattering noise caused by the two‐way wave equation and can facilitate the robust estimation for energy fluxes of wavefields required for the seismic illumination analysis. Then, based on the estimation of the respective wavefield propagation vectors and associated directions, we evaluate the illumination energy for each subsurface location as a function of image depth point and subsurface azimuth and reflection angles. Thus, the final directional‐oriented wavefield imaging kernel is a cross‐correlation of the decomposed source and receiver wavefields weighted by the illuminated energy estimated at each depth location. The application of the directional‐oriented wavefield imaging condition can be employed during the generation of both depth‐stacked images and azimuth–reflection angle‐domain common image gathers. Numerical examples using synthetic and real data demonstrate that the new imaging condition can properly image complex wave paths and produce high‐fidelity depth sections.     

Logging effects on sediment flux observed in a pollen‐based record of overbank deposition in a northern California catchment

A palynological approach was used to estimate overbank deposition rates in a forested catchment affected by logging. The palynological approach uses downcore variations in total fossil pollen and fossil pollen assemblage to calculate rates of overbank deposition and has a distinct advantage over radioisotopic approaches in that it is not limited by radioactive decay. Using this approach, we determined that overbank deposition rates increased over 400 per cent within years of logging events and that the increased rates persisted for less than 4 years. After logging‐induced deposition peaked, overbank deposition decreased over 60 per cent relative to the pre‐logging background values. The decreased deposition rates persisted for over 40 years. The immediate effect of logging in this catchment was to induce mass‐wasting events in hollows that produced rapidly travelling sediment pulses. In the subsequent recovery period, reduced sediment loading occurred as a result of a reduction in the volume of sediment available for transport. The reduction in sediment load led to a reduction in overbank deposition rates until subsequent logging disturbances destabilized and emptied other hollows. Copyright © 2005 John Wiley & Sons, Ltd.