The effects of radiation and variable viscosity on natural convection heat transfer from a vertical stretching sheet with surface mass transfer is analysed. A three dimensional similarity solution to the governing momentum and energy equations is presented. Numerical data for the friction and Nusselt number have been tabulated.

     Transient free convection flow of an incompressible viscous fluid past a semi-infinite vertical plate has been studied by using an implicit finite-difference technique, which is always stable and convergent. The effect of the dissipation parameter ? on time to reach steady-state is studied. It is observed that time required to reach steady-state temperature increases with the increasing Prandtl number of fluid. Also there is a rise in the average skin-friction and a fall in the average Nusselt number due to greater viscous dissipative heat.

     An analysis is performed to study the flow and heat transfer characteristics of laminar combined forced and free convection, with the effects of radiation, of a viscous incompressible and optically dense fluid from a horizontal cylinder. The transformed convection equations of non-similar boundary layers obtained for both the forced convection flow and the natural convection flow dominated regimes are solved by the implicit finite difference method. Numerical results are obtained for different physical parameters, such as the Rosseland parameter, R_d and the surface temperature parameter, __ , for fluids having the Prandtl number of 0.7. Results are presented in terms of local skin-friction and the local Nusselt number against the angular displacement, _ ,as well as the buoyancy parameter, __ . It is observed that both the local shear and the Nusselt number increase due to an increase in the values of R_d and __ , in the entire pure forced to pure free convection regimes.

     The thermodynamical model, derivation of the field equations, proper boundary conditions, analytical and numerical solutions to an elasto-viscoelasto-elastic problem in a layered structure have been presented. Three different kinds of loading have been applied to the structure. A space-time analysis of displacements in each layer has been done.

     The instability of plane interface between two superposed Walters' B' elastico-viscous fluids in porous medium has been studied to include the effects of suspended (dust) particles. It is found that the system is stable or unstable if the kinematic viscoelasticity (assumed equal for both fluids) is less than or greater than the medium permeability divided by medium porosity, for the potentially stable arrangement whereas the system is unstable for the potentially unstable arrangement. This problem has also been studied for electrically conducting Walters' fluid in the presence of a variable horizontal magnetic field. For a potentially stable case, the results are the same as in a hydrodynamic case. However, for a potentially unstable case, the magnetic field has got stabilizing effect and completely stabilizes certain wave-number band if kinematic viscoelasticity is less than the medium permeability divided by medium porosity.

     This paper presents an analytical study of the fully developed forced convection in a parallel-plate channel. The central part of the channel is occupied by a porous core and the peripheral part of the channel is occupied by a clear fluid. A uniform heat flux is imposed at the walls of the channel. The Brinkman-Forchheimer-extended Darcy equation is utilized as a momentum equation in the porous region. This formulation accounts for both viscous and quadratic drag effects in the porous region. Analytical solutions for the velocity and temperature distributions, as well as for the Nusselt number are obtained.

     A concise outline is presented of the principle of self-regulated flow rate in hydrostatic lubrication and some interesting applications are described on kinematic pairs and their systems. The principle permits a continuous and automatic distribution of the total flow in partial constant flows in the recesses of hydrostatic bearings due to their special shape. Studies and results obtained and presented here refer to:
- generalities of the principle, pointed out by a bearing,
- self-regulated hydrostatic screw and nut,
- self regulated hydrostatic screw and nut with rectangular threads,
- system of self-regulated hydrostatic pads.

     The paper deals with a new method (Prosnak and Kosma, 1991) for the determination of unsteady, plane flows of viscous incompressible fluids. The characteristic feature of the method consists in elimination of pressure from the system of the Navier-Stokes equations governing the flow - in such a manner that the order of the resulting system is not increased in comparison with the original one. Furthermore, the mathematical problem posed for the resulting system is reduced in the frame of the method to an initial problem for a system of first order ordinary differential equations, wherein time represents the only independent variable.
    The nonlinearities of the Navier-Stokes equations do not cause any difficulties by virtue of such an approach.
    In this paper, the method has been applied to flows in plane, finite rectangular domains, and domains composed of rectangles. Numerical solutions to such problems are presented in the paper in graphical form, and some conclusions are drawn concerning the results as well as the method.

     The steady laminar flow of molten polymer modelled by viscoplastic fluid is considered, through a narrow space between two fixed surfaces of revolution. The problem is described by boundary layer equations. Using the method of averaged inertia one obtains the formulae expressing the pressure distribution. Generally, the flow of viscoplastic fluids given by the nonlinear model of Shulman is considered. The flows of polymers modelled by the viscoplastic fluids of Vočadlo, Herschel - Bulkley, Ostwald - de Waele and Newtonian are discussed in detail. Numerical examples of pressure distributions in the clearance between parallel disks and concentric spherical surfaces are presented.

     Couple-stresses affect lubrication problem when additives are used or lubricant contains long-chain molecules. Such couple-stresses may have a considerable effect on the behaviour of bearings in practice. The influence of couple-stress and lubricant inertia on mechanical parameters of the curvilinear thrust bearing with squeeze film is considered. The simulation of the squeeze-film behaviour of a human joint modelled as a spherical bearing is discusses as an example. It is shown that the influence of couple-stresses on the characteristics of the discussed bearing is considerable but the inertia effect is significant and not negligible. Both the effects - couple-stress and inertia - provide an enhancement in the pressure distribution and in the load-carrying capacity.

     In this work the authors consider laminar flows of generalized second grade fluids in rectilinear channels. These fluids do exhibit both viscoplastic and viscoelastic behaviour. In the model of fluid under analysis the viscoplastic behaviour is described by Shulman equation, and the viscoelastic behaviour by Rivlin-Ericksen equation for a second grade fluid. A connection between the above mentioned behaviours is modelled in the following way: the constant coefficent of shear viscosity of a fluid of second grade fluid is replaced by a shear viscosity taken from a viscoplastic fluid modelled by Shulman equation. If the normal stress coefficients are set equal to zero, this model reduces to the standard viscoplastic Shulman model. The solutions are obtained for Poiseuille and Couette flows in the case of constant values of material coefficients.