External Pressure Forces Research Articles

On the [formula omitted](2) symmetric deformation of rotating rings with shear deformation

We study the SO (2) symmetric deformation of a circular ring, modeled using the beamshell theory of Libai and Simmonds. The equations of motion are based on general partial differential equations governing the elastodynamics of geometrically exact rings, which have been formulated by Dempsey (Proc. Roy. Soc. London A 452 (1996) 1927–1943). Thus, the formulation is valid for arbitrary pressure forcing and large deformations, although the results are tempered by a linear constitutive relation and an assumption of plane strain. With the assumption that the deformation retains SO (2) symmetry, the partial differential equations are reduced to a set of coupled ordinary differential equations. Within this restricted space of solutions, we study the existence and stability of relative equilibria and discuss the effects of constant hydrostatic pressure on the dynamical response. Specifically, interaction between inertial effects arising from rotational motion and the combined elastic and external pressure forces can produce unexpected behavior, including the existence of a non-trivial state which retains the symmetry, yet physically implies that material planes do not lie in the radial direction. Such a state is shown to affect the large-amplitude response of the system in a singular limit of the governing equations.

RoHo Dry Floatation system: an alternative means of pressure relief

Pressure sores are believed to occur as a result of external pressure and disruptive shearing forces. One of the main principles of pressure sore prevention is relief or reduction of pressure. Scandinavian Mobility produces a range of systems including cushions and mattresses that can reduce pressure. These systems have been shown to be cost-effective in the clinical setting.

A CO(J=2−1) line survey of the Galactic center

We have observed the Galactic center in 12CO( J=2−1) and 13CO( J=2−1) lines using the Tokyo-Onsala-ESO-Calán 60-cm telescope and compared the data with the 12CO( J=1−0) Columbia survey (Bitran et al. 1997). The 12CO J = 2−1 J = 1−0 intensity ratio is very high (0.96) in the central 900 pc of the Galaxy, although it is about 0.6-0.7 for typical molecular clouds in the Galactic disk. The observed intensity ratios, 12CO J = 2−1 J = 1−0 and 13CO/ 12CO ( J = 2−1), indicate that even the 12CO line is not very optically thick: τ12 CO( J=1−0) ∼ 1 or smaller in the Galactic center. This may be due to high temperature and large velocity dispersion. The large velocity dispersion is probably caused by large external pressure and tidal forces.

Effects of small‐scale wind on coastal upwelling with application to Point Conception

A three‐dimensional, limited‐area, ocean general circulation model is used to study the response of the coastal ocean to small‐scale wind forcing. The model considers an idealized forcing of a semi‐infinite, upwelling‐favorable wind with a large positive wind curl at one end. Such a wind pattern is commonly found in the vicinity of capes and points where the synoptic‐scale wind is disturbed by coastal mountain ranges. The model study includes both process and hindcast studies. The process study considers the upwelling spinup and relaxation, driven by a positive wind curl. It also examines the model sensitivity to wind patterns and an external pressure force. The model results show that a strong poleward alongshore pressure gradient is set up by a positive wind curl. This pressure force drives an inshore poleward current into the upwelling region during active wind forcing, and it causes a surge of warm water into the upwelling zone during wind relaxation. The hindcast study is applied to the 1983 Organization of Persistent Upwelling Structures (OPUS) observations near Point Conception off central California. The simulation uses an idealized geometry but with observed winds. During OPUS, a sudden reversal of the surface equatorward currents occurred in the midst of a steady upwelling‐favorable wind. This unexpected coastal flow feature was faithfully reproduced in the model simulation.

Dynamics of a two-dimensional ribbon of shallow water on an f-plane

An analytical solution is derived for the motion of an infinitely long fluid ribbon of shallow water, with parabolic depth and linear velocity distribution. The solution is of intrinsic dynamical interest and also offers a “bench-mark” for testing numerical methods which handle the intersection of a free surface with a horizontal boundary. The development is expressed in terms of the translation and deformation of the hump. The former component is determined uniquely by the initial velocity at the symmetry axis and the externally imposed uniform pressure gradient. On the other hand, the deformation component depends on the initial shear and divergence, but is unaffected by a uniform external pressure force. Relative to its axis, the fluid ribbon alternately expands and contracts, with a pulsation frequency (≥ the inertial frequency f) that is a function of a Rossby, and a rotational Froude number, defined in terms of the initial velocity, and height field. DOI: 10.1034/j.1600-0870.1993.00004.x

Failure of spheroplastics under axial tension and hydrostatic compression

The problem of determination of the external pressure and axial tensile force, which fail spheroplastics — a structurally inhomogeneous material consisting of hollow glass microspheres of different diameters, the latter being distributed in a random manner in a continuous epoxy matrix — is examined.

A study of helically reinforced cylinders under axially symmetric loads and application to strand mathematical modelling

Locally transversely isotropic elastic material can be used in a cylinder configuration in which the material principal direction is helically wound. Stress-strain equations are obtained for such a cylinder under axially symmetric loading: axial force, twisting moment and uniform radial internal and external pressure forces. The equations are generalized to a system of n coaxial tubes with an isotropic core. Finally, it is shown how this can be used to represent the global mechanical behaviour of strand-like systems such as overhead electrical conductors and cables.

Stability of ribbed conical shells under the action of external pressure or axial compressive forces

In the review [2] it was noted that the effect of a discrete arrangement of stiffening ribs on the parameters of the critical stresses up to the present time has been sufficiently studied only for cylindrical shells. The indicated question has been investigated less for conical shells. From [2], it follows that in the modern literature a limited number of publications devoted to investigations of this kind are known. They refer mainly to shells with annular reinforcement. In separate studies, the stability of conical shells with crossed ribs has been studied. However, the discrete arrangement of the reinforcing elements was taken into account there only for annular ribs, and for meridional ribs the theory of constructive-orthotropic shells is used.

Unsteady flow in a collapsible tube subjected to external pressure or body forces

Flows in thin-walled, collapsible tubes are of fundamental importance to various physiologic phenomena and clinical devices.A one-dimensional, unsteady theory is developed for flows generated either by externally applied pressures or by body forces. Part 1 deals with small-amplitude, linearized flows, part 2 with large amplitude, nonlinear flows. Experimental results for a tube collapsing under external pressure are given in part 3, together with theoretical interpretations and comparative results of numerical simulations.Several new and unanticipated phenomena are revealed. These are in part associated with the highly nonlinear ‘equation of state’ (transmural pressure versus area) for a partially collapsed tube, and in part with whether the flow speed is sub- or supercritical relative to the speed of area waves. For instance, a flow produced by a spatially uniform external pressure applied to a limited region becomes choked at a flow-limiting throat at which point the fluid speed reaches the local wave speed. This throat forms at the edge of the pressurized region. The critical velocity can be exceeded with the application of certain types of spatially graded external pressures.

Relation between a rheological and a microscopical model of sintering

The results obtained point to a relation between the possibility of employing the rheological model (1) for describing the viscous flow of a material and the validity of the Mackenzie equation. Two limiting cases may arise. In one of them, where pore healing in a material occurs through a mechanism of viscous change in shape of the nonporous part of the material, the Mackenzie relation (2) or its generalized variant (15) is applicable. The rheological model for an isotropic solid is described by Eq. (1). When the Nabarro-Herring diffusional creep mechanism is operative, the coefficients of viscosity are calculated with Eqs. (19) and (20). In the other limiting case the nonporous part of a material possesses no viscosity (n = ∞), which rules out the possibility of employing the rheological model (1) and the Mackenzie equation. To describe the kinetics of shrinkage under the action of external pressure and surface tension forces, recourse must be had to Eq. (14). The coefficients of volume viscosity for typical porous structures (one pore and an assembly of pores in a polycrystal grain and pores at grain boundaries) are given by Eqs. (22), (27), and (31).

Finite cylindrical deformations of a reinforced elastic tube

We consider the deformation of an elastic and incompressible cylindrical tube of circular cross-section into a cylinder of a given shape under the action of end forces, uniform external pressure and normal forces applied on the inside surface. The outside surface of the undeformed tube is reinforced by a two parameter family of inextensible helical cords. The only restriction imposed on the cross-section of the deformed inside surface is that it has a continuous curvature. The equations are given for the general strain-energy function. Subsequently, it is assumed that the thickness of the tube is small compared to its overall diameter and the solutions to the equations are obtained, to the second order, for a neo-Hookean solid. Since the deformations considered are plane, the analysis is also valid for a Mooney-Rivlin material. Applications to specific boundary value problems are also given.

Stability of a cylindrical shell loaded by external pressure and axial tensile forces

stability of cylindrical shell loaded by external pressure. Simpie formulas in closed form are obtained in the usual way for the linearized problem. It is shown that the influence of the axial tensile forces can be described by parameters whose graphs have been constructed. An extensive series of experiments were carried out on shells to study the joint effect of axial tensile forces and external pressure. The experimental results agree quite well with calculations based on the solution in [2]. The results show that axial tensile forces can increase the load-carrying capacity of a shell by a factor of 2 to 2.5. The question of the stability of a cylindrical shell loaded by outward pressure and axial tensile forces leads, as is well known [1], to the integration of the differential equation D 2 2 ~ - . EO~w --h-- v v v v~w + ~-~ T qR ~ ~ 2 0~w ~ ~ 0~' -V-~-) v v -b-~.~ + ~

Effects of boundary conditions on the stability of cylinders subject to lateral and axial pressures

This paper presents an analysis of critical combinations of uniform external lateral pressure and central axial compressive force for circular cylindrical shells with arbitrarily prescribed boundary conditions. Numerical results are presented for the special case of hydrostatic pressure loading for eight different sets of boundary conditions. The numerical results show that prevention of edge rotation of a generator has little effect on the critical pressure unless the cylinder is very short. On the other hand, it is shown that the presence of axial restraint at the boundaries significantly increases critical pressures for intermediate-length cylinders.

Impact Theory Calculations of the Lift and Drag of Ducted Bodies

SummaryNewtonian impact theory has been used to estimate the external pressure forces on ducted bodies of rectangular, circular and semi-circular cross section. With an allowance for skin friction (or other incidence-independent drag) it is shown that maximum lift/drag ratios of about three to four are possible with little effect of body geometry provided that short and very divergent, or tall and narrow rectangular ducts are avoided.Large changes occur in the separate lift and drag at maximum lift/drag ratio and these have to be made compatible with the weight and thrust capacity of the configurations. The maximum lift/drag ratio of a duct can be improved by the addition of wings, especially for a duct with poor lift/drag ratio, but the best overall performance is with a duct of good lift/drag ratio and low drag.It is anticipated that impact theory underestimates the pressure forces, and hence the lift/drag ratios, which would obtain at finite Mach numbers but the trends with geometrical changes should be reasonably reliable.