| Summary: | 碩士 === 國立臺灣大學 === 土木工程學研究所 === 102 === Lift-up problem is a process during which an object immersed in fluid is initially extricated from a bed (Fig. 2-1). In ocean and offshore engineering, it has many applications such as removing hydraulic structures, salvaging sunken ships, and operating submarines. Based on field experience (Liu (1969)), the lifting force is considerably large and the operation time is extremely long. It is a process that slowly increases the gap between the object and the bed until a turning point when the object
is abruptly unfastened. This is called breakout phenomenon. Because of it and the lifting force, it is helpful to ocean and offshore engineering if the mechanics of the problem can be understood.
This study applies Stokes flow to the gap, investigating a lift-up problem with a rigid impermeable bed. Also, this study applies Stokes flow to the gap and Song and Huang’s (2000) laminar poroelasticity theory to the porous medium, analyzing problems with a rigid porous bed and a hard poroelastic bed, respectively. Stokes flow can react to the horizontal and vertical velocities, Song and Huang’s (2000) laminar poroelasticity theory can respond to the viscous effect of pore flow, and the complete boundary conditions can react to the continuity of velocity and stress.
The first stage of this study provides the exact solution to the two-dimensional lift-up problem with a rigid impermeable bed. The exact solution reveals the tiny error
of the pressure in adhesion approximation (Acheson (1990)), and verifies that the tiny error does not influence the kinematics of the flow and the dynamic force acting on
the object.
The second stage of this study proposes a more general solution to the two-dimensional lift-up problem with a rigid porous bed. The solution has been verified by Mei’s (1985) experiments, and reveals many mechanics of the problem.
The dynamic force acting on the object and the breakout phenomenon are displayed, and other mechanics are demonstrated. In fact, they are the mechanics that cannot be
presented by Mei’s (1985) approach, and suggest that adhesion approximation (Acheson (1990)), Darcy’s law, and Beavers and Joseph’s (1967) partial-slip flow might not be suitable to problems with porous beds.
This study finally offers the solution to the two-dimensional lift-up problem with a hard poroelastic bed. The fluid and solid parts in the porous medium can be decoupled. The mechanics of the fluid is exactly the same as those in the problem with a rigid porous bed. Deformation lines, deformations and effective stresses of the solid in a hard poroelastic bed are revealed. The porous medium can be influenced to the depth of L , half the length of the object, and the solid effective stresses are only
influenced by the pressure distribution in the porous medium.
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