Oscillatory Flow Induced Hydrodynamic Forces upon a Pipeline near Erosive Sandy Seabed

An experimental study of the properties of hydrodynamic forces upon a marine pipeline is presented in this paper, in the equilibrium scour conditions for various Keulegan-Carpenter numbers and various initial relative gaps between pipeline and the erosive sandy seabed. The tests are conducted in a U-shaped oscillatory water tunnel with a sand box located at the bottom of the test section. According to the experimental results, the maximum horizontal forces on the pipelines with an initial gap to seabed will decrease to some extent due to scouring process. For engineering appliances, it seems safer to estimate wave induced forces on pipelines under the assumption that seabed is plane. However, it should be noticed that great changes would be brought to the frequency properties of lift forces because of the sandy scour beneath the pipeline, which occurs for certain KC numbers.

Molecular dynamics simulation of adhesion forces in a dipalmitoylphosphatidylcholine membrane

Adhesion forces of Dipalmitoylphosphatidylcholine ( DPPC) membrane in the gel phase are investigated by molecular dynamics ( MD) simulation. In the simulations, individual DPPC molecules are pulled out of DPPC membranes with different rates and we get the maximum adhesion forces of DPPC membrane. We find that the maximum adhesion forces increase with pull rate, from about 400 to 700 pN when pull rates are from 0.001 to 0.03 nm/ps. We analyze the relationship between pull rate and adhesion forces of different origins using Brownian dynamics and notice that viscosity of solvent plays an important role in adhesion forces. Then we simulate the motion of a single DPPC molecule in solvent and it elucidates that the maximum drag force is almost linear with respect to the pull rate. We use Stokes' relation to describe the motion of a single DPPC molecule and deduce the effective length of a DPPC molecule. Conformational analyses indicate that the free energy variation of a DPPC molecule inside and outside of the DPPC membrane is an essential part of adhesion energy.