Incipient sediment motion in vegetated open-channel flows predicted by critical flow velocity

Incipient sediment motion plays a key role in scouring and bed load transport. However, the incipient sediment motion in the vegetated open-channel flows has yet to be fully understood. This study aims to quantify the critical conditions of the sediment particle movement in the presence of emergent and submerged vegetation. A new formula of the critical flow velocity was proposed to predict the incipient sediment motion based on the force balance equation of a sediment particle and the assumption that the velocity distribution in the bed roughness boundary layer fits the logarithmic law. Analysis of the derived formula revealed that the critical flow velocity for incipient sediment motion decreases with the increase in vegetation density. The proposed formula agrees well with the experimental data in the literature, thereby implying that the critical flow velocity can effectively quantify the incipient sediment motion in the vegetated open channel flows.

VIBRATION AND STABILITY OF VERTICAL UPWARD-FLUID-CONVEYING PIPE IMMERSED IN RIGID CYLINDRICAL CHANNEL

A theoretical model is developed for the vibration and stability of a vertical pipe subjected concurrently to two dependent axial flows. The external fluid, after exiting the outer annular region between the pipe and a rigid cylindrical channel, is conveyed upwards inside the pipe. This configuration thus resembles of a pipe that aspirating fluid. The equation of planar mo- tion is solved by means of the differential quadrature method （DQM）. Calculations are conducted for a slender drill-string-like and a bench-top-size system, for different confinement conditions of the outer annular channel. It is shown that the vibrations of these two systems are closely related to the degree of confinement of the outer annular channel. For a drill-string-like system with narrow annuli, buckling instability may occur in the second and third modes. For a bench-top-size system, however, both buckling and flutter may occur in the lowest three modes. The form of instability depends on the annuli size.

New insight into the stability and dynamics of fluid-conveying supported pipes with small geometric imperfections

In several previous studies,it was reported that a supported pipe with small geometric imperfections would lose stability when the internal flow velocity became sufficiently high.Recently,however,it has become clear that this conclusion may be at best incomplete.A reevaluation of the problem is undertaken here by essentially considering the flow-induced static deformation of a pipe.With the aid of the absolute nodal coordinate formulation(ANCF)and the extended Lagrange equations for dynamical systems containing non-material volumes,the nonlinear governing equations of a pipe with three different geometric imperfections are introduced and formulated.Based on extensive numerical calculations,the static equilibrium configuration,the stability,and the nonlinear dynamics of the considered pipe system are determined and analyzed.The results show that for a supported pipe with the geometric imperfection of a half sinusoidal wave,the dynamical system could not lose stability even if the flow velocity reaches an extremely high value of 40.However,for a supported pipe with the geometric imperfection of one or one and a half sinusoidal waves,the first-mode buckling instability would take place at high flow velocity.Moreover,based on a further parametric analysis,the effects of the amplitude of the geometric imperfection and the aspect ratio of the pipe on the static deformation,the critical flow velocity for buckling instability,and the nonlinear responses of the supported pipes with geometric imperfections are analyzed.

Dynamical Stability of Cantilevered Pipe Conveying Fluid with Inerter-Based Dynamic Vibration Absorber

Cantilevered pipe conveying fluid may become unstable and flutter instability would occur when the velocity of the fluid flow in the pipe exceeds a critical value.In the present study,the theoretical model of a cantilevered fluid-conveying pipe attached by an inerter-based dynamic vibration absorber(IDVA)is proposed and the stability of this dynamical system is explored.Based on linear governing equations of the pipe and the IDVA,the effects of damping coefficient,weight,inerter,location and spring stiffness of the IDVAon the critical flow velocities of the pipe system is examined.It is shown that the stability of the pipe may be significantly affected by the IDVA.In many cases,the stability of the cantilevered pipe can be enhanced by designing the parameter values of the IDVA.By solving nonlinear governing equations of the dynamical system,the nonlinear oscillations of the pipe with IDVA for sufficiently high flow velocity beyond the critical value are determined,showing that the oscillation amplitudes of the pipe can also be suppressed to some extent with a suitable design of the IDVA.

Simulation of the effect of hydrate adhesion properties on flow safety in solid fluidization exploitation

During the solid fluidization exploitation of marine natural gas hydrates,the hydrate particles and cuttings produced via excavation and crushing are transported by the drilling mud.The potential flow safety issues arising during the transport process,such as the blockage of pipelines and equipment,have attracted considerable attention.This study aims to investigate the impact of hydrate adhesion features,including agglomeration,cohesion,and deposition,on the flow transport processes in solid fluidization exploitation and to provide a reference for the design and application of multiphase hydrate slurry transport in solid fluidization exploitation.We established a numerical simulation model that considers the hydrate adhesion properties using the coupled computational fluid dynamics and discrete element method(CFD-DEM)for the multiphase mixed transport in solid fluidization exploitation.An appropriate model to simulate the adhesion force of the hydrate particles and the corresponding parameter values were obtained.The conclusions obtained are as follows.Under the same operating conditions,a stationary bed is more likely to form in the transport process due to the hydrate adhesion forces;adhesion forces can increase the critical deposition velocity of the mixture of hydrate particles and cuttings.Hydrate adhesion lowers the height of the solid-phase moving bed,while the agglomeration and cohesion of particles can intensify the aggregation and deposition of hydrate debris and cuttings at the bottom of the pipe.These particles tend to form a deposit bed rather than a moving bed,which reduces the effective flow area of the pipeline and increases the risk of blockage.

FREE VIBRATION ANALYSIS OF FLUID-CONVEYING CARBON NANOTUBE VIA WAVE METHOD

The wave method is introduced to vibration analysis of the fluid-conveying carbon nanotube. The constitutive relation of carbon nanotube on micro-scale is founded using the non- local elastic theory. The governing equation on micro-scale is obtained. And the first five orders of the natural frequency of the carbon nanotube conveying fluid with various speeds are calculated through the wave method. Besides, the critical flow velocity when the carbon nanotube loses stability is obtained. Meanwhile, a contrast is made between the result obtained through tile wave method and that in previous researches.