Suction Effects on Sediment Transport in Closed-Conduit Flow

Source: ICHE Conference Archive - https://mdi-de.baw.de/icheArchiv

Harmonic structure of the nonlinear force on a fixed ship-shaped floating production, storage and offloading vessel under dispersive phase-focused wave groups

This paper presents a numerical investigation on the harmonic structure of hydrodynamic forces on a fixed and simplified representative floating production, storage and offloading (FPSO) vessel hull under dispersive phase-focused wave groups. The high-fidelity numerical model utilizes the two-phase flow solver in the open-source toolbox OpenFOAM. A series of cases were computed using the numerical model, where the effects of wave steepness, bow diameter, and length of the FPSO are investigated. It is found that given an FPSO under different wave steepness, the non-dimensional inline force exhibits remarkable similarity in terms of the temporal development. The harmonic structure of the inline force is only weakly dependent on the steepness of the incident wave group and the bow diameter, but strongly dependent on the FPSO length. When [Formula: see text], where L is the length of the FPSO and kp is the wave number at peak frequency, the incident wave group is diffracted significantly by the FPSO. The entire wave–structure interaction process is largely linear, where transfer between different harmonics is rarely seen. However, when kpL is further reduced to 0.57, globally the disturbance of the FPSO on the far field incident wave group is reduced, but locally a strongly nonlinear flow occurs at the rear of the FPSO, where severe run-up occurs at the downstream stagnation point. Higher-order harmonics of inline forces are excited, and the interaction process becomes much more nonlinear

Towards modelling wave-induced forces on an armour layer unit of rubble mound coastal revetments

Wave-induced forces on an armour layer unit are key parameters for assessing the stability of rubble mound coastal revetment, but how to predict them accurately and efficiently remains an open question. This study explores the feasibility of using the Morison-type equation to convert numerically simulated porous media flow in an armour layer into the forces on a single armour unit. Wave flume tests are conducted, in which the forces on a cuboid placed in the armour layer of a sloped revetment were measured. In conjunction, numerical simulations were performed using an OpenFOAM solver, which treats the revetment as a porous media. The validated flow simulation was synchronized with the force measurement to illustrate the correlations between the predicted porous media flow and the impact force. Based on these correlations, a Morison-type predictor, which consists of inertial force, drag force, pressure gradient force and lift force, is proposed. The calibrated model can reasonably approximate the temporal variation of wave-induced force. However, it is found that the inertial coefficients vary significantly with the dynamic stability number and the initial submergence of the armour unit. Additional research is required to give a sufficiently large dataset for calibrating empirical formulae

Simultaneous dissipation of trichloroethene and arsenic from co-contaminated groundwater by coupling biodechlorination and biodetoxification with assistance of biochar

Co-contamination of groundwater with trichloroethene (TCE) and arsenic (As) is a widespread problem in industrial sites. The simultaneous biological removal of As and TCE has not yet been developed. This study incorporated biochar into anaerobic dechlorination system to achieve a greatly accelerated dissipation and co-removal of TCE and As. Biochar eliminated microbial lag (6 days) and achieved a 100% TCE removal within 12 days even at a relatively high initial concentration (TCE: 30 mg L−1; As(V): 4 mg L−1), while without biochar, only 75% TCE was removed until day 18. Biochar adsorbed TCE and the intermediate products allowing them to be degraded on its surface gradually, maintaining a high metabolic activity of microbes. Biochar facilitated the preferential colonization of its surfaces by dechlorinating microorganisms (Clostridium and Dehalococcoides) and suppressed hydrogen-competing microorganisms (Desulfovibrio) in water. Biochar itself cannot adsorb As, however, separation of biochar carrying the As-laden microorganisms achieved 50–70% As-removal from groundwater. The biochar-amended incubations were found to be enriched with microbes possessing more crucial As-transforming genes (K00537-arsC and K07755-AS3MT), and upregulated amino acid metabolism, thus enhancing the self-detoxification ability of microorganisms to transform As(V) to As(III) or volatile organic As. This study proposes a strategy of regulating microbes’ metabolic activity by biochar to achieve simultaneous removal of coexisting contaminations, which is an important step prior to examining the feasibility of biochar application for enhanced bioremediation. Graphical Abstract: [Figure not available: see fulltext.]</p

Symmetry guaranteed Dirac-line semimetals in two-dimensions against strong spin-orbit coupling

Several intriguing electronic phenomena and electric properties were discovered in three-dimensional Dirac nodal line semimetals (3D-DNLSM), which are, however, easy to be perturbed under strong spin-orbit coupling (SOC). While two-dimensional (2D) layers are an emerging material category with many advantages, 2D-DNLSM against SOC is yet to be uncovered. Here, we report a 2D-DNLSM in odd-atomic-layer Bi (the brick phase, another Bi allotrope), whose robustness against SOC is protected by the little co-group C_2v \times Z^T_2, the unique protecting symmetry we found in 2D.Specially, (4n+2) valence electrons fill the electronic bands in the brick phase, so that the Dirac nodal line with fourfold degeneracy locates across the Fermi level. There are almost no other low energy states close to the Fermi level; this allows to feasibly observe the neat DNLSM-induced phenomena in transport measurements without being affected by other bands. In contrast, Other VA-group elements also form the brick phases, but their DNL states are mixed with the extra states around the Fermi level. This unprecedented category of layered materials allows for exploring nearly isolated 2DDNL states in 2D.Comment: Totally 25 pages including main text, methods and supporting information, 4 figures, 8 SI figure

Effect of suction on flow characteristics and sediment transport in closed-conduit flows

This study presents experimental studies on suction effects on sediment transport and flow characteristics such as velocity profile, turbulence and shear stress distribution, etc. The results show that the bedload transport rate increases with suction rate (with a constant undisturbed oncoming flow rate, Q0). A conceptual model with consideration of the near-bed vertical and horizontal forces is presented. Particle Image Velocimetry (PIV) measurement of the flow field shows that the flow characteristics in the near-bed region changes significantly in the presence of suction, i.e., the near-bed velocity increases significantly and deviates from traditional log-law but fits the modified law very well. In addition, the shear stress first increases before marginally decreasing with suction intensity, Is (= Vs/U0, where Vs and U0 are the suction velocity and mean streamwise velocity, respectivley). Consequently, a turning point at Is = 2.32% is noticed and a possible explanation is given. The experimental results also show that the suction zone length, like suction intensity in the form of Is, can influence the sediment transport rate. Combining the effect of Is and suction zone length, the study shows that suction intensity in the form of Qs/Q0 (Qs is the suction flow rate) is a superior variable to use in the analysis of sediment transport rate. Moreover, PIV measurement of the velocity profile shows that the streamwise velocity at the bed level increases with increasing Vs/U0 at fixed suction zone length and also with increasing suction zone length at fixed Vs/U0, which is used to explain how the suction zone length affects the sediment transport rate.Doctor of Philosophy (CEE

Inline force on human body due to non-impulsive wave overtopping at a vertical seawall

Wave overtopping endangers pedestrians on a seawall’s crest area, but quantitative assessment of the risk is still a challenge. In this study, a circular cylinder is taken as a schematisation of human body. Physical model tests and numerical simulations were conducted to investigate the inline force on the cylinder due to overtopping flow at a vertical seawall. It is found that the overtopping flow is well reproduced by the numerical model. When the overtopping flow encounters the cylinder, a high pressure zone is developed on the front surface of the cylinder due to the local flow stagnation, while a shallow wake flow occurs behind the cylinder due to the blockage effect. The asymmetric pressure distribution produces the inline force on the cylinder. When the cylinder is further moved inland, the depth of the local overtopping flow it faces is smaller. However, the inline force does not decay accordingly, which is due to the variation of the flow velocity along the seawall’s crest. Finally, a predictor is developed for desktop computation of the maximum inline force. A prototype-scale numerical case study is carried out to demonstrate the capability of this predictor

Analysis of convergence behaviour for the overset mesh based numerical wave tank in OpenFOAM

This paper presents a solution verification and validation study for an overset mesh based numerical wave tank in OpenFOAM, which considers the coupling between a free-surface hydrodynamic flow model, a rigid body motion model and an overset mesh. The coupling between the rigid body motion solver and the free surface flow solver was achieved in a segregated manner. Free decay of roll motion of a barge was modelled using the numerical wave tank, and the damping coefficient was selected as the target quantity for solution verification. The least square based solution verification procedure was adopted, where one of the four types of error estimators was fit to the data in the least square sense. Both structured and unstructured mesh were tested, and their effects on the convergence order, numerical uncertainty and error were carefully investigated. From the numerical tests, it is found that the numerical wave tank exhibits a very good convergence property for the floating body problems with structured mesh, i.e. nearly second order in space and first order in time. However, when switching the body-fitted mesh to unstructured mesh, the grid convergence is reduced to first order. Unstructured mesh does not significantly affect the convergence order in time domain, but results in a larger uncertainty due to data scattering