High Flow Velocity Conditions Research Articles

Numerical Study of Vortex-Induced Vibration Characteristics of a Long Flexible Marine Riser

In ocean engineering, interactions between ocean currents and risers lead to regular vortex shedding on both sides of the riser, causing structural deformation. When the frequency of vortex shedding approaches the natural frequency of the structure, resonance occurs, significantly increasing deformation. This phenomenon is a critical cause of riser failure. Therefore, the dynamic response of flexible risers to vortex-induced vibrations (VIV) is crucial for their structural safety. This paper employs the finite-volume method to integrate over control volumes to solve for forces, such as pressure and shear stress, on the surface of the riser, while the finite-element method discretizes the continuous structural body into elements and nodes to solve for structural displacements and stresses. A strongly coupled method is utilized at each timestep to iteratively transfer load-displacement data between the fluid and structural fields, updating the boundary conditions of the fluid domain to achieve a bidirectional fluid–structure interaction simulation of vortex-induced vibrations in a seawater environment for flexible risers. The study finds that the three-dimensional flexible riser exhibits multi-frequency vibration phenomena and broadband vibration response characteristics under high flow velocity conditions. As the flow velocity increases, the vortex-shedding mode is observed to transition from the simple two single (2S) mode to the more complex pair + single (P + S) and two pair (2P) modes. In addition, the stiffness at the ends is enhanced by the fixed boundary conditions, and the coupling between the natural frequency of the ends and the vortex-shedding frequency triggers complex vortex-shedding phenomena in these regions. At higher flow velocities, these boundary effects result in more complex vortex-shedding modes and stronger vibration responses at both ends of the riser.

Hydrological conditions can change the effects of major nutrients and dissolved organic matter on phytoplankton community dynamics in a eutrophic river

In river ecosystems with diverse hydrological conditions, major nutrients and dissolved organic matter (DOM) play significant roles in governing the abundance and composition of phytoplankton communities. Understanding the significance of these factors is crucial for mitigating phytoplankton blooms. An extensive study was conducted to explore this relationship including an examination of 108 phytoplankton communities in the mainstream and eight tributaries of the Dongjiang River during both rainy and dry seasons. The results demonstrated the profound impact of both major nutrients and DOM on phytoplankton communities (p < 0.05). Under high-flow velocity conditions, major nutrients dominated, explaining 20 % and 2.5 % of variance during the rainy and dry seasons, respectively. DOM had a smaller influence, contributing −2.4 % and 1.1 % during these seasons. Conversely, under low-flow velocity conditions, major nutrients explained less than 8.0 % of the variation, while DOM became prominent, explaining over 11.8 % of the phytoplankton community dynamics. The dominant genera exhibited pronounced blooms in response to increased major nutrient levels across all hydrological conditions. Specific DOM components (C2, C3, and C4) were positively correlated with phytoplankton in the low flow streams (p < 0.05). These findings indicate the dual influence of major nutrients and DOM on phytoplankton communities, with their relative significance contingent upon spatial variations in hydrological conditions. As the flow velocity decreased and the potential nutrient retention time increased, the importance of DOM surpassed that of the major nutrients. These insights have substantial implications for managing stream ecosystems and preventing phytoplankton blooms in eutrophic river systems.

Numerical simulation of ultrasonic velocity in wet steam

Wet steam widely exists in thermal power plants, nuclear power plants and other power engineering. The accurate measurement of steam wetness is very important for the safety and economy of power engineering. Ultrasonic method is feasible to measure the concentration of two-phase flow in particle system. In order to obtain a theoretical model of ultrasonic velocity which can be applied to high flow velocity conditions (Reynolds number > 4000), the numerical results of several ultrasonic propagation characteristics theoretical models are compared with the experimental results. The calculated results of U-A coupled phase model show good agreement with the experimental results (the max error is 3.8%), and the model is more suitable for the numerical calculation of ultrasonic sound velocity in droplet two- phase flow with high flow velocity. The ultrasonic sound velocity of steam system in nuclear power plant is calculated numerically. When the droplet size is less than 8 μm, the sound velocity is sensitive to the variation of the droplet size, and when the droplet size is larger than 8 μm, the sound velocity is almost unaffected (velocity variation <0.5%) by the fluctuation of the droplet size. When the effect of the droplet size change on sound velocity can be ignored, the ultrasonic sound velocity method combined with U-A coupled phase model can be used to measure the steam wetness online.

Experimental study of liquid spreading and atomization due to jet impingement in liquid–liquid systems

Liquid spreading and atomization due to jet impingement in liquid–liquid systems is considered to be crucial for understanding cooling behavior of high temperature molten martial into shallow water pool. This phenomenon takes place where a liquid jet enters a pool filled with another immiscible liquid. The jet spreads radially after impinging on the floor while forming a thin liquid film and atomizing droplets. In this study, we employed three-dimensional Laser-Induced Fluorescence (3D-LIF) measurements and three-dimensional (3D) reconstruction to quantify its unsteady three-dimensional behavior. Under high flow velocity conditions, atomization occurred along with the spreading of the liquid film. To evaluate the spreading behavior of the liquid film, a comparison was made with the existing theory of gas–liquid systems. The spreading of the liquid film was suppressed compared with that of the gas–liquid system. Furthermore, the particle tracking velocimetry method was successfully used to measure the velocity boundary layer and velocity profile in the liquid film, which are important factors that affect the spreading mechanism of the liquid film. The results revealed that in liquid–liquid systems, shear stress at the liquid–liquid interface causes a decrease in the flow velocity and suppressed development of the velocity boundary layer. To evaluate the atomization behavior, the number and diameter distribution of the atomized droplets were measured from the acquired three-dimensional shape data of the jet. The number of droplets increased with the flow velocity. The results show that the jump radius is affected by such atomization behavior, and since the jump radius was lower than the theoretical value, we can conclude that the location of the hydraulic jump is expected to undergo an upstream transition.

Chemistry and corrosion research and development for the water cooling circuits of European DEMO

The European DEMO design will potentially use single-phase water cooling in various components that require protection against corrosion. Coolant conditions will be similar to those of fission plant but with additional considerations arising from materials choices (Eurofer-97, CuCrZr) and 14 MeV neutron irradiation. Presently, many aspects of the water chemistry and corrosion behaviour are not well defined, and several strands of work, reported here, are ongoing to address these challenges under the EUROfusion framework in collaboration with industrial partners to leverage knowledge and expertise from elsewhere in the nuclear industry.Starting with the water-cooled lithium-lead blanket concept and considering the interaction of water with Eurofer-97, the foundation of this work has been the definition of a working water chemistry specification, supported by a review of relevant operating experience from light water fission reactors to understand the potential for technology transfer. Radiolysis modelling has been used to assess options for suppression of oxidising species under high energy neutron irradiation as these can be corrosive to components within the plant. High temperature water corrosion testing facilities have also been employed to expand the corrosion database and supplement existing experimental activities in the EUROfusion programme.In-vessel cooling of the divertor will use CuCrZr tubes under lower-temperature, high flow velocity conditions, which will lead to different considerations compared to the blanket and the potential for flow-accelerated corrosion. Additionally, high, unidirectional, heat fluxes lead to a radial temperature profile and the possibility of sub-nucleate boiling. A separate test setup, currently under construction, to expand this corrosion database is described.

The role of flow velocity combined with habitat complexity as a top–down regulator in seagrass meadows

Large‐scale losses of seagrass areas have been associated with eutrophication events, which have led to an overproduction of photosynthetic organisms including epiphytes. Grazers that feed on epiphytes can exert a significant top–down control in the system, but the effects of physical factors on grazing activity and feeding behaviour have been rarely examined. We addressed the combination of hydrodynamic regime and seagrass shoot density can alter the feeding and foraging behaviours of mesograzers. A full factorial experiment, with flow velocity (high, medium and low) and shoot density (high versus low) as main factors, was conducted in a racetrack flume using artificial seagrass plots. The results showed that when high flow velocity conditions were combined with low shoot density, consumption of epiphytes by mesograzers was strongly reduced. In contrast, when flow velocity was low or shoot density was high, mesograzers exhibited high feeding rates and vigorous swimming behaviour. These results clearly indicate that hydrodynamic stress reduces the time that mesograzers can spend feeding, since it inhibits their swimming behaviour, and thus indirectly affecting to the density of epiphytes. Therefore, the triggering of trophic cascade effects in seagrass communities under these experimental conditions depended on the interrelationship and feedbacks among shoot density, abiotic (flow velocity) and biotic (epiphytes and mesograzers) compartments, with flow velocity exerting a top–down control on seagrass ecosystems.

Abiotic influences on bicarbonate use in the giant kelp, Macrocystis pyrifera, in the Monterey Bay.

In the Monterey Bay region of central California, the giant kelp Macrocystis pyrifera experiences broad fluctuations in wave forces, temperature, light availability, nutrient availability, and seawater carbonate chemistry, all of which may impact their productivity. In particular, current velocities and light intensity may strongly regulate the supply and demand of inorganic carbon (Ci) as substrates for photosynthesis. Macrocystispyrifera can acquire and utilize both CO2 and bicarbonate (HCO3- ) as Ci substrates for photosynthesis and growth. Given the variability in carbon delivery (due to current velocities and varying [DIC]) and demand (in the form of saturating irradiance), we hypothesized that the proportion of CO2 and bicarbonate utilized is not constant for M.pyrifera, but a variable function of their fluctuating environment. We further hypothesized that populations acclimated to different wave exposure and irradiance habitats would display different patterns of bicarbonate uptake. To test these hypotheses, we carried out oxygen evolution trials in the laboratory to measure the proportion of bicarbonate utilized by M.pyrifera via external CA under an orthogonal cross of velocity, irradiance, and acclimation treatments. Our Monterey Bay populations of M.pyrifera exhibited proportionally higher external bicarbonate utilization in high irradiance and high flow velocity conditions than in sub-saturating irradiance or low flow velocity conditions. However, there was no significant difference in proportional bicarbonate use between deep blades and canopy blades, nor between individuals from wave-exposed versus wave-protected sites. This study contributes a new field-oriented perspective on the abiotic controls of carbon utilization physiology in macroalgae.

Microfluidic hydrodynamic focusing for high-throughput applications

Microfluidic hydrodynamic focusing is critical for chip-based bioanalytical systems to increase throughput and sensitivity, especially for microflow cytometers, enabling a sample flow to be confined to the center of a microchannel with a narrow cross-section. Current microfluidic hydrodynamic focusing designs are usually unable to maintain stable focusing in high flow velocity conditions, resulting in a large cross-section or even failed focusing. To overcome this challenge, this paper aims to develop a design that can achieve effective microfluidic hydrodynamic focusing at high velocity with favorable performance. For this purpose, specially designed structures and arc-shaped channels are used. Two focusing regions are modeled and optimized mathematically, and flow behavior is investigated using numerical simulations. The functional relationship between flow rates and the cross-sectional dimensions of the focused sample flow is explored, and a measurement method for testing the dimensions is developed. The design is implemented in glass chips and characterized experimentally. In a rectangular channel with a cross-section of 300 μm × 150 μm the sample flow can be focused down to 5–11 μm horizontally and 10–21 μm vertically at a roughly constant velocity of 4.4 m s−1 when the sample flow rate varies between 10 and 60 μl min−1. Effective focusing is accessible within a wide velocity range from 0.7 to 10 m s−1. The experimental results validate that the focusing design performs better than existing microfluidic designs at high velocities, while its performance is close to that of the designs used in conventional flow cytometers with much less volume and a simpler structure. The focusing design can serve as the basis for microflow cytometers or it can be integrated into various microfluidic systems where complete focusing is needed.

Influence of artificial ecological floating beds on river hydraulic characteristics

The artificial ecological floating bed is widely used in rivers and lakes to repair and purify polluted water. However, the water flow pattern and the water level distribution are significantly changed by the floating beds, and the influence on the water flow is different from that of aquatic plants. In this paper, based on the continuous porous media model, a moveable two-layer combination model is built to describe the floating bed. The influences of the floating beds on the water flow characteristics are studied by numerical simulations and experiments using an experimental water channel. The variations of the water level distribution are discussed under conditions of different flow velocities (v=0.1 m/s, 0.2 m/s, 0.30 m/s, 0.4 m/s), floating bed coverage rates (20%, 40%, 60%) and arrangement positions away from the channel wall (D = 0 m, 0.1 m, 0.2 m). The results indicate that the flow velocity increases under the floating beds, and the water level rises significantly under high flow velocity conditions in the upstream region and the floating bed region. In addition, the average rising water level value (ARWLV) increases significantly with the increase of the floating bed coverage rate, and the arrangement position of floating beds in the river can also greatly influence the water level distribution under a high-flow velocity condition (v ≥ 0.2 m/s).

Aquatic macrophytes and water current velocity in the Danube River

Water currents are among the strongest environmental variables determining the occurrence and/or abundance of aquatic plants and the European Water Framework Directive (WFD) considers river morphology and current conditions important aspects of reference conditions. We used the database of the Joint Danube Survey JDS-2, organised by the International Commission for the Protection of the Danube River (ICPDR, Vienna), to assess the relationship between water flow velocity and the occurrence and abundance of aquatic macrophyte to be species along the navigable reach of the Danube River. Species abundance was assessed in accordance with European Standard EN 14184-2003. Flow velocities were assessed close to the plant stands and estimated in 4 classes, worked out in earlier studies in combination with flow meter measurements, which is appropriate for the conditions in this large river. Statistical analysis revealed that only survey units with either no visible flow or those with fast flow were characterised by indicator species with high significance. Still water conditions or barely visible flow (flow class 1) were indicated by Potamogeton friesii, Nitellopsis obtusa and Trapa natans, and by Potamogeton perfoliatus, Potamogeton nodosus and Stuckenia pectinata. Lemna minor, indicative for flow class 1 as well, was detected in all the other flow classes, too: in all these cases it established its stands closely intermingled with other aquatic species. For survey units with slow (flow class 2) or intermediate flow (flow class 3). No statistically significant indicator species could be determined. The highest flow velocities (flow class 4) were significantly indicated by Fissidens rufulus, Cinclidotus fontinaloides and Cinclidotus riparius. Occurrence of aquatic mosses was also highly related to the Berger–Parker dominance index in NMDS analysis. In the same analysis, but towards the smallest flow velocities, the majority of vascular species and the Simpson diversity index were closely related. The spectrum of life forms was considerably reduced under medium and high flow velocity conditions. With regard to Simpson species diversity, our analysis indicates an agglomeration of diversity in the Middle Danube reach that includes the Iron Gates impoundment. Our results offer a first look at the macrophyte/water flow relationship of a very large river in Europe, the Danube.

High Velocity Flow in and Around Long Perforation Tunnels

In oil industries, wells are mostly cased and perforated rather than completing the producing formation as open-hole. This study concentrates on the steady-state flow behaviour of a single-phase fluid in and around perforated completion tunnels (up to 50 inches long) including inertial effects (Non-Darcy flow). It is shown that the pressure drop inside long perforated tunnels under high flow velocity conditions is negligible compared to that around the perforated region within the porous medium. The results also indicate that the impact of perforation parameters varies with increasing fluid velocity but approaches an asymptotic value at very high flow velocity. The perforation length is the most important parameter whereas perforation radius, rock and fluid properties have little impact on the perforation performance.

Characterization of the thermal and fluid flow behavior of industrial ribbon burners

This study focuses on the key parameters that enhance the stability of ribbon burners used in various industrial processes, characterizing the flame environment and flow regimes that the burner creates under changing operating conditions. The research includes the exploration of ribbon-pack configurations in order to define the effects of separation distance and port arrangement on flame stability and thermal and fluid flow. Flow visualization studies reveal that burners having a 1.5–2.0mm division between rows appear to be the most stable. More than four rows of ports in the ribbon-pack are required to resist entrainment of ambient air as a result of impinging surface motion. The flame environment created by the ribbon burner experiences higher levels of mixing, at a lower Reynolds number in comparison to a single non-reacting jet due to combustion-induced turbulence and jet interaction. Under these high flow velocity conditions there is little entrainment of ambient air along the impingement surface and a very stable flame environment is created.

The effect of abiotic environmental variables on the Danube macrozoobenthic communities

160 taxa of macrozoobenthos were recorded in the Danube River (R. km 1880-1704) riparian zone. Many typical Danubian and Pontocaspian representatives were found. The Slovak section of the Danube represents the border between its epipotamal and metapotamal stretches with different flow velocity and other environmental variables related to it. This paper characterises some abiotic ecological factors influencing the distribution of benthic taxa in five sampling sites along the longitudinal profile of the Danube. The benthic community composition was more strongly influenced by hydrological parameters than by oxygen and temperature regimes, as well as by the water quality conditions. In other words, the bottom substrate, flow velocity, water level fluctuation and discharge changes had the predominant effect on the taxocoenoses of macrozoobenthos. Three characteristic groups of macrozoobenthic organisms were recorded, as far as corresponding trophic guilds are concerned. Shredders and scrapers predominated in high flow velocity conditions with coarse substrate particles. Collectors of fine organic matter preferred slow-flowing conditions caused by damming or by a reduction of current due to the presence of bedrock. Filterers prevailed in the lower section of the Danube stretch monitored. Predators occurred independently of the environmental variables studied.