Quiescent Water Research Articles

Melting of olive oil in immiscible surroundings: experiments and theory

We report on the melting dynamics of frozen olive oil in quiescent water for Rayleigh numbers up to $10^9$ . The density difference results in an upward buoyancy-driven flow of liquid oil forming a thin film around the frozen oil. We experimentally investigate flat, cylindrical and spherical shapes and we derive theoretical expressions for the local film thickness, velocity and the local melt rate for these three canonical geometries. Our theoretical models predict the correct order of magnitude and the correct scaling as compared with our experimental findings.

Environmental Life Cycle Assessment of Innovative Ejectors Plant Technology for Sediment by-Pass in Harbours and Ports

Sedimentation is the natural process of sediment transportation and deposition in quiescent water conditions. Sedimentation can affect the functionality of ports, harbours and navigation channels by reducing water depth, making navigation difficult, if not impossible. Different solutions are available to guarantee infrastructure functionality against sedimentation, with maintenance dredging being the most widely adopted. Alternative technologies for dredging have been developed and tested to reduce the environmental concerns related to dredging operations. Among other solutions, applying a sediment by-pass system based on a jet pump emerged as one of the most promising. While the existing literature covers the techno-economic aspects of sediment by-pass systems, the environmental impacts must be better evaluated and assessed. This paper aims to resolve this gap by evaluating, through the ReCiPe2016 life cycle assessment (LCA) methodology, the environmental impact of an innovative sediment by-pass system called an “ejectors plant”. The LCA results are based on the demonstrator established in Cervia Harbour in Italy, which was extensively monitored for 15 months during its operation. This paper shows how energy consumption during the operation phase highly affects the considered midpoint and endpoint categories. For example, the GWP100 of the ejectors plant, considering the Italian electricity mix, equals 1.75 million tons of equivalent CO2 over 20 years, while under a low-carbon scenario, it is reduced to 0.17. In that case, material consumption in the construction phase becomes dominant, thus highlighting the importance of eco-innovation of ejectors plants to minimise oxidant formation. Finally, this paper compares the ejectors plant and traditional dredging through environmental LCA. The ejectors plant had a lower impact in all categories except for GWP-related categories. The sensitivity analysis showed how such a conclusion may be mitigated by considering different electricity mixes and maintenance dredging working cycles.

Volumetric Measurements Of Particle-Wake Interactions

Research on freely falling particles has primarily focused on wake dynamics and vortex shedding of individual particles in quiescent flow. When these particles fall collectively, the wakes of surrounding particles alter the flow fields. Hence, we conducted volumetric experiments to investigate how the settling and wake dynamics of particles are affected by the wakes of other settling particles. Negatively buoyant 12 mm particles of (sphere, flat cuboid, circular and square cylinders) are first released individually into quiescent water. Then, the particles are released individually into the bulk wakes of 20 mono-dispersed particles. Using four high-speed cameras and LEDs, we simultaneously capture both 3D particle and fluid motions in the terminal velocity regime. The imaging domain measures 90 mm x 90 mm x 40 mm. Our results show that all trailing particles settling through the bulk wakes gain additional downward momentum from the turbulent wakes, falling faster than in the quiescent flow. Upstream of the particle, the vortices in the bulk wake interact with the developing shear layer along the particle. The wake downstream of the trailing particle also appears more chaotic than that in a quiescent flow.

Effect of aspect ratio on flow-induced vibration of oblate spheroids and implications for energy generation

This study experimentally investigates the influence of aspect ratio on cross-flow flow-induced vibration (FIV) of elastically mounted oblate spheroids. The aspect ratio (ϵ=b/a) of an oblate spheroid, defined as the ratio of the major diameter (b) in the cross-flow direction to the minor diameter (a) in the streamwise direction, was varied between 1.00 and 3.20. The FIV response was characterized over a range of reduced velocity, 3.0⩽U∗=U/(fnwb)⩽12.0, where U is the free-stream velocity and fnw is the natural frequency of the system in quiescent water. The corresponding Reynolds number varied over the range 4730⩽Re⩽20120. It was found that in addition to the vortex-induced vibration (VIV) Mode I and Mode II responses observed for a sphere, on increasing the aspect ratio to ϵ=1.53 and 2.0, a galloping-dominated response, denoted by G-I, was encountered at high reduced velocities. With a further increase in aspect ratio to ϵ=2.50, the body vibration exhibited an additional VIV-like response (V-I) following the sequential appearance of Mode I, Mode II and G-I, with smooth transitions between these modes. In the case of the largest aspect ratio considered in the present study, ϵ=3.20, the spheroid intriguingly exhibited only a pure VIV Mode I before transitioning to a VIV-dominated mode, namely V-II. The largest vibration amplitude observed was 2.17b, occurring at the highest tested reduced velocity of U∗=12.0 for ϵ=2.5. Furthermore, the maximum time-averaged power coefficient was observed to be 0.165 for the thinnest oblate spheroid tested, ϵ=3.20, approximately 660% higher than that observed for VIV of a sphere. This shows the relevance of geometry for FIV energy harvesting from oblate spheroids. The findings highlight the distinctive nature of FIV responses of 3D oblate spheroids compared to 2D bluff bodies such as elliptical, D-section, and square cylinders.

Micromechanical analysis of contact erosion under cyclic loads using the coupled CFD‒DEM method

Different layers of soil often have distinct particle sizes. When exposed to the natural environment, soil is easily affected by natural rainfall, rising groundwater levels, and human activities, leading to particle contact erosion, which reduces the safety and service performance of the soil structure. In this paper, a coupled computational fluid dynamics–discrete element method (CFD–DEM) model was employed to investigate the particle migration phenomena, mechanical response of contact interfaces, variations in flow fields, and macroscopic deformation during the contact erosion process under cyclic loads at different frequencies and amplitudes. The conclusions are presented as follows: (1) Within one cycle of cyclic loading, both compression during loading and stress relaxation during unloading are the main factors triggering the migration of fine particles. (2) The migration and loss of fine particles mainly occur in the early stages of cyclic loading, where strong contact force chains are formed within the fine particle layer, leading to significant plastic deformation of the soil at the macroscopic level. (3) Under cyclic loading, changes in the soil pore structure cause an upwards hydraulic gradient in the initial quiescent water flow field. This hydraulic gradient can rupture weak contact force chains and cause particle pumping. (4) Increasing the frequency and amplitude of cyclic loading intensifies the erosion of fine particles, causing greater axial deformation of the soil. Compared to cyclic loading frequency, the amplitude of cyclic loading has a greater impact on contact erosion.

Nonlinear static and dynamic behaviors of partially and fully submerged rod pendulums in quiescent water

Nonlinear static and dynamic behaviors of partially and fully submerged rod pendulums in quiescent water

Stratified water columns: homogenization and interface evolution

Stratified water columns are often found in lakes and oceans. Stratifications result from differences in density due to salt concentration, temperature, solid content and oxygenation. The stability of stratifications affects bioactivity, sedimentation, contaminant transport and environmental remediation. This study investigates the evolution of 6 stratified water columns created by differences in salinity, suspended minerals and the presence of a bottom heat source. We use acoustic wave reflection, photography, and both electrical conductivity and temperature profiles to track changes in stratification. Results show that multiple concurrent processes emerge across layers in otherwise quiescent water bodies. Dissimilar chemo-thermo conditions give rise to chemical and thermal diffusion, convection, and double-diffusion convection. When stratification involves suspended particles, interlayer processes include diffusiophoresis, flocculation/aggregation, sedimentation, osmosis, and chemo-consolidation; in this case, the specific surface and surface charge of suspended particles, and the salt concentration in contiguous layers determine aggregation-sedimentation-consolidation patterns. The interlayer transition zone acts as a high-pass filter that preferentially reflects low-frequency long-wavelength P-waves; invasive thermal and electrical conductivity probes provide complementary information and may identify stratification even when it is undetected by acoustic signals.

Settling Velocities of Small Microplastic Fragments and Fibers.

There is only sparse empirical data on the settling velocity of small, nonbuoyant microplastics thus far, although it is an important parameter governing their vertical transport within aquatic environments. This study reports the settling velocities of 4031 exemplary microplastic particles. Focusing on the environmentally most prevalent particle shapes, irregular microplastic fragments of four different polymer types (9-289 μm) and five discrete length fractions (50-600 μm) of common nylon and polyester fibers are investigated, respectively. All settling experiments are carried out in quiescent water by using a specialized optical imaging setup. The method has been previously validated in order to minimize disruptive factors, e.g., thermal convection or particle interactions, and thus enable the precise measurements of the velocities of individual microplastic particles (0.003-9.094 mm/s). Based on the obtained data, ten existing models for predicting a particle's terminal settling velocity are assessed. It is concluded that models, which were specifically deduced from empirical data on larger microplastics, fail to provide accurate predictions for small microplastics. Instead, a different approach is highlighted as a viable option for computing settling velocities across the microplastics continuum in terms of size, density, and shape.

Evolution of Vortex Structures Generated by a Rigid Flapping Wing with a Winglet in Quiescent Water

This study aims to the utilization of vortex structures generated through wing flapping for achieving sustainable flight, and the motivation is elicited by the phenomenon observed in natural flyers. The vortex structures in the flow field generated by a flapping rigid wing are captured using vorticity and the LAMDA2 criterion. The study investigates a comparative analysis between a wing both with and without a winglet. Moreover, the influence of flapping frequency is examined as well. For the experiments, particle image velocimetry (PIV) measurements are employed for the flow field around mechanical flapping motion in a quiescent water condition. The flapping mechanism has one-degree freedom, showing a 1:3 ratio in motion, and tested wings at 1.5 and 2.0 Hz. A “modified” vortex filamentation and fragmentation phenomenon is proposed as a significant finding in the present study, based on a comprehensive analysis of the flow field around the wing with a winglet.

Experimental study on the effects of sediment size gradation and suspended sediment concentration on the settling velocity, ws

The settling velocity of non-uniform sediment is of great importance for the sediment transport on coasts and estuaries. Although existing settling velocity formulas have already considered factors such as the Suspended Sediment Concentration (SSC), temperature, salinity, turbulence, and the effect of size gradation on the settling velocity, more studies are needed. In this study, the fiber optic sensors and Particle Track Velocimetry (PTV) for high SSC were used to measure the settling velocity of two size gradations with different SSCs in a settling column in laboratory. The size gradation curves and settling velocities of these two non-cohesive sediment with SSCs ranged from 0.1 kg/m3 to 50 kg/m3 were obtained. When the SSC is less than or equal to 0.1 kg/m3, the settling velocity is close to the grain size distribution weighted average settling velocity. With the increase of SSC, the distance between sediment particles decreases, and the mutual influence between different components become important, resulting in a decrease in the settling velocity of large-sized particles and an increase in the settling velocity of small-sized particles. This phenomenon is more pronounced for natural sediments. Based on the experimental results, a particle size distribution correction coefficient was proposed to fit the fine-grained settling velocity in quiescent water considering size gradation and sediment concentration.

Biologically generated turbulent energy flux in shear flow depends on tensor geometry.

It has been proposed that biologically generated turbulence plays an important role in material transport and ocean mixing. Both experimental and numerical studies have reported evidence of the nonnegligible mixing by moderate Reynolds number swimmers, such as zooplankton, in quiescent water, especially at aggregation scales. However, the interaction between biologically generated agitation and the background flow, as a key factor in biologically generated turbulence that could reshape our previous knowledge of biologically generated turbulence, has long been ignored. Here, we show that the geometry between the biologically generated agitation and the background hydrodynamic shear can determine both the intensity and direction of biologically generated turbulent energy flux. Measuring the migration of a centimeter-scale swimmer-as represented by the brine shrimp Artemia salina-in a shear flow and verifying through an analog experiment with an artificial jet revealed that different geometries between the biologically generated agitation and the background shear can result in spectral energy transferring toward larger or smaller scales, which consequently intensifies or attenuates the large-scale hydrodynamic shear. Our results suggest that the long ignored geometry between the biologically generated agitation and the background flow field is an important factor that should be taken into consideration in future studies of biologically generated turbulence.

Measurement of interfacial mass transfer of single bubbles rising in homogeneous turbulence

Accurate quantification of the mass transfer coefficient kl of bubbles under high Reynolds number conditions has been in desperate need for decades to provide insights into its fundamental physics and reach a general mass transfer model. In this paper, a state-of-the-art SI-LIF technique is developed to quantify kl indirectly based on the law of conservation of mass. The reconstruction algorithms of the required physical quantities are validated numerically, suggesting an overall uncertainty of less than ± 5 % in the estimation of kl. We perform mass transfer experiments of single oxygen bubbles with deq = 1.5 mm rising in both quiescent water and homogeneous turbulence, with turbulent Reynolds number ReT ranging from 0 to 1114. The measured kl under turbulent conditions are independent of ReT, and all collapse to the analytical solution of potential flow approximation, revealing that the flow in the immediate vicinity of the bubble interface remains undisturbed by the fluctuating liquid motions across all the turbulence settings examined in this paper.

NUMERICAL MODELING OF BUBBLE DYNAMICS USING INTERFACE CAPTURING METHOD

In the present numerical study, a dynamics of single gas bubble (circular in 2D and spherical in 3D) rising in a stagnant viscous liquid due to the buoyancy is presented using various volume of fluid (VOF) based computational fluid dynamics (CFD) solvers such as commercial Converge and Star CCM+, and open source OpenFOAM<sup>®</sup> platform. To capture the interface dynamics, either an interpolated curved interface based on the high-resolution interface framework or a mass conservative VOF approach with a planar sharp interface based geometric reconstruction of the piecewise-linear interface calculation (PLIC) scheme was used. Both qualitative and quantitative analysis of air bubble rising upward inside the quiescent water column at ratios of low density, ρ<sub>r</sub> = 10 and high density, ρ<sub>r</sub> = 1000 are simulated and evaluated similar to report by Hysing et al. The proposed numerical models can simulate a wide range of density and viscosity ratios. In this study, a robustness and accuracy of the solvers are evaluated and comparative study between open source OpenFOAM<sup>®</sup> solver with commercial solvers such as Converge and Star CCM+. Based on the present numerical results, the gas bubble base undergoes severe deformations for the high density ratio, ρ<sub>r</sub> = 1000 and high viscosity ratio, μ<sub>r</sub> = 100 compared to low density ratio, ρ<sub>r</sub> = 10 and low viscosity ratio, μ<sub>r</sub> = 10. Any of the solvers can be used to simulate complex multiphase flow situations encountered in many industrial applications.

Experimental investigation on diffusion and fountain behavior of bubble plumes in quiescent water

Bubble plumes are a common phenomenon, which are simultaneously hazardous and exploitable. Diffusion and fountain behavior, regarded as significant characteristics of bubble plumes, were investigated in this experiment utilizing high-speed photography and image processing techniques within a rectangular bubble column crafted from organic glass. The diffusion, coalescence, and collapse phenomena of bubble plumes were visualized during their ascent under varying gas flow rates in quiescent water. Furthermore, the influence of gas supply pressure, flow rate, and water depth on the fountain height was examined. The research findings demonstrate that an increase in gas supply pressure and flow rate leads to a subsequent rise in fountain height once the bubble plume reaches the surface. Conversely, a reduction in the fountain height is observed with an increase in the depth of gas release. Through the application of a dimensionless analysis method, the factors influencing the fountain height were correlated, resulting in the proposal of a relationship formula for predicting fountain height. The predicted outcomes exhibited good consistency with the data reported in this experiment.

Dynamics of single bubble ascension in stagnant liquid: An investigation into multiphase flow effects on hydrodynamic characteristics using computational simulation

Industrial operations often involve multiphase flows, where the motion of bubbles plays a crucial role in determining hydrodynamic properties. In the context of modern practices, advanced computational solvers are increasingly employed to simulate the interactions and trajectories of bubbles within complex flows. This study stands out in its unique examination of the computational software COMSOL's capability to accurately simulate the upward motion of a single bubble within quiescent water, akin to a liquid-like environment. The novelty lies in the comprehensive coverage of various container diameters (ranging from 10 to 80 mm) and heights (from 25 to 300 mm) relative to the air bubble diameter (ranging from 0.5 to 8 mm). Through meticulous comparisons between nine empirical formulas and numerically projected bubble ascent through a water column, a remarkable level of agreement emerges, underscoring the precision and consistency of the simulations. These simulations unveil intriguing findings, shedding light on the intricate interplay of forces governing bubble behavior. Notably, variations in drag forces induce changes in bubble shapes as a function of diameter, while the ascent of bubbles is accompanied by distinctive vortices, resulting in fascinating asymmetry. Furthermore, vorticity concentrates within the bubble, particularly in lighter fluid regions characterized by reduced pressure. The study also unveils how larger aspect ratios minimize flow drag, consequently boosting ascent velocities, and demonstrates the influence of container diameter on the rising velocity. Gravitational forces are found to reduce ascent velocities at greater column heights, while the rate of air bubble rise escalates with its size. This meticulous exploration of bubble dynamics in multiphase flows yields invaluable insights for diverse industrial applications, ultimately enhancing our understanding of this complex phenomenon. The strong alignment observed between empirical formulations and numerical simulations within the COMSOL framework underscores the utility of such computational tools for the study and design of multiphase flows' intricate dynamics.

The role of turbulence in the deposition of intrinsically buoyant MPs

Intrinsically floating microplastics (MP) such as polyethene (PE) or polypropylene (PP) are among the most common MPs found in aquatic sediments. There must hence be mechanisms that cause lighter-than-water MPs to deposit despite them being buoyant. How these MPs end up in the sediment bed is only partly understood. This study explores how turbulence in the water can affect the vertical movement of buoyant MP and bring them in contact with the bed. The deposition of PE (995 kg m−3) in slow-flowing water (average flow velocities of 1.85 and 4.17 cm s−1) was measured by tracking them and analyzing their motion in an open, rectangular, glass-sided flume. Flow characteristics in terms of turbulent kinetic energy and shear velocity were measured by particle image velocimetry. Experiments were conducted at a water depth of 27 cm and at various hydraulic conditions created by adjusting inflow speeds and using different bed materials: medium gravel, fine gravel, medium sand, cohesive sediment, and glass. The results showed that the vertical velocity of the MPs in the turbulent flow regimes varied over 4 orders of magnitude from their predicted rising velocity in quiescent water (laminar flow). Turbulence mixing resulted in distribution throughout the water column with a substantial quantity consistently subject to downward vertical transport, which in turn increased the chance of the PE particles encountering the bed and potentially getting immobilized. This work provides a plausible explanation and further experimental validation for the concept of mixing induced transfer of MPs from the water surface to the sediments of shallow waters.

A numerical study of the settling of non-spherical particles in quiescent water

Settling of non-spherical particles is poorly understood with previous studies having focused mainly on spherical particles. Here, a series of particle-resolved direct numerical simulations are conducted using FLOW-3D (commercial computational fluid dynamics software) for spheres and five regular, non-spherical shapes of sediment particles, i.e., prolate spheroid, oblate spheroid, cylinder, disk, and cube. The Galileo number varies from 0.248 to 360, and the particle Reynolds number Rep ranges from 0.002 77 to 562. The results show that a non-spherical particle may experience larger drag and, consequently, attain a lower terminal velocity than an equivalent sphere. If Rep is sufficiently small, the terminal velocity is less affected by particle shape as characterized by the particle aspect ratio. For relatively large Rep, the shape effect (represented by the Corey shape factor) becomes more significant. Empirical correlations are derived for the dimensionless characteristic time t95∗ and displacement s95∗ of particle settling, which show that t95∗ remains constant in the Stokes regime (Rep < 1) and decreases with increasing Rep in the intermediate regime (1 ≤ Rep < 103), whereas s95∗ increases progressively with increasing Rep over the simulated range. It is also found that in the Stokes regime, particle orientation remains essentially unchanged during settling, and so the terminal velocity is governed by the initial orientation. In the intermediate regime, a particle provisionally settling at an unstable orientation self-readjusts to a stable equilibrium state, such that the effect of initial orientation on the terminal velocity is negligible. Moreover, an unstable initial orientation can enhance the vertical displacement and may promote vortex shedding.

Settling processes of cylindrical microplastics in quiescent water: A fully resolved numerical simulation study

The settling process of marine microplastics (MPs) is crucial research concerning the transport and movement of MPs. The settling processes of MP fibers that possess a cylindrical geometry are affected by environmental factors and properties. In this study, a three-dimensional numerical model for the still water settling of MPs with complex shapes was constructed using the lattice Boltzmann method (LBM) and the immersed boundary method (IBM). The fully resolved settling simulation of cylindrical MPs was achieved, and the model results demonstrated good agreement with the semi-empirical settling velocity formulas. Based on the simulation results, the critical aspect ratio of the cylindrical MP was found to be between 0.93 and 0.94. Near this critical aspect ratio, there is a decline in the drag force. Additionally, it was found that the angular displacement and aspect ratio influence horizontal movement but not the vertical settling velocity, while the density only affects vertical movement.

Using convective mixing in mesocosms to study climate-driven shifts in phytoplankton community distributions

With climate change predicted to alter water column stability and mixing across the world’s oceans, a mesocosm experiment was designed to ascertain how a natural phytoplankton community would respond to these changes. As a departure from other mesocosm experiments, we used heating and cooling to produce four different climate-inspired mixing scenarios ranging from well-mixed water columns representative of typical open turbulence (ϵ = 3 x 10-8 m2/s3) through to a quiescent water column with stable stratification (ϵ = 5 x 10-10 m2/s3). This method of turbulence generation is an improvement on previous techniques (e.g., grid, shaker, and aeration) which tend to produce excessive dissipation rates inconsistent with oceanic turbulence observations. Profiles of classical physical parameters used to describe turbulence and mixing (turbulent dissipation rate, buoyancy frequency, turbulent eddy diffusivity, Ozmidov scale) were representative of the profiles found in natural waters under similar mixing conditions. Chlorophyll-a profiles and cell enumeration showed a clear biological response to the different turbulence scenarios. However, the responses of specific phytoplankton groups (diatoms and dinoflagellates) did not conform to the usual expectations: diatoms are generally expected to thrive under convective, turbulent regimes, while dinoflagellates are expected to thrive in converse conditions, i.e., in stable, stratified conditions. Our results suggest that responses to mixing regimes are taxon-specific, with no overwhelming physical effect of the turbulence regime. Rather, each taxon seemed to very quickly reach a given vertical distribution that it managed to hold, whether actively or passively, with a high degree of success. Future studies on the effects of climate change on phytoplankton vertical distribution should thus focus on the factors and mechanisms that combine to determine the specific distribution of species within taxa. Our convection-based mesocosm approach, because it uses a primary physical force that generates turbulence in open waters, should prove a valuable tool in this endeavor.

Assessment of Chloride Salt Release Flux from Fluidized Cohesive Sediments in Quiescent Water

Assessment of Chloride Salt Release Flux from Fluidized Cohesive Sediments in Quiescent Water