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Flagellates are key components of aquatic microbial food webs. Their flagella propel the cell through the water and/or generate a feeding current from which bacterial prey is harvested, but the activity of the flagella also disturbs the ambient water, thereby attracting the flagellate's flow-sensing predators. Here we use computational fluid dynamics to explore the optimality and fluid dynamics of the diverse arrangements, beat patterns, and external morphologies of flagella found among free-living flagellates in light of the fundamental propulsion–foraging–predation-risk trade-off. We examine 5-µm-sized representative model organisms with different resource acquisition modes: autotrophs relying on photosynthesis and uptake of nutrient molecules, phagotrophs that feed on bacteria, and mixotrophs that employ both strategies. For all types, the transport of inorganic molecules is diffusion dominated, and the flagellum in autotrophic species therefore mainly serves propulsion purposes. Flagellates with a single, naked flagellum found among nonforaging swarmer stages have a waveform (less than one wave) that is optimized for swimming and stealth but inefficient for feeding. Flagellates with a hairy flagellum typically have many waves, which optimizes swimming and stealth but is suboptimal for foraging, leading to a design trade-off. However, when compared with naked flagella, the presence of hairs allows a very efficient feeding current, making these primarily phagotrophic flagellates the most efficient and dominant bacterivores in the ocean. Autotrophic biflagellates have wave patterns optimized for both propulsion and foraging but conflicting weakly with stealth. Finally, the mixotrophic haptophytes are optimized for foraging, conflicting with both stealth and propulsion. This is largely due to the long, slender filament (haptonema) that improves prey collection but at the cost of stealth and propulsion. We use dimensional analysis to rationalize our findings. We conc

Ocean wave energy has the potential of contributing significantly to the industrial energy production and export, both as a stand alone solution by using wave energy converters (WECs) in wave energy parks, and in combination with offshore wind energy. For comparison the power intensity of wind is 0.4 0.6 kW/m2 , solar is 0.1 0.2 kW/m2 and wave power intensity is about 2 3 kW/m 2 [1, Wave energy converter Crestwing is an attenuator type of WEC with proven technology and simple feasibi lity originated from shipbuilding industry [3]. Although it seems to be a feasible technology it still has many avenues for optimizing efficiency and minimizing costs to drive down levelized cost of electricity. E ffective energy harvesting is only conceiva ble by interpretation of underlying physics and optimization of key parameters of a WEC such as hull shape. For these goals we use state of the art CFD simulation tools based on fifth order Stokes wave theory, overset mesh technique and dynamic fluid body interaction. At the symposium we will present verification and validation of the CFD model relying on existing model tests and use the CFD results to highlight the key components of the Crestwing WEC [4, 5].

In this study, by performing all-atom Non-Equilibrium Molecular Dynamics simulations, we propose a graphene-based thermal nanopump which produces controlled and continuous water flow in nanoslit channels. Upon systematically imposing thermal gradients, a significant net fluid flow towards the lowtemperature zone is measured in all cases. The nanopump produces water flow with average velocities up to 3 m/s. We find that the water flow rates increase due to an enhancement of the wall thermal rippling as higher temperature gradients are imposed. Moreover, vibrational and phonon Density-of-States analyses on graphene reveal that the out-of-plane flexural phonons in graphene are responsible for the net flow production. We notice that lower frequency phonon branches are activated with higher imposed temperature gradients. This study provides the basis for developing a functional thermal pumping system based on sheets of graphene, which is capable to produce continuous water flow in nanofluidic conduits, opening the door to practical exploitation of thermal gradients for fluid transport in integrated nanofluidic devices.

Ocean wave energy has the potential of contributing significantly to the industrial energy production and export, both as a standalone solution by using wave energy converters (WECs) in wave energy parks, and in combination with offshore wind energy. For comparison the power intensity of wind is 0.4 - 0.6 kW/m2, solar is 0.1 - 0.2 kW/m2 and wave power intensity is about 2 - 3 kW/m2 [1, 2]. The wave energy converter Crestwing is an attenuator type of WEC with proven technology and simple feasibility originated from the shipbuilding industry [3]. Although it seems to be a feasible technology it still has many avenues for optimizing efficiency and minimizing costs to drive down levelized cost of electricity. Effective energy harvesting is only conceivable by interpretation of underlying physics and optimization of key parameters of a WEC such as hull shape. For these goals we use state-of-the-art CFD simulation tools based on fifth-order Stokes wave theory, overset mesh technique and dynamic fluid body interaction. At the symposium we will present verification and validation of the CFD model relying on existing model tests and use the CFD results to highlight the key components of the Crestwing WEC [4, 5].

Ship-to-ship bunkering of liquid fuel, e.g., LNG, outside the port can be a flexible and cost-efficient solution without having to upgrade the infrastructures at the port. The occurrence of fluid resonance in the narrow gap between side-by-side receiving vessel and bunkering vessel under wave actions may greatly influence the dynamic responses of the vessels through hydrodynamic interaction. The proximity of marine structures can generate drastic wave elevation in the narrow gap, which may reduce the operational time window of the bunkering and even risk the safety of the crew. In this paper, the fluid resonance inside the gap between two non-identical ship cross-sections in side-by-side configuration is studied under various wave conditions. The bunkering scenario that a larger cross-section on the upstream is fixed while a small cross-section free to heave in the downstream is considered as more realistic. A numerical wave tank is set up based on the commercial CFD package STAR-CCM+. The unsteady Reynolds averaged Navier-Stokes turbulence model is required to consider viscous dissipation. The volume of fluid method is applied to capture the position of the free surface, and regular waves are generated and absorbed using forcing zones at upstream and downstream boundaries, respectively. The overset meshing technique is employed near the downstream section to enable the heave motion. The heave response of the downstream section and its effect on the linear and nonlinear harmonics of wave elevation and resonant frequency inside the gap are investigated and compared with the fixed-sections scenario. It can be concluded that the heave motion reduces the gap resonance to a large extent by modulating the frequency of wave elevation inside the gap, thus avoiding the occurrence of second-order excitation.

The importance of CFD is increasing in marine hydrodynamics in studying seakeeping and added resistance of ships. While extensive numerical studies have been reported for various ships in head seas in the literature, much fewer CFD studies are found for oblique waves, which in practice is very important in, for instance, estimating required power and maneuverability of ships in realistic sea states. In this paper, the added resistance and motion responses for the KCS container ship in regular waves are studied and validated systematically for two wave headings and six wavelengths using CFD. The ship is free to heave, pitch, and roll. User-defined implementations in the commercial CFD code are made to effectively constrain the surge and yaw. Results of the CFD model are compared with up to three sets of experimental data sets, Potential Flow (PF) and existing CFD results from the literature. In general, the present CFD results show significantly better agreement with the experiments than previously published CFD results. The present study shows that CFD simulations can accurately predict motion responses and added resistance in oblique regular waves. With these results, designers of ship hulls can get an insight of where to focus the optimization work in the pursuit of fuel-efficient vessels.

The important role of unicellular flagellated micro-organisms in aquatic food webs is mediated by their flagella, which enable them to swim and generate feeding currents. The flagellum in many predatory flagellates is equipped with hairs (mastigonemes) that reverse the direction of thrust compared to the thrust due to a smooth flagellum. Conventionally, the mechanism of such reversal has been attributed to drag-based thrust of individual hairs, neglecting their hydrodynamic interactions. However, at natural densities of hairs, hydrodynamic interactions are important. In fact, using fully resolved three-dimensional computational fluid dynamics, we show here that hydrodynamic interactions are key to thrust-generation and reversal in hairy flagellates, making their hydrodynamics fundamentally different from the slender-body theory governing smooth flagella. We reveal the significant role of the curvature of the flagellum, and using model analysis we demonstrate that strongly curved flagellar waveforms are optimal for thrust-generation. Our results form a basis for understanding the diverse flagellar architectures and feeding modes of predatory flagellates.

The present paper focuses on the numerical studies of droplets impinging onto metallic meshes, aiming to provide further insight, identify and quantify droplet impact characteristics that are difficult to be evaluated experimentally. For this purpose, an enhanced Volume-Of-Fluid (VOF) based numerical simulation framework, previously developed in the general context of OpenFOAM CFD Toolbox is utilised. In more detail, initially, validation studies of droplets impacting onto solid surfaces, previously reported in the literature are presented, for relatively high We numbers, in comparison to the ones tested in the past with the same model. Then, specific in-house experimental droplet impacts on metallic meshes are reproduced numerically, with satisfactory degree of agreement. Finally, the numerical model allows us to probe/study parameters difficult to reach experimentally, and perform a series of parametric numerical investigations in order to isolate, identify and quantify the effect of fundamental controlling parameters, such as the fluid viscosity, surface tension as well as the metallic surface wettability characteristics, on the resulting droplet impact dynamics.

Nanofluidics has become interesting as the basis for further device miniaturization. Different from macro and microfluidics, nanoconfined flows are significantly influenced by fluid-wall interaction. In this context, recent studies have reported the potential exploitation of imposed thermal gradients as mechanism to transport water in nanoconduits. Moreover, graphene-based materials have attracted increasing attention in nanofluidic applications due to their unique thermal, structural and hydrodynamic properties. Here, we conduct atomistic simulations to investigate water transport in graphene nanoslit channels driven by thermal gradients. The study is focused in understanding therelation between phonon currents induced in the walls by imposed thermal gradients and the corresponding measured flow rates. Furthermore, a comprehensive analysis of the influence of wettability, multi-layer graphene in the walls and geometrical asymmetries is performed. Our results provide valuable information for the design of thermal graphene-based nanopumps and contribute to the understanding of suitable driving mechanisms for liquids in nanoconduits.

We present continuum simulations of water flow past fullerene molecules. The governing Navier-Stokes equations are complemented with the Navier slip boundary condition with a slip length that is extracted from related molecular dynamics simulations. We find that several quantities of interest as computed by the present model are in good agreement with results from atomistic and atomistic-continuum simulations at a fraction of the computational cost. We simulate the flow past a single fullerene and an array of fullerenes and demonstrate that such nanoscale flows can be computed efficiently by continuum flow solvers, allowing for investigations into spatiotemporal scales inaccessible to atomistic simulations.

The transport of water in nanopores is of both fundamental and practical interest. Graphene Channels (GCs) are potential building blocks for nanofluidic devices dueto their molecularly smooth walls and exceptional mechanical properties. Numerous studies have found a significant flow rate enhancement, defined as the ratio of the computed flow rate to that predicted from the classical Poiseuille model. Moreover, these studies point to the fact that the flow enhancement is a function of channel height and the fluid-wall physical-chemistry. In spite of the intensive research, an explicit relation between the chirality of the graphene walls and the slip length has not been established. In this study, we perform non-equilibrium molecular dynamics simulations of water flow in single- and multi-walled GCs. We examine the influence on the flow rates of dissipating the viscous heat produced by connecting the thermostat to the water molecules, the CNT wall atoms or both of them. From the atomic trajectories, we compute the fluid flow rates in GCs with zig-zag and armchair walls, heights from 1 to 4 nm and different number of graphene layers on the walls. A relation between the chirality, slip length, and flow enhancement is found.

In this study, a novel concept of nanoscale pump fabricated using Carbon Nanotubes (CNTs) is presented. The development of nanofluidic systems provides unprecedented possibilities for the control of biology and chemistry at the molecular level with potential applications in low energy cost devices, novel medicaltools, and a new generation of sensors. CNTs offer a number of attractive features for the fabrication of fluidic nanodevices including fast flow, useful electronic and thermal properties, high mechanical strength and biocompatibility. Therefore, the transport of liquids in CNTs is now of great interest in nanofluidics. Thermophoresisis the phenomenon observed when a mixture of two or more types of motile objects experience a force induced by a thermal gradient and the different types of objects respond to it differently, inducing a motion and segregation of the objects. Using molecular dynamics simulations, we explore the possibility to design thermophoretic pumping devices fabricated of CNTs for water transport in nanoconduits. The design of the nanopumps is based on the concept of the Feynman-Smoluchowski ratchet.

We present continuum simulations of water flow past fullerene molecules. The governing Navier-Stokes equations are complemented with the Navier slip boundary condition with a slip length that is extracted from related molecular dynamics simulations. We find that several quantities of interest as computed by the present model are in good agreement with results from atomistic and atomistic-continuum simulations at a fraction of the computational cost. We simulate the flow past a single fullerene and an array of fullerenes and demonstrate that such nanoscale flows can be computed efficiently by continuum flow solvers, allowing for investigations into spatiotemporal scales inaccessible to atomistic simulations.

We present large-scale parallel direct numerical simulations using particle vortex methods of the instability of the helical vortices. We study the instability of a single helical vortex and find good agreement with inviscid theory. We outline equilibrium configurations for three double helical vortices - similar to those produced by three blade wind turbines. The simulations confirm the stability of the inviscid model, but predict a breakdown of the vortical system due to viscosity.