Fluids

13 pages, 3014 KiB

Open AccessEditor’s ChoiceArticle

On the Effect of the Respiratory Droplet Generation Condition on COVID-19 Transmission

by Ali Hosseinpour Shafaghi, Farzad Rokhsar Talabazar, Ali Koşar and Morteza Ghorbani

Fluids 2020, 5(3), 113; https://doi.org/10.3390/fluids5030113 - 16 Jul 2020

Cited by 31 | Viewed by 6594

Coronavirus (COVID-19) is a highly infectious viral disease and first appeared in Wuhan, China. Within a short time, it has become a global health issue. The sudden emergence of COVID-19 has been accompanied by numerous uncertainties about its impact in many perspectives. One [...] Read more.

Coronavirus (COVID-19) is a highly infectious viral disease and first appeared in Wuhan, China. Within a short time, it has become a global health issue. The sudden emergence of COVID-19 has been accompanied by numerous uncertainties about its impact in many perspectives. One of major challenges is understanding the underlying mechanisms in the spread of this outbreak. COVID-19 is spread similar to the majority of infectious diseases through transmission via relatively large respiratory droplets. The awareness of the dispersal of these droplets is crucial in not only improving methods for controlling the dispersion of COVID-19 droplets, but also in discovering fundamental mechanisms of its transmission. In this study, a numerical model is developed to study the motion of droplets expelled through the respiratory system. Based on the source of these droplets, different sizes of droplets such as large ones and aerosols, which behave differently in the environment, can be generated. In this regard, diverse sources of droplets, namely breathing, coughing, and sneezing, are considered in this analysis. Besides, the time for a single droplet to fall from a height of 1.8 m is also obtained. The results reveal that the traditional distances suggested by different sources for keeping the social distance are not enough, which is linked to different nature of the droplet generation. Full article

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19 pages, 14668 KiB

Open AccessEditor’s ChoiceArticle

Data-Driven Pulsatile Blood Flow Physics with Dynamic Mode Decomposition

by Milad Habibi, Scott T. M. Dawson and Amirhossein Arzani

Fluids 2020, 5(3), 111; https://doi.org/10.3390/fluids5030111 - 14 Jul 2020

Cited by 32 | Viewed by 6443

Abstract

Dynamic mode decomposition (DMD) is a purely data-driven and equation-free technique for reduced-order modeling of dynamical systems and fluid flow. DMD finds a best fit linear reduced-order model that represents any given spatiotemporal data. In DMD, each mode evolves with a fixed frequency [...] Read more.

Dynamic mode decomposition (DMD) is a purely data-driven and equation-free technique for reduced-order modeling of dynamical systems and fluid flow. DMD finds a best fit linear reduced-order model that represents any given spatiotemporal data. In DMD, each mode evolves with a fixed frequency and therefore DMD modes represent physically meaningful structures that are ranked based on their dynamics. The application of DMD to patient-specific cardiovascular flow data is challenging. First, the input flow rate is unsteady and pulsatile. Second, the flow topology can change significantly in different phases of the cardiac cycle. Finally, blood flow in patient-specific diseased arteries is complex and often chaotic. The objective of this study was to overcome these challenges using our proposed multistage dynamic mode decomposition with control (mDMDc) method and use this technique to study patient-specific blood flow physics. The inlet flow rate was considered as the controller input to the systems. Blood flow data were divided into different stages based on the inlet flow waveform and DMD with control was applied to each stage. The system was augmented to consider both velocity and wall shear stress (WSS) vector data, and therefore study the interaction between the coherent structures in velocity and near-wall coherent structures in WSS. First, it was shown that DMD modes can exactly represent the analytical Womersley solution for incompressible pulsatile flow in tubes. Next, our method was applied to image-based coronary artery stenosis and cerebral aneurysm models where complex blood flow patterns are anticipated. The flow patterns were studied using the mDMDc modes and the reconstruction errors were reported. Our augmented mDMDc framework could capture coherent structures in velocity and WSS with a fewer number of modes compared to the traditional DMD approach and demonstrated a close connection between the velocity and WSS modes. Full article

(This article belongs to the Special Issue Classical and Modern Topics in Fluid Dynamics and Transport Phenomena)

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18 pages, 4027 KiB

Open AccessEditor’s ChoiceReview

An Introduction of Droplet Impact Dynamics to Engineering Students

by Sara Moghtadernejad, Christian Lee and Mehdi Jadidi

Fluids 2020, 5(3), 107; https://doi.org/10.3390/fluids5030107 - 2 Jul 2020

Cited by 62 | Viewed by 10850

Abstract

An intensive training course has been developed and implemented at the California State University Long Beach based on 8 years of experience in the multiphase flow area with the specific focus on droplet–solid interactions. Due to the rapid development of droplet-based equipment and [...] Read more.

An intensive training course has been developed and implemented at the California State University Long Beach based on 8 years of experience in the multiphase flow area with the specific focus on droplet–solid interactions. Due to the rapid development of droplet-based equipment and industrial techniques, numerous industries are concerned with understanding the behavior of droplet dynamics and the characteristics that govern them. The presence and ensuing characteristics of the droplet regimes (spreading, receding, rebounding, and splashing) are heavily dependent on droplet and surface conditions. The effect of surface temperature, surface wettability, impact velocity, droplet shape and volume on droplet impact dynamics, and heat transfer are discussed in this training paper. Droplet impacts on moving solid surfaces and the effects of normal and tangential velocities on droplet dynamics are other topics that are discussed here. Despite the vast amount of studies into the dynamics of droplet impact, there is still much more to be investigated as research has expanded into a myriad of different conditions. However, the current paper is intended as a practical training document and a source of basic information, therefore, the scope is kept sufficiently broad to be of interest to readers from different engineering disciplines. Full article

(This article belongs to the Special Issue Teaching and Learning of Fluid Mechanics, Volume II)

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23 pages, 587 KiB

Open AccessEditor’s ChoiceArticle

On the Data-Driven Modeling of Reactive Extrusion

by Ruben Ibañez, Fanny Casteran, Clara Argerich, Chady Ghnatios, Nicolas Hascoet, Amine Ammar, Philippe Cassagnau and Francisco Chinesta

Fluids 2020, 5(2), 94; https://doi.org/10.3390/fluids5020094 - 15 Jun 2020

Cited by 15 | Viewed by 3895

Abstract

This paper analyzes the ability of different machine learning techniques, able to operate in the low-data limit, for constructing the model linking material and process parameters with the properties and performances of parts obtained by reactive polymer extrusion. The use of data-driven approaches [...] Read more.

This paper analyzes the ability of different machine learning techniques, able to operate in the low-data limit, for constructing the model linking material and process parameters with the properties and performances of parts obtained by reactive polymer extrusion. The use of data-driven approaches is justified by the absence of reliable modeling and simulation approaches able to predict induced properties in those complex processes. The experimental part of this work is based on the in situ synthesis of a thermoset (TS) phase during the mixing step with a thermoplastic polypropylene (PP) phase in a twin-screw extruder. Three reactive epoxy/amine systems have been considered and anhydride maleic grafted polypropylene (PP-g-MA) has been used as compatibilizer. The final objective is to define the appropriate processing conditions in terms of improving the mechanical properties of these new PP materials by reactive extrusion. Full article

(This article belongs to the Special Issue Advances in Experimental and Computational Rheology, Volume II)

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15 pages, 4717 KiB

Open AccessEditor’s ChoiceArticle

A Numerical Study of Metachronal Propulsion at Low to Intermediate Reynolds Numbers

by Shawtaroh Granzier-Nakajima, Robert D. Guy and Calvin Zhang-Molina

Fluids 2020, 5(2), 86; https://doi.org/10.3390/fluids5020086 - 31 May 2020

Cited by 18 | Viewed by 3388

Abstract

Inspired by the forward swimming of long-tailed crustaceans, we study an underwater propulsion mechanism for a swimming body with multiple rigid paddles attached underneath undergoing cycles of power and return strokes with a constant phase-difference between neighboring paddles, a phenomenon known as metachronal [...] Read more.

Inspired by the forward swimming of long-tailed crustaceans, we study an underwater propulsion mechanism for a swimming body with multiple rigid paddles attached underneath undergoing cycles of power and return strokes with a constant phase-difference between neighboring paddles, a phenomenon known as metachronal propulsion. To study how inter-paddle phase-difference affects flux production, we develop a computational fluid dynamics model and a numerical algorithm based on the immersed boundary method, which allows us to simulate metachronal propulsion at Reynolds numbers (RE) ranging from close to 0 to about 100. Our main finding is that the highest average flux is generated when nearest-neighbor paddles maintain an approximate 20%–25% phase-difference with the more posterior paddle leading the cycle; this result is independent of stroke frequency across the full range of RE considered here. We also find that the optimal paddle spacing and the number of paddles depend on RE; we see a qualitative transition in the dynamics of flow generated by metachronal propulsion as RE rises above 80. Roughly speaking, in terms of average flux generation, a tight paddle spacing is preferred when RE is less than 10, but a wider spacing becomes clearly favored when RE is close to or above 100. In terms of efficiency of flux generation, at RE 0.1 the maximum efficiency occurs at two paddles, and the efficiency decreases as the number of paddles increases. At RE 100 the efficiency increases as the number of paddles increases, and it appears to saturate by eight paddles, whereas using four paddles is a good tradeoff for both low and intermediate RE. Full article

(This article belongs to the Special Issue Advances in Biological Flows and Biomimetics)

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24 pages, 9530 KiB

Open AccessEditor’s ChoiceArticle

Verification and Validation of openInjMoldSim, an Open-Source Solver to Model the Filling Stage of Thermoplastic Injection Molding

by João Pedro, Bruno Ramôa, João Miguel Nóbrega and Célio Fernandes

Fluids 2020, 5(2), 84; https://doi.org/10.3390/fluids5020084 - 29 May 2020

Cited by 13 | Viewed by 4463

Abstract

In the present study, the simulation of the three-dimensional (3D) non-isothermal, non-Newtonian fluid flow of polymer melts is investigated. In particular, the filling stage of thermoplastic injection molding is numerically studied with a solver implemented in the open-source computational library [...] Read more.

In the present study, the simulation of the three-dimensional (3D) non-isothermal, non-Newtonian fluid flow of polymer melts is investigated. In particular, the filling stage of thermoplastic injection molding is numerically studied with a solver implemented in the open-source computational library

O p e n F O A M^{®}

. The numerical method is based on a compressible two-phase flow model, developed following a cell-centered unstructured finite volume discretization scheme, combined with a volume-of-fluid (VOF) technique for the interface capturing. Additionally, the Cross-WLF (Williams–Landel–Ferry) model is used to characterize the rheological behavior of the polymer melts, and the modified Tait equation is used as the equation of state. To verify the numerical implementation, the code predictions are first compared with analytical solutions, for a Newtonian fluid flowing through a cylindrical channel. Subsequently, the melt filling process of a non-Newtonian fluid (Cross-WLF) in a rectangular cavity with a cylindrical insert and in a tensile test specimen are studied. The predicted melt flow front interface and fields (pressure, velocity, and temperature) contours are found to be in good agreement with the reference solutions, obtained with the proprietary software

M o l d e x 3 D^{®}

. Additionally, the computational effort, measured by the elapsed wall-time of the simulations, is analyzed for both the open-source and proprietary software, and both are found to be similar for the same level of accuracy, when the parallelization capabilities of

O p e n F O A M^{®}

are employed. Full article

(This article belongs to the Special Issue Advances in Experimental and Computational Rheology, Volume II)

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28 pages, 18030 KiB

Open AccessEditor’s ChoiceArticle

Droplet Impact on Suspended Metallic Meshes: Effects of Wettability, Reynolds and Weber Numbers

by Konstantinos Vontas, Cristina Boscariol, Manolia Andredaki, Anastasios Georgoulas, Cyril Crua, Jens Honoré Walther and Marco Marengo

Fluids 2020, 5(2), 81; https://doi.org/10.3390/fluids5020081 - 22 May 2020

Cited by 29 | Viewed by 5366

Abstract

Liquid penetration analysis in porous media is of great importance in a wide range of applications such as ink jet printing technology, painting and textile design. This article presents an investigation of droplet impingement onto metallic meshes, aiming to provide insights by identifying [...] Read more.

Liquid penetration analysis in porous media is of great importance in a wide range of applications such as ink jet printing technology, painting and textile design. This article presents an investigation of droplet impingement onto metallic meshes, aiming to provide insights by identifying and quantifying impact characteristics that are difficult to measure experimentally. For this purpose, an enhanced Volume-Of-Fluid (VOF) numerical simulation framework is utilised, previously developed in the general context of the OpenFOAM CFD Toolbox. Droplet impacts on metallic meshes are performed both experimentally and numerically with satisfactory degree of agreement. From the experimental investigation three main outcomes are observed—deposition, partial imbibition, and penetration. The penetration into suspended meshes leads to spectacular multiple jetting below the mesh. A higher amount of liquid penetration is linked to higher impact velocity, lower viscosity and larger pore size dimension. An estimation of the liquid penetration is given in order to evaluate the impregnation properties of the meshes. From the parametric analysis it is shown that liquid viscosity affects the adhesion characteristics of the drops significantly, whereas droplet break-up after the impact is mostly controlled by surface tension. Additionally, wettability characteristics are found to play an important role in both liquid penetration and droplet break-up below the mesh. Full article

(This article belongs to the Special Issue Classical and Modern Topics in Fluid Dynamics and Transport Phenomena)

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12 pages, 7396 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Rotational Maneuvers of Copepod Nauplii at Low Reynolds Number

by Kacie T. M. Niimoto, Kyleigh J. Kuball, Lauren N. Block, Petra H. Lenz and Daisuke Takagi

Fluids 2020, 5(2), 78; https://doi.org/10.3390/fluids5020078 - 21 May 2020

Cited by 7 | Viewed by 3593

Abstract

Copepods are agile microcrustaceans that are capable of maneuvering freely in water. However, the physical mechanisms driving their rotational motion are not entirely clear in small larvae (nauplii). Here we report high-speed video observations of copepod nauplii performing acrobatic feats with three pairs [...] Read more.

Copepods are agile microcrustaceans that are capable of maneuvering freely in water. However, the physical mechanisms driving their rotational motion are not entirely clear in small larvae (nauplii). Here we report high-speed video observations of copepod nauplii performing acrobatic feats with three pairs of appendages. Our results show rotations about three principal axes of the body: yaw, roll, and pitch. The yaw rotation turns the body to one side and results in a circular swimming path. The roll rotation consists of the body spiraling around a nearly linear path, similar to an aileron roll of an airplane. We interpret the yaw and roll rotations to be facilitated by appendage pronation or supination. The pitch rotation consists of flipping on the spot in a maneuver that resembles a backflip somersault. The pitch rotation involved tail bending and was not observed in the earliest stages of nauplii. The maneuvering strategies adopted by plankton may inspire the design of microscopic robots, equipped with suitable controls for reorienting autonomously in three dimensions. Full article

(This article belongs to the Special Issue Fluid Mechanics of Plankton)

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16 pages, 1027 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Modeling Acoustic Cavitation Using a Pressure-Based Algorithm for Polytropic Fluids

by Fabian Denner, Fabien Evrard and Berend van Wachem

Fluids 2020, 5(2), 69; https://doi.org/10.3390/fluids5020069 - 13 May 2020

Cited by 18 | Viewed by 3492

Abstract

A fully coupled pressure-based algorithm and finite-volume framework for the simulation of the acoustic cavitation of bubbles in polytropic gas-liquid systems is proposed. The algorithm is based on a conservative finite-volume discretization with collocated variable arrangement, in which the discretized governing equations are [...] Read more.

A fully coupled pressure-based algorithm and finite-volume framework for the simulation of the acoustic cavitation of bubbles in polytropic gas-liquid systems is proposed. The algorithm is based on a conservative finite-volume discretization with collocated variable arrangement, in which the discretized governing equations are solved in a single linear system of equations for pressure and velocity. Density is described by the polytropic Noble-Abel stiffened-gas model and the interface between the interacting bulk phases is captured by a state-of-the-art algebraic Volume-of-Fluid (VOF) method. The new numerical algorithm is validated using representative test-cases of the interaction of acoustic waves with the gas-liquid interface as well as pressure-driven bubble dynamics in infinite and confined domains, showing excellent agreement of the results obtained with the proposed algorithm compared to linear acoustic theory, the Gilmore model and high-fidelity experiments. Full article

(This article belongs to the Special Issue Modelling of Reactive and Non-reactive Multiphase Flows)

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28 pages, 4940 KiB

Open AccessEditor’s ChoiceReview

Kinematic and Dynamic Scaling of Copepod Swimming

by Leonid Svetlichny, Poul S. Larsen and Thomas Kiørboe

Fluids 2020, 5(2), 68; https://doi.org/10.3390/fluids5020068 - 11 May 2020

Cited by 23 | Viewed by 4084

Abstract

Calanoid copepods have two swimming gaits, namely cruise swimming that is propelled by the beating of the cephalic feeding appendages and short-lasting jumps that are propelled by the power strokes of the four or five pairs of thoracal swimming legs. The latter may [...] Read more.

Calanoid copepods have two swimming gaits, namely cruise swimming that is propelled by the beating of the cephalic feeding appendages and short-lasting jumps that are propelled by the power strokes of the four or five pairs of thoracal swimming legs. The latter may be 100 times faster than the former, and the required forces and power production are consequently much larger. Here, we estimated the magnitude and size scaling of swimming speed, leg beat frequency, forces, power requirements, and energetics of these two propulsion modes. We used data from the literature together with new data to estimate forces by two different approaches in 37 species of calanoid copepods: the direct measurement of forces produced by copepods attached to a tensiometer and the indirect estimation of forces from swimming speed or acceleration in combination with experimentally estimated drag coefficients. Depending on the approach, we found that the propulsive forces, both for cruise swimming and escape jumps, scaled with prosome length (L) to a power between 2 and 3. We further found that power requirements scales for both type of swimming as L³. Finally, we found that the cost of transportation (i.e., calories per unit body mass and distance transported) was higher for swimming-by-jumping than for cruise swimming by a factor of 7 for large copepods but only a factor of 3 for small ones. This may explain why only small cyclopoid copepods can afford this hydrodynamically stealthy transportation mode as their routine, while large copepods are cruise swimmers. Full article

(This article belongs to the Special Issue Fluid Mechanics of Plankton)

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21 pages, 3233 KiB

Open AccessEditor’s ChoiceArticle

Soliton Turbulence in Approximate and Exact Models for Deep Water Waves

by Dmitry Kachulin, Alexander Dyachenko and Vladimir Zakharov

Fluids 2020, 5(2), 67; https://doi.org/10.3390/fluids5020067 - 10 May 2020

Cited by 12 | Viewed by 3138

Abstract

We investigate and compare soliton turbulence appearing as a result of modulational instability of the homogeneous wave train in three nonlinear models for surface gravity waves: the nonlinear Schrödinger equation, the super compact Zakharov equation, and the fully nonlinear equations written in conformal [...] Read more.

We investigate and compare soliton turbulence appearing as a result of modulational instability of the homogeneous wave train in three nonlinear models for surface gravity waves: the nonlinear Schrödinger equation, the super compact Zakharov equation, and the fully nonlinear equations written in conformal variables. We show that even at a low level of energy and average wave steepness, the wave dynamics in the nonlinear Schrödinger equation fundamentally differ from the dynamics in more accurate models. We study energy losses of wind waves due to their breaking for large values of total energy in the super compact Zakharov equation and in the exact equations and show that in both models, the wave system loses 50% of energy very slowly, during few days. Full article

(This article belongs to the Collection Geophysical Fluid Dynamics)

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10 pages, 448 KiB

Open AccessEditor’s ChoiceArticle

Multiple Soliton Interactions on the Surface of Deep Water

by Dmitry Kachulin, Alexander Dyachenko and Sergey Dremov

Fluids 2020, 5(2), 65; https://doi.org/10.3390/fluids5020065 - 28 Apr 2020

Cited by 11 | Viewed by 2449

Abstract

The paper presents the long-time dynamics with multiple collisions of breathers in the super compact Zakharov equation for unidirectional deep water waves. Solutions in the form of breathers were found numerically by the Petviashvili method. In the terms of envelope and the assumption [...] Read more.

The paper presents the long-time dynamics with multiple collisions of breathers in the super compact Zakharov equation for unidirectional deep water waves. Solutions in the form of breathers were found numerically by the Petviashvili method. In the terms of envelope and the assumption of the narrow spectral width the super compact equation turns into the well known exact integrable model—nonlinear Schrödinger equation, and the breather solution in this case turns into envelope soliton. The results of numerical simulations show that two main scenarios of long-time dynamics occur during numerous collisions of breathers. In the first case, one of the breathers regularly takes a number of particles from the other one at each collision and in the second one a structure resembling the bi-soliton solution of nonlinear Schrödinger equation arises during the collision. Despite these scenarios, it is shown that after numerous collisions the only one breather having initially a larger number of particles remains. Full article

(This article belongs to the Collection Geophysical Fluid Dynamics)

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22 pages, 602 KiB

Open AccessEditor’s ChoiceArticle

An Experimental Study on Human Milk Rheology: Behavior Changes from External Factors

by Diana Alatalo and Fatemeh Hassanipour

Fluids 2020, 5(2), 42; https://doi.org/10.3390/fluids5020042 - 27 Mar 2020

Cited by 14 | Viewed by 5712

Abstract

The influence of external factors, including temperature, storage, aging, time, and shear rate, on the general rheological behavior of raw human milk is investigated. Rotational and oscillatory experiments were performed. Human milk showed non-Newtonian, shear-thinning, thixotropic behavior with both yield and flow stresses. [...] Read more.

The influence of external factors, including temperature, storage, aging, time, and shear rate, on the general rheological behavior of raw human milk is investigated. Rotational and oscillatory experiments were performed. Human milk showed non-Newtonian, shear-thinning, thixotropic behavior with both yield and flow stresses. Storage and aging increased milk density and decreased viscosity. In general, increases in temperature lowered density and viscosity with periods of inconsistent behavior noted between 6–16

^{\circ}

C and over 40

^{\circ}

C. Non-homogeneous breakdown between the yield and flow stresses was found which, when coupled with thixotropy, helps identify the source of nutrient losses during tube feeding. Full article

(This article belongs to the Special Issue Advances in Experimental and Computational Rheology, Volume II)

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20 pages, 3102 KiB

Open AccessEditor’s ChoiceArticle

Influence of Oxidation Degree of Graphene Oxide on the Shear Rheology of Poly(ethylene glycol) Suspensions

by Yago Chamoun F. Soares, Elyff Cargnin, Mônica Feijó Naccache and Ricardo Jorge E. Andrade

Fluids 2020, 5(2), 41; https://doi.org/10.3390/fluids5020041 - 26 Mar 2020

Cited by 15 | Viewed by 4122

Abstract

This work studies the influence of the concentration and oxidation degree on the rheological behavior of graphene oxide (GO) nanosheets dispersed on polyethylene glycol (PEG). The rheological characterization was fulfilled in shear flow through rotational rheometry measurements, in steady, transient and oscillatory regimes. [...] Read more.

This work studies the influence of the concentration and oxidation degree on the rheological behavior of graphene oxide (GO) nanosheets dispersed on polyethylene glycol (PEG). The rheological characterization was fulfilled in shear flow through rotational rheometry measurements, in steady, transient and oscillatory regimes. Graphene oxide was prepared by chemical exfoliation of graphite using the modified Hummers method. The morphological and structural characteristics originating from the synthesis were analyzed by X-ray diffraction, Raman spectroscopy, thermogravimetric analysis, Fourier transform infrared spectroscopy, and atomic force microscopy. It is shown that higher oxidation times increase the functional groups, which leads to a higher dispersion and exfoliation of GO sheets in the PEG. Moreover, the addition of GO in a PEG solution results in significant growth of the suspension viscosity, and a change of the fluid behavior from Newtonian to pseudoplastic. This effect is related to the concentration and oxidation level of the obtained GO particles. The results obtained aim to contribute towards the understanding of the interactions between the GO and the polymeric liquid matrix, and their influence on the suspension rheological behavior. Full article

(This article belongs to the Special Issue Advances in Experimental and Computational Rheology, Volume II)

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32 pages, 3066 KiB

Open AccessFeature PaperEditor’s ChoiceReview

A Review of Topology Optimisation for Fluid-Based Problems

by Joe Alexandersen and Casper Schousboe Andreasen

Fluids 2020, 5(1), 29; https://doi.org/10.3390/fluids5010029 - 4 Mar 2020

Cited by 170 | Viewed by 16943

Abstract

This review paper provides an overview of the literature for topology optimisation of fluid-based problems, starting with the seminal works on the subject and ending with a snapshot of the state of the art of this rapidly developing field. “Fluid-based problems” are defined [...] Read more.

This review paper provides an overview of the literature for topology optimisation of fluid-based problems, starting with the seminal works on the subject and ending with a snapshot of the state of the art of this rapidly developing field. “Fluid-based problems” are defined as problems where at least one governing equation for fluid flow is solved and the fluid–solid interface is optimised. In addition to fluid flow, any number of additional physics can be solved, such as species transport, heat transfer and mechanics. The review covers 186 papers from 2003 up to and including January 2020, which are sorted into five main groups: pure fluid flow; species transport; conjugate heat transfer; fluid–structure interaction; microstructure and porous media. Each paper is very briefly introduced in chronological order of publication. A quantititive analysis is presented with statistics covering the development of the field and presenting the distribution over subgroups. Recommendations for focus areas of future research are made based on the extensive literature review, the quantitative analysis, as well as the authors’ personal experience and opinions. Since the vast majority of papers treat steady-state laminar pure fluid flow, with no recent major advancements, it is recommended that future research focuses on more complex problems, e.g., transient and turbulent flow. Full article

(This article belongs to the Special Issue Flow-Based Optimization of Products or Devices)

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25 pages, 7416 KiB

Open AccessEditor’s ChoiceArticle

Breaking the Kolmogorov Barrier in Model Reduction of Fluid Flows

by Shady E. Ahmed and Omer San

Fluids 2020, 5(1), 26; https://doi.org/10.3390/fluids5010026 - 18 Feb 2020

Cited by 15 | Viewed by 4195

Abstract

Turbulence modeling has been always a challenge, given the degree of underlying spatial and temporal complexity. In this paper, we propose the use of a partitioned reduced order modeling (ROM) approach for efficient and effective approximation of turbulent flows. A piecewise linear subspace [...] Read more.

Turbulence modeling has been always a challenge, given the degree of underlying spatial and temporal complexity. In this paper, we propose the use of a partitioned reduced order modeling (ROM) approach for efficient and effective approximation of turbulent flows. A piecewise linear subspace is tailored to capture the fine flow details in addition to the larger scales. We test the partitioned ROM for a decaying two-dimensional (2D) turbulent flow, known as 2D Kraichnan turbulence. The flow is initiated using an array of random vortices, corresponding to an arbitrary energy spectrum. We show that partitioning produces more accurate and stable results than standard ROM based on a global application of modal decomposition techniques. We also demonstrate the predictive capability of partitioned ROM through an energy spectrum analysis, where the recovered energy spectrum significantly converges to the full order model’s statistics with increased partitioning. Although the proposed approach incurs increased memory requirements to store the local basis functions for each partition, we emphasize that it permits the construction of more compact ROMs (i.e., of smaller dimension) with comparable accuracy, which in turn significantly reduces the online computational burden. Therefore, we consider that partitioning acts as a converter which reduces the cost of online deployment at the expense of offline and memory costs. Finally, we investigate the application of closure modeling to account for the effects of modal truncation on ROM dynamics. We illustrate that closure techniques can help to stabilize the results in the inertial range, but over-stabilization might take place in the dissipative range. Full article

(This article belongs to the Special Issue Recent Numerical Advances in Fluid Mechanics)

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10 pages, 887 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Metachronal Swimming with Rigid Arms near Boundaries

by Rintaro Hayashi and Daisuke Takagi

Fluids 2020, 5(1), 24; https://doi.org/10.3390/fluids5010024 - 14 Feb 2020

Cited by 17 | Viewed by 2828

Abstract

Various organisms such as crustaceans use their appendages for locomotion. If they are close to a confining boundary then viscous as opposed to inertial effects can play a central role in governing the dynamics. To study the minimal ingredients needed for swimming without [...] Read more.

Various organisms such as crustaceans use their appendages for locomotion. If they are close to a confining boundary then viscous as opposed to inertial effects can play a central role in governing the dynamics. To study the minimal ingredients needed for swimming without inertia, we built an experimental system featuring a robot equipped with a pair of rigid slender arms with negligible inertia. Our results show that directing the arms to oscillate about the same time-averaged orientation produces no net displacement of the robot each cycle, regardless of any phase delay between the oscillating arms. The robot is able to swim if the arms oscillate asynchronously around distinct orientations. The measured displacement over time matches well with a mathematical model based on slender-body theory for Stokes flow. Near a confining boundary, the robot with no net displacement every cycle showed similar behavior, while the swimming robot increased in speed closer to the boundary. Full article

(This article belongs to the Special Issue Advances in Biological Flows and Biomimetics)

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18 pages, 3553 KiB

Open AccessEditor’s ChoiceArticle

Fluid Dynamics of Ballistic Strategies in Nematocyst Firing

by Christina Hamlet, Wanda Strychalski and Laura Miller

Fluids 2020, 5(1), 20; https://doi.org/10.3390/fluids5010020 - 8 Feb 2020

Cited by 8 | Viewed by 9855

Abstract

Nematocysts are stinging organelles used by members of the phylum Cnidaria (e.g., jellyfish, anemones, hydrozoans) for a variety of important functions including capturing prey and defense. Nematocysts are the fastest-known accelerating structures in the animal world. The small scale (microns) coupled with rapid [...] Read more.

Nematocysts are stinging organelles used by members of the phylum Cnidaria (e.g., jellyfish, anemones, hydrozoans) for a variety of important functions including capturing prey and defense. Nematocysts are the fastest-known accelerating structures in the animal world. The small scale (microns) coupled with rapid acceleration (in excess of 5 million g) present significant challenges in imaging that prevent detailed descriptions of their kinematics. The immersed boundary method was used to numerically simulate the dynamics of a barb-like structure accelerating a short distance across Reynolds numbers ranging from 0.9–900 towards a passive elastic target in two dimensions. Results indicate that acceleration followed by coasting at lower Reynolds numbers is not sufficient for a nematocyst to reach its target. The nematocyst’s barb-like projectile requires high accelerations in order to transition to the inertial regime and overcome the viscous damping effects normally encountered at small cellular scales. The longer the barb is in the inertial regime, the higher the final velocity of the projectile when it touches its target. We find the size of the target prey does not dramatically affect the barb’s approach for large enough values of the Reynolds number, however longer barbs are able to accelerate a larger amount of surrounding fluid, which in turn allows the barb to remain in the inertial regime for a longer period of time. Since the final velocity is proportional to the force available for piercing the membrane of the prey, high accelerations that allow the system to persist in the inertial regime have implications for the nematocyst’s ability to puncture surfaces such as cellular membranes or even crustacean cuticle. Full article

(This article belongs to the Special Issue Advances in Biological Flows and Biomimetics)

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15 pages, 5241 KiB

Open AccessEditor’s ChoiceArticle

Stability of Soft Magnetic Helical Microrobots

by Kiarash Samsami, Seyed Amir Mirbagheri, Farshad Meshkati and Henry Chien Fu

Fluids 2020, 5(1), 19; https://doi.org/10.3390/fluids5010019 - 5 Feb 2020

Cited by 11 | Viewed by 3537

Abstract

Nano/microrobotic swimmers have many possible biomedical applications such as drug delivery and micro-manipulation. This paper examines one of the most promising classes of these: rigid magnetic microrobots that are propelled through bulk fluid by rotation induced by a rotating magnetic field. Propulsion corresponds [...] Read more.

Nano/microrobotic swimmers have many possible biomedical applications such as drug delivery and micro-manipulation. This paper examines one of the most promising classes of these: rigid magnetic microrobots that are propelled through bulk fluid by rotation induced by a rotating magnetic field. Propulsion corresponds to steadily rotating and translating solutions of the dynamics of such microrobots that co-rotate with the magnetic field. To be observed in experiments and be amenable to steering control, such solutions must also be stable to perturbations. In this paper, we analytically derive a criterion for the stability of such steadily rotating solutions for a microrobot made of soft magnetic materials, which have a magnetization that depends on the applied field. This result generalizes previous stability criteria we obtained for microrobots with a permanent magnetization. Full article

(This article belongs to the Special Issue Advances in Biological Flows and Biomimetics)

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19 pages, 2635 KiB

Open AccessEditor’s ChoiceArticle

On the Deterministic Prediction of Water Waves

by Marco Klein, Matthias Dudek, Günther F. Clauss, Sören Ehlers, Jasper Behrendt, Norbert Hoffmann and Miguel Onorato

Fluids 2020, 5(1), 9; https://doi.org/10.3390/fluids5010009 - 7 Jan 2020

Cited by 44 | Viewed by 5289

Abstract

This paper discusses the potential of deterministic wave prediction as one basic module for decision support of offshore operations. Therefore, methods of different complexity—the linear wave solution, the non-linear Schrödinger equation (NLSE) of two different orders and the high-order spectral method (HOSM)—are presented [...] Read more.

This paper discusses the potential of deterministic wave prediction as one basic module for decision support of offshore operations. Therefore, methods of different complexity—the linear wave solution, the non-linear Schrödinger equation (NLSE) of two different orders and the high-order spectral method (HOSM)—are presented in terms of applicability and limitations of use. For this purpose, irregular sea states with varying parameters are addressed by numerical simulations as well as model tests in the controlled environment of a seakeeping basin. The irregular sea state investigations focuses on JONSWAP spectra with varying wave steepness and enhancement factor. In addition, the influence of the propagation distance as well as the forecast horizon is discussed. For the evaluation of the accuracy of the prediction, the surface similarity parameter is used, allowing an exact, quantitative validation of the results. Based on the results, the pros and cons of the different deterministic wave prediction methods are discussed. In conclusion, this paper shows that the classical NLSE is not applicable for deterministic wave prediction of arbitrary irregular sea states compared to the linear solution. However, the application of the exact linear dispersion operator within the linear dispersive part of the NLSE increased the accuracy of the prediction for small wave steepness significantly. In addition, it is shown that non-linear deterministic wave prediction based on second-order NLSE as well as HOSM leads to a substantial improvement of the prediction quality for moderate and steep irregular wave trains in terms of individual waves and prediction distance, with the HOSM providing a high accuracy over a wider range of applications. Full article

(This article belongs to the Special Issue Nonlinear Wave Hydrodynamics)

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19 pages, 9501 KiB

Open AccessFeature PaperEditor’s ChoiceReview

Fast Models of Hydrocarbon Migration Paths and Pressure Depletion Based on Complex Analysis Methods (CAM): Mini-Review and Verification

by Ruud Weijermars, Aadi Khanal and Lihua Zuo

Fluids 2020, 5(1), 7; https://doi.org/10.3390/fluids5010007 - 5 Jan 2020

Cited by 13 | Viewed by 3586

Abstract

A recently developed code to model hydrocarbon migration and convective time of flight makes use of complex analysis methods (CAM) paired with Eulerian particle tracking. Because the method uses new algorithms that are uniquely developed by our research group, validation of the fast [...] Read more.

A recently developed code to model hydrocarbon migration and convective time of flight makes use of complex analysis methods (CAM) paired with Eulerian particle tracking. Because the method uses new algorithms that are uniquely developed by our research group, validation of the fast CAM solutions with independent methods is merited. Particle path solutions were compared with independent solutions methods (Eclipse). These prior and new benchmarks are briefly summarized here to further verify the results obtained with CAM codes. Pressure field solutions based on CAM are compared with independent embedded discrete fracture method (EDFM) solutions. The CAM method is particularly attractive because its grid-less nature offers fast computation times and unlimited resolution. The method is particularly well suited for solving a variety of practical field development problems. Examples are given for fast optimization of waterflood patterns. Another successful application area is the modeling of fluid withdrawal patterns in hydraulically fractured wells. Because no gridding is required, the CAM model can compute the evolution of the drained rock volume (DRV) for an unlimited (but finite) number of both hydraulic and natural fractures. Such computations of the DRV are based on the convective time of flight and show the fluid withdrawal zone in the reservoir. In contrast, pressure depletion models are based on the diffusive time of flight. In ultra-low permeability reservoirs, the pressure depletion zones do not correspond to the DRV, because the convective and diffusive displacement rates differ over an order of magnitude (diffusive time of flight being the fastest). Therefore, pressure depletion models vastly overestimate the drained volume in shale reservoirs, which is why fracture and well spacing decisions should be based on both pressure depletion and DRV models, not pressure only. Full article

(This article belongs to the Special Issue Recent Advances in Fluid Mechanics: Feature Papers)

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