Fluid Dynamic Phenomena Research Articles

A combined morphological and electrochemical characterization of carbon electrodes in vanadium redox flow batteries: Insights into positive and negative electrode performance

Reduced power density is one of the main critical issues of vanadium redox flow batteries and it is mainly due to critical electrolyte distribution over the porous electrodes and sluggish kinetics. Therefore, it is crucial to elucidate the interplay between morphological and electrochemical features of carbon electrodes and their influence on battery performance. In this work, three considerably different carbon electrodes were characterized in order to highlight the most relevant electrode properties governing system operation, distinguishing between positive and negative electrode performance, that are differently affected by kinetic and mass transport losses, especially at local level. Scanning electron microscopy and Raman spectroscopy provided insights on the electrodes structure, while mercury intrusion porosimetry and thermogravimetric analyses permitted to evaluate porous volume, surface area, skeletal density and pores diameter. Local electrochemical characterization was performed by means of through-plate hydrogen reference electrodes: innovative in-situ cyclic voltammetry allowed to decouple electrode operation from fluid-dynamic phenomena, whose influence on battery performance was evaluated with polarization curves at different flow rates. The best electrode design is the result of a proper combination between high surface area and pores diameter, that on one hand has to be sufficiently large to promote electrolyte permeation from distribution channel, but on the other hand has to be small to reduce mass transport losses at pore scale. The negative electrode performance is strongly affected by kinetic losses, while the operation of positive electrode is mainly influenced by mass transport phenomena.

Modeling Fluid dynamics and Aerodynamics by Second Law and Bejan Number (Part 1 - Theory)

Two fundamental questions are still open about the complex relation between fluid dynamics and thermodynamics. Is it possible (and convenient) to describe fluid dynamic in terms of second law based thermodynamic equations? Is it possible to solve and manage fluid dynamics problems by mean of second law of thermodynamics? This chapter analyses the problem of the relationships between the laws of fluid dynamics and thermodynamics in both first and second law of thermodynamics in the light of constructal law. In particular, taking into account constructal law and the diffusive formulation of Bejan number, it defines a preliminary step through an extensive thermodynamic vision of fluid dynamic phenomena.

Solitary-wave and new exact solutions for an extended (3+1)-dimensional Jimbo–Miwa-like equation

This paper is focusing in studying an extended (3+1)-dimensional Jimbo–Miwa-like equation. By the Hirota’s method, we construct the solitary-wave solutions and new exact solutions, and discuss the dynamical behavior of those solutions with graphical illustration. The results can be expected to model some phenomena in fluid dynamics.

Characteristics of the breather waves, rogue waves and solitary waves in an extended (3 + 1)-dimensional Kadomtsev–Petviashvili equation

PurposeThe purpose of this paper is to study the breather waves, rogue waves and solitary waves of an extended (3 + 1)-dimensional Kadomtsev–Petviashvili (KP) equation, which can be used to depict many nonlinear phenomena in fluid dynamics and plasma physics.Design/methodology/approachThe authors apply the Bell’s polynomial approach, the homoclinic test technique and Hirota’s bilinear method to find the breather waves, rogue waves and solitary waves of the extended (3 + 1)-dimensional KP equation.FindingsThe results imply that the extended (3 + 1)-dimensional KP equation has breather wave, rogue wave and solitary wave solutions. Meanwhile, the authors provide the graphical analysis of such solutions to better understand their dynamical behavior.Originality/valueThese results may help us to further study the local structure and the interaction of solutions in KP-type equations. The authors hope that the results provided in this work can help enrich the dynamic behavior of such equations.

Thermographic visualization of a flow instability in an electromagnetically driven electrolyte layer

Visualization of complex flows is one of the most challenging problems in fluid dynamics. In this paper, we analyze experimentally the free surface swirling flow of a shallow layer of a weak electrolyte driven electromagnetically in a cylindrical container using a thermographic camera. The flow is produced by a force created by the interaction of a radial electric current and the vertical magnetic field of a permanent magnet. For given values of the applied current and fluid layer thickness, the azimuthal flow develops an instability that leads to the formation of long-lived, traveling anticyclonic vortices close to the walls of the container. Under isothermal conditions, a colorant is used to visualize the onset of the flow instability. Thermographic images provide evidence that the instability is also present under non-isothermal conditions and reveal the thermal effect of the vortices in the global flow. Our results encourage the use of non-intrusive electromagnetic and thermographic methods to explore complex phenomena in fluid dynamics and heat transfer.

Effect of Gravity Expressed as a Fluid Dynamic Phenomenon on Space Curve

Effect of Gravity Expressed as a Fluid Dynamic Phenomenon on Space Curve

Multi-exponential wave solutions to two extended Jimbo–Miwa equations and the resonance behavior

Resonance phenomena occur widely in fluid, physics and other fields, e.g., they are related with the optical elements, the well-balanced scheme for shallow water with discontinuous topography, and some phenomena in chaotic dynamics and fluid dynamics. Application of the principle of linear superposition to the Hirota bilinear equation gives rise to a sufficient and necessary condition for the existence of multi-exponential wave solutions. We study the resonance behavior based on the construction of multi-exponential wave solution to two extended (3 + 1)- dimensional Jimbo–Miwa equations. The resonance characteristics are analyzed and simulated for some resonant two-wave and three-wave solutions.

Two new integrable modified KdV equations, of third-and fifth-order, with variable coefficients: multiple real and multiple complex soliton solutions

ABSTRACT In this work, we develop two new integrable modified KdV equations, of third and fifth orders, with time-dependent coefficients, which can be used to describe many nonlinear phenomena in fluid dynamics and solitary waves theory. We systematically investigate the complete integrability of each equation by exhibiting the Painlevé test. With the aid of new complex forms of the simplified Hirota's method, we show that each equation admits multiple real and multiple complex soliton solutions. The influence of the new terms with variable coefficients on solitonic structures and interaction properties are investigated. The two developed equations are likely to be of applicative relevance, because it may be considered an application of a large class of nonlinear equations with variable coefficients.

New analytical study of water waves described by coupled fractional variant Boussinesq equation in fluid dynamics

The main objective of this paper is to introduce an analytical study for the water wave solutions of coupled fractional variant Boussinesq equation, which is modelled to investigate the waves in fluid dynamics. Wave transformation in fractional form is applied to convert the original fractional-order nonlinear partial differential equation into another nonlinear ordinary differential equation. The strategy here is to use the unified method to obtain a variety of exact solutions. The unified method works well and reveals distinct exact solutions which are classified into two different types, namely polynomial function and rational function solutions. The results are also depicted graphically for different values of fractional parameter. These findings are highly encouraging and have significant importance for some special physical phenomena in fluid dynamics

A Three-Zone Scavenging Model for Large Two-Stroke Uniflow Marine Engines Using Results from CFD Scavenging Simulations

The introduction of modern aftertreatment systems in marine diesel engines call for accurate prediction of exhaust gas temperature, since it significantly affects the performance of the aftertreatment system. The scavenging process establishes the initial conditions for combustion, directly affecting exhaust gas temperature, fuel economy, and emissions. In this paper, a semi-empirical zero-dimensional three zone scavenging model applicable to two-stroke uniflow scavenged diesel engines is updated using the results of CFD (computational fluid dynamics) simulations. In this 0-D model, the engine cylinders are divided in three zones (thermodynamic control volumes) namely, the pure air zone, mixing zone, and pure exhaust gas zone. The entrainment of air and exhaust gas in the mixing zone is specified by time varying mixing coefficients. The mixing coefficients were updated using results from CFD simulations based on the geometry of a modern 50 cm bore large two-stroke marine diesel engine. This increased the model’s accuracy by taking into account 2-D fluid dynamics phenomena in the cylinder ports and exhaust valve. Thus, the effect of engine load, inlet port swirl angle and partial covering of inlet ports on engine scavenging were investigated. The three-zone model was then updated and the findings of CFD simulations were reflected accordingly in the updated mixing coefficients of the scavenging model.

Investigation of Combustion Mode Control in a Mach 8 Shape-Transitioning Scramjet

Accelerating scramjet engines for an access to space system should optimally anchor dual-mode combustion at one end of the trajectory and supersonic combustion at the other. Conventionally, dual-mode combustion engines flamehold with physical obstructions, causing prohibitive losses at high speed. Past experiments of an airframe-integrated scramjet engine at Mach 8 showed that dual-mode combustion was initiated and sustained with an unobstructed flowpath by varying the fueling rate. This flowfield is examined numerically to ascertain the fluid dynamic phenomena leading to the separated combustion modes. Two regimes of dual-mode combustion occurred above stoichiometric fueling: a separation-train-dominated flowfield downstream of the injectors and a flowfield dominated by a large separation upstream of the body-side injectors at higher fueling rates. The first mode occurs when two shocks combine and create a separation on the body-side wall that merges with the body-side injector wake separation, causing a train of separations. The second regime is from the upstream movement of combustion with increasing equivalence ratio: when combustion reaches the rear of the body-side injector, the near injector flow structure changes, allowing fuel and combustion to enter a large separation upstream. The performance of the different modes is explored.

Influence of Turbulence and Thermophysical Fluid Properties on Cavitation Erosion Predictions in Channel Flow Geometries

<div class="section abstract"><div class="htmlview paragraph">Cavitation and cavitation-induced erosion have been observed in fuel injectors in regions of high acceleration and low pressure. Although these phenomena can have a large influence on the performance and lifetime of injector hardware, questions still remain on how these physics should be accurately and efficiently represented within a computational fluid dynamics model. While several studies have focused on the validation of cavitation predictions within canonical and realistic injector geometries, it is not well documented what influence the numerical and physical parameters selected to represent turbulence and phase change will have on the predictions for cavitation erosion propensity and severity.</div><div class="htmlview paragraph">In this work, a range of numerical and physical parameters are evaluated within the mixture modeling approach in CONVERGE to understand their influence on predictions of cavitation, condensation and erosion. Particular attention is paid to grid resolution, turbulence model and near-wall treatment, fuel surrogate properties, and non-condensable gas content. Assessment of cavitation predictions are conducted through comparison of measured and predicted mass flow rates and cavitation probability distributions for flow through a channel with a sharp inlet. Predictions for hydrodynamic impact loading and cavitation erosion are compared with the experimentally measured incubation period and critical site for erosion. Based on these findings, recommendations are provided for modeling turbulent cavitating flows, using the single fluid mixture modeling approach, to improve predictions for cavitation-induced erosion. In particular, to capture the fluid dynamic phenomena characterizing cavitation cloud formation, development and shedding, a Large Eddy simulation with grid resolution as fine as 2.50 μm is recommended. The assumed concentration of non-condensable gas content is observed to have a strong influence on the predicted cavitation erosion severity, which motivates the need for dissolved gas concentration measurements for future cavitation erosion experimental studies. Using the best practices established in this work, good agreement is achieved between the measured and predicted cavitation parameters, as well as the critical site for cavitation-induced erosion.</div></div>

Abound rogue wave type solutions to the extended (3+1)-dimensional Jimbo–Miwa equation

Under investigation in this article is an extended (3+1)-dimensional Jimbo–Miwa equation (JM), which can be used to describe many nonlinear phenomena in fluid dynamics. By using the Hirota bilinear form of the extended (3+1)-dimensional JM equation, thirty classes of rogue wave type solutions are found with the help of symbolic computations. The rogue wave type solutions contain two important parameters a and b. When a and b get different values, we can present obvious rogue wave type solutions. For example, (i) taking a=b=0, then we have algebraic solitary waves (lump); (ii) if one of a and b is fixed at 0 and the other is nonzero, then we have an interaction solution between an algebraically decayed soliton and exponentially decayed soliton (lumpoff); (iii) let a≠0 and b≠0, then we have an interaction solution between lump and exponentially localized twin soliton wave (instant or rogue wave). The new rogue wave type solutions help us to know different physical worlds.

Relating the Lyapunov exponents to transport coefficients in kinetic theory

In this contribution, we report our recent findings on the phenomenological applications of non-equilibrium attractors to transport phenomena in fluid dynamics. Within the kinetic theory description, we study the non-linear hydrodynamization processes of a relativistic fluid undergoing Bjorken flow. The mathematical problem of solving the Boltzmann equation with a time-dependent relaxation time is recast into an infinite set of nonlinear ordinary differential equations for the moments of the one-particle distribution function. The constitutive relations of each non-hydrodynamic mode can be written as a multi-parameter trans-series that encodes the non-perturbative dissipative contributions quantified by the Knudsen Kn and inverse Reynolds Re−1 numbers. At a given order in the gradient expansion, we show that summing over all the non-perturbative sectors leads to a renormalized transport coefficient. The universal behavior of the renormalized shear viscosity is determined by the Lyapunov exponent and the anomalous dimension of the first moment at the stable fixed point. We comment on the relation between our findings and the physics of non-Newtonian fluids.

Force and flowfield measurements to understand unsteady aerodynamics of cycloidal rotors in hover at ultra-low Reynolds numbers

This paper provides a fundamental understanding of the unsteady fluid-dynamic phenomena on a cycloidal rotor blade operating at ultra-low Reynolds numbers (Re ∼ 18,000) by utilizing a combination of instantaneous blade force and flowfield measurements. The dynamic blade force coefficients were almost double the static ones, indicating the role of dynamic stall. For the dynamic case, the blade lift monotonically increased up to ±45° pitch amplitude; however, for the static case, the flow separated from the leading edge after around 15° with a large laminar separation bubble. There was significant asymmetry in the lift and drag coefficients between the upper and lower halves of the trajectory due to the flow curvature effects (virtual camber). The particle image velocimetry measured flowfield showed the dynamic stall process during the upper half to be significantly different from the lower half because of the reversal of dynamic virtual camber. Even at such low Reynolds numbers, the pressure forces, as opposed to viscous forces, were found to be dominant on the cyclorotor blade. The power required for rotation (rather than pitching power) dominated the total blade power.

Low-intensity ultrasound induced cavitation and streaming in oxygen-supersaturated water: Role of cavitation bubbles as physical cleaning agents

A number of acoustic and fluid-dynamic phenomena appear in ultrasonic cleaning baths and contribute to physical cleaning of immersed surfaces. Propagation and repeated reflection of ultrasound within cleaning baths build standing-wave-like acoustic fields; when an ultrasound intensity gradient appears in the acoustic fields, it can in principle induce steady streaming flow. When the ultrasound intensity is sufficiently large, cavitation occurs and oscillating cavitation bubbles are either trapped in the acoustic fields or advected in the flow. These phenomena are believed to produce mechanical action to remove contaminant particles attached at material surfaces. Recent studies suggest that the mechanical action of cavitation bubbles is the dominant factor of particle removal in ultrasonic cleaning, but the bubble collapse resulting from high-intensity ultrasound may be violent enough to give rise to surface erosion. In this paper, we aim to carefully examine the role of cavitation bubbles from ultrasonic cleaning tests with varying dissolved gas concentration in water. In our cleaning tests using 28-kHz ultrasound, oxygen-supersaturated water is produced by oxygen-microbubble aeration and used as a cleaning solution, and glass slides spin-coated with silica particles of micron/submicron sizes are used to define cleaning efficiency. High-speed camera recordings and Particle Image Velocimetry analysis with a pressure oscillation amplitude of 1.4 atm at the pressure antinode show that the population of cavitation bubbles increases and streaming flow inside the bath is promoted, as the dissolved oxygen supersaturation increases. The particle removal is found to be achieved mainly by the action of cavitation bubbles, but there exists optimal gas supersaturation to maximize the removal efficiency. Our finding suggests that low-intensity ultrasound irradiation under the optimal gas supersaturation in cleaning solutions allows for having mild bubble dynamics without violent collapse and thus cleaning surfaces without cavitation erosion. Finally, observations of individual bubble dynamics and the resulting particle removal are reported to further support the role of cavitation bubbles as cleaning agents.

Open-source toolbox for electromigrative separations

We present a novel simulation toolbox for electromigrative problems. The formulation presented includes all the fields, interaction and effects that configure both electrokinetic and electroosmotic phenomena for studying capillary and chip electromigrative separation methods. This simulation toolbox was developed by using the Finite Volume Method (FVM) in the platform OpenFOAM®. This implementation defines a new scenario for numerical simulations of electromigrative separations in terms of four main novel and outstanding characteristics offered by OpenFOAM®: (i) native 3D support, (ii) electroosmotic fluid flow solution by using Navier–Stokes equation, (iii) automatic parallel and supercomputing support, and (iv) GNU-GPL license. In this way, we put into consideration of the electromigrative separation community, for the first time, a simulation tool for a wide range of experimental methods such as capillary zone electrophoresis, isotachophoresis, and isoelectric focusing, among others.After presenting the main characteristics of the implementation, different validation and application examples are provided in order to illustrate the capabilities and potentialities of the library, but also trying to motivate a discussion about its future. Program summaryProgram title: electroMicroTransportProgram Files doi:http://dx.doi.org/10.17632/9wpvypzj9y.1Licensing provisions: GNU General Public License v3External routines: OpenFOAM®v17.12, v18.06Nature of problem: 3D multiphysics problems involving fluid dynamic and electromigrative phenomena in capillaries and microfluidic chips.Solution method: Navier–Stokes, charge conservation and transport equations solved via the Finite Volume Method running in shared and distributed memory parallel platforms.

Flexibility for Absorption and Distillation Columns

The flexibility of absorption and distillation columns is limited by macroscopic fluid dynamic phenomena like flooding, entrainment, weeping or dewetting. The design of those columns is a crucial task and depends on the separation task, the needed capacity and the fluid dynamic boundaries. An enhancement in flexibility is necessary to face upcoming challenges in chemical industry, including increasing uncertainties in material and energy supply or product demand. Results show that an increased flexibility is either possible by means of internal structuring or modularization of the columns. Both measures have a potential to enhance capacity flexibility of absorption and distillation columns.

The Inveterate Tinkerer 15. Miscellaneous Fluid Instabilities

In this series of articles, the author discusses various phenomena in fluid dynamics, which may be investigated via tabletop experiments using low-cost or home-made instruments. The fifteenth article in this series demonstrates some fluid instabilities.

Very large eddy simulation of swirling flow in the Dellenback abrupt expansion tube

There is usually a fluid dynamic phenomenon which involves a strong swirling flow between runner and draft tube of Francis turbines at part load condition. It is characterized by highly unsteady, large scale vortices, intense turbulence production, etc. The adverse pressure gradient in the draft tube cone can lead to vortex breakdown, which is recognized now as the main cause of hydro plant’s instabilities. Therefore, it is very significant to predict the swirling flow field exactly in design period. However, the popular Reynolds-Averaged Navier-Stokes (RANS) or large eddy simulation (LES) method also has some shortcomings in CFD analysis, such as inaccurate large structures resolution or restrictive grid requirement. Very large eddy simulation(VLES), as a hybrid RANS/LES methodology, could combine the advantages of different turbulence approaches of RANS, LES, which has been proved by many authors. This paper presents a VLES case-study, based on the experimental studies of the swirling flow in the abrupt expansion by Dellenback et al. It is validated that VLES and LES model are much more accurate than the RANS models compared with test data. The standard k-ε and k-ω models are unable to accurately model the effect of the large-scale unsteadiness, while VLES almost has similar ability as LES in resolving that problem. In comparison with LES, it is demonstrated that VLES can give satisfactory convergence with a coarse mesh.