Flow In Such Systems Research Articles

Boundary Layer Flow of Dusty Ferrofluid: A Comparative Analysis of Stagnation Flow Influence

This research examines the characteristics of the boundary layer in the occurrence of dust particles within the ferrofluid boundary layer, aiming to understand the impact of stagnation flow or without stagnation flow in such systems. For this purpose, ferroparticles, namely magnetite (Fe3O4), are taken into consideration with kerosene and water as base fluids. The governing partial differential equations of the problem under consideration are converted into ordinary differential equations (ODEs) through the utilization of similarity transformations. Here, the equations obtained are then numerically solved utilizing MATLAB's built-in bvp4c solver. Moreover, the parameters’ effects, namely the dust particle loading, volume fraction of ferroparticles, and Eckert number to the flow with and without stagnation flow are computed and shown through tables and graphs. The findings indicate that the skin friction coefficient values for the stagnation-point flow are higher than those without stagnation-point flow. The Eckert number increases temperature profiles for both flows but more prominent in the flow without stagnation-point.

Visualization of the two-phase flow in the air gap of an optically accessible generic electric motor and its effect on torque

Electric motors with high-power densities are required for the implementation of electromobility. To achieve this, direct liquid cooling methods are increasingly being considered, in which oil is injected into the motor compartment. This results in a two-phase flow that can be used for efficient cooling. However, the oil, which can also penetrate the air gap between the rotor and stator, can also lead to additional losses due to increased friction. Since little is known about the two-phase flow in such systems, especially in the air gap, it is investigated by means of simple optical visualizations and high-speed laser-induced fluorescence imaging as well as torque measurements. The measurements are carried out in the air gap of an optically accessible generic model of a directly cooled electric motor. Speed variations were performed from 100 to 2000 rpm, and three different two-phase flow regimes were observed. At low speeds (Flow Regime 1), the air gap is filled locally with oil in radial direction, in the medium speed range (Flow Regime 2) with foam, while at high speeds (Flow Regime 3) separated films were observed on the rotor and stator. The torque difference between the two-phase and single-phase operation, which quantifies the mechanical losses due to the injected oil, increased continuously due to the oil in the air gap until it reached a maximum in Flow Regime 2 due to foam formation. In Flow Regime 3, the torque difference was negative. This was attributed to the fact that the grooves in the stator were filled with oil, thus reducing the turbulence generation of the air flow.

A Cautionary Perspective on Hydrogel-Induced Concentration Gradient Generation for Studying Chemotaxis.

The achievement of consistent and static chemical gradients is critically important in the study of diffusion and chemotaxis at the micro- and nanoscales. In this context, a number of groups have reported on hydrogel-based systems for generating concentration gradients. Here, we analyze the behavior of agarose and gelatin-based hydrogels in hybridization chambers of different heights. Our focus is on the issues that are caused by the presence of robust bulk fluid flows in such systems due to the solutes present in the hydrogel and/or the surrounding fluid. We describe the key insights derived from these experiments, offering practical guidelines for establishing gradients using hydrogel-based systems and make the community aware of different variables that can make the experiments nonreproducible and prone to misinterpretations.

Investigation of turbine cooling using semi-analytical methods in non-Newtonian fluid flow with porous wall

This study explores non-Newtonian fluid dynamics in an axisymmetric channel with a porous boundary, focusing on its relevance to turbine cooling systems in engineering and industry. Managing fluid flow in such systems is crucial. To address the complexity, mathematical tools like the Homotopy Perturbation Method, Variational Iteration Method, and Runge-Kutta 4th numerical method are used to solve nonlinear differential equations governing momentum and heat transfer. The investigation primarily aims to elucidate relationships in system parameters, including the Power Law index, Reynolds number, and Prandtl number. These relationships are compared with numerical techniques, assessing the precision and simplicity of the employed methods. The inquiry goes beyond conventional research paradigms, exploring constant parameters and trial function steps. The proposed solution reveals a theme of precision, simplicity, and efficient convergence. The correlation between the Reynolds number and the thermal boundary layer is a significant finding. Increasing the Reynolds number and adjusting the Power Law index results in a noticeable reduction in the thermal boundary layer. This has substantial implications for temperature profiles in coupled systems, potentially enhancing cooling effectiveness in various industrial applications.

Processing of experimental results for super-cavitating flow past cone by local polynomial regression (LOESS)

The aim of the study is to define the correlations describing the flow parameters during super-cavitating flow past an obstacle, often found in various elements of thermal power systems and units, as well as to offer a simple and reliable method for analysing experimental datasets for the flows in such systems. Full-scale modelling of cavitation processes was carried out using a circulating hydrodynamic set-up. The process of super-cavitation flow past cones with base diameters of15.45 and 21.75 mm and opening angles of 154° and 127°, respectively, in a working section having a diameter of 30 mm, was investigated. The obtained experimental data comprises a four-dimensional array that describes the dependence of the cavity length arising behind the obstacle and the pressure inside the cavity on the flow rate and temperature. Due to the complexity of processing and visual representation, this array was divided into two three-dimensional arrays. The approximation of the obtained data was carried out by locally estimated scatterplot smoothing (LOESS). The results demonstrated that the transition from vapour–gas to vapour cavitation is independent of the geometric dimensions of the obstacle. In addition, the dependence corresponding to the transition process to vapour cavitation was obtained by processing the experimental data. An empirical equation describing such a transition is proposed. Therefore, the method of smoothing a locally estimated scatter plot (local polynomial regression) illustrates the correlation between the processed data. The proposed empirical equation allows the critical length of the cavity to be determined that corresponds to the transition from vapour–gas to vapour cavitation and can be used for the design and operation of thermal power equipment.

A Sufficient Condition for $k$-Contraction of the Series Connection of Two Systems

The flow of contracting systems contracts 1-dimensional parallelotopes, i.e., line segments, at an exponential rate. One reason for the usefulness of contracting systems is that many interconnections of contracting sub-systems yield an overall contracting system. A generalization of contracting systems is $k$-contracting systems, where $k\in\{1,\dots,n\}$. The flow of such systems contracts the volume of $k$-dimensional parallelotopes at an exponential rate, and in particular they reduce to contracting systems when $k=1$. It was shown by Muldowney and Li that time-invariant $2$-contracting systems have a well-ordered asymptotic behaviour: all bounded trajectories converge to the set of equilibria. Here, we derive a sufficient condition guaranteeing that the system obtained from the series interconnection of two sub-systems is $k$-contracting. This is based on a new formula for the $k$th multiplicative and additive compounds of a block-diagonal matrix, which may be of independent interest. As an application, we find conditions guaranteeing that $2$-contracting systems with an exponentially decaying input retain the well-ordered behaviour of time-invariant 2-contracting systems.

Specification of systems with parameterised events: An institution-independent approach

Event-based systems operate in an environment that signals events upon which the system reacts. Besides the control flow of such systems also their data flow is of major importance. We present an event/data-based institution which is generic in the underlying data state institution. The logic is based on previous developments [14,8] and is now extended to take into account event parameters, quantification over data, and non-deterministic choice of arguments in an institution-independent way. We show that the resulting framework forms again an institution.

An impact of the line resistance on the power flow calculations with installed phase-shifting transformer in different voltage levels power systems

In modern power systems, phase-shifting transformers are installed not only in the extra high voltage lines, but some of these devices can be built for lower voltage levels (e.g., 40 kV). Due to big voltage difference, in the power flow analyses, not only phase-shifting transformer properties, but also line parameters, including resistance, should be considered. For high voltage power systems, the line reactance is much greater than its resistance (the ratio is even greater than 10), while it is smaller in lower voltages. This fact should be taken into account during any calculations of power flows in such systems. This article considers 400 kV, 220 kV, and 70 kV transmission lines with installed asymmetrical phase-shifting transformers to present the line resistance impact on the final power flow calculations. Also, the IEEE 5-bus power system is tested to prove the dependency of line resistance on power flow calculations and bus voltage values. The highest voltage levels calculation results show that the assumption of omitting line resistance is correct (the relative error is equal to 2%), while for the 70 kV system, the same error results even 30%. What is more, the line resistance for medium voltage power systems cannot be omitted. What is more, the resistance effect is intentionally not taken into account in some power flow calculations presented in the literature.

Insights into scale translation of methane transport in nanopores

Accurate prediction of flow behavior in shale matrix is critical for efficient development of shale gas reservoirs. In these systems, the majority of pores are in the nano-size range. As a result, continuum-based approaches may not be appropriate to simulate flow in such systems. Molecular dynamics (MD) simulations are capable of capturing the relevant microscale physics. Their relatively high computational expense, however, restricts MD simulations to rather small systems and domains. This limitation creates a gap between computational need of macroscale systems and capabilities of MD simulations. The lattice Boltzmann method (LBM) is a suitable candidate to bridge this gap. In this work, the multiple-relaxation-time (MRT)-LBM is used to study methane transport in nano-size pores. Adsorption effects near solid boundaries, as well as non-ideal behavior of fluids, are accounted for via incorporating appropriate force terms in LBM. Parameters associated with the force terms in the equation of state are studied in detail, and a workflow is proposed to determine optimal values of these parameters for gas flow in slit pores. Specifically, we establish these parameters such that the range of density values that the model is able to simulate is maximized. We demonstrate this workflow by simulating gas flow where velocity and density profiles from MD simulations are used as reference data. Results from LBM simulations are in good agreement with MD reference data for pores that are 4 nm in width or larger. Moreover, we propose a preconditioning scheme to improve the stability of LBM in dealing with complex geometries. The robustness of this scheme is demonstrated by simulating several roughness geometries. This work motivates the use of LBM in scale translation of the physics of mass transport in more complex permeable media.

Dynamics of shallow wakes on gravel-bed floodplains: dataset from field experiments

Abstract. Natural dynamics of river floodplains are driven by the interaction of flow and patchy riparian vegetation, which has implications for channel morphology and diversity of riparian habitats. Fundamental mechanisms affecting the dynamics of flow in such systems are still not fully understood due to a lack of experimental data collected in natural environments that are free of the unavoidable scaling effects in laboratory studies. Here we present a detailed dataset on the hydrodynamics of shallow wake flows that develop behind solid and porous obstructions. The dataset was collected during a field experimental campaign carried out in a side branch of the gravel-bed Tagliamento River in northeast Italy. The dataset consists of 30 experimental runs in which we varied the diameter of the surface-mounted obstruction, its solid volume fraction, its porosity at the leading edge, the object's submergence and the approach velocity. Each run included: (1) measurements of mean velocity and turbulence in the longitudinal transect through the centreline of the flow with up to 25–30 sampling locations and from 8 to 10 lateral profiles measured at 14 locations, (2) detailed surveys of the free surface topography and (3) flow visualizations and video recordings of the wake patterns using a drone. The field scale of the experimental setup, precise control of the approach velocity, configuration of models and natural gravel-bed context for this experiment makes this dataset unique. Besides enabling the examination of scaling effects, these data also allow the verification of numerical models and provide insight into the effects of driftwood accumulations on the dynamics of wakes. Data are available with open access via the Zenodo portal (Shumilova et al., 2020) with DOI https://doi.org/10.5281/zenodo.3968748.

Development of a novel two-phase flow solver for nuclear reactor analysis: algorithms, verification and implementation in OpenFOAM

A porous medium-based representation of nuclear reactors and complex engineering systems more in general can significantly reduce simulation and modelling costs, while preserving a reasonable degree of accuracy via regime map-based correlations for modelling physical interactions. This paper presents a segregated algorithm for the simulation of dispersed two phase flows in such systems treated as porous media in an Eulerian framework. The global algorithm pertaining to the coupling between the mass, momentum and energy conservation equations solution is discussed and implemented via the finite volume OpenFOAM programming library. In the context of pressure–velocity coupling, a novel implementation of the Partial Elimination Algorithm for the treatment of the inter-phase momentum transfer term is developed. It is found to perform better than existing implementations for a number of cases with important momentum coupling between phases. A conclusive verification of the overall solution algorithm is performed with the Method of Manufactured Solutions and order-of-accuracy testing. From an implementation perspective, the performance of the algorithm in parallel strong scaling up to 4096 cores is assessed and proves to be in line with OpenFOAM-based code standards.

МОДЕЛЮВАННЯ ПРОЦЕСІВ ВОЛОГОПЕРЕНОСУ В БАГАТОШАРОВИХ АПЛІКАЦІЙНИХ МАТЕРІАЛАХ МЕДИЧНОГО ПРИЗНАЧЕННЯ

To carry out modeling of moisture transfer processes in multilayer medical application materials for forecasting of targeted transport of drugs of a certain effective concentration into the lesion center, based on the geometric parameters of the wounds. Using digital image processing techniques in a computer environment have developed methods for designing three-dimensional fluid propagation effects in multilayer medical wound dressings. Modern wound dressings have been found to be multilayer multidimensional compositions with complex fluid kinetics. Additionally, the wounds on the human body itself have a complex space geometric shape. This requires taking into account the volumetric effects when analyzing and forecasting the processes of wetting and fluid flow in such systems. An algorithm for ordering the boundary of a wound by digital photography with the subsequent processing of a wound image in a computer environment is proposed. To evaluate the moisture transport properties for the controlled release of medicinal substances at the desired concentration in different layers of wound dressings, an experiment was made with wetting from two and three sources, followed by three-dimensional modeling. It has been experimentally found that the parameters of the layered distribution of liquid moisture in multilayer dressings vary significantly.. The results of the simulation are suitable for making prompt decisions about the type, the required geometric parameters of medical materials and wound dressings. The proposed method of establishing the real geometry of the wound, together with three-dimensional modeling, allows to predict the exact boundaries of the application of the drug, to calculate its required amount and time of movement to the wound. Using 3D computer graphics of discrete objects, methods have been developed to predict the conditions for the reliable functioning of multilayer dressings. Investigated processes of transport and distribution of liquid moisture can be used in the selection of components of the raw material composition, structure and number of layers of multilayer wound dressings, taking into account the real geometric profile of the wound.

Comparative Study of Physics-Based Modeling and Neural Network Approach to Predict Cooling in Vehicle Integrated Thermal Management System

Vehicle integrated thermal management system (VTMS) is an important technology used for improving the energy efficiency of vehicles. Physics-based modeling is widely used to predict the energy flow in such systems. However, physics-based modeling requires several experimental approaches to get the required parameters. The experimental approach to obtain these parameters is expensive and requires great effort to configure a separate experimental device and conduct the experiment. Therefore, in this study, a neural network (NN) approach is applied to reduce the cost and effort necessary to develop a VTMS. The physics-based modeling is also analyzed and compared with recent NN techniques, such as ConvLSTM and temporal convolutional network (TCN), to confirm the feasibility of the NN approach at EPA Federal Test Procedure (FTP-75), Highway Fuel Economy Test cycle (HWFET), Worldwide harmonized Light duty driving Test Cycle (WLTC) and actual on-road driving conditions. TCN performed the best among the tested models and was easier to build than physics-based modeling. For validating the two different approaches, the physical properties of a 1 L class passenger car with an electric control valve are measured. The NN model proved to be effective in predicting the characteristics of a vehicle cooling system. The proposed method will reduce research costs in the field of predictive control and VTMS design.

A Network-Topology-Based Approach for the Load-Flow Solution of AC–DC Distribution System With Distributed Generations

The ac–dc distribution systems have recently gained huge popularity due to advancements in power converters, high penetration of renewable energy resources, and wide usages of dc loads. However, load flow in such systems is a challenging task due to nonlinear characteristics of power converters. This paper presents a novel load-flow algorithm for ac–dc distribution systems, utilizing the concept of graph theory and matrix algebra. Four developed matrices, loads beyond branch matrix, the path impedance matrix, the path drop matrix, and the slack bus to other buses drop matrix, and simple matrix operations are utilized to obtain load-flow solutions. These matrices reveal the network topology and relevant information about the behavior of ac–dc distribution network during load-flow studies. In contrast with traditional load-flow methods for HVDC systems, the proposed technique does not require any lower–upper decomposition, matrix inversion, and forward–backward substitution of the Jacobian matrix. Because of the aforementioned reasons, the developed technique is computationally efficient. The proposed method has been tested using several case studies of ac–dc distribution network, which includes different operating modes of various power converters. Results show the feasibility and authenticity of the proposed method.

An Efficient Load Flow Algorithm for AC–DC Distribution Systems

The AC–DC distribution systems have recently gained huge popularity due to advancements in power converters, high penetration of renewable energy resources and widespread use of DC loads. However, load flow in such systems is a challenging task, due to non-linear characteristics of power converters. This paper presents a new and computationally efficient load flow algorithm for AC distribution systems using graphical search technique along with matrix algebra and has been further extended for solving AC–DC distribution systems load flow problem by making use of proposed per-unit equivalent model of various power converter devices. In contrast with traditional methods, the proposed technique does not require any LU decomposition, matrix inversion and forward backward substitution of admittance matrix or Jacobian matrix. The significant feature of the proposed algorithm lies in formulation of path impedance matrix and loads beyond branch matrix, which will remain unaltered for the entire operation. Reconfiguration of these matrices takes place automatically in accordance with the change in input data. Because of the aforementioned reasons, the developed technique is computationally potent when applied on large sized distribution systems. The proposed method has been validated on various standard test systems. Results show feasibility and authenticity of the proposed method.

Wave-induced setup and wave-driven current over Quasi-2DH reef-lagoon-channel systems

The primary action of near-shore waves at many field reef sites is to produce a wave-induced setup and 2DH reef-lagoon-channel circulation cells. Instead of using wave basin to study the behaviors of the wave-induced setup and wave-driven flow in such systems, an experimental technique was introduced to achieve the same objective through wave-flume tests. A pipe with a valve was installed underneath the wave flume to connect the lagoon and the ocean side, which makes it possible to simulate a rip channel connecting the lagoon and the offshore side. Experimental results for two reef systems with a fore-reef slope (1V:6H) were obtained under a series of monochromatic wave conditions: one simulating a closed lagoon and the other simulating an open lagoon. Compared with the reef system with a closed lagoon, connecting the lagoon to the ocean side was found to have the following effects: (i) producing a wave-driven current on the reef flat, (ii) reducing the maximum wave setup on the reef flat by 10%–40%, and (iii) causing the wave setup to decrease continuously along the reef flat all the way to the lagoon. For the reef system with an open lagoon, the wave-driven current was found to increase with increasing wave height and wave period, and there appeared to be a peak value of the current within the tested range of reef-flat submergences. The experimental results for the two reef systems were compared with a set of existing wave-basin results for similar reef systems, but with a much steeper fore-reef slope (1V:1H). The dimensionless parameters used in the literature were found to be insufficient to describe both the wave setup and wave-driven current on the reef flat for the reef system with an open lagoon. A semi-analytical quasi-2DH model, which was devised according to the balances of momentum and mass, was also included to understand how the reef and channel morphology and the bottom roughness affect the wave setup and wave-driven current in a reef-lagoon-channel system. The semi-analytical model was validated by our laboratory data as well as field observations reported in the literature.

Variational discretization of the nonequilibrium thermodynamics of simple systems

In this paper, we develop variational integrators for the nonequilibrium thermodynamics of simple closed systems. These integrators are obtained by a discretization of the Lagrangian variational formulation of nonequilibrium thermodynamics developed in (Gay-Balmaz and Yoshimura 2017a J. Geom. Phys. part I 111 169–93; Gay-Balmaz and Yoshimura 2017b J. Geom. Phys. part II 111 194–212) and thus extend the variational integrators of Lagrangian mechanics, to include irreversible processes. In the continuous setting, we derive the structure preserving property of the flow of such systems. This property is an extension of the symplectic property of the flow of the Euler–Lagrange equations. In the discrete setting, we show that the discrete flow solution of our numerical scheme verifies a discrete version of this property. We also present the regularity conditions which ensure the existence of the discrete flow. We finally illustrate our discrete variational schemes with the implementation of an example of a simple and closed system.

Design of a bubble collecting section in a high speed water tunnel for ventilated supercavitation experiments

The main objective of the present study is to design a bubble collecting section for use in ventilated supercavitation experiments. The large amounts of air ventilated around a cavitator split into small bubbles that follow the water’s passage through the water tunnel. The presence of these bubbles in the test section of the water tunnel prevents effective observation of supercavitation. To enable the clear observation of cavitation shape, a bubble collecting section with large volume is necessary upstream of the test section to collect bubbles. The buoyancy of bubbles provides a simple means for their collection. However, the bubbly flows in such systems have rather high velocities and a non-uniform velocity distribution, which degrades the buoyancy effect. In the present study, a bubble collecting section with three porous plates that produce a uniformly low velocity distribution is designed and analyzed with numerical methods. The effectiveness of this approach is assessed experimentally with the 1/10 miniature model. The reduction of the void fraction downstream of the bubble collecting section was also assessed in the test section. The bubble collecting section in the full-scale water tunnel was also eventually found to be well designed through flow speed measurements and bubble visualization in the test section.

Recycle Flows in Lab-on-Chip Applications Using Electroosmotic Effects

Several lab-on-chip applications deal with the flow of fluids through microchannels. Some applications require recycle flows in such systems. In this work we show how this can be achieved in microfluidic systems using electroosmosis. The system consists of flow through a main channel and a side channel with a provision for recycle. The main flow is pressure-driven, and the recycle flow is achieved by imposing an electric field across the side channel. When the applied electric field is greater than a critical value, the flow in the side channel is reversed and recycle is induced. The critical applied electric field is investigated as a function of zeta potential. The effect of the applied electric field on pressure and flow rate in the main and side channels is analyzed. The influence of radius and position of the side channel on flow behavior has also been studied.

Combustion of Boron Particles in Premixed Methane/Air Flames

AbstractBecause of its high energy density, boron particles have been a subject of interest for the use as propellant in propulsion systems for many years. A cheap and fast opportunity to investigate multiphase reacting flows in such systems is offered by numerical simulations. Therefore, a detailed knowledge of the chemistry and kinetics of boron combustion in different gaseous surroundings is required. The main topic of this contribution is the experimental investigation of the influence of the equivalence ratio of reacting methane‐air mixtures on the combustion time of boron particles. Additionally, numerical calculations were performed using a combustion model proposed by Yeh et al. [1] and modified by Hussmann et al. [2, 3]. The experimental results show that for small particles there exists an optimal stoichiometry, at which the combustion time of boron particles is minimized. High equivalence ratios yield larger burning times because of a low concentration of oxygen. Low equivalence ratios are accompanied by low flame temperatures also leading to large burning times, because of a slow reactive evaporation of the boron‐oxide layer. With increasing particle size the burning process is dominated by the evaporation process. Numerical results are in a good agreement with the experiments for small particles. For larger particles, the predicted burning time is too high due to the fact that boron particles cannot be treated as a sphere as assumed in the original model. By implementing a sphericity factor good agreement with the experiments can be achieved.