Non-zero Flow Research Articles

Wave Radiation from Bottom Vibrations in Open-Channel Flow with Uniform Vorticity

The linearized water-wave radiation problem for a 2D oscillating source in an inviscid shear flow with a free surface is investigated analytically. The singular source is located at the bottom, with constant fluid depth. The velocity of the basic flow varies linearly with depth, assuming uniform vorticity. The far-field surface waves radiated out from the 2D bottom source are calculated, based on Euler’s equation of motion with the application of radiation conditions. The present analysis extends the work by Tyvand and Sveen to include a nonzero surface flow. Doppler effects will arise in a system at rest with the undisturbed free surface. Resonance with zero group velocity will occur at a critical Froude number for the surface flow. In the presence of a basic surface flow, there are in total four radiated waves when the surface flow is subcritical. With supercritical flow, there are only two emitted waves, with downstream propagation. The critical Froude number depends on the surface velocity, the shear rate and the oscillation frequency of the bottom source.

Distributed transfer function approach for one-dimensional thermoacoustic combustor models

This paper proposes a new approach for one-dimensional thermoacoustic combustor models. Our new model is a transfer function estimated from the frequency response of the linearised Euler equation to a spatially normalised and temporally impulsive input. The proposed approach can deal with combustors with varying cross-sectional areas under a non-zero mean flow, distributed heating/cooling, and outlet boundary conditions involving entropy waves, overcoming limitations of the popular network model. In addition our new approach can provide a more reliable thermoacoustic model for combustors with entropy-related boundary conditions, remedying the inaccurate entropy model of the network model. Numerical comparisons of our new model with a network model show apparent similarities between the two, validating the new model. It is also observed that, compared to our new model, the network model is more sensitive to mean flow and significantly overestimates the entropy wave effects on combustor acoustics.

Gravity-resisting colloidal collectives.

Self-assembly of dynamic colloidal structures along the vertical direction has been challenging because of gravity and the complexity in controlling agent-agent interactions. Here, we present a strategy that enables the self-growing of gravity-resisting colloidal collectives. By designing a unique dual-axis oscillating magnetic field, time-varying interparticle interactions are induced to assemble magnetic particles against gravity into vertical collectives, with the structures continuing to grow until reaching dynamic equilibrium. The collectives have swarm behavior and are capable of height reconfiguration and adaptive locomotion, such as moving along a tilted substrate and under nonzero fluidic flow condition, gap and obstacle crossing, and stair climbing.

Effect of the Stokes boundary layer on the dynamics of particle pairs in an oscillatory flow

The alignment of a pair of spherical particles perpendicular to a horizontally oscillating flow is attributed to a non-zero residual flow, known as steady streaming. This phenomenon is the basis of complex patterns in denser systems, such as particle chains and the initial stages of rolling-grain ripples. Previous studies on such self-organization processes used two distinct systems: an oscillating box filled with viscous fluid and an oscillating channel flow, where the fluid oscillates relative to the bottom boundary. In this paper, we show that particle pair dynamics in these two systems are fundamentally different, due to the presence of a Stokes boundary layer above the bottom in the oscillating channel flow. The results are obtained from direct numerical simulations in which the dynamics of a pair of particles are simulated using an immersed boundary method. The oscillating box and the oscillating channel flow are only equivalent in a limited region of the parameter space, where both the normalized Stokes boundary layer thickness and the normalized relative particle excursion length are small. Overall, the particle dynamics in the oscillating channel flow, compared to the oscillating box, are governed by an additional dimensionless parameter, that is, the particle–fluid density ratio.

Stability of yield stress fluid flows, Part 1

<h2>Abstract</h2> Yield stress fluids are idealized simple models (e.g., Bingham, Casson, Herschel-Bulkley), describing fluids for which there is no deformation wherever the deviatoric stress lies below the yield stress. This simple rheological change results in hydrodynamic stability features that are quite different from those of purely viscous fluids. We can classify the flows into those that have a non-zero base flow <b>U</b>, and those for which <b>U</b> = 0. Unlike purely viscous fluids, flows with <b>U</b> = 0 have meaningful applications beyond hydrostatics. It is these flows that we explore here, focusing on general features of stability, control and determining the critical limits.

Investigation of the acoustics of full-scale perforated liners in gas turbine combustors

In conventional gas turbine combustors, the combustion chamber linings are perforated and used for cooling. To cool the liner evenly, bias flows are introduced into the combustor at rates that depend on the operating condition. It has been found that airflow through the liner not only provides cooling but also improves sound absorption and acoustic instability. This experiment reports a unique comprehensive investigation of the influences of single and double-layer cylindrical full-scale gas turbine combustor liner sound absorption properties based on the no flow and non zero bias flows. In particular it is shown that combustor liner porosity (determined by orifice diameter and axial pitch distance) has an important influence on non-zero bias flow in that it increases the peak absorption or dissipation compared with that which occurs in the absence of flow. It is shown that the main influence of bias flow is to increase absorption compared with no flow above 600 Hz and to decrease the transmission loss measured in the absence of flow below 600 Hz but to increase it above 600 Hz. Internal resonance in the combustion liner test section influences both absorption and transmission loss spectra near 600 Hz. To create higher damping, and decrease in acoustic instability during the combustion process, gas turbine combustors require mapping between the inner and outer liner perforation to increase efficiency and lower the hydrocarbon emission. The calculated pressure ratio versus mass flow and combined discharge coefficient effect explain the non-linear distribution of the absorptive and dissipative energy measured at the gas turbine combustor.

Effect of Yaw Angle on Flutter of Rectangular Plates at Low Supersonic Speeds

The stability of the infinite series of thin elastic rectangular plates simply supported along all edges and exposed to a supersonic flow is investigated. The nonzero flow yaw angle with supersonic leading edge is considered. To derive expression for the unsteady aerodynamic pressure distribution over the oscillating plate, potential flow theory was used, and an integrodifferential eigenvalue problem for finding complex eigenvalues was obtained. Flutter boundaries for the first and second modes are computed. Changing of the single-mode and coupled-mode flutter boundaries with the change of the yaw angle was shown. While they are smooth at zero angle, the boundaries become irregular, and additional isolated regions of stability and instability appear when increasing the yaw angle. This irregularity occurs due to the interaction of multiple spanwise modes, which takes place at nonzero yaw angle due to their aerodynamic coupling.

Experimental study on drag reduction of the turbulent boundary layer via porous media under nonzero pressure gradient

The complex surface of an aircraft generates a nonzero pressure gradient flow. In this study, the boundary conditions of favorable and adverse pressure gradients are constructed in a small low-turbulence wind tunnel test section. Hot-wire anemometers and time-resolved image velocimetry are used to analyze the flow structure in a fully developed turbulent boundary layer with porous media. The effects of the porous surface on the statistical characteristics of the turbulent flow field and turbulent flow structure are analyzed and discussed. The results show that porous media reduce the velocity gradient in the linear layer, and the friction drag reduction effect is higher downstream of the porous wall. The drag reduction effect decreases along the flow direction. A wall with a 10 pores per inch produces a slightly better drag reduction effect than smooth wall. The maximum local drag reduction effect of a 10-pores-per-inch porous wall is 43.7% under a favorable pressure gradient and 42.3% under an adverse pressure gradient. The velocity streaks in the inner layer show that the porous wall widens the low-velocity streaks, making them more stable, while the high-speed streaks decrease in size under the pressure gradient. In the case of the adverse pressure gradient, the structure of the streaks becomes blurred, and their strength weakens. Under both favorable and adverse pressure gradients, the porous media lift up the coherent structures near the wall, thus weakening the large-scale coherent wall structures.

Numerical study of a pair of spheres in an oscillating box filled with viscous fluid

When two identical spherical particles are submerged in an oscillating fluid, they align themselves perpendicularly to the direction of the flow, leaving a small gap between them. The formation of this structure is attributed to a non-zero residual flow known as steady streaming. We performed direct numerical simulations of a fully resolved, oscillating flow in which the pair of particles is modeled using an immersed boundary method. In equilibrium, the particles oscillate both parallel and perpendicularly to the oscillating flow. Two scaling regimes for the particle dynamics, which are related to changes in the (average) flow field, are observed.

Synthesis of new microfluidics models in the research in the “Robotics and Control in Technical Systems” laboratory

Robotics is an interdisciplinary scientific direction, therefore, when solving a number of problems, it becomes necessary to conduct research that is typical for other areas of science. So, when synthesizing new types of equipment in the „Robotics and Control in Technical Systems” laboratory, more accurate characteristics of individual components and assemblies were required, which ultimately led to a whole cycle of work on microfluidics. Of particular interest in the work performed is the development of new microdevices, since this involves the construction of a single complex of new mathematical and computer models that link hydrodynamics, elasticity theory, heat transfer process, parametric optimization and control theory. The first models were obtained that describe the movement of a piston fixed by an elastic spring in a pipe, taking into account the influence of dry viscous types of friction. By the method of factorial computational experiment, analytical dependencies were obtained for the main part of the fuel dosing unit — servo piston. Next, mathematical and computer models were built to analyze the fluid flow through a channel with variable geometry. Based on the simulation results, the design of a new micromechanical device — a piezoelectric micropump consisting of an elastic tube and annular piezoelements located on it, the sequential oscillation of which creates a non-zero average fluid flow, was refined. In the course of the adaptation of the model, an effective design of the device for smoothing fluid flow pulsations was obtained. These devices were developed and calculated for use in the liquid cooling system of capillary microgripper. A parametric analysis of the process of cooling the hot side of the Peltier element of the microgripper was carried out with a change in the following gripper parameters: the size and shape of the radiator, the coolant flow rate and the heat transfer coefficient. The results of the analysis of temperature fluctuations of the radiator under unsteady fluid flow confirmed the expected dependences of the cooling efficiency on the chosen geometry of the radiator and low-frequency fluid flow fluctuations.

Modifications of Limiting Current and Magnetic Insulation in a Crossed-Field Diode by a Series Resistor

Adding an orthogonal magnetic field to a diode is crucial for multiple applications, including high power generation. These crossed-field diodes (CFDs) have curved electron trajectories that store current in the gap. Above a critical magnetic field referred to as the Hull cutoff, electrons emitted from the cathode fail to cross the anode-cathode gap, leading to magnetic insulation and the storage or transport of all the current. Normally, the Hull cutoff may be easily calculated from diode geometry and boundary conditions; however, dissipative effects in the circuit or the addition of a protective shunt resistor may introduce an external resistance in series that causes a mismatch between the applied voltage and the voltage drop across the gap. For non-magnetically-insulated CFDs, non-zero net current flows in the circuit due to space-charge limited current (SCLC) in the gap. In this paper, we examine several models for crossed-field SCLC below the Hull cutoff to determine the impedance of the CFD. We find that the resistor reduces the voltage drop across the gap, reducing the magnetic field necessary for magnetic insulation below the typical Hull cutoff, which in turn lowers the SCLC. One-dimensional particle-in-cell simulations show that adding the series resistor causes electron trajectories to oscillate between insulated and non-insulated states for CFDs operating within 30% of the conventional Hull cutoff; these findings are validated by further examination of the theory. Extensions to other perturbations, such as AC modulation and magnetic field tilts, are discussed.

Angular momenta in fields from a rotational mechanical antenna‡ ‡JPCO-102088.R3, Submitted to J. Phys. D: Appl. Phys. on 13th May, transferred to J. Phys. Commun. 3rd August, revised 17th Nov. and 5th Dec., accepted 8th Dec. 2021.

Mechanic antennas provide opportunities for human portable, VLF communications, where a rotational dipole emits EM signals with angular momenta. In this paper we analytically derive the electromagnetic fields from a rotational electric dipole using Fourier transform method, and find that the radiated fields from the rotational electric dipole carries nonzero energy flow density in both orbital and spin angular momentum (AM) parts by their flux tensors. Intuitively, a dipole circulating on the transverse plane induces a longitudinal orbital angular momentum and a longitudinal spin angular momentum. And the binding force for the rotational electric dipole is then shown to result mainly from the Coulomb fields. We believe that our work will contributes to novel communication designs for portable mechanic antennas.

ON SINGULAR SOLUTIONS OF THE STATIONARY NAVIER-STOKES SYSTEM IN POWER CUSP DOMAINS

The boundary value problem for the steady Navier–Stokes system is considered in a 2D bounded domain with the boundary having a power cusp singularity at the point O. The case of a boundary value with a nonzero flow rate is studied. In this case there is a source/sink in O and the solution necessarily has an infinite Dirichlet integral. The formal asymptotic expansion of the solution near the singular point is constructed and the existence of a solution having this asymptotic decomposition is proved.

Acoustic and hydrodynamic power of wave scattering by an infinite cascade of plates in mean flow

We study the balance of outgoing and incoming power for acoustic wave scattering by a cascade of flat blades in uniform subsonic mean flow. In nonzero mean flow the Kutta condition at the trailing edge of the cascade plates leads to the production of vorticity in the form of unsteady vortex sheets attached at the trailing edges, which results in a hydrodynamic power flux and the exchange of energy between the unsteady vorticity and the acoustic field. Although the scattering problem itself has been subject to much previous research, a comprehensive study of the composition of the outgoing energy flux for cascade scattering has received little previous attention. As an application of our detailed study we show how the time-averaged energy balance can be used to explain certain symmetries of the field in the angle of incidence with respect to the cascade face when there is no mean flow and the effect of zero acoustic reflection at certain angles of incidence with mean flow. We also show that, depending on the angle of incidence of the acoustic wave and the velocity of the mean flow, the vortex sheets can either absorb or generate acoustic energy. This can, for certain parameter values, result in a net increase in outgoing sound power over the incident sound power, as we demonstrate on a number of numerical examples that are computed using the semi-analytical solution described in recent work by Maierhofer and Peake (J. Sound Vib., vol. 481 (2020), 115418).

A Nonhomogeneous Boundary Value Problem for Steady State Navier-Stokes Equations in a Multiply-Connected Cusp Domain

The boundary value problem for the steady Navier–Stokes system is considered in a 2D multiply-connected bounded domain with the boundary having a power cusp singularity at the point O. The case of a boundary value with nonzero flow rates over connected components of the boundary is studied. It is also supposed that there is a source/sink in O. In this case the solution necessarily has an infinite Dirichlet integral. The existence of a solution to this problem is proved assuming that the flow rates are “sufficiently small”. This condition does not require the norm of the boundary data to be small. The solution is constructed as the sum of a function with the finite Dirichlet integral and a singular part coinciding with the asymptotic decomposition near the cusp point.

Perspective—On the thermodynamics of perfect unconditional security

A secure key distribution (exchange) scheme is unconditionally secure if it is unbreakable against arbitrary technological improvements of computing power and/or any development of new algorithms. There are only two families of experimentally realized and tested unconditionally secure key distribution technologies: quantum key distribution (QKD), the base of quantum cryptography, which utilizes quantum physical photonic features, and the Kirchhoff-Law–Johnson-Noise (KLJN) system that is based on classical statistical physics (fluctuation–dissipation theorem). The focus topic of this paper is the thermodynamical situation of the KLJN system. In all the original works, the proposed KLJN schemes required thermal equilibrium between the devices of the communicating parties to achieve perfect security. However, Vadai et al., in (Nature) Sci. Rep. 5, 13653 (2015) show a modified scheme, where there is a non-zero thermal noise energy flow between the parties, yet the system seems to resist all the known attack types. We introduce an attack type against their system. The attack utilizes coincidence events between the line current and voltages. We show that there is a non-zero information leak toward the Eavesdropper, even under idealized conditions. As soon as the thermal equilibrium is restored, the system becomes perfectly secure again. In conclusion, perfect unconditional security requires thermal equilibrium.

Zero Flow Rate Detection of Ultrasonic Water Meter Using Machine Learning Techniques

This paper presents a classification-based method to detect false-positive ultrasonic sensor measurements when the flow rate is zero while the pipeline is full of water caused by the physical disturbances in flow metering. Due to the outlined disturbances, these false-positive readings of the meters may cause the wrong billing. To overcome this problem, ultrasonic sensor measurements are collected as timeseries data at variously different water flow rates. Numerical and statistical measures of the timerseries are computed to construct an input-output relation. Hence, the feature extraction process is performed. For the modeling phase, both zero flow rate and non-zero flow rate parts of the dataset, and its corresponding features are labeled. Logistic Regression (LR), Support Vector Machine (SVM), and Linear Discriminant Analysis (LDA) algorithms are employed to classify the flow status in the MATLAB environment. Model performances are compared in terms of accuracy, sensitivity, specificity, and precision. For the investigation of the availability of the implementation of an embedded system, model complexities are discussed. Selected model parameters are embedded in a water meter, and consumption values are compared to a water meter without the detection algorithm in the same test bench underfilled pipeline with zero flow rate condition. To simulate the physical disturbance conditions, and observe the effect of the false-positive detection algorithm on flow metering, the flow pipeline is vibrated periodically. Consumption loggings of the water meters are tabled, model performance results are discussed, and test results are shared. According to the results, the radial basis kernel SVM algorithm performs better in terms of all metrics. LR algorithm is employed for the real plant experiment when its model complexity is considered.

Para-Kenmotsu metric as a $\eta$-Ricci soliton

The purpose of the paper is to study of Para-Kenmotsu metric as a $\eta$-Ricci soliton. The paper is organized as follows: * If an $\eta$-Einstein para-Kenmotsu metric represents an $\eta$-Ricci soliton with flow vector field $V$, then it is Einstein with constant scalar curvature $r = -2n(2n+1)$. * If a para-Kenmotsu metric $g$ represents an $\eta$-Ricci soliton with the flow vector field $V$ being an infinitesimal paracontact transformation, then $V$ is strict and the manifold is an Einstein manifold with constant scalar curvature $r = -2n(2n+1)$. * If a para-Kenmotsu metric $g$ represents an $\eta$-Ricci soliton with non-zero flow vector field $V$ being collinear with $\xi$, then the manifold is an Einstein manifold with constant scalar curvature $r = -2n(2n+1)$. Finally, we cited few examples to illustrate the results obtained.

Estimation of Unit Process Data for Life Cycle Assessment Using a Decision Tree-Based Approach.

Lacking unit process data is a major challenge for developing life cycle inventory (LCI) in life cycle assessment (LCA). Previously, we developed a similarity-based approach to estimate missing unit process data, which works only when less than 5% of the data are missing in a unit process. In this study, we developed a more flexible machine learning model to estimate missing unit process data as a complement to our previous method. In particular, we adopted a decision tree-based supervised learning approach to use an existing unit process dataset (ecoinvent 3.1) to characterize the relationship between the known information (predictors) and the missing one (response). The results show that our model can successfully classify the zero and nonzero flows with a very low misclassification rate (0.79% when 10% of the data are missing). For nonzero flows, the model can accurately estimate their values with an R2 over 0.7 when less than 20% of data are missing in one unit process. Our method can provide important data to complement primary LCI data for LCA studies and demonstrates the promising applications of machine learning techniques in LCA.

Non-stationary Navier–Stokes equations in 2D power cusp domain

Abstract The initial boundary value problem for the non-stationary Navier-Stokes equations is studied in 2D bounded domain with a power cusp singular point O on the boundary. The case of the boundary value with a nonzero flow rate is considered. In this case there is a source/sink in O and the solution necessary has infinite energy integral. In the first part of the paper the formal asymptotic expansion of the solution near the singular point is constructed. The justification of the asymptotic expansion and the existence of a solution are proved in the second part of the paper.