Wall Excitation Research Articles

Direct numerical simulation of two-dimensional wall-bounded turbulent flows from receptivity stage

Deterministic route to turbulence creation in 2D wall boundary layer is shown here by solving full Navier-Stokes equation by dispersion relation preserving (DRP) numerical methods for flow over a flat plate excited by wall and free stream excitations. Present results show the transition caused by wall excitation is predominantly due to nonlinear growth of the spatiotemporal wave front, even in the presence of Tollmien-Schlichting (TS) waves. The existence and linear mechanism of creating the spatiotemporal wave front was established in Sengupta, Rao and Venkatasubbaiah [Phys. Rev. Lett. 96, 224504 (2006)] via the solution of Orr-Sommerfeld equation. Effects of spatiotemporal front(s) in the nonlinear phase of disturbance evolution have been documented by Sengupta and Bhaumik [Phys. Rev. Lett. 107, 154501 (2011)], where a flow is taken from the receptivity stage to the fully developed 2D turbulent state exhibiting a k(-3) energy spectrum by solving the Navier-Stokes equation without any artifice. The details of this mechanism are presented here for the first time, along with another problem of forced excitation of the boundary layer by convecting free stream vortices. Thus, the excitations considered here are for a zero pressure gradient (ZPG) boundary layer by (i) monochromatic time-harmonic wall excitation and (ii) free stream excitation by convecting train of vortices at a constant height. The latter case demonstrates neither monochromatic TS wave, nor the spatiotemporal wave front, yet both the cases eventually show the presence of k(-3) energy spectrum, which has been shown experimentally for atmospheric dynamics in Nastrom, Gage and Jasperson [Nature 310, 36 (1984)]. Transition by a nonlinear mechanism of the Navier-Stokes equation leading to k(-3) energy spectrum in the inertial subrange is the typical characteristic feature of all 2D turbulent flows. Reproduction of the spectrum noted in atmospheric data (showing dominance of the k(-3) spectrum over the k(-5/3) spectrum in Nastrom et al.) in laboratory scale indicates universality of this spectrum for all 2D turbulent flows. Creation of universal features of 2D turbulence by a deterministic route has been established here for the first time by solving the Navier-Stokes equation without any modeling, as has been reported earlier in the literature by other researchers.

Discharge Speed Control of In-Pipe Detector in Gas Pipeline

The in-pipe detector with magnetic flux leakage runs along the pipeline under the effect of gas flow field, completing the pipeline wall excitation, magnetic flux leakage testing, etc. Excitation within the detector on the wall and the adequacy of magnetic flux leakage signal acquisition is complete are decided by speed of in-pipe detector. In this paper, mathematical model simulation is established and the actual experiment is proposed. Simulation and experiment results show that after a long time running, in-pipe detector with different discharge area have respective upper speed limit. According to references and experiment results, the in-pipe detector with 40% discharge area is meet the detection accuracy and have the maximum detection efficiency under work conditions described in this paper

Spin-wave excitations of domain walls in bubble-state magnetic nanoelements

Domain wall (DW) excitation spectrum for a bubble-state element has been investigated using three-dimensional micromagnetic simulations within a wide frequency range (0.1--100 GHz). Multiple DW resonance lines have been identified; localized modes at the DW in the low-frequency part of the spectrum and a set of high-frequency DW excitations above the frequency gap of domains spreading into the adjacent domains. In the last case, various DW modes with and without rotational symmetry coupled to multinodal domain modes are revealed. Due to the geometrical confinement leading to a strongly nonuniform static effective field, high-frequency DW modes with the same number of nodes but different localizations and frequencies exist, which prevents the use of a mode indexation with independent quantization numbers. The effects of the size of the element and a longitudinal polarizing magnetic field on the DW spectrum are then analyzed. In particular, the softening of the fundamental DW mode is evidenced at the first-order phase transitions between the bubble state and the saturated states. These numerical findings are discussed in light of theoretical models.

Surface plasmon and photonic mode propagation in gold nanotubes with varying wall thickness

Gold nanotube arrays are synthesized with a range of wall thicknesses (15 to >140 nm) and inner diameters of \ensuremath{\sim}200 nm using a hard-template method. A red spectral shift (>0.39 eV) with decreasing wall thickness is observed in dark-field spectra of nanotube arrays and single nanowire/nanotube heterostructures. Finite-difference-time-domain simulations show that nanotubes in this size regime support propagating surface plasmon modes as well as surface plasmon ring resonances at visible wavelengths (the latter is observed only for excitation directions normal to the nanotube long axis with transverse polarization). The energy of the surface plasmon modes decreases with decreasing wall thickness and is attributed to an increase in mode coupling between propagating modes in the nanotube core and outer surface and the circumference dependence of ring resonances. Surface plasmon mode propagation lengths for thicker-walled tubes increase by a factor of \ensuremath{\sim}2 at longer wavelengths (>700 nm), where ohmic losses in the metal are low, but thinner-walled tubes (30 nm) exhibit a more significant increase in surface plasmon propagation length (by a factor of more than four) at longer wavelengths. Additionally, nanotubes in this size regime support a photonic mode in their core, which does not change in energy with changing wall thickness. However, photonic mode propagation length is found to decrease for optically thin walls. Finally, correlations are made between the experimentally observed changes in dark-field spectra and the changes in surface plasmon mode properties observed in simulations for the various gold nanotube wall thicknesses and excitation conditions.

Nonlinear and Nonparallel Receptivity of Zero-pressure Gradient Boundary Layer

Nonlinear and nonparallel receptivity of zero pressure gradient boundary layer have been investigated by solving the full two-dimensional Navier-Stokes equation with respect to wall excitation at fixed frequency. The excitation is triggered through a blowing-suction strip at a location near the leading edge of the plate. Results are reported for cases which are linearly stable in a spatial formulation, for different amplitudes of the excitation of the strip.Presented results for a lower amplitude of excitation for the chosen frequency of excitation help explain the displayed nonlinearity of the streamwise component of the disturbance velocity at intermediate heights inside the shear layer. The disturbance field is noted to be neutrally stable at heights close to the plate and in the outer part of the boundary layer. Overall, such behaviour of the disturbance field is explained in terms of the presence of multiple modes and their spatiotemporal interactions and the importance of nonlinearity at specific he...

Stability characteristics of suspension flow through wavy-walled channels

A numerical study of the linear temporal stability characteristics of particulate suspension flow through a converging-diverging symmetric wavy-walled channel is considered. The basic flow is a superposition of plane channel flow of particulate suspension and periodic flow components arising due to the small amplitude sinusoidal waviness of the channel walls. The disturbance equations are derived within the framework of Floquet theory and solved using the spectral collocation method. The effects of small amplitude sinusoidal waviness of the channel walls and those of the presence of particles on the initial growth of the disturbances are examined. Two-dimensional stability calculations for particulate suspensions indicate the presence of fast growing unstable modes that arise due to the waviness of the walls. Neutral stability calculations are performed in the disturbances wavenumber-Reynolds number (αs−Re) plane, for the wavy channel with representative values of wavenumber (λ) and the wall amplitude to semi-channel height ratio (∈w) for different values of volume fraction density of the particles (C). It is observed that the critical Reynolds number for transition decreases with increase of ∈w and C. However, the flow can be modulated by suitable wall excitation which in turn can stabilize the flow.

BY-PASS MECHANISM OF TRANSITION TO TURBULENCE

The by-pass mechanism of transition for a wall-bounded shear layer is explained for the case when an infinite row of convecting vortices migrate over a boundary layer at a specific speed range. Such a mechanism is important for noisy flows over bluff bodies, flows inside turbomachinery and flows over helicopter rotor blades. By solving the Navier–Stokes equation, it is shown that this by-pass transition is a consequence of vortices migrating at convection speeds that are significantly lower than the free-stream speed. This situation is commonly found in flows that are affected by the presence of periodic wakes. Whenever the speed of migrating vortices is in a certain critical range, there is a local instability of the underlying shear layer with a very high-growth rate as compared to the growth of pure Tollmien–Schlichting waves created by wall excitation. The above interpretation is supported by solving the linearized and full Navier–Stokes equation for disturbance quantities under the parallel flow approximation in two dimensions. Various ramifications of such a by-pass route of transition are discussed in this paper.

Breathing domain wall mode of an interface pinned Nèel wall

A step defect at the interface between a two – sublattice antiferromagnetic substrate and a uniaxial ferromagnetic thin film stabilizes a Nèel wall. Breathing domain wall modes (BDWM) are studied with the external field in the plane of the film. The restoring force of the domain wall excitations originates from the interface exchange energy within the domain wall. For low anisotropy ferromagnets there is a significant reduction of the domain wall width and the frequency of the domain wall excitations is determined from the interface exchange coupling in a small interface area. For strong interface coupling, the width of the domain wall excitation spectrum scales with the square root of the ratio between the interface effective exchange field and the anisotropy field of the ferromagnetic film.

Resistivity due to Domain Wall Excitation in Thin Wires of Ferromagnetic Metals

We study the influence of thermal excitation of a domain wall (DW) on resistivity in thin wires of disordered ferromagnetic metals. The negative correction for local conductivity δσ due to the DW e...

Magneto-optical current sensors of high bandwidth

A new current transformer is described by means of new results concerning the domain wall excitation in orthoferrites. The measured parameter is the geometrical position of the boundary between domains with opposite magnetizations. In a wide frequency band, including DC currents, the measurement results are a linear function of the measured current due to the very high domain wall velocity. They are not affected by temperature changes and mechanical factors.

Magneto-optical current sensor by domain wall motion in orthoferrites

A brief review of the latest developments on optical current measurements based on the Faraday effect is presented. In all existing magneto-optical current transformers, the main measured parameter is the polarization state of the light transmitted by a sensor element. A new transformer is described by means of new results concerning the domain wall excitation. The measured parameter is the geometrical position of the boundary between domains with opposite magnetizations. In a wide frequency band, including DC currents, the measurement results are a linear function of the measured current. They are not affected by temperature changes and mechanical factors.

Stability characteristics of wavy walled channel flows

The linear temporal stability characteristics of converging–diverging, symmetric wavy walled channel flows are numerically investigated in this paper. The basic flow in the problem is a superposition of plane channel flow and periodic flow components arising due to the small amplitude sinusoidal waviness of the channel walls. The disturbance equations are derived within the frame work of Floquet theory and solved using the spectral collocation method. Two-dimensional stability calculations indicate the presence of fast growing unstable modes that arise due to the waviness of the walls. Neutral stability calculations are performed in the disturbance wavenumber–Reynolds number (αs−R) plane, for the wavy channel with wavenumber λ1=0.2 and the wall amplitude to semi-channel height ratio, εw, up to 0.1. It is also shown that the two-dimensional wavy channel flows can be modulated by a suitable frequency of wall excitation ωg, thereby stabilizing the flow.

Spectrum of elementary excitations of a single domain wall in garnet films with in-plane magnetization

Epitaxial garnet films with in-plane magnetization are studied by microwave technique. Resonances related to different branches of domain wall excitations are observed: a low-frequency Goldstone branch of translational wall vibrations in the MHz band and a high-frequency Gilinskii branch in the GHz band. A strong coupling between neighbouring domain walls is observed, if their distance becomes comparable to the wall thichkness. In this case each resonance is split into two coupled resonant modes, their frequencies being larger at smaller wall distances. Spin wave excitations of a single wall are analyzed by numerical solution of Landau-Lifshitz and magnetostatic equations for the case of bulk orthorhombic ferromagnetic material. Unidirectional Gilinskii branch and asymmetric Goldstone branch are shown to exist in the spectrum of the wall excitations for the case of the propagation direction perpendicular to the magnetization in domains. Unexpected wall excitation spectra are predicted at some values of the anisotropy parameters when the wall becomes unstable. Such spectrum shows a gap in the wave number and a loop in the dispersion of the Goldstone branch.

On the theory of supercompression of a gas bubble in a liquid-filled flask

The spherically symmetric problem of the oscillations of a gas bubble in the center of a spherical flask filled with a compressible liquid that is excited by pressure oscillations on the flask wall is considered. A generalization of the Rayleigh–Plesset equation for a compressible liquid is given in the form of two ordinary difference-differential equations that take into account the pressure waves that are reflected from the bubble and those that are incident on the bubble from the flask wall. The initial value problem for the initiation of bubble oscillations due to flask wall excitation and for the evolution of these oscillations is considered. Linear and nonlinear periodic bubble oscillations are analyzed analytically. Nonlinear resonant and near-resonant solutions for the bubble nonharmonic oscillations, which are excited by harmonic pressure oscillations on the flask wall, are obtained. The influence of heat transfer phenomena on the bubble oscillations is analyzed.

THE RESONANT SUPERCOMPRESSION AND SONOLUMINESCENCE OF A GAS BUBBLE IN A LIQUID-FILLED FLASK

The spherically-symmetric problem of the oscillations of a small gas bubble in the center of a spherical flask filled with a compressible liquid that is excited by small radial displacement of the flask's wall is considered. Two asymptotic solutions have been found for the low Mach number stage. The first one is an asymptotic solution for the field far from the bubble, and it corresponds to linear wave theory. The second one is an asymptotic solution for the boundary layer near the bubble and it corresponds to an incompressible fluid. In the analytical solution of the low Mach number step matching of these asymptotic solutions is done. A generalization of the Rayleigh = Plesset equation for a compressible liquid is given in the form of two ordinary difference-differential equations that take into account the pressure waves which are reflecting from the bubble and those that are incident on the bubble from the flask wall. The initial value problem for the initiation of the bubble oscillations due to flask wall excitation and for the evolution of these oscillations was considered. Linear and non-linear periodic bubble oscillations were analyzed analytically, and resonant frequencies were identified. Non-linear resonant and near-resonant solutions for the bubble's non-harmonic oscillations, which are excited by harmonic pressure or velocity oscillations on the flask wall, are obtained.

Détermination du débit dans une conduite à partir de mesures de pression acoustique

An original technique is presented for measurement of flow rate in a duct, based on acoustic pressure measurements on the tube wall and single frequency excitation. A relation between the cross-section average velocity and two acoustic pressure phase differences can be obtained for particular frequencies. This method is carried out in an hydraulic circuit. The results are in good agreement with measurements using an industrial flowmeter.

ANISOTROPIC HYPERFINE INTERACTIONS IN Fe4N ALLOY

The nuclear magnetic resonance spectrum of 57Fe nuclei in γ - Fe4N alloy has been observed by the coherent spin-echo method. The peculiar line shape of nuclear magnetic resonance of Fe Ⅱ arises from the anisotropic hyperfine interactions and the domain wall excitation.

Dynamical characteristics of wave-excited channel flow

This paper is part of a study on the receptivity characteristics of the shear flow in a channel whose walls are subjected to a wave-like excitation. The small amplitude forced wavy wall motion is characterised by a wave number vectorλ 1,λ 2 and a frequencyω g . The basic flow in the problem is a superposition of the Poiseuille flow and a periodic component that corresponds to the wave excitation of the wall. The aim of the study is to examine the susceptibility of this flow to transition. The problem is approached through studying the stability characteristics of the basic flow with respect to small disturbances. The theoretical framework for this purpose is Floquet theory. The solution procedure for solving the eigenvalue problem is the spectral collocation method. Preliminary results showing the influence of the amplitude and the wave number of the wall excitation on the stability boundary of the flow are presented.

Magnetization Process of a Dilute Ising-Like Chain

Magnetization process at zero temperature is studied numerically for an Ising-like magnetic chain diluted with nonmagnetic impurities, which break up a magnetic chain into independent segments with various lengths. Calculations are made of the weighted contributions to the magnetization from each segment. The magnetization curve thus obtained shows successive metamagnetic transitions at lower fields in addition to the metamagnetic transition corresponding to the one in the pure case. These successive transitions are attributed to the instabilities of a single domain wall excitation including end-spin reversal process in various segments consisting of even numbers of spins. The characteristic features of the magnetization curve are consistent with recent experimental results on CsCo 1- x Mg x Cl 3 at very low temperatures.

A New Type of Domain Wall Excitation in Magnetic Systems

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