Oscillatory Displacement Research Articles

Acoustic streaming in the Cochlea resulting from periodic excitation

This paper presents a model describing acoustic streaming in the scala vestibuli and the scala tympani of the cochlea. The magnitude and spatial features of the acoustic streaming are a function of the vibration of the cochlear partition. Hence, spatial adjustment of the acoustic streaming field can be accomplished by suitable harmonic adjustment of the excitation. It is shown that the dynamic behavior of the fluid is a function of the values of two nondimensional parameters S and R. The Strouhal number S is the ratio of the typical length to the oscillatory particle displacement and the oscillatory Reynolds number R is the ratio of typical length to the oscillatory boundary layer thickness. The method of matched asymptotic expansions is used to obtain a solution that is valid in the limit as 1/S tends to zero. The solution is shown for the slow streaming R/S = O(1) and moderate streaming R/S2 = O(1) cases.

Study of Typical Ruptured and Unruptured Intracranial Aneurysms Based on Fluid–Structure Interaction

Most intracranial aneurysms (IAs) will be abnormal bulges on the walls of intracranial arteries that result from the dynamic interaction of geometric morphology, hemodynamics, and pathophysiology. Hemodynamics plays a key role in the origin, development, and rupture of IAs. In the past, hemodynamic studies of IAs were mostly based on the rigid wall hypothesis of computational fluid dynamics, and the influence of arterial wall deformation was ignored. We used fluid-structure interaction (FSI) to study the features of ruptured aneurysms, because it can solve this problem very well and the simulation will be more realistic. A total of 12 IAs, 8 ruptured and 4 unruptured, at the middle cerebral artery bifurcation were studied using FSI to better identify the characteristics of ruptured IAs. We studied the differences in the hemodynamic parameters, including the flow pattern, wall shear stress (WSS), oscillatory shear index (OSI), and displacement and deformation of the arterial wall. Ruptured IAs had a larger low WSS area and more complex, concentrated, and unstable flow. Also, the OSI was higher. In addition, the displacement deformation area at the ruptured IA was more concentrated and larger. A large aspect ratio; a large height/width ratio; complex, unstable, and concentrated flow patterns with small impact areas; a large low WSS region; large WSS fluctuation, high OSI; and large displacement of the aneurysm dome could be risk factors associated with aneurysm rupture. If similar cases are encountered when simulation is used in the clinic, priority should be given to diagnosis and treatment.

Wells turbine efficiency improvements: experimental application of a speed control strategy

Systems based on the OWC principle allow to convert the sea-wave energy into electrical energy. The potential energy contained in the wave motion is first converted into the pneumatic energy of a bi-directional airflow, generated by the oscillatory displacement of a column of water inside a chamber. Then, an air turbine, such as the Wells turbine, placed at the top of the chamber, converts the pneumatic energy into mechanical energy at its shaft, which can be coupled to an electric motor to produce energy. The turbine’s performance is strongly affected by the non-stationary behavior of the airflow, which continuously changes its intensity. Control solutions based on the variation of the turbine rotational speed can overcome this limitation.This work aims to study a speed-controlled Wells turbine with an experimental approach, making use of a turbine coupled to an Oscillating Water Column (OWC) simulator. A control law for the rotational speed has been defined and applied to the turbine in order to move the turbine’s operating conditions as close as possible to its maximum efficiency point, for the majority of the piston period.The experimental results prove the effectiveness of the proposed control strategy for moving the operating conditions and for obtaining operating conditions close to the best efficiency point for the majority of the wave period.

Analysis and Control of Combustion Instabilities in Rocket Engines

Sound is the transmission of energy that is produced when two particles or objects undergo collision. It is one of the forms in which energy modulates and travels in a medium. Vibration is a phenomenon in which an object of mass executes a periodic oscillatory displacement when certain energy is transferred to it. Combustion is a chemical reaction in which a lot of molecules collide. Combustion instabilities occur in a reacting flow. It is a physical phenomenon. Combustion instabilities have mostly been studied in a particular flow but they also occur in real life. Real engines often feature specific unstable modes such as azimuthal instabilities in gas turbines or transverse modes in rocket chambers. Eddy simulation has been the major tool to study this type of instabilities so far but recently it has been proved to be insufficient to completely understand the complex nature of these instabilities [3]. These instabilities involve large Reynolds number, high pressure, densities in real engines. Combustion instabilities can occur at any part of the rocket propulsion system like nozzle, combustion chamber, injectors, feed systems and lines. Theory plays an important role in understanding and analysing these instabilities and the amount of damage they can cause to a particular object. In this paper we will look at different types or modes of combustion instabilities and active and passive ways to control them in real situations.

Modeling Local Oscillatory Shear Dynamics of Functionalized Polymer Grafted Nanoparticles

AbstractThis study examines local oscillatory shear dynamics of spherical polymer grafted nanoparticles (PGNs) composed of a rigid nanoparticle core of fixed radius, nm, and coronas of varying number of grafted polymer arms, . The grafted polymer arms are end‐functionalized such that the interacting PGNs form labile bonds when their coronas overlap. The effect of PGN structure on local dynamics is examined by imposing controlled oscillatory displacement on simple systems constructed using three identical interacting PGNs with bonded interactions under mechanical equilibrium. The displacement‐induced bond evolution between PGNs in this system is captured employing a master equation for determining probability of finding n bonds between PGNs as a function of oscillation frequency (). A multi‐component model combined with master equation approach is employed for modeling the effect of varying number of grafted arms ( and 1200) and strain amplitude () on the local shear response of the PGN system. The resulting shear response is analyzed using both linear and non‐linear rheological measures obtained using the Chebyshev polynomial framework. The rheological measures thus obtained are found to vary strongly with the number of grafted arms and strain amplitude.

Fracture analysis of ultra-thin coating/substrate structures with interface cracks

This paper presents a fracture analysis of ultra-thin coating/substrate structures with interface cracks. For this purpose, an advanced boundary element method (BEM) is adopted. To correctly describe the oscillatory displacement and stress fields near the interfacial crack-tip, a new set of novel special crack-tip elements are implemented. The complex interfacial stress intensity factors (SIFs) can be directly and accurately computed at the collocation points extremely close to the crack-tip by using the proposed computational strategy based on the novel special crack-tip elements. Furthermore, the troublesome nearly-singular boundary integrals, which are crucial in the application of the BEM for both crack-like and ultra-thin structural problems, are calculated accurately by using a nonlinear coordinate transformation method. It is shown that the advanced BEM based on the novel special crack-tip elements and appropriate technique for evaluating nearly-singular boundary integrals provides an accurate and robust numerical tool for interfacial crack analysis of ultra-thin coating structures. Accurate and reliable BEM results with only a small number of boundary elements can be obtained for a relative coating-to-substrate thickness as small as 10-9, which is important for modeling ultra-thin coating structures as used in smart materials and micro/nano-electro-mechanical systems (MEMS/NEMS). Several representative numerical examples will be presented and discussed to reveal the effects of the key geometrical and material parameters, the crack configuration and the loading conditions on the interfacial fracture parameters such as energy release rate and complex SIFs

PARAMETERS FOR MODELING PASSIVE SUSPENSIONS OF SPRAY BARS

The uniformity and quality of spraying depend on the stability of the spray boom, defined as the suspension system between the boom and the machine chassis. This paper presents a procedure for improving the performance of passive spray bar suspensions through parameter adjustment. Two multibody dynamics models of a tractor sprayer set were developed to evaluate their suspension systems: a rigid body dynamics model (RBDM) and a finite element model (FEM) using deformable bodies. To calibrate the models in the experiment, an accelerating force was applied to the suspension, and the displacements of the shock absorber and the rubber springs were monitored. The FEM is more suitable for the evaluation of the horizontal oscillations of the bar, based on root mean square (RMS) values and a standard curve used to evaluate the stability of the bar. The horizontal stiffness of the bar significantly influences the oscillatory displacement and must be included in the simulation models. Resizing the structure can reduce the horizontal oscillations of the bar.

Fracture mechanics analysis of bimaterial interface cracks using the generalized finite difference method

This paper makes the first attempt to apply the generalized finite difference method (GFDM), a recently developed meshless collocation method, for fracture mechanics analysis of dissimilar elastic materials with interfacial cracks. The main idea of the GFDM is to divide the entire computational domain into a set of overlapping small subdomains in which the local Taylor series expansion and moving-least square approximation are employed for generating the local systems of linear equations. Since the method is meshless and no element connectivity is needed, the burdensome remeshing procedures associated with the finite element method (FEM) is avoided. The multi-domain GFDM technique is used to handle the non-homogeneity of the cracked dissimilar materials. The displacement extrapolation method (DEM), which avoids the direct calculation of the oscillatory near-tip displacement and stress fields, is employed to compute the complex stress intensity factors (SIFs) for cracked composite bimaterials. Several representative numerical examples are presented and discussed to demonstrate that the present method is highly accurate and relatively robust for interface crack analysis of composite bimaterials.

Generalized composite noncertainty-equivalence adaptive control of a prototypical wing section with torsional nonlinearity

The paper presents a generalized composite noncertainty-equivalence adaptive control system for the control of a prototypical aeroelastic wing section using a single trailing-edge control surface. The plunge–pitch (two-degree-of-freedom) dynamics of this aeroelastic system include torsional pitch-axis nonlinearity. The open-loop system exhibits limit cycle oscillations beyond a critical free-stream velocity. It is assumed that parameters of the model are not known. The objective is to suppress the oscillatory responses of the system. Based on the immersion and invariance approach, a generalized composite noncertainty-equivalence adaptive (NCEA) control system for regulation of the pitch angle is designed. The control system consists of a control module and a composite parameter identifier—designed independently. The composite integral parameter estimation law is based on (1) the immersion and invariance (I&I) theory, (2) gradient-based adaptation algorithm, and (3) classical certainty-equivalence adaptive (CEA) update rule. Besides the composite integral component, the full parameter estimate also includes a judiciously chosen nonlinear algebraic function. This composite identifier inherits stronger stability properties. Using the Lyapunov analysis, asymptotic suppression of the limit cycle oscillations and the boundedness of system trajectories are established. Interestingly, in the closed-loop system including the composite update rule, there exist two attractive manifolds to which the system’s trajectories converge. Simulation results are presented which show the suppression of the oscillatory plunge displacement and pitch angle responses despite uncertainties in the model parameters. Furthermore, the performance and stability properties of this composite NCEA control system—including the gradient-based adaptation and the update rule of the CEA system—are better than the simple NCEA system.

Novel special crack-tip elements for interface crack analysis by an efficient boundary element method

This paper presents a new set of novel special crack-tip elements for analysis of interface cracks in linear elastic composite bimaterials by using the boundary element method (BEM). Such interface crack problems present some simulation difficulties based on the conventional numerical methods because the asymptotic crack-tip fields in this case exhibit an oscillatory behavior which is quite different from that in a cracked homogeneous material. The novel special crack-tip elements developed in this study contain the correct local asymptotic behavior of the displacement and stress fields at the tip of an interface crack, and can thus appropriately describe the oscillatory near-tip displacement and stress fields for cracked composite bimaterials. The novel special crack-tip elements are implemented into a BEM, which can significantly improve the computational accuracy of the complex stress intensity factor (SIF) for an interface crack even with a relatively coarse mesh. The proposed computational strategy for interface crack analysis based on the novel special crack-tip elements also enables a direct and accurate evaluation of the complex SIF at the collocation point extremely close to the crack-tip. Several numerical examples are presented and discussed to demonstrate that the present method based on the novel special crack-tip elements is pretty simple, highly accurate, and relatively robust with respect to the size of the used crack-tip elements.

Optimization of Orifice Shape and Oscillatory Displacement of an Ultrasonic Microbubble Generator to Increase Microbubble Yield

Optimization of Orifice Shape and Oscillatory Displacement of an Ultrasonic Microbubble Generator to Increase Microbubble Yield

Large shaking table tests of pile-supported structures in different ground conditions

This study investigates the dynamic response of structures and the effect of soil-structure interaction for flexible structures supported by pile foundations located in non-liquefiable and liquefiable sites as well as a rigid foundation. A series of large-scale shaking table experiments were performed, and accelerations and dynamic displacements of model superstructures were recorded during the shaking events. It was observed that the structure in non-liquefiable soil deposits had the highest accelerations due to the amplification of ground motion within the soil. On the other hand, the accelerations of the structure in liquefied site was small due to the increased dissipation of seismic energy. The structures founded on soil experienced residual (permanent) lateral displacements, with the permanent lateral displacement and tilting of the structure in the liquefied site being much larger. The structure on rigid foundation (i.e. foundation on rock or fixed base) experienced high accelerations and oscillatory displacement as the seismic energy could not be dissipated. Therefore, structures in liquefiable soil are prone to overturning damage due to failure of their foundations. Furthermore, the base shear force of the structure in the non-liquefied site was significantly larger than that in the liquefied site and fixed-base, indicating it would be more susceptible to shear failure. On the other hand, the fixed-base structure experienced relatively large acceleration and dynamic displacement responses, which indicates increased seismic demands on its structural members.

Proper Orthogonal Decomposition for the prediction of fretting wear characteristics

Fretting happens when there is an oscillatory displacement between two contact parts. In gross slip regime, wear is a dominant failure mode, which has a detrimental effect on the contact parts. To understand the wear behaviour in the fretting process, many experiments and simulations have been done. This phenomenal analysis is time-consuming, especially when fretting wear investigation is required under certain loading cases or during certain number of cycles. Under these conditions, fast prediction tools provide an alternative for this kind of problem. In this paper, a model reduction based on Proper Orthogonal Decomposition with Radial Basis Function (POD-RBF) is used to predict the wear characteristics of specimens under different slip amplitudes and at different fretting wear cycles based on simulation results. To demonstrate the advantages and disadvantages of POD-RBF, Grey Model and Curve Fitting are used to verify POD-RBF for one-parameter prediction, while Artificial Neural Network is applied to verify POD-RBF for three-parameter prediction. The results show that the approach of POD-RBF can predict the wear characteristics under different slip amplitudes and normal loads at different cycles with high accuracy and high computational efficiency.

A fretting test apparatus for measuring friction hysteresis of bolted joints

This paper focuses on an experimental investigation of the mechanical behavior of bolted joints. A new test apparatus to measure friction hysteresis was designed to provide a reliable experimental database for the calibration of contact models. This apparatus uses a piezoelectric actuator to provide the contact interfaces with stable oscillatory relative displacement. The friction force and the bolt preload were continuously measured during the test, using a load cell and a force washer, respectively. The relative motion between the interfaces was measured using a single laser vibrometer. The influence of the bolt preload and excitation amplitude on friction hysteresis was investigated. A numerical model was developed to extract the tangential contact stiffness from the measured data.

Analysis of Cyclic Response of Bucket Foundations Based on Simplified Kinematic Hardening Model

The response of bucket foundations for offshore wind turbines subjected to cyclic loading in saturated clay is explored through three‐dimensional finite element numerical analyses. In the analyses, nonlinear cyclic hysteretic behavior of clay under undrained condition is modeled through a simple kinematic hardening constitutive model embedded in ABAQUS. The finite element model is validated against published in situ tests of bucket foundations under quasistatic cyclic loading in Bothkennar clay. The computed results agreed generally with those from in situ tests. The behavior of bucket foundations with different aspect ratios under displacement‐controlled cyclic loading mode is investigated. Then, the evolution of foundation displacement with increasing number of cycles is studied subjected to wind and wave combined loading. The results show that, for the cycles of low‐amplitude rotation, dimensionless moment‐rotation curve is approximately elastic; however, the curve engenders obvious hysteresis loop, whose shape is influenced by soil‐sidewall interface condition, during high‐amplitude cycles. Under thousands of loading cycles, for bucket foundations of low aspect ratio, the oscillatory displacement component is smaller; however, the residual component will accumulate gradually until the serviceability rotation is exceeded. For foundations of high aspect ratio, the oscillatory component is relatively larger, but the accumulation rate of residual displacement decreases gradually.

Investigation of oscillatory displacement of the hydraulic presses moving parts during their braking in the upper initial position

The investigations revealed that, despite the usage of individual servo drive of the inlet valve of return cylinders, ram overrun upwards after its being shut down which is accompanied by intensive oscillatory phenomena is inevitable. By analyzing the experimental diagrams it is stated that the existing mechanism of ram braking in upper starting position is unacceptable from the point of view of fast and non-impact process realization, as well as the accuracy of stoppage. The mathematical model of ram damped oscillations in the upper position after the inlet valve of return cylinders shut down, which takes into account the parameters of oscillatory system consisting of ram and fluid masses in power and return cylinders, is developed. A numerical analysis of the developed mathematical model established that when using counterbalance cylinders in press design, the period of oscillations increases with decreasing the amplitude. Besides, the complete damping of the oscillations occurs earlier than in the case of counterbalance cylinders absence.

Magnetic Reconnection in Three Dimensions: Observations of Electromagnetic Drift Waves in the Adjacent Current Sheet

AbstractMagnetic reconnection at the subsolar magnetopause is persistently accompanied by strong fluctuations of the magnetic field (B), plasma density (n), and all components of the electric field (E) and current (J). The strongest fluctuations are at frequencies below the lower hybrid frequency and observed in a thin, intense current sheet adjacent to the electron diffusion region. In this current sheet, the background magnitudes of B and n are changing considerably, creating an inhomogeneous plasma environment. We show that the fluctuations in B and n are consistent with an oscillatory displacement of the current sheet in the surface normal direction. The displacement is propagating parallel to the magnetic reconnection X line. Wavelengths are on the order of or longer than the thickness of the current sheet to which they are confined, so we label these waves electromagnetic drift waves. E and J fluctuations are more complex than a simple displacement. They have significant variations in the component along B, which suggest that the drift waves also may be confined along B. The current sheet is supported by an electron drift driven by normal electric field, which, in turn, is balanced by an ion pressure gradient. We suggest that wave growth comes from an instability related to the drift between the electrons and ions. We discuss the possibility that drift waves can displace or penetrate into the electron diffusion region giving magnetic reconnection three‐dimensional structure. Drift waves may corrugate the X line, possibly breaking the X line and generating turbulence.

Expanding ring-shaped Bose-Einstein condensates as analogs of cosmological models: Analytical characterization of the inflationary dynamics

We analytically study the expansion of a Bose-Einstein condensate in a ring-shaped trap with an increasing central radius. The evolution of the ground state is described using a scaling transform. Additionally, the dynamics of excited azimuthal modes over the varying ground state is analyzed through a generalization of the Bogoliubov-de Gennes approach. Our results explain some of the features observed in recent experiments focused on testing the applicability of the system as a parallel of cosmological inflationary models. The radial dynamics, which corresponds to the inflaton field of the cosmological counterpart, is analytically characterized: The expansion is found to induce the oscillatory displacement of the condensate as well as the coupled variation of the radial and vertical widths. Our findings account also for the observed redshift and emergence of the Hubble friction in the evolution of initially-prepared azimuthal modes. Our description, which traces the role of the different components of the setup in the expansion, enhances the controllability, and, therefore, the potential of the system as a ground for emulating the inflationary dynamics of cosmological models.

Analysis of the application of ZrN coatings for the mitigation of the development of fretting wear processes at the surfaces of push fit joint elements

The purpose of this article is to assess the influence of a ZrN coating on the development of fretting wear in a model of a wheel/axle push fit joint in a rail vehicle. The examinations of the fretting wear process in rail vehicle wheel sets were conducted on two groups of models consisting of a shaft and sleeve connected by means of a push-fit joint, in compliance with the rules for the representation of the real condition. Wear tests of a joint loaded with a vertical force were conducted on a fatigue testing machine permitting oscillatory tangential displacement, which is responsible for the development of fretting wear. Uncoated shafts were the first model, and shafts coated with a ZrN coating were the second one. The tolerance was 0.02 mm, the model loading force was 550 N and the number of fatigue cycles was 7 × 106. The destruction of models was evaluated in, for example, the computer image analysis process with the use of the Image Pro Plus 7.0 software. Macroscopic observations showed wear traces for both model groups. In the first group, wear traces were observed on either side of the axle seat, and those traces had the form of a 2–3 mm wide ring comprising the entire shaft circumference. The random location of the traces of wear was observed in the second model. The microscopic observations of the destruction areas demonstrated mainly the presence of material build-ups, the local surface abrasion and micropits. The analysis of the chemical composition of material build-ups showed the presence of iron and oxygen, a fact which confirms the shearing of the microirregularities of the sleeve top layer. Plastically deformed material build-ups become oxidised, hence the brown colour of the worn-out areas can be observed at the sample surface during macroscopic observations. The significant mitigation of fretting wear on samples with ZrN coatings has been demonstrated. This shows the need for fretting wear tests to be performed on a real object to confirm the scale of improvement of the life of forced-in joints, thus contributing to the higher reliability of rail vehicle wheel sets.

Heat transfer enhancement of air-cooled heat sink channel using a piezoelectric synthetic jet array

In the last decade, active devices, such as synthetic jets have been intensively studied for electronics cooling. The present study experimentally and computationally investigates heat transfer performance of synthetic jet arrays used in heat sink channels. The heat sink of the present study consists of multiple flow channels that are parallel to one another. The channel walls behave like fins in that their tops are exposed to the flow. The jets are designed to impinge on the tops and sides of those fins to augment heat transfer. The current study employs a single channel of the heat sink to investigate heat transfer augmentation performance by the jets. The oscillating diaphragms that create the jets are driven by a piezo-bow operating at its second resonant vibrational mode to generate a large oscillatory displacement at a high working frequency. The frequency for this study is 1240 Hz and the measured displacement of the jet-driving diaphragm is 0.5 mm. The corresponding peak velocity of each jet is around 45 m/s and the total power consumption is 1.6 W when operating with 20 jets. Heat transfer experiments using jet arrays of different jet orifice configurations are conducted in a single narrow channel that represents one of the channels of a multi-channel heat sink. With a through-flow velocity of 14.7 m/s driven by a centrifugal fan, the synthetic jet arrays with square orifices achieve 9.3% enhancement on heat transfer coefficient averaged over the entire fin (channel wall) surface, compared to the value for a case with through-flow only. When the channel through-flow velocity decreases to 8 m/s spatially-averaged heat transfer enhancement by the jets is 21.7%; again, averaged over the entire fin (channel wall) surface. In a computational study, the jet diaphragm movement is realized with a dynamic mesh available within the commercial software package ANSYS Fluent. Computed surface-average Nusselt numbers show good agreement with the experimental data, differing by no more than 10%. The numerical study was performed to quantify the effects of system parameters, such as the jet and channel flow rates and orifice-to-fin-surface distance, on heat transfer performance over different sections of the channel (fin) wall. According to the results of the numerical study, the synthetic jets have a strong cooling effect on the channel wall tip region, the nearest surface to the jet orifice, and a weaker effect on the channel side surfaces. It is found that the synthetic jet can enhance locally-averaged heat transfer coefficients at the fin tip by up to 413%, compared to a case with cooling by channel through-flow only.