Constant Excitation Amplitude Research Articles

Control of flow separation over an aerofoil by external acoustic excitation at a high Reynolds number

The effectiveness of acoustic excitation as a means of flow control at high Reynolds number turbulent flows is investigated numerically by using improved delayed detached eddy simulations (IDDES). Previous studies on low Reynolds number laminar flows have shown that acoustic excitation can substantially suppress flow separation for specific effective frequency and amplitude ranges. However, the effect of acoustic excitation on higher Reynolds number turbulent flow separation has not yet been explored due to limitations on appropriate fidelity computational methods or experimental facility constraints. Therefore, this paper addresses this research gap. A NACA0015 airfoil profile at 1 × 106 Reynolds number based on the airfoil chord length is used for the investigations. Acoustic excitation is applied to the baseline flow field in the form of transient boundary conditions at the computational domain inlet. A parametric study revealed that the effective sound frequency range shows a Gaussian distribution around the frequency of the dominant disturbances in the baseline flow. A maximum of ∼43% increase in lift-to-drag ratio is observed for the most effective excitation frequency F+=1.0 at a constant excitation amplitude of Am=1.8%. The effect of excitation amplitude follows an asymptotic trend with a maximum effective excitation amplitude above which the gains are not significant. A fully reattached flow is observed for the highest excitation level considered (Am=10%) that results in ∼120% rise in airfoil lift-to-drag coefficient. Overall, the findings of the current work demonstrate the higher Reynolds number effectiveness of acoustic excitation on separated turbulent flows, thereby paving the way for application in realistic flow scenarios observed in aircraft and gas turbine engine flow fields.

Dynamical characterization of a Duffing–Holmes system containing nonlinear damping under constant excitation

A Duffing-Holmes system containing nonlinear damping is used to investigate some dynamical properties of the system under the combined action of constant excitation and simple harmonic excitation. The harmonic balance method is used to find the main vibration equation of the system and obtain the amplitude-frequency response relationship. In the analysis of the global characteristics of the system, the Melnikov method is applied to analyze the necessary, insufficient conditions for the analysis of the system chaos, and the correctness of the analytical solution is verified by numerical calculations; the effect of the constant excitation amplitude on the global characteristics of the system is analyzed by the cell mapping method. The paper shows: gradually increasing the constant excitation amplitude causes the system amplitude-frequency response to appear rigidly asymptotic phenomenon; the boundary curve of the system generating chaos in the sense of Smale's horseshoe, with the increase of excitation frequency ω, the curve first decreases, then rises, finally tends to infinity, and the possibility of chaos occurs most near the resonance frequency of the system; with the change of the constant excitation amplitude, The number of attractor domains and attractors in the global attraction domain of the system changes substantially with the change of the constant excitation amplitude, and the attractor domains are entangled with each other.

Physics-informed data-driven reduced-order models for Dynamic Induction Control

In this work, we find a reduced-order model for the wake of a wind turbine controlled with dynamic induction control. We use a physics-informed dynamic mode decomposition algorithm to reduce the model complexity in a way such that the physics of the wake mixing can be investigated and that the model itself can be easily embedded into control-oriented frameworks.After discussing the advantage of forcing the linear system resulting from the algorithm to be conservative (as a consequence of the periodicity of the pitch excitation) and the choice of observables, we describe a procedure for calculating the energy associated with individual modes. The considered data-set is composed of large eddy simulation (LES) results for a single DTU 10 MW wind turbine in uniform flow. Simulations were performed first with baseline control (for reference) and then with the Pulse and the Helix approaches with constant excitation amplitude and different excitation frequencies. The frequencies and energies associated with the resulting modes are discussed.

Influence of the Selected Physical Modifier on the Dynamical Behavior of the Polymer Composites Used in the Aviation Industry.

In this research, an analysis of polymer composite with the matrix of L285-cured hardener H286 and six reinforcement layers of carbon fabric GG 280 T was provided. It involved a comparison of the dynamical behavior responses for three cases of composite structures in the context of the presence of the mass share modifier. The samples with the addition of a physical modifier with varying mass percentages were investigated by being subjected to dynamic tests with specific parameters, i.e., constant excitation amplitude and vibration frequency in the vicinity of the base resonance zone. The analysis allowed for indicating the relationship between the composition of the prepared composites and their dynamic response via stiffness characteristics. In addition, the investigation resulted in determining the range of harmful dynamical operating conditions, which may contribute to damage to the composite structures.

The numerical and experimental investigations of a gimballed pendulum energy harvester

A simple or single-degree-of-freedom (1-DOF) pendulum system is a typical mechanism that has been used for vibration energy harvesting. Generally, a 1-DOF pendulum system is constrained to move on a single plane which limits the system when excited by multi-directional disturbances. In order to overcome this limitation, this paper considers the application of a 2-axis gimbal pendulum system. The gimballed pendulum system consists of two perpendicular horizontal pivots which allow the pendulum to achieve spherical or orbital motion around the pivots. Theoretical and experimental investigations of a gimballed pendulum energy harvesting system for multi-directional excitation are described in this paper. A mathematical model of the mechanical arrangement is derived based on the Lagrange equation and a simplified assumption that the perpendicular pivots are treated as two coupled rotating references with geometric relationships. This derived model is able to realistically reflect and identify the asymmetric properties between the coupled references created by a gimbal mechanism. The theoretical and experimental investigations of 1-DOF and coupled-DOF (2-DOF) pendulum dynamics are presented using a constant excitation amplitude over a range of frequencies with different heading angles representing the directions of the external disturbance. The numerical and experimental results show generally good correlation and the dominant coupling effect of the coupled rotating references that modulates the pendulum responses is theoretically and practically captured. Interestingly, the measured simultaneous electrical power production generated by the coupled dynamics of the gimballed pendulum achieves less power compared to when it dynamically performs as a 1-DOF system at the same disturbing condition.

Vibration lifetime estimation of PBGA solder joints using Steinberg model

The work studies the vibration lifetime modeling of Sn37Pb soldered plastic ball grid array (PBGA) assemblies on the basis of vibration tests, finite element analysis (FEA) and Steinberg's empirical formulation. The test vehicles for the vibration tests consist of twelve PBGA components with built-in daisy-chained circuits, which are assembled on a printed circuit board (PCB) symmetrically. First, the natural frequencies of the test vehicle were determined by modal tests. The first three natural frequencies were obtained using FEA, which were compared with those from the modal test. Then, narrow-band sinusoid vibration tests were conducted at the first natural frequency of the test vehicle using constant-amplitude excitation, and the number of failure cycles was recorded by monitoring the overall electrical resistance. FEA was performed to obtain the stress of the critical solder joints under various levels of sinusoidal vibration loadings. To construct the stress versus failure cycles (SN) curve of the solder joints, the stresses of solder joints were used in conjunction with the lifetime obtained from the sinusoidal vibration tests. The random vibration test was conducted on the test vehicle to assess the lifetime of the solder joints subjected to broad-band excitation. Random vibration analyses were performed numerically to obtain the displacement responses of the PBGA assembly. Finally, a fatigue life prediction based on Steinberg's model was deduced from all these procedures. A comparison of the lifetime between the experimental results and the prediction suggests that the methodology is valid and practicable in predicting the vibration lifetime of PBGA solder joints.

Employment of sawtooth-shaped-function excitation signal and oversampling for improving resistance measurement accuracy.

In order to achieve higher measurement accuracy of routine resistance without increasing the complexity and cost of the system circuit of existing methods, this paper presents a novel method that exploits a shaped-function excitation signal and oversampling technology. The excitation signal source for resistance measurement is modulated by the sawtooth-shaped-function signal, and oversampling technology is employed to increase the resolution and the accuracy of the measurement system. Compared with the traditional method of using constant amplitude excitation signal, this method can effectively enhance the measuring accuracy by almost one order of magnitude and reduce the root mean square error by 3.75 times under the same measurement conditions. The results of experiments show that the novel method can attain the aim of significantly improve the measurement accuracy of resistance on the premise of not increasing the system cost and complexity of the circuit, which is significantly valuable for applying in electronic instruments.

Optimal Damped Vibration Absorber: Including Multiple Modes and Excitation Due to Rotating Unbalance

Optimal vibration absorbers are designed using the definition of H-infinity norm and numerical optimization. The cases of both constant amplitude excitation and rotating unbalance excitation are considered. The design procedure is quite general, and can easily handle multiple modes of vibration. A general procedure to construct a root loci plot is also developed. Using a cantilever beam, numerical results are presented.

Directivity maximization and optimal far-field pattern of time modulated linear antenna arrays using evolutionary algorithms

In this paper evolutionary algorithm based design of time modulated 16-element linear antenna array with improved directivity and side lobe level (SLL) with low radiated power at sideband harmonics has been dealt with. In time modulated linear antenna array time is used as fourth dimensional parameter. The comparative study has been made with all the possible combinations of control parameters like constant excitation amplitude, inter-element spacing, and “time” as the fourth dimension. The same array radiates at various harmonic frequencies. In this paper the authors have considered only two harmonic frequencies; first sideband frequency and second side band frequency. Various simulation results are presented showing better side lobe performance, better side band performance and improved directivity with respect to the uniform case. The statistical analysis and t-test have been done to prove the superior performance of differential evolution with wavelet mutation (DEWM) better than real coded genetic algorithm (RGA), particle swarm optimization (PSO), and differential evolution (DE).

Bimodal atomic force microscopy driving the higher eigenmode in frequency-modulation mode: Implementation, advantages, disadvantages and comparison to the open-loop case

We present an overview of the bimodal amplitude–frequency-modulation (AM-FM) imaging mode of atomic force microscopy (AFM), whereby the fundamental eigenmode is driven by using the amplitude-modulation technique (AM-AFM) while a higher eigenmode is driven by using either the constant-excitation or the constant-amplitude variant of the frequency-modulation (FM-AFM) technique. We also offer a comparison to the original bimodal AFM method, in which the higher eigenmode is driven with constant frequency and constant excitation amplitude. General as well as particular characteristics of the different driving schemes are highlighted from theoretical and experimental points of view, revealing the advantages and disadvantages of each. This study provides information and guidelines that can be useful in selecting the most appropriate operation mode to characterize different samples in the most efficient and reliable way.

Interactions of lock-on wake behind side-by-side cylinders of unequal diameter at Reynolds number 600

The flow characteristics, mutual interactions and downstream evolutions of the wakes behind the side-by-side cylinders of unequal diameter are illustrated by flow visualization and Laser Doppler Anemometry. All the experiments are conducted in a recirculating water channel at Reynolds number 600. The gap ratio is 0.75 and the diameter ratio is 2.0. Periodic excitations are applied to the large cylinder with constant excitation amplitude. Within the excitation frequency range studied, the wake behind the large cylinder experiences the primary and the one third sub-harmonics lock-on; whereas the wakes behind the small cylinder are not locked-on. For the wake behind the large cylinder experiencing the primary lock-on, the gap flow becomes unbiased and the gap vortices are observed to interact and shed alternately into the wake behind each cylinder in synchronization with the excitation frequency (the natural vortex shedding frequency of large cylinder). The synchronized flow patterns exist only in the near wake region; in the downstream region, only one wider wake dominated by the excitation frequency is detected. While the wake behind large cylinder is locked-on at the one third sub-harmonics of the excitation frequency, the gap vortices are observed to entrain alternately into the wake behind each cylinder in synchronization with the one third of the excitation frequency (e.g., natural vortex shedding frequency of large cylinder). The synchronized flow patterns exist only in the near wake region; in the downstream region, the wake is relatively weak, less organized and turbulent. For both lock-on cases, the near and the far wake structures change suddenly within a short distance.

Conditioning-induced elastic nonlinearity in hysteretic media

The definition and measurement of the nonlinear elastic properties of a sample is of great importance for a large number of applications, including characterization of material performances and damage detection. However, such measurements are often influenced by spurious effects due to a combination of nonlinearity and nonequilibrium phenomena. We will present experimental data to show how nonlinearity due to small cracks in concrete samples increases as a consequence of conditioning, i.e., after having perturbed them with a constant amplitude excitation. In addition, our experimental data highlight “memory effects,” i.e., they show that when the excitation is removed, the elastic modulus does not return instantaneously to the initial value.

Vibration Durability Assessment of Sn3.0Ag0.5Cu and Sn37Pb Solders Under Harmonic Excitation

In this paper, the vibration durability of both SAC305 and Sn37Pb interconnects are investigated with narrow-band harmonic vibration tests conducted at the first natural frequency of the test, printed wiring board, using constant-amplitude excitation. A time-domain approach, reported by Upadhyayula and Dasgupta (1998, “Guidelines for Physics-of-Failure Based Accelerated Stress Test,” Proceedings, Reliability and Maintainability Symposium, pp. 345–357), was adapted in this study for the fatigue analysis. The test board consists of daisy-chained components, to facilitate real-time failure monitoring. The response of the test specimens was characterized, and accelerated fatigue tests were conducted at different loading amplitudes to obtain a mix of low-cycle fatigue (LCF) and high-cycle fatigue data points. The SAC305 interconnects were found to have lower fatigue durability than comparable Sn37Pb interconnects, under the narrow-band harmonic excitation levels used in this study. This trend is consistent with most results from broadband vibration tests by Zhou et al. (2006, “Vibration Durability Comparison of Sn37Pb vs. SnAgCu Solders,” Proceedings of ASME International Mechanical Engineering Congress and Exposition, Chicago, IL, Paper No. 13555), Zhou and Dasgupta (2006, “Vibration Durability Investigation for SnPb and SnAgCu Solders With Accelerated Testing and Modeling,” IEEE-TC7 Conference on Accelerated Stress Testing & Reliability, San Francisco, CA), and Woodrow (2005, “JCAA/JG-PP No-Lead Solder Project: Vibration Test,” Boeing Electronics Materials and Processes Technical Report) and from repetitive mechanical shock tests by Zhang et al. (2005, “Isothermal Mechanical Durability of Three Selected Pb-Free Solders: Sn3.9Ag0.6Cu, Sn3.5Ag and Sn0.7Cu,” ASME J. Electron. Packag., 127, pp. 512–522), but counter to findings from quasistatic, LCF, and mechanical cycling studies by Cuddalorepatta and Dasgupta (2005, “Cyclic Mechanical Durability of Sn3.0Ag0.5Cu Pb-Free Solder Alloy,” Proceedings of the ASME International Mechanical Engineering Congress and Exposition, Orlando, FL, Paper No. 81171). Failure analysis revealed two competing failure modes, one in the solder and another in the copper trace under the component. Thus solder fatigue properties extracted with the help of finite element simulation of the test article should be treated as lower-bound estimates of the actual fatigue curves.

Structural Damping with Friction Beams

In the last several years, there has been increasing interest in the use of friction joints for enhancing damping in structures. The joints themselves are responsible for the major part of the energy dissipation in assembled structures. The dissipated work in a joint depends on both the applied normal force and the excitation force. For the case of a constant amplitude excitation force, there is an optimal normal force which maximizes the damping. A ‘passive’ approach would be employed in this instance. In most cases however, the excitation force, as well as the interface parameters such as the friction coefficient, normal pressure distribution, etc., are not constant. In these cases, a ‘semi-active’ approach, which implements an active varying normal force, is necessary. For the ‘passive’ and ‘semi-active’ approaches, the normal force has to be measured. Interestingly, since the normal force in a friction joint influences the local stiffness, the natural frequencies of the assembled structure can be tuned by adjusting the normal force. Experiments and simulations are performed for a simple laboratory structure consisting of two superposed beams with friction in the interface. Numerical simulation of the friction interface requires non-linear models. The response of the double beam system is simulated using a numerical algorithm programmed inMATLABwhich models point-to-point friction with the Masing friction model. Numerical predictions and measurements of the double beam free vibration response are compared. A practical application is then described, in which a friction beam is used to damp the vibrations of the work piece table on a milling machine. The increased damping of the table reduces vibration amplitudes, which in turn results in enhanced surface quality of the machined parts, reduction in machine tool wear, and potentially higher feed rates. Optimal positioning of the friction beams is based on knowledge of the mode shapes, which are obtained from experimental modal analysis. The modal damping and the natural frequencies for the two dominant modes are measured for several combinations of excitation force and normal force.

Dynamic force microscopy with quartz tuning forks at high oscillation amplitudes

Dynamic force microscopy (DFM) with the self-oscillator (SO)method allows reasonably high scanning rates even with highQ-factors of the resonant force sensor, typical of cantilevers in ultra-high vacuum and ofquartz tuning forks. However, due to simpler interpretation of force spectroscopymeasurements, small oscillation amplitudes (sub-nm level) are generally preferred. Inapplications like ‘apertureless’ scanning near-field optical microscopy (SNOM), oscillationamplitudes of the order of 5–10 nm are needed to increase optical sensitivity and toapply standard optical artefact suppression methods. This motivates the studyof the behaviour of tuning forks driven at such high amplitudes, as comparedto usual air-operated cantilevers. Both constant-excitation-amplitude (CE) andconstant-oscillation-amplitude (CA) modes of SO-DFM are analysed, since the CA mode ismore convenient for SNOM applications, denoting remarkable differences. In particular,possible instability effects, previously found in CE mode, are not anticipated for CAmode. It is shown how resonance and approach (‘isophase’) curves in both modescan be conveniently described in terms of the usual ‘normalized frequency shift’γ and of a‘normalized gain’ η, defined as a measurement of surface dissipation.

Stability of dynamic force microscopy with the self-oscillator method

Dynamic force microscopy (DFM) with the self-oscillator (SO) method is not generallysubjected to the instability effects typical of tapping-mode DFM, as confirmedexperimentally. The inherent stability of SO-DFM is related to phase locking of thecantilever oscillation to the excitation signal. Such phase locking determines univocally theoscillation state (i.e. amplitude and frequency) on the resonance curve, even when multipleamplitude values are compatible with a given frequency. By modelling the behaviour of anair-operated DFM system, it is found that, while stabilizing tip/surface distance for DFMimaging at constant frequency shift, and beyond a certain critical phase value,instabilities are possible in the SO constant-excitation amplitude mode. However, suchinstabilities cannot affect dynamic force spectroscopy approach curves, because of phaselocking. By extension to vacuum operation, this result can confirm the origin ofjumps in frequency shift found on some experimental DFM approach curves, forinstance between non-passivated silicon tips to specific surface atomic sites ofreconstructed silicon, since instrumental effects of the SO method can be ruledout.

Dynamics of Magnetic Fluid-Permanent Magnet System Subjected to Vertical Vibration

This paper is concerned with the dynamic behavior of a magnetic fluid-permanent magnet system subjected to the vertical vibration. The surface response of magnetic fluid adsorbed to a disc permanent magnet is investigated experimentally. Experiment is performed on a vibration-testing system. The electrodynamic shaker is operated at a given frequency, displacement, and acceleration. The dynamic behavior of magnetic fluid surface is analyzed by a three-dimensional motion analysis system. The displacement of the permanent magnet is also measured with an optical displacement detector system. It is shown that the response amplitude depends on the excitation frequency at the constant excitation amplitude. The 1=2-sub harmonic response on the surface of magnetic fluid is observed at higher frequencies. This response is observed as polygonal modes with large amplitude azimuthal waves. The response of the system such as jumping motion is also shown at the resonance frequency of the system.

Dynamic AFM using the FM technique with constant excitation amplitude

Dynamic atomic force microscopy using the frequency modulation technique is investigated for the case that the excitation amplitude is kept constant. This mode of operation has very unique properties. A computer simulation is used to investigate the distance dependence of the measurement signals frequency and amplitude using various conservative and non-conservative interaction forces. It is shown how the two measurement channels are interlinked and influenced by both conservative and dissipative interactions. Further, discontinuous frequency shift versus distance curves as reported in the literature were not obtained.

The application of a biquadratic programming method to phase only optimization of antenna arrays

Antenna array optimization by a biquadratic programming method is formulated with variable-phase (analog and digital) and constant amplitude excitation. The method can optimize an antenna performance index expressible as a ratio of Hermitian quadratic forms. Power gain optimization and least-squares pattern synthesis with a specified gain are illustrated. Linear, planar, and circular arrays with dipole or circular loop elements are optimized, taking mutual coupling effects between antenna elements into account. The method shows much more rapid convergence characteristics than the Rosenbrock (1960) and Fletcher-Power (1963) methods. >

Sidelobe reduction of circular arrays with a constant excitation amplitude

It is shown that the sidelobe level of circular arrays with a constant excitation amplitude can be reduced by adjusting the excitation phases of array elements. The resulting radiation pattern has equal-ripple sidelobes similar to a Chebyshev pattern. Examples are given.