Primary Frequency Components Research Articles

Theoretical and experimental study on the self-excited vibration of a flexible rotor system with floating spline

The internal friction of floating spline can cause self-excited vibration of a supercritical flexible rotor system. To address this issue, a high-efficiency dynamic modeling method is proposed to investigate the self-excited vibration behavior and instability evolution of the rotor. Experiments are conducted to validate the theoretical results. The coupled dynamic equations for the rotor system connected with the floating spline are derived through the combination of finite element method and lumped parameter model. A hybrid numerical approach of precise integration and Runge-Kutta method is adopted to examine the effects of the friction coefficient of spline’s tooth surface, torque, and eccentricity on the self-excited vibration of the rotor system. The results show that the spline friction leads to negative damping and inputs energy into the rotor system under supercritical conditions, triggering self-excited vibration when the input energy exceeds a specific level. With the same parameters, the experimentally obtained axial trajectory and primary frequency components are consistent with the theoretical results, verifying the accuracy of the proposed theoretical model. This study can serve as a useful theoretical guide for the dynamic stability design of flexible rotor systems with the floating spline.

A Liquid-Metal Wearable Sensor for Respiration Monitoring: Biomechanical Requirements, Modeling, Design, and Characterization

Liquid metals are metals in the liquid state at room temperature; they mostly belong to the alloys of Ga and In. These alloys have attracted attention because of their capacity to conduct electricity and heat, coupled with unique mechanical properties, which allow them to undergo extreme deformations, and self-heal. Their many possible applications range from microfluidics to soft robotics. They are particularly interesting for the fabrication of wearable sensing devices. We design and develop a liquid-metal-based hybrid sensor to study the qualitative and semiquantitative aspects of respiratory mechanics, monitoring the expansion and contraction of chest movements, exploiting the coupling of eutectic Ga–In alloy (EGaIn) and polydimethylsiloxane (PDMS), which show optimal characteristics to build a sensor for this application. The liquid metal has a suitable ohmic response and PDMS, due to its hyperelastic behavior and low viscosity, can be deformed easily and can transmit, without distortion, the primary frequency components useful to characterize different respiratory patterns, from slow and deep breathing to fast and shallow breathing. The time course and spectrum of the electric resistance signal from the sensor correspond closely to a standard reference obtained simultaneously by measuring chest deformation via optoelectronic stereophotogrammetry. The fabrication process is easy, cheap, and robust and can be industrialized.

A Gold Diaphragm-Based Fabry-Perot Interferometer With a Fiber-Optic Collimator for Acoustic Sensing

In this paper, a high sensitive fiber-optic Fabry-Perot interferometer sensor based on gold diaphragm for acoustic detection has been proposed and experimentally demonstrated. The extrinsic Fabry-Perot interferometer (EFPI) comprises a 140-nm-thick gold diaphragm and the end-face of a fiber-optic collimator, both of which are enclosed into a structure made of glass tubes. A fiber-optic collimator is first adopted in acoustic sensing to reduce the light loss and improve the sensitivity. The experimental results show that the proposed sensor has a wide flat response range from 400 Hz to 12 kHz, almost covering the primary frequency component of audible sound. The pressure sensitivity and the minimum detectable acoustic pressure level (MDP) are measured to be −175.7 dB re 1rad/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula> Pa@150Hz and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$95.3~\mu $ </tex-math></inline-formula> Pa/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> @2kHz, respectively. The proposed sensor has advantages of high sensitivity, wide flat response, low cost, and simple fabrication, which shows its potential as a fiber optical microphone with high sensitivity and high acoustic quality in practical applications.

Sediment transport and beach profile evolution induced by bi-chromatic wave groups with different group periods

In this paper, large-scale experimental data are presented showing the beach profile morphological evolution induced by four different bi-chromatic wave conditions characterized by very similar energy content between them but varying the modulation period. Important differences were observed in the resultant beach profiles as a function of the wave group periods. Larger variability in the profile evolution is generally observed for larger wave group periods and, more importantly, as the wave group period increases the distance between the generated breaker bar and the shoreline increases. The measured primary wave height to depth ratio (γ) increases with the wave group period, which is consistent with the observed larger wave height at the breaking location. The primary wave breaking location is also observed at increasing distances with respect to the initial shoreline as the wave group period increases. The variation in γ with wave group period is related to the selective energy dissipation of the higher primary frequency component (f1) during the wave group shoaling. Broad bandwith conditions (reduced wave group period) lead to larger dissipation of wave heights at the f1 component relative to f2 resulting in a reduction in the wave modulation and primary wave height at the breaking location. Suspended sediment fluxes obtained from collocated velocity and sediment concentration measurements in the surf zone showed a consistently larger contribution of the mean return flow to the suspended sediment fluxes compared with the wave group and primary wave components. The distinct beach profile evolution in terms of bar location is interpreted from an increasing distance of the mean breakpoint location and the location of maximum return flow with respect to the shoreline as the wave group period increases.

An image filter based on primary frequency analysis to improve the bit error rate in holographic data storage systems

The primary frequency analysis method is proposed to design an image filter for applications in holographic data storage systems that use a Nyquist aperture. We designed the filter by analyzing the frequency of the data patterns, requiring neither the point spread function of the system nor iteration. The component frequencies that constitute the data pattern were calculated, and the modulation transfer function (MTF) determines that the primary frequency components was boosted. This filter boosts the primary frequency region where the modulation transfer function (MTF) <0.5 to reduce the bit error rate (BER). The performance of the designed image filter was verified by both simulation and experimentation. The proposed system exhibited an improvement in BER from 0.0215 to 0.0070 for a Nyquist factor of 1.1 and from 0.0061 to 0.0017 for a Nyquist factor of 1.2. These results are expected to contribute to the design of holographic data image filters.

Trailing-edge dynamics of a morphing NACA0012 aileron at high Reynolds number by high-speed PIV

Particle image velocimetry (piv) measurements are made at the trailing edge of a piezoelectric actuated aileron in order to investigate the physical effect on the flow via high-frequency low-amplitude actuation at high Reynolds numbers. The measurements at different actuation frequencies show the modification of the primary frequency components of the flow with the actuation frequency. A statistical analysis reveals the reduction of the Reynolds stress components which increases with the actuation frequency. Proper orthogonal decomposition (pod) analysis shows the modification of the spatial modes illustrating the vortex breakdown in the shear-layer and the reduction of the temporal mode spectral energy depending on the actuation. It has been shown that a specific low amplitude actuation frequency produces a significant reduction of the predominant shear-layer frequency.

Numerical investigation of two-degree-of-freedom vortex-induced vibration of a circular cylinder in oscillatory flow

Two-degree-of-freedom (2dof) vortex-induced vibration (VIV) of a circular cylinder in oscillatory flow is investigated numerically. The direction of the oscillatory flow is perpendicular to the spanwise direction of the circular cylinder. Simulations are carried out for the Keulegan–Carpenter (KC) numbers of 10, 20 and 40 and the Reynolds numbers ranging from 308 to 9240. The ratio of the Reynolds number to the reduced velocity is 308. At KC=10, the amplitude of the primary frequency component is much larger than those of other frequency components. Most vibrations for KC=20 and 40 have multiple frequencies. The primary frequency of the response in the cross-flow direction decreases with the increasing reduced velocity, except when the reduced velocity is very small. Because the calculated primary frequencies of the response in the cross-flow direction are multiple of the oscillatory flow frequency in most of the calculated cases, the responses are classified into single-frequency mode, double-frequency mode, triple frequency mode, etc. If the reduced velocity is in the range where the VIV is transiting from one mode to another, the vibration is very irregular.For each KC number the range of the reduced velocity can be divided into a cross-flow-in-phase regime (low Vr), where the response and the hydrodynamic force in the cross-flow direction synchronize, and a cross-flow-anti-phase regime (high Vr), where the response and the hydrodynamic force in the cross-flow direction are in anti-phase with each other. The boundary values of Vr between the cross-flow-in-phase and the cross-flow-anti-phase regimes are 7, 9 and 11 for KC=10, 20 and 40, respectively. For KC=20, another cross-flow-anti-phase regime is found between 15≤Vr≤19. Similarly the in-line-in-phase and the in-line-anti-phase regimes are also identified for the response in the in-line direction. It is found that the boundary value of Vr between the in-line-in-phase and the in-line-anti-phase regimes is greater than that in the cross-flow direction. They are 14 and 26 for KC=10 and 20, respectively. Maximum amplitude occurs at the boundary value of the reduced velocity between in-phase regime and anti-phase regime in both the x- and the y-directions.

A cascaded second-order approach to computing third-order scattering of noncollinear acoustic beams

A theoretical development of the third-order nonlinear scattering of sound from two noncollinear ultrasonic beams produced by baffled piston sources is presented. The third-order intermodulation (IM3) frequency components are derived by exploiting cascaded second-order nonlinear effects where the quadratic nonlinear interaction of second-order frequency components with first-order (primary) frequency components is considered. It is shown that cascaded second-order interactions generate intermodulation frequency components that are equivalent to those generated by cubic nonlinear effects. Comparison of measured and modeled amplitude sweeps demonstrate the three-to-one gain in decibels of the amplitudes of the third-order intermodulation frequency components to that of the primary frequency components. Measurements are also presented for the farfield interaction of distantly spaced sources, which results in a highly focused ultrasonic parametric array. Also considered are the nearfield interaction of closely spaced sources, which results in scattering, with good agreement to the theory.

PERIODICITY OF AVERAGED HISTORIES OF CHAOTIC OSCILLATORS

This study is dedicated to demonstrate the periodicities embedded in the averaged responses of chaotic systems with periodic excitations. Recent studies in the field of non-linear oscillations often found random-like responses for some deterministic non-linear systems with periodic excitations, which were then named “chaotic systems”. However, in this study, by discretizing the initial conditions on a chosen domain and averaging the corresponding responses, the averaged response can be calculated for the chaotic motions of Duffing, van der Pol and piecewise linear systems. These averaged responses exhibit near-periodicities with primary frequency components at excitation frequency, odd multiples or half multiples of excitation frequency. It is also found that this periodicity becomes more evident as the number of discretized initial conditions over a fixed domain. These results were obtained and validated by simulations.

Ultrasound production by pregnant rats

At Day 21 of gestation, individual, pregnant, Sprague-Dawley rats were continuously monitored for ultrasound with a Holgate bat detector until delivery of the first pup. Nonpregnant controls were monitored over similar time periods. Pregnant animals produced more ultrasound than nonpregnant. The primary frequency component of the calls was 40 kHz. On the day of delivery, pregnant rats emitted significantly more ultrasound (15 to 60 kHz) than on the day preceding delivery. All rats produced some ultrasound which exhibited diurnal fluctuations. This ultrasound might be related to stress during labor or be a signal to conspecifics.

Auditory-nerve fiber encoding of two-tone approximations to steady-state vowels.

Responses to two harmonically related tones, approximating the lowest formants of nine American English vowels, were recorded from single auditory-nerve fibers. Data were compiled as period histograms for tones presented singly and in combination using the fundamental frequency of the two-tone complex as the time base. The amplitudes of the primary frequency components present in a histogram were estimated by least-squares fitting a half-wave rectified sum of the stimulating sinusoids plus a constant. Nonlinear interactions resulted for most two-tone stimuli: one tone dominated the response. When one tone was equal to best frequency, that tone always controlled discharge timing, usually suppressing the response to the second tone. Complicated interactions took place when the stimulating frequencies bracketed best frequency. The tone nearest best frequency was most effective near threshold, while higher stimulus levels usually favored the low-frequency tone. Nevertheless, the suppression mechanisms appear to provide an effective spatial separation in the cochlea for the response components to each vowel approximation. Fourier analysis of the period histograms yielded qualitatively similar results.