Presence Of Low-frequency Oscillations Research Articles

Effects of back pressure perturbation on shock train oscillations in a rectangular duct

The nature of flow field inside the scramjet engine consists of shock-shock interaction and the shock boundary layer interaction. For a sustained combustion, a device called isolator plays a critical role in providing adequate pressure to the combustion chamber by a series of bifurcated shock waves called “shock train”. A small downstream pressure perturbation can cause upstream movement of this shock train and results in engine unstart condition. The propagation speed of downstream disturbance can influence the shock train oscillation. In the present study, computational fluid dynamics analysis is conducted to understand the oscillatory characteristic of shock train in a rectangular duct at M =1.75. The impact of downstream perturbation frequency (fb = 10 – 50 Hz) and the amplitude (A = 0.01 – 0.1) are simulated and discussed. The presence of low-frequency oscillation is observed without back pressure perturbation. The downstream pressure perturbation has a noticeable effect on the shock train excursion length i.e., the maximum upstream to downstream distance moved by shock train. The increase in perturbation frequency leads to decrease in disturbance propagation speed. Similarly, the propagation speed increases with the increase in the perturbation amplitude.

Understanding the interplay between baroreflex gain, low frequency oscillations, and pulsatility in the neural baroreflex

The neural baroreflex, which regulates mean arterial pressure (MAP) via the action of the brain, consists of baroreceptors which measure MAP, and actuators that can produce a change in MAP, such as the heart and parts of the peripheral resistance containing innervated smooth muscle. The brain is the controlling unit, maintaining an appropriate MAP in spite of various disturbances. Under certain circumstances, including haemorrhage and other states of distress, the gain of the neural baroreflex can change, causing low frequency (LF) oscillations (sometimes termed Mayer waves) in blood pressure (BP). Though their purpose is unclear, the origins of these LF oscillations has previously been explained via a nonlinear feedback model, though focusing on the peripheral resistance as an MAP actuator only. The present paper now includes analytical and simulation results explaining the LF oscillation phenomenon for the full neural baroreflex, containing both peripheral resistance (PR) and cardiac branches. However, the main contribution of the paper is to examine the effect of blood pulsatility, or a lack of pulsatility, on the neural baroreflex, and how it's effect can manifest in the presence of LF oscillations. This may have importance in cases where pulsatility is reduced (for example where left-ventricular assist devices are present), or completely absent (for example in turbine-based artificial hearts).

The features of the characteristic electromagnetic plasma bunches oscillations in the long magnetic mirror

The aim of this work was to study the spectrum of LF and HF oscillations generated by plasma bunches created and confined in the volume of a microwave cavity immersed in the magnetic field of a mirror trap. The registration of electrostatic oscillations in the plasma was carried out using two flat electrodes mounted diametrically opposite in the central part of the cavity close to its wall. This diagnostic showed the presence of low-frequency oscillations with frequencies of 130 kHz and 450 kHz. The oscillation spectrum in the microwave range was recorded at the minimum of the magnetic trap using a real-time spectrometer and a loosely coupled loop antenna. The registration of the spectra in the 40 MHz band revealed a regular change in the frequency of the fundamental oscillation mode of the cavity and the presence of two harmonics of the synchrotron radiation of the plasma bunch at frequencies of 2.25 GHz and 4.52 GHz, respectively. According to the obtained data, the parameters of the formed bunch (density, shape, volume, energy spectra of plasma components) can be restored.

Series Compensated Line Parameters Estimation Using Synchrophasor Measurements

This paper proposes a robust approach to accurately estimate parameters for compensated lines in the presence of low-frequency oscillation (LFO) that arises in measurements during a fault close to the series capacitor. The estimation uses measurements available from phasor measurement units (PMUs) and considers both cases of transposed and untransposed lines. The robustness toward measurement errors or departures from the fundamental frequency model due to LFO present in PMU signals is introduced by exploiting the least trimmed square (LTS) estimator. The least trimmed squares is a highly robust estimator that treats the bad effects of outliers. Mutual line parameters between the phases are calculated as well. Simulations were performed with the included sensor errors, and the results show the superiority of the LTS-based estimation compared to the classical estimation using the weighted least squares. The performance of the proposed approach was also assessed with a real-time digital simulator.

Experimental evidence of warm electron populations in magnetron sputtering plasmas

This work report on the results obtained using the Langmuir probe (LP) measurements in high-power dc magnetron sputtering discharges. Data show clear evidence of two electron components, such as warm and bulk electrons, in the sputtering plasma in a magnetic trap. We have also used optical emission spectroscopy diagnostic method along with LP to investigate the plasma production. Data show that there is a presence of low-frequency oscillations in the 2–3 MHz range, which are expected to be generated by high-frequency waves. Analysis also suggests that the warm electrons, in the plasmas, can be formed due to the collisionless Landau damping of the bulk electrons.

Study of jet precession, recirculation and vortex breakdown in turbulent swirling jets using LES

Large eddy simulations (LES) are used to investigate turbulent isothermal swirling flows with a strong emphasis on vortex breakdown, recirculation and instability behaviour. The Sydney swirl burner configuration is used for all simulated test cases from low to high swirl and Reynolds numbers. The governing equations for continuity and momentum are solved on a structured Cartesian grid, and a Smagorinsky eddy viscosity model with the localised dynamic procedure is used as the sub-grid scale turbulence model. The LES successfully predicts both the upstream first recirculation zone generated by the bluff body and the downstream vortex breakdown bubble. The frequency spectrum indicates the presence of low frequency oscillations and the existence of a central jet precession as observed in experiments. The LES calculations well captured the distinct precession frequencies. The results also highlight the precession mode of instability in the center jet and the oscillations of the central jet precession, which forms a precessing vortex core. The study further highlights the predictive capabilities of LES on unsteady oscillations of turbulent swirling flow fields and provides a good framework for complex instability investigations.

Nonrandom variability in respiratory cycle parameters of humans during stage 2 sleep

We analyzed breath-to-breath inspiratory time (TI), expiratory time (TE), inspiratory volume (VI), and minute ventilation (Vm) from 11 normal subjects during stage 2 sleep. The analysis consisted of 1) fitting first- and second-order autoregressive models (AR1 and AR2) and 2) obtaining the power spectra of the data by fast-Fourier transform. For the AR2 model, the only coefficients that were statistically different from zero were the average alpha 1 (a1) for TI, VI, and Vm (a1 = 0.19, 0.29, and 0.15, respectively). However, the power spectra of all parameters often exhibited peaks at low frequency (less than 0.2 cycles/breath) and/or at high frequency (greater than 0.2 cycles/breath), indicative of periodic oscillations. After accounting for the corrupting effects of added oscillations on the a1 estimates, we conclude that 1) breath-to-breath fluctuations of VI, and to a lesser extent TI and Vm, exhibit a first-order autoregressive structure such that fluctuations of each breath are positively correlated with those of immediately preceding breaths and 2) the correlated components of variability in TE are mostly due to discrete high- and/or low-frequency oscillations with no underlying autoregressive structure. We propose that the autoregressive structure of VI, TI, and Vm during spontaneous breathing in stage 2 sleep may reflect either a central neural mechanism or the effects of noise in respiratory chemical feedback loops; the presence of low-frequency oscillations, seen more often in Vm, suggests possible instability in the chemical feedback loops. Mechanisms of high-frequency periodicities, seen more often in TE, are unknown.

Multifrequency Bunching in Reflex Klystrons

Webster's simple bunching theory is extended to include the simultaneous oscillation of a reflex klystron at two or more frequencies. General expressions for the power and electronic admittance are derived that show the intermodulation effects of the oscillations upon each other. It is shown that in the presence of a vigorous low-frequency oscillation, oscillations may be obtained simultaneously at a higher frequency if the transit time is (n+1/4) r.f. cycles. The "normal" oscillations obtained for (n+3/4) r.f. cycles are shown to be usually unstable in the presence of the low-frequency oscillation. An analysis is given of the power and electronic admittances obtained when the oscillation frequencies are exactly in the same ratio as two integers. The theory is found to explain the intermodulation effects previously reported by the author.