Broadband Fluctuations Research Articles

Measurements of Strain on 310 mm Torque Converter Turbine Blades

An automotive torque converter was tested in order to determine the effect of converter operating condition and turbine blade design on turbine blade strain in the region of the inlet core tab restraint. The converter was operated over a wide range of speed ratios (0 to 0.95) at constant input torque and a stall condition for two input torques. Foil‐type strain gages in combination with wireless microwave telemetry were used to measure surface strain on the turbine blade. Strain measurements were made on two turbine blade designs.The steady component of strain over the range of speed ratios suggests the effect of both torque loading and centrifugal loading on the turbine blade tip. The unsteady strain was greatest at stall condition and diminished as speed ratio increased. Greater input torque at stall condition resulted in both greater steady strain and greater unsteady strain. The spectral distribution of strain over the range of tested speed ratios displayed an increase in low‐frequency broadband fluctuations near stall condition. A blade‐periodic event is observed which correlates to the pump‐blade passing frequency relative to the turbine rotating frame. Reducing the blade‐tip surface area and increasing the inlet‐tab root radius reduced the range of steady strain and magnitude of unsteady strain imposed near the inlet core tab restraint over the range of operating conditions.

High-confinement-mode edge stability of Alcator C-mod plasmas

For steady state high-confinement-mode (H-mode) operation, a relaxation mechanism is required to limit build-up of the edge gradient and impurity content. Alcator C-Mod [Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] sees two such mechanisms—EDA (enhanced D-alpha H mode) and grassy ELMs (edge localized modes), but not large type I ELMs. In EDA the edge relaxation is provided by an edge localized quasicoherent (QC) electromagnetic mode that exists at moderate pedestal temperature T<500 eV, high pedestal density, and high edge safety factor, q95>3.5, and does not limit the buildup of the edge pressure gradient. The q boundary of the operational space of the mode depends on plasma shape, with the q95 limit moving down with increasing plasma triangularity. At high edge pressure gradients and temperatures the mode is replaced by broadband fluctuations ( f<50 kHz) and small irregular ELMs are observed. Ideal MHD (magnetohydrodynamic) stability analysis that includes both pressure and current driven edge modes shows that the discharges where the QC mode is observed are stable. The ELMs are identified as medium n (10<n<50) coupled peeling/ballooning modes. The predicted stability boundary of the modes as a function of pedestal current and pressure gradient is reproduced in experimental observations. The measured dependence of the ELMs’ threshold and amplitude on plasma triangularity is consistent with the results of ideal MHD analysis performed with the linear stability code ELITE [Wilson et al., Phys. Plasmas 9, 1277 (2002)].

Time-varying properties of renal autoregulatory mechanisms

In order to assess the possible time-varying properties of renal autoregulation, time-frequency and time-scaling methods were applied to renal blood flow under broad-band forced arterial blood pressure fluctuations and single-nephron renal blood flow with spontaneous oscillations obtained from normotensive (Sprague-Dawley, Wistar, and Long-Evans) rats, and spontaneously hypertensive rats. Time-frequency analyses of normotensive and hypertensive blood flow data obtained from either the whole kidney or the single-nephron show that indeed both the myogenic and tubuloglomerular feedback (TGF) mechanisms have time-varying characteristics. Furthermore, we utilized the Renyi entropy to measure the complexity of blood-flow dynamics in the time-frequency plane in an effort to discern differences between normotensive and hypertensive recordings. We found a clear difference in Renyi entropy between normotensive and hypertensive blood flow recordings at the whole kidney level for both forced (p < 0.037) and spontaneous arterial pressure fluctuations (p < 0.033), and at the single-nephron level (p < 0.008). Especially at the single-nephron level, the mean Renyi entropy is significantly larger for hypertensive than normotensive rats, suggesting more complex dynamics in the hypertensive condition. To further evaluate whether or not the separation of dynamics between normotensive and hypertensive rats is found in the prescribed frequency ranges of the myogenic and TGF mechanisms, we employed multiresolution wavelet transform. Our analysis revealed that exclusively over scale ranges corresponding to the frequency intervals of the myogenic and TGF mechanisms, the widths of the blood flow wavelet coefficients fall into disjoint sets for normotensive and hypertensive rats. The separation of the scales at the myogenic and TGF frequency ranges is distinct and obtained with 100% accuracy. However, this observation remains valid only for the whole kidney blood pressure/flow data. The results suggest that understanding of the time-varying properties of the two mechanisms is required for a complete description of renal autoregulation.

Dynamics of air avalanches in the access pit of an underground quarry.

Temperature measurements have been performed in the vertical access pit of an underground quarry. During autumn, air avalanches induce an initial thermal feedback and a stationary mixing state characterized by spatially coherent broad-band fluctuations with a standard deviation of about 0.2 degrees C, linearly increasing with the inside-minus-outside temperature difference. Phase changes of water are shown to contribute to the onset condition, the feedback, and the stationary mixing state. This experiment may give insight on turbulent thermal and compositional convection with nonadiabatic boundaries.

Experimental study of lower-hybrid drift turbulence in a reconnecting current sheet

The role of turbulence in the process of magnetic reconnection has been the subject of a great deal of study and debate in the theoretical literature. At issue in this debate is whether turbulence is essential for fast magnetic reconnection to occur in collisionless current sheets. Some theories claim it is necessary in order to provide anomalous resistivity, while others present a laminar fast reconnection mechanism based on the Hall term in the generalized Ohm’s law. In this work, a thorough study of electrostatic potential fluctuations in the current sheet of the magnetic reconnection experiment (MRX) [Yamada et al., Phys. Plasmas 4, 1936 (1997)] was performed in order to ascertain the importance of turbulence in a laboratory reconnection experiment. Using amplified floating Langmuir probes, broadband fluctuations in the lower hybrid frequency range (fLH∼5–15 MHz) were measured which arise with the formation of the current sheet in MRX. The frequency spectrum, spatial amplitude profile, and spatial correlation characteristics of the measured turbulence were examined carefully, finding consistency with theories of the lower-hybrid drift instability (LHDI). The LHDI and its role in magnetic reconnection has been studied theoretically for decades, but this work represents the first detection and detailed study of the LHDI in a laboratory current sheet. The observation of the LHDI in MRX has provided the unique opportunity to uncover the role of this instability in collisionless reconnection. It was found that: (1) the LHDI fluctuations are confined to the low-beta edge of current sheets in MRX; (2) the LHDI amplitude does not correlate well in time or space with the reconnection electric field, which is directly related to the rate of reconnection; and (3) significant LHDI amplitude persists in high-collisionality current sheets where the reconnection rate is classical. These findings suggest that the measured LHDI fluctuations do not play an essential role in determining the reconnection rate in MRX.

Lyapunov spectrum from time series using moving boxes.

We present a very fast algorithm (few seconds) for estimating full Lyapunov spectrum from time series. The method requires a smaller number of parameters than other time-average algorithms, and is tested for data with different numerical precision, sampling frequencies, total sampling times, and presence of noise. We report conclusive results for the electron density broadband fluctuations of a plasma at the edge of tokamak TBR-1.

Acoustic normal mode fluctuation statistics in the 1995 SWARM internal wave scattering experiment

In order to understand the fluctuations imposed upon low frequency (50 to 500 Hz) acoustic signals due to coastal internal waves, a large multilaboratory, multidisciplinary experiment was performed in the Mid-Atlantic Bight in the summer of 1995. This experiment featured the most complete set of environmental measurements (especially physical oceanography and geology) made to date in support of a coastal acoustics study. This support enabled the correlation of acoustic fluctuations to clearly observed ocean processes, especially those associated with the internal wave field. More specifically, a 16 element WHOI vertical line array (WVLA) was moored in 70 m of water off the New Jersey coast. Tomography sources of 224 Hz and 400 Hz were moored 32 km directly shoreward of this array, such that an acoustic path was constructed that was anti-parallel to the primary, onshore propagation direction for shelf generated internal wave solitons. These nonlinear internal waves, produced in packets as the tide shifts from ebb to flood, produce strong semidiurnal effects on the acoustic signals at our measurement location. Specifically, the internal waves in the acoustic waveguide cause significant coupling of energy between the propagating acoustic modes, resulting in broadband fluctuations in modal intensity, travel-time, and temporal coherence. The strong correlations between the environmental parameters and the internal wave field include an interesting sensitivity of the spread of an acoustic pulse to solitons near the receiver.

Plasma effects driven by electromotive force of spacecraft solar array

SFU(Space Flyer Unit), a 4‐ton class experimental platform, was on orbit at the height of 480 km circular for 10 months from 1995 to 1996. It carried environment diagnostic packages which contained plasma probes and wave receivers to study the plasma environment surrounding the SFU. The potential of the SFU main body during the daytime reached about 40 V negative with respect to the ambient plasma, which was driven by the electromotive force of the SFU solar array paddles(52 V). It was found that the negative potential resulted in the formation of ion flow towards the SFU main body with fluctuations in a frequency range below 100 kHz. In the sunlight, the wave receiver detected broadband fluctuations extending beyond 1 MHz. This study provides the first observation on the plasma effects of spacecraft potential biased by the solar array voltage.

Visible radiation sliced image emission velocimeter interferometer fluctuation diagnostic

An instrument designed to measure k-filtered plasma fluctuations has been used to look at edge fluctuations in Alcator C-Mod tokamak plasmas. The instrument utilizes a sieve technique, whereby an optical image is split and filtered through two out-of-phase, striped transmission masks, and then imaged onto photodiode detectors. The difference signal is then digitized and analyzed to give the frequency characteristics of the fluctuations at a particular value of k, as determined by the wavelength of the mask stripes. Measurements have been made looking at the edge plasma with hydrogen Balmer-alpha filtering, and k both parallel and perpendicular to the magnetic field, with |k| in the range from 5 to 15 cm−1. Broad band (ν&amp;lt;500 kHz) fluctuations are seen for k perpendicular to B, and characteristics in ohmic and L-mode plasmas are described.

Alfvénic turbulence associated with density and temperature filaments

A systematic laboratory study of controlled density and temperature filaments having transverse scale length comparable to the electron skin-depth has been performed in the large plasma device (LAPD) at UCLA. It is found that large amplitude shear Alfvén waves develop spontaneously and are localized within the filaments. As the plasma conditions change (e.g., lowering the plasma beta parameter or increasing the heating power) the highly coherent eigenmodes develop into broad band Alfvénic turbulence. A kinetic description that includes the effect of coulomb collisions has been developed to understand the linear properties of the modes. Excellent agreement with the measured eigenfunctions is found for the density filaments in the higher beta regime in which the modes remain strongly coherent. The similarity between the broad band fluctuation spectra generated in a variety of plasma configurations suggest the possibility of a universal process involving filamentary structures and spontaneously generated Alfvénic turbulence.

Dynamic autoregulation and renal injury in Dahl rats.

The Dahl salt-sensitive (Dahl S) rat develops hypertension and renal injuries when challenged with a high salt diet and has been considered to be a model of chronic renal failure. Renal injuries appear very early in life compared with the spontaneously hypertensive rat (SHR). During the course of hypertension, a gradual impairment of autoregulatory control of renal blood flow might expose the glomerular circulation to periods of elevated pressure, resulting in renal injuries in Dahl S rats. Dynamic autoregulatory capacity was assessed in Dahl S and Dahl salt-resistant (Dahl R) rats, SHR, and Sprague-Dawley rats by inducing broad-band fluctuations in the arterial blood pressure and simultaneously measuring renal blood flow. Dynamic autoregulation was estimated by the transfer function using blood pressure as the input and renal blood flow as the output. Renal morphological injuries were evaluated in Dahl S rats and SHR and were scored semiquantitatively. Dynamic autoregulation was efficient and comparable in the low-frequency range (<0.015 Hz) in Dahl R rats, SHR, and Sprague-Dawley rats. The response in Dahl S rats depended strongly on the initiation time of the high salt diet. Autoregulation was preserved during a low salt diet and in rats exposed to a late-onset hypertension of short duration, only partly preserved if the late-onset hypertension was of a longer duration, and abolished in early-onset hypertension. All Dahl S rats on a high salt diet showed severe morphological changes in the kidney. In conclusion, autoregulatory capacity in the kidney of Dahl S rats is gradually impaired when rats are rendered hypertensive with a high salt diet. Renal morphological injuries develop before loss of dynamic autoregulation. Impaired autoregulation appears to be the result, not the cause, of the process that ultimately leads to renal failure in the Dahl S rat.

Measurements of broadband fluctuations and plasma potential with the 2 MeV heavy ion beam probe on the TEXT-U Tokamak (oral)

Broadband density fluctuations and plasma potential profiles are measured with a 2 MeV heavy ion beam probe (HIBP) diagnostic system on the TEXT-U tokamak. Measurements are performed in various locations including the edge and the core, as well as the high and low field side regions of the plasma. Results show a poloidal asymmetry in the core density fluctuation power spectra in which a distinct mode in the frequency range 70–170 kHz is present in the low field side region only. Furthermore, the HIBP provides a direct measurement of the local plasma potential from which the electric field profile across the plasma is inferred. Results showing the poloidal asymmetry as well as the potential and electric field profiles are presented.

Characterization of fluctuations in the JET Divertor plasmas with Langmuir probes

Langmuir probe tips embedded in the divertor and X-point sweeping provide JET with high spatial resolution profiles of fluctuations. Broadband fluctuation spectra have been observed in the divertor plasma strike zones with the absolute and relative level of fluctuations increasing when crossing the separatrix from the scrape-off layer into the private flux region. Asymmetries in the fluctuation levels measured at the inner and outer divertor plates have been observed with a higher level of fluctuations at the outer target. Fluctuation levels are altered by the L - H transition, with the strongest modification in the proximity of the separatrix. A significant rise in the relative level of fluctuations has also been observed in detached plasmas in comparison with attached plasmas.

Heart rate variability in isolated rabbit hearts.

The presence of heart rate variability (HRV) in patients with cardiac denervation after heart transplantation raised our interest in HRV of isolated, denervated hearts. Hearts from seven adult white ELCO rabbits were transferred to a perfusion apparatus. All hearts were perfused in the working mode and in the Langendorff mode for 20 minutes each. HRV was analyzed in the frequency domain. A computer simulated test ECG at a constant rate of 2 Hz was used for error estimation of the system. In the isolated, denervated heart, HRV was of random, broadband fluctuations, different from the well-characterized oscillations at specific frequencies in intact animals. Mean NN was 423 +/- 51 ms in the Langendorff mode, 406 +/- 33 ms in the working heart mode, and 500 ms in the test ECG. Total power was 663 +/- 207 ms2, 817 +/- 318 ms2, and 3.7 ms2, respectively. There was no significant difference in any measure of HRV between Langendorff and working heart modes. The data provide evidence for the presence of HRV in isolated, denervated rabbit hearts. Left atrial and ventricular filling, i.e., the working heart mode, did not alter HRV, indicating that left atrial or ventricular stretch did not influence the sinus nodal discharge rate.

Completely quantized theory of homogeneous line broadening in long-lived nuclear states.

Long-lived nuclei incorporated in a lattice are submitted to fluctuating interactions exerted on them by the lattice atoms surrounding them. Two models are considered: Gaussian Markovian processes and Gaussian broadband fluctuations. The frequency spectrum of the electromagnetic radiation emitted by nuclei is the Fourier transform of the first-order correlation function, which can be expressed solely in terms of two-time and one-time averages of nuclear operators. The one-time averages, which depend on the fluctuations, can be calculated. Using the quantum regression theorem, the two-time averages are calculated as well and hence the frequency spectrum. For both models this spectrum is Lorentzian with width equal to the sum of the natural linewidth and a lattice-induced width.

Magnetohydrodynamic and hybrid simulations of broadband fluctuations near interplanetary shocks

We present results of a theoretical study of evolution of a spectrum of finite amplitude right‐hand elliptically polarized magnetohydrodynamic (MHD) waves. The analysis includes use of one‐and‐a‐half‐dimensional solutions of the equations that describe compressible MHD together with one‐and‐a‐half‐dimensional hybrid simulation of the phenomenon. The motivation of the study is to understand the origin and properties of finite amplitude waves often observed in the vicinity of collisionless shocks in the heliosphere. The solutions of the MHD equations are compared with both the results of the hybrid simulations and observations previously reported by Viñas et al. (1984) in the vicinity of a quasi‐parallel interplanetary shock. The initial conditions of the MHD solutions were constructed to model the observed spectrum of magnetic and velocity fluctuations; plasma parameters were also chosen to replicate the observed parameters. For the typical parameters of β = 0.5, δB/B0 = 0.25 and a spectrum of parallel propagating, circularly polarized dispersive waves, initially the density and magnetic energy density correlations grow due to the (nonlinear) ponderomotive effect. The spectral features below the ion cyclotron frequency are established quickly on the Alfvénic timescale but then persist and match closely the observed fluctuations. The parametric decay instabilities that subsequently appear further enhance the density fluctuations and produce a high‐frequency magnetic power spectrum consistent with the spacecraft observation. The MHD and hybrid simulations extend the previous picture of wave generation by a beam‐driven ion cyclotron instability to the fully nonlinear stage.

Nonlinear phenomena and plasma turbulence in fusion plasmas

Recent progress in the characterization of the nonlinear nature of the broadband fluctuations in magnetically confined devices has provided a missing link between experimental measurements and theoretical turbulence models. At the same time, the study of influence of atomic physics drives (ionization and radiation) on plasma turbulence has stimulated new experimental developments. A view of the latest experimental results in these areas is presented.

Correlation between broad-band noise and frequency fluctuations of narrow-band noise in the charge-density wave in NbSe3.

The broad-band noise (BBN) and the frequency fluctuations of the narrow-band noise (NBN) were simultaneously measured in small ${\mathrm{NbSe}}_{3}$ samples. The two variables were highly correlated. Some discrete switching events show up in both noises. When multiple NBN peaks were present, some large fluctuations appearing in the BBN did not appear in the frequency of the biggest NBN peak, indicating that parallel uncorrelated charge-density-wave conduction paths were present.

Internal-wave effects on 1000-km oceanic acoustic pulse propagation: Simulation and comparison with experiment

A recent 1000-km acoustic pulse transmission experiment in the Pacific revealed unexpected fluctuations on received wavefronts, including a dominant rapid variation, called the broadband fluctuation, with time scales less than 10 minutes and spatial scales of less than 60 m; a distinct breakdown of the geometrical optics wavefront pattern and broadening of the wavefront near the transmission finalé; and a coherent wavefront motion with a timescale near the semi-diurnal tidal period. Parabolic-equation numerical simulations have been carried out which utilize environmental data and which take into account internal-wave-induced sound-speed perturbations obeying the Garrett–Munk (GM) spectral model. It is shown that the effects of internal waves can account for the broadband fluctuations, the breakdown of the geometrical optics pattern, and the wavefront broadening. The sensitivity of these fluctuations to internal-wave energy and modal content is examined. The spectral energy in the GM model at tidal periods proves insufficient to explain the tidal period coherent fluctuations, strongly suggesting the influence of an internal tide during the experiment. The simulations allow the estimation of the average travel-time bias caused by internal waves. The simulation results for travel-time wander and bias are compared with analytic calculations based on the path-integral technique.

Optical Bloch Equations in the Presence of Stochastic Broad-band Vacuum

Abstract The atomic Bloch equations driven by a quantum noise are investigated in the framework of c-number multiplicative stochastic Rabi amplitudes. It is shown that the stochastic broad-band vacuum fluctuations can be incorporated in such a way that spontaneous absorption is removed. It is shown that the multiplicative stochastic noise with no spontaneous absorption is equivalent to the quantum vacuum noise. A stochastic model of the broad-band squeezed vacuum is introduced and investigated. From such a stochastic description the fast and the slow damping rates of the Bloch polarizations are obtained. The strong-field resonance fluorescence spectrum and the steady-state atomic inversion for the stochastic model of squeezed vacuum fluctuations are derived.