Strong Low-frequency Component Research Articles

Nonlinear regimes of inertial wave attractors generated by a precessing lid: Zonal flows and Rossby waves

Fluid flow excited by inertial waves in a rotating annulus with a trapezoidal cross section is experimentally investigated. The forcing is introduced by the precession of a flat annular lid bounding the cavity. Such a setup allows to investigate interactions between inertial waves and zonal flows in the presence of equivalent topographic beta-effect, and its dynamic behavior under weak and strong forcing is experimentally studied for the first time. Due to the specific dispersion relation of inertial waves, a cavity shape supports wave attractors that appear only for retrograde lid precession. At a relatively low wave intensity, a zonal flow is generated in the vicinity of a “virtual” axial cylinder, the radius of which coincides with the radius of the focusing reflection from the conical bottom. A succession of non-linear regimes is observed as forcing increases, starting with a clearly identifiable case of triadic resonance. Further, the frequency spectrum is progressively enriched by emergence of additional discrete components, gradually reaching the state of “overheated” wave turbulence with a significant continuous background of the frequency spectrum. Finally, a strong low-frequency component develops in the frequency spectrum, and the continuous background of the spectrum decreases dramatically. The low-frequency component corresponds to azimuthally periodic structures identified as Rossby waves. In the case of strong forcing, a long-term variability of zonal-flow velocity at the timescale of tens of forcing periods is observed, which is likely a consequence of the interaction of azimuthal structures corresponding to a rich wave number set.

Understanding Key Roles of Two ENSO Modes in Spatiotemporal Diversity of ENSO

AbstractEl Niño–Southern Oscillation (ENSO) events exhibit a diversity of amplitudes, spatial patterns, and life cycles, with the main ENSO periods concentrated in the 3–7-yr [low-frequency (LF)] and 2–3-yr [quasi-biennial (QB)] bands. In this study, the spatiotemporal diversity of ENSO is quantitatively examined by extracting the two ENSO modes, namely, the LF and QB components of ENSO, from the traditional Niño-3.4 index and connecting them with the spatial types of ENSO. El Niño events can be regrouped as the QB-dominated central-Pacific ENSO-like (QB-CP), LF-dominated eastern-Pacific ENSO-like (LF-EP), and LF-dominated mixing (LF-mixing) types. La Niña events with vague spatial patterns can also have the same categorization. The QB-CP and LF-EP El Niño types both have a high-amplitude QB component. Meanwhile, the former is less affected by its powerless LF component, but the latter is controlled by its strong LF component. Ocean dynamics of the two El Niño types are distinct from each other. The thermocline feedback dominates the growth of the two El Niño types and contributes to the phase transition of the LF-EP type, while the zonal advective feedback is of increasing importance in the QB-CP El Niño and mainly contributes to the phase transitions of the two El Niño types. Additionally, the LF-mixing type with ambiguous spatial features and complex life cycles is distinguished from the other two types. These results indicate that the two ENSO modes coexist in the tropical Pacific air–sea system, and their combination with changing amplitude is the key to explaining the spatiotemporal diversity of ENSO.

Sliding-band dynamic range compression for use in hearing aids

Sensorineural hearing loss is associated with elevated hearing thresholds, reduced dynamic range, and loudness recruitment. Signal processing with dynamic range compression is used in hearing aids for presenting all the sounds comfortably within the limited dynamic range of the listener. A dynamic range compression technique, named as ‘sliding-band compression’, is proposed to overcome the shortcomings of the commonly used single-band and multiband compression techniques. The gain at each frequency index is calculated as a function of the short-time power in an auditory critical band centered at it. The technique avoids the attenuation of high-frequency components due to the presence of strong low-frequency components, which may occur in single-band compression. Further, it avoids distortions in the shape of spectral resonances and discontinuities during the resonance transitions, which may occur in multiband compression. The proposed technique was implemented for offline processing and compared with single-band compression and multiband compression using objective measures and different test inputs. For two-tone inputs, the single-band compression showed attenuation of the high-frequency tone with an increase in the level of the low-frequency tone. For swept-frequency single-tone input, the multiband compression showed errors in the tone amplitude of 1.5–2.5 dB for compression ratio of 2–10. Sliding-band compression did not show either type of error. The technique has been also implemented using a fixed-point processor for real-time processing with audio latency acceptable for face-to-face communication.

Detection and recognition of mechanical, digging and vehicle signals in the optical fiber pre-warning system

This paper presents detection and recognition method to locate and identify harmful intrusions in the optical fiber pre-warning system (OFPS). Inspired by visual attention architecture (VAA), the process flow is divided into two parts, i.e., data-driven process and task-driven process. At first, data-driven process takes all the measurements collected by the system as input signals, which is handled by detection method to locate the harmful intrusion in both spatial domain and time domain. Then, these detected intrusion signals are taken over by task-driven process. Specifically, we get pitch period (PP) and duty cycle (DC) of the intrusion signals to identify the mechanical and manual digging (MD) intrusions respectively. For the passing vehicle (PV) intrusions, their strong low frequency component can be used as good feature. In generally, since the harmful intrusion signals only account for a small part of whole measurements, the data-driven process reduces the amount of input data for subsequent task-driven process considerably. Furthermore, the task-driven process determines the harmful intrusions orderly according to their severity, which makes a priority mechanism for the system as well as targeted processing for different harmful intrusion. At last, real experiments are performed to validate the effectiveness of this method.

Countermeasures against noise and vibrations in lightweight wooden buildings caused by outdoor sources with strong low frequency components

Low frequency sound may contribute to human annoyance by inducing building vibration involving both rattling and whole body vibrations. The basic mechanism, verified by a series of field measurements, is that transmitted low frequency noise excites floor vibrations. Hence, improved sound insulation at the lowest frequencies will also mitigate floor vibrations. While techniques for sound insulation at higher frequencies are well known, there is a lack of knowledge on how to design countermeasures providing good low frequency sound insulation, especially for lightweight structures. To close this knowledge gap, a broad research program on low frequency sound transmission and sound induced vibrations has been performed. The aim of the study has primarily been to develop cost-effective countermeasures against low frequency noise for lightweight building structures, which also has a good effect in the frequency range that determines the A-weighted sound level. A countermeasure with stiffening of the walls and roof using steel sheet profiles screwed to the existing studs in combination with plywood panels was proposed. The study involved laboratory measurements, Finite Element simulations, proposal and design of countermeasures, as well as full-scale field testing of the proposed countermeasures in a typical Norwegian wooden building. In this paper we present an overview of the results from the laboratory measurements and the full scale field testing. The study showed that the proposed countermeasure combined with improvements of windows and balanced ventilation gives about 10 dB reduction of the A-weighted indoor sound level from F-16 aircrafts and 30-70% reduction of the floor vibrations.

Adjoint analysis for noise control in a two-dimensional compressible mixing layer

The adjoint of the compressible Navier–Stokes equations are solved on a two-dimensional mixing layer using Direct Numerical Simulation. The goal is to study the sensitivity of aeroacoustic noise radiated by this flow to various local perturbations of mass and energy, which is the first step to possible optimal control applications. The adjoint field shows the forcings that can be applied in order to achieve a given effect. Two test cases are presented. The first sensitivity analysis is based on a localized sinusoidal source in the dual space, corresponding to a localized sinusoidal source in the real space. The second one obtained from optimal control theory allows any frequency signature of the forcing. The adjoint perturbation, once it has reached the shear layer, propagates upstream until the inflow. The pressure perturbation exhibits a high sensitivity at the inflow as was found by Cervifio et al. [Cervifio L, Bewley TR, Freund JB, Lele SK. Perturbation and adjoint analyses of flow-acoustic interactions in an unsteady 2D jet. In: CTR Proceedings of the summer program, 2002. p. 27–40], and in the location of high shear in the mixing layer. Interactions between the direct field and the adjoint field lead to a large broadening of the adjoint variables spectra, with a strong low-frequency component, indicating a possibility for low-frequency control of higher frequency sound. Comparisons of sensitivities at the inflow and near the dual source show a particular amplification of the adjoint pressure near the inflow, indicating a high sensitivity to a forcing of the continuity equation. This gives a good basis for noise control by mass suction/blowing at the inflow.

Low‐frequency oscillations in precipitation, temperature, and runoff on a west facing mountain front: A hydrogeologic interpretation

This paper examines the space‐time patterns of annual, interannual, and decadal components of precipitation, temperature, and runoff (P‐T‐R) using long‐record time series across the steep topographic gradient of the Wasatch Front in northern Utah. This region forms the major drainage area to the Great Salt Lake. The approach is to use multichannel singular spectrum analysis as a means of detecting dominant oscillations and spatial patterns in the data and to discuss the relation to the unique mountain and basin hydrologic setting. Results of the analysis show that high‐elevation runoff is dominated by the annual and seasonal harmonics, while low‐elevation runoff exhibits strong interannual and decadal oscillations. For precipitation and temperature, only the annual/seasonal spectral peaks were found to be significantly different from the underlying noise floor, and these components increase with altitude similar to the mean orographic pattern. Spectral peaks in runoff show a more complex pattern with altitude, with increasing low‐frequency components at intermediate and lower elevation. This pattern is then discussed in terms of basin storage effects and groundwater‐stream interaction. A conceptual hydrogeologic model for the mountain and basin system proposes how losing streams and deep upwelling groundwater in the alluvial aquifer could explain the strong low‐frequency component in streams entering the Great Salt Lake. The phase‐plane trajectories of the dominant components for P‐T‐R are reconstructed as a function of altitude showing the relation of hydrogeologic conditions to the strongest oscillations in mountain runoff and discharge to the Great Salt Lake. The paper shows that weak interannual and decadal oscillations in the climate signal are strengthened where groundwater discharge dominates streamflow.

Climatic and geomagnetic influences on the Lac du Bouchet palaeomagnetic SV record through the last 110 000 years

Abstract Palaeomagnetic secular variation records stacked from five cores ∼ 20 m long extend back through the last glacial cycle to ∼ 100 000 years ago. The time-scale has been constructed from conventional and accelerator radiocarbon ages back to ∼ 45 Ka BP and beyond this by comprehensive pollen studies which identify forest and steppe cycles that have been related to marine oxygen isotopic stages 4 and 5. Spectral analyses of the intensity and directional records using both Fourier and maximum entropy methods reveal strong low-frequency components which fall in the range of the Milankovitch obliquity and precession periodicities (40-20 Ka). It is concluded that although part of this signal may have fed indirectly into the natural remanence via the influence of climatic change on sediment facies, part has been impressed directly by perturbation of the geomagnetic dynamo process at the core-boundary by variations in the Earth's obliquity and precession. These long periods envelop a well-defined pattern, registered also by the independent ‘Mackereth’ core stack back to 50 Ka BP, of irregular secular oscillations of the geodynamo with periods of a few thousands of years.

Low-frequency Wiener spectra of radiographic screen-film systems

Wiener spectra of radiographic screen-film systems were measured in the low-frequency range from 0.05 to 0.8 mm-1 with a special photometric set-up capable of scanning large film areas with a very long slit. It appears that inhomogeneities in radiographic film and film processing produce a strong low-frequency component of the Wiener spectrum below approximately 0.4 mm-1. Similar disturbances over the whole measured frequency range 1 mm-1 can be introduced by technologically-caused inhomogeneities in radiographic screens (low-frequency screen noise). Low-frequency film and screen noise must be considered for calculation of the detectability, especially of large low-contrast details from Wiener spectral data.