Unsteady Pressure Measurements in a Heated Rotating Cavity

The flow in the heated rotating cavity of an aero-engine compressor is driven by buoyancy forces, which result in pairs of cyclonic and anticyclonic vortices. The resultant cavity flow field is three-dimensional, unsteady and unstable, which makes it challenging to model the flow and heat transfer. In this paper, properties of the vortex structures are determined from novel unsteady pressure measurements collected on the rotating disc surface over a range of engine-representative parameters. These measurements are the first of their kind with practical significance to the engine designer and for validation of computational fluid dynamics. One cyclonic/anticyclonic vortex pair was detected over the experimental range, despite the measurement of harmonic modes in the frequency spectra at low Rossby numbers. It is shown that these modes were caused by unequal size vortices, with the cyclonic vortex the larger of the pair. The structures slipped relative to the discs at a speed typically around 10% to 15% of that of the rotor, but the speed of precession was often unsteady. The coherency, strength and slip of the vortex pair increased with the buoyancy parameter, due to the stronger buoyancy forces, but they were largely independent of the rotational Reynolds number.

Acoustic and vibrational signaling in true katydid Nesoecia nigrispina: three means of sound production in the one species

The males of Mexican katydids Nesoecia nigrispina (Stal) produce calling songs and protest sounds using the same stridulatory apparatus as in most of the other Ensifera at the base of the elytra. It includes pars stridens on the upper elytron and plectrum on the lower. Calling sounds are 2−pulse series, repeated with a frequency of 2−3 per sec. Protest signals in the form of short trills from the same pulse duration males produce with tactile stimulation. The pulse repetition rate is almost three times higher than that of the calling sounds - up to 10 per sec. The frequency spectra of these signals have maxima in the band of 14−15 kHz. However, in addition to the sounds described, both males and females are capable to produce protest signals of the second type with the help of another sound apparatus, namely with the help of the wings. Insects with removed elytra are unable to produce an audible sound. Thus, the sound is produced by the friction of the wings on the elytra, but there are no specialized stridulatory structures on them. In females, in response to tactile stimulation, short clicks are recorded, which they make, apparently, by the mandibles. Vibrational signals at tremulation are emitted by individuals of both sexes during courtship and males, completing the calling signal cycle and after copulation. It is possible that vibrational signals are an additional factor in reproductive isolation in sympatric species, since the calling sound signals in representatives of the genus Nesoecia are similar and exhibit significant variability.

Automatic quality detection system for structural objects using dynamic output method: Case study Vilnius bridges

Paper provides an attempt to create a methodology for automated structure health monitoring procedures using vibration spectrum analysis. There is an option to use autoregressive (AR) spectral analysis to extract information from frequency spectra when conventional Fast Fourier transformation (FFT) analysis cannot give relevant information. An autoregressive spectrum analysis is widely used in optics and medicine; however, it can be applied for different purposes, such as spectra analysis in electronics or mechanical vibration. This paper presents an automated structural health monitoring approach based on the algorithm-driven definition of the first resonant frequency value from a noisy signal, acquired from traffic-created bridge vibrations. We implemented the AR procedure and developed a peak detection algorithm for experimental data processing. The functionality of the proposed methodology was evaluated by performing research on six bridges in Vilnius (Lithuania). We compared three methods of data processing: FFT, filtered FFT and AR. Bridges vibrations under different excitation conditions (wind, impulse and traffic) in normal direction were measured using accelerometers. AR provided one peak representing the lowest resonant frequency in all cases, while FFT and filtered FFT provided up to 12 peaks with similar frequency values. Such results allow implementing our method for remote automated structures health monitoring and ensure structures safety using a convenient and straightforward diagnostic method.

Tissue Characterization Using an Electrical Bioimpedance Spectroscopy-Based Multi-Electrode Probe to Screen for Cervical Intraepithelial Neoplasia

The successful management of cervical intraepithelial neoplasia (CIN) with proper screening and treatment methods could prevent cervical cancer progression. We propose a bioimpedance spectroscopic measurement device and a multi-electrode probe as an independent screening tool for CIN. To evaluate the performance of this screening method, we enrolled 123 patients, including 69 patients with suspected CIN and 54 control patients without cervical dysplasia who underwent a hysterectomy for benign disease (non-CIN). Following conization, the electrical properties of the excised cervical tissue were characterized using an electrical bioimpedance spectroscopy-based multi-electrode probe. Twenty-eight multifrequency voltages were collected through the two concentric array electrodes via a sensitivity-optimized measurement protocol based on an electrical energy concentration method. The electrical properties of the CIN and non-CIN groups were compared with the results of the pathology reports. Reconstructed resistivity tended to decrease in the CIN and non-CIN groups as frequency increased. Reconstructed resistivity from 625 Hz to 50 kHz differed significantly between the CIN and non-CIN groups (p < 0.001). Using 100 kHz as the reference, the difference between the CIN and non-CIN groups was significant. Based on the difference in reconstructed resistivity between 100 kHz and the other frequencies, this method had a sensitivity of 94.3%, a specificity of 84%, and an accuracy of 90% in CIN screening. The feasibility of noninvasive CIN screening was confirmed through the difference in the frequency spectra evaluated in the excised tissue using the electrical bioimpedance spectroscopy-based multi-electrode screening probe.

INVESTIGATION ON MODE CHARACTERISTICS OF ROTATING INSTABILITY AND ROTATING STALL IN AN AXIAL COMPRESSOR

Rotating instability (RI) and rotating stall (RS) are two types of aerodynamic instability in axial compressors. The former features the side-by-side peaks below the blade passing frequency (BPF) in frequency spectra, and the latter represents one or more stall cells rotating in the compressor. This paper presents an experimental on the nearfield pressure and farfield acoustic characteristics of RI phenomenon in a low-speed axial compressor rotor, which endures both RI and RS at several working conditions. In order to obtain the high-order modes of RI and other aerodynamic instability, a total of 9 or 20 Kulites are circumferentially mounted on the casing wall to measure the nearfield pressure fluctuation using a mode order calibration method. Meantime in the farfield 16 microphones are planted to measure the acoustic mode order using the compressive sensing method. Through calibration the experiments acquire the mode orders generated by RI and the interaction between RI and BPF, which is higher than the number of transducers. As for RS, the mode decomposition shows a mode order of 1, indicating one single stall cell rotating in the compressor. This experiment also shows that amplitude of RI modes is decreased when RS occurs, but RS modes and RI modes will both be enhanced if the flow rate is further reduced. This experiment reveals that RI experiences three stages of “strengthen-weaken-strengthen”, and hence RI may not be regarded only as “prestall” disturbance.

Comparison of the Natural Vibration Frequencies of Timoshenko and Bernoulli Periodic Beams

This paper deals with the linear natural vibrations analysis of beams where the geometric and material properties vary periodically along the beam axis. In contrast with homogeneous prismatic beams, the frequency spectra of such beams are irregular as there exist enlarged intervals between some adjacent frequencies. Presented here are two averaged models of beams based on the tolerance modelling approach. The assumptions of classical Euler–Bernoulli and Timoshenko–Ehrenfest beam theories are adopted as the foundations. The resulting mathematical models are systems of differential equations with constant, weight-averaged coefficients. This makes it possible to apply any classical method of solution suitable for homogeneous beams, such as Galerkin orthogonalization. Here, emphasis is placed on the comparison of natural frequencies neighbouring the frequency band-gaps that are obtained from these two theories. Two basic cases of material and geometric property distribution in a periodicity cell are studied, and the natural frequencies and mode shapes are obtained for a simply supported beam. The results are supported by a comparison with the finite element method and partially exact solutions.

High-Precision and High-Resolution Synchrosqueezing Transform via Time-Frequency Instantaneous Phases

Synchrosqueezing transform (SST) can effectively improve time-frequency precision and resolution by squeezing time-frequency spectra via instantaneous frequencies, and it has been applied in many diverse disciplines; however, the precision of estimated instantaneous frequencies during SST is usually affected by the time-sample interval of the inputted signal; this usually leads to low-precision or inaccurate SST results and limits its further application. To obtain high-precision and high-resolution SST results with high efficiency, we propose a high-precision and high-resolution SST via time-frequency instantaneous phases (HSST); in HSST, time-frequency instantaneous phases with period-jumps removal are used for high-precision instantaneous frequencies estimation and SST. Two synthetic signal examples show that HSST can minimize the impact of the time-sample interval to achieve high-precision and high-resolution SST results with high efficiency. A real 3D seismic data application demonstrates that HSST has fantastic performance in time-frequency precision and resolution enhancement, and it can be widely used in digital signals processing and interpretation fields.

Full characterization of a phase-locked DUV double pulse generated in an all-reflective shaping setup working under grazing incidence in a broad spectral range

Controlling the temporal and spectral properties of ultrashort laser pulses in the visible and near-infrared spectral range by means of a femtosecond pulse-shaping device is a powerful tool with many applications in ultrafast spectroscopy. A major and successful concept is known as the 4f design, which has a symmetric zero-dispersion-compressor geometry. Most 4f pulse shapers rely on using transmissive optics in their beam path limiting the operational wavelength ranges. In the present contribution, we use an all-reflective shaping setup to generate a phase-locked 266 nm double pulse to benchmark its performance in the limit of short wavelengths. The setup comprises the complete spectral amplitude and phase diagnostics for quantitative analysis of the pulse properties before and after the shaper using the technique of frequency-resolved optical gating. The measured time–frequency spectra are in good agreement with optical simulations. The geometry and hardware of the device including the optical components are designed, such that all harmonics of the deep UV pulses travel the same path, giving the instrument the ability to work with soft X-ray pulses, under vacuum conditions, down to the few-nanometer wavelength scale.

Stability and dynamics of a spectral graph model of brain oscillations

We explore the stability and dynamic properties of a hierarchical, linearized, and analytic spectral graph model for neural oscillations that integrates the structuring wiring of the brain. Previously we have shown that this model can accurately capture the frequency spectra and the spatial patterns of the alpha and beta frequency bands obtained from magnetoencephalography recordings without regionally varying parameters. Here, we show that this macroscopic model based on long-range excitatory connections exhibits dynamic oscillations with a frequency in the alpha band even without any oscillations implemented at the mesoscopic level. We show that depending on the parameters, the model can exhibit combinations of damped oscillations, limit cycles, or unstable oscillations. We determined bounds on model parameters that ensure stability of the oscillations simulated by the model. Finally, we estimated time-varying model parameters to capture the temporal fluctuations in magnetoencephalography activity. We show that a dynamic spectral graph modeling framework with a parsimonious set of biophysically interpretable model parameters can thereby be employed to capture oscillatory fluctuations observed in electrophysiological data in various brain states and diseases.

Flow regimes in a multi-gap circular Couette-Taylor system with opposite rotating cylinders

This work investigates the flow structure in the gaps of a multi-cylinder circular Couette-Taylor system, which is a model of a two-rotor heat generator. The initial information for studying the flow structure was data on the magnitude of the resistance torque to rotors opposed rotation, as well as on the nature of the amplitude-frequency spectrum of pulsations of this torque, depending on the viscosity of the working fluid and the rotational speed of the heat generator rotors. The obtained data allow comparing the nature of hydrodynamic processes in the single and obtained multi-gap circular space of Couette-Taylor and calculating the parameters of structural formations in the multi-gap working space of the heat generator. At relative rotational speeds of rotors (3-50) rad/s, the main energy of flow pulsations (up to 90%) is found in the amplitude-frequency spectra in the frequency range (12-70) Hz. It is associated with vortices first described by Taylor, which are extended low-frequency regularly alternating spirals and vortex structures with right and left rotation in the region of higher frequencies (200– 500) Hz; their frequency is determined by the width of the annular gaps of the multi-cylinder system.