Analysis Of Spectral Content Research Articles

Dynamics of cavity structures and wall-pressure fluctuations associated with shedding mechanism in unsteady sheet/cloud cavitating flows

The physics and mechanism of sheet/cloud cavitation in a convergent–divergent channel are investigated using synchronized dynamic surface pressure measurement and high-speed imaging in a water tunnel to probe the cavity shedding mechanism. Experiments are conducted at a fixed Reynolds number ofRe = 7.8 × 105for different values of the cavitation numberσbetween 1.20 and 0.65, ranging from intermittent inception cavitation, sheet cavitation to quasi-periodic cloud cavitation. Two distinct cloud cavitation regimes, i.e. the re-entrant jet and shockwave shedding mechanism, are observed, accompanied by complex flow phenomenon and dynamics, and are examined in detail. An increase in pressure fluctuation intensity at the numbers 3 and 4 transducer locations are captured during the transition from re-entrant jet to shockwave shedding mechanism. The spectral content analysis shows that, in cloud cavitation, several frequency peaks are identified with the dominant frequency caused by the large-scale cavity shedding process and the secondary frequency related to re-entrant jet/shockwave dynamics. Statistical analysis based on defined grey level profiles reveals that, in cloud cavitation, the double-peak behaviours of the probability density functions with negative skewness values are found to be owing to the interactions of the re-entrant jet/shockwave with cavities in the region of 0.25 ~ 0.65 mean cavity length (Lc). In addition, multi-scale proper orthogonal decomposition analysis with an emphasis on the flow structures in the region of 0.25 ~ 0.65Lcreveals that, under the shockwave shedding mechanism, both the re-entrant jet and shockwave are captured and their interactions are responsible for the dynamics and statistics of cloud shedding process.

Data-driven performance evaluation of a low-cost seismograph

The design and implementation of a performance protocol for local seismicity monitoring, is presented. A low-cost seismograph was installed in an area of high seismic activity in Evgiros, Lefkada Island, Greece, collocated with a high resolution 24-bit digitizer equipped with a broadband seismometer. A testing list of 28 local events with different epicenters and magnitudes has been compiled while acquisition data from the conventional seismograph and the proposed one were analyzed. Stochastic data analysis was used to compare these recordings on the same test site with different logging devices. The obtained results showed a satisfactory outcome in the performance of the proposed low-cost seismograph. Even though noise was present, P and S waves were clearly recorded with distinct amplitudes and therefore this arrival time difference, when compared with the one originating from the conventional seismograph, was found to be insignificant. Moreover, through a known magnitude equation, it manages to calculate earthquake’s local magnitude with a deviation of ± 0.4, a result that can be further improved. Lastly, spectral content analysis revealed almost identical waveforms with equivalent relative frequency distributions between the devices being compared.

Automatic Frames Extraction and Visualization From Noisy Fringe Sequences for Data Recovering in a Portable Digital Speckle Pattern Interferometer for NDI

We report on a method for recovering data from a simple portable Digital Speckle Pattern Interferometer, we used for monitoring structural behavior of a painting on wood, hanging on a wall, outside of laboratory conditions, without anti-vibration devices. In such a situation, fringes produced by the object displacements were affected by unpredictable distortions caused by environment vibrations. However, an sufficient number of suitable, i.e., undistorted or barely distorted, fringe patterns usable for processing was found. We performed multiple acquisitions at a frame rate as high as possible. The main task was picking out usable interferograms from large amount of frames. We developed a PC-based method based on jointly analysis of spectral content and fringe image sharpness as selection rules. The selected frames were utilized for off-line processing by using an approach based on Hilbert Transform and Phase Unwrapping via MAx-flow (PUMA) algorithm. We obtained a collection of displacement-maps, that allowed for evaluating the whole structure deformations, caused by environmental thermo-hygrometric fluctuations.

Electromagnetic fields and EEG spiking rate in patients with focal epilepsy

ObjectiveDespite the increase in mobile telephone technology use and possible effects on brain excitability, no studies have investigated the impact of GSM like (Global System for Mobile Communications) signal on the ongoing spiking activity in human epileptic patients. MethodsBrain electrical (electroencephalogram, EEG) activity of 12 patients with focal epilepsy has been recorded under both Real and Sham exposure following a double-blind, crossover, counterbalanced design: before the exposure (pre-exposure/baseline session), during the Real or Sham 45min exposure (during-exposure session), and after the exposure (post-exposure session). As dependent variables both spiking activity (spikes count) and EEG quantitative indices (spectral power and coherence data) have been considered. ResultsSpiking activity tended to be lower under Real than under Sham exposure. EEG spectral content analysis indicated a significant increase of Gamma band under Real exposure, mainly evident in Parieto-occipital and Temporal areas. Connectivity data indicated increased interhemispheric (left temporal to right frontal Regions of Interest, ROIs) instantaneous coherence, in the Beta frequency band during-exposure with respect to baseline session. No significant modification of lagged coherence was observed. ConclusionsAcute GSM exposure in epileptic patients slightly influences their EEG properties, without reaching any clinical relevance. SignificanceNo signs were found of an increased risk of incoming seizures for these patients as a consequence of using mobile phones.

A critical assessment of automatic photoelastic methods for the analysis of edge residual stresses in glass

The measurement of residual stresses is of great importance in the glass industry. The analysis of residual stresses in the glass is usually carried out by photoelastic methods since the glass is a photoelastic material. This article considers the determination of membrane residual stresses of glass plates by digital photoelasticity. In particular, it presents a critical assessment concerning the automated methods based on gray-field polariscope, spectral content analysis, phase shifting, RGB photoelasticity, “test fringes” methods and “tint plate” method. These methods can effectively automate manual methods currently specified in some standards.

Effect of cascade methods on vibration defects detection

Within the framework of monitoring rotating machines, vibration analysis remains an effective tool for fault detection. This analysis generally consists of measuring acceleration signals from critical and judiciously chosen points of a machine with the help of piezoelectric sensors. However, relevant information concerning the machine health can be masked by disturbances such as noise. The detection reliability will then be conditioned directly by the quality of the collected signal. Signal preprocessing methods, in particular denoising methods, can significantly improve the detection quality in terms of reliability. In this paper we aim to compare two methods of denoising based on signal spectral content analysis: discrete wavelet transform and empirical mode decomposition. A first study is carried out in order to optimize specific parameters related to each of the two methods, starting from experimental data obtained on degraded bearings. In fact for each parameter, one has to define conditions which allow the best detection of periodic pulses in vibration signals thanks to indicators such as kurtosis and crest factor. The second study consists of assessing the effectiveness of each denoising method on a vibration signal measured on a failed bearing. This signal is then disturbed by various noises simulated with variable levels. This study aims to show the effectiveness of each of these two methods on the early detection of impulse defects.

Pavement characteristics affecting the frequency content of tire/pavement noise

Tire/pavement noise is a major contributor to traffic noise at highway speeds. Annoyance is more closely related to frequency content than to overall A-weighted noise levels, therefore spectral content analysis of tire/pavement noise is an important consideration. A study conducted in California measured the noise levels of different mix types and the pavement characteristics affecting noise levels as determined by on-board sound intensity (AASHTO TP 76-08). Data was collected on dense-graded asphalt concrete mixes (DGAC), conventional open-graded mixes (OGAC), open-graded rubberized asphalt concrete mixes (RAC-O), and gap-graded rubberized asphalt concrete mixes (RAC-G) with ages less than 8 years old. This paper evaluates the effects of pavement characteristics including the air-void content, gradation properties, rubber inclusion, roughness, texture, pavement surface condition, and age on one-third octave band noise levels. This research confirmed that macrotexture and IRI, which are associated with the impact and shock mechanisms, increase the noise levels at lower frequencies. Higher air-void content, associated with the air pumping mechanism, reduces the noise levels at higher frequencies.

Phase shifting photoelasticity in white light

The availability of image acquisition systems has led to the development of digital photoelasticity both in monochromatic and white light. In particular white light has been used mainly with the following methods: Spectral Content Analysis, RGB photoelasticity and phase shifting photoelasticity. Phase shifting photoelasticity in the colour domain has been used effectively for the determination of the isoclinic parameter in order to reduce the influence of the isochromatic fringes. The method has been also proposed for the determination of the isochromatic fringe order. This paper concerns the general characteristics of phase shifting photoelasticity in the colour domain. Special attention is drawn towards the influence of spectral content of the light source, the spectral response of the colour camera filters, the dispersion of the birefringence and the error of quarter wave plates.

Problems in the Analysis of a Shock Sequence with Application to Gunfire Analysis and Simulation

In the planning of a laboratory simulation based on a transient sequence, non-conventional techniques must be applied to the analysis of the sequence, due to the non-stationary character of the signals. This paper deals with the specific analysis problems of a transient sequence. The RMS and spectral content analysis of a single transient elucidates the distortion in the characterization of the sequence, when stationary time-series analysis methods are implemented. The way this distortion affects the simulation of the sequence is presented next. From this presentation, it can be concluded that the best method for the laboratory simulation of a transient sequence is the application of a measured time-history, while preserving the statistical features of the sequence. This conclusion has been implemented in the planning of the laboratory simulation of a gunfire-generated sequence of transients, produced by a gun installed on a helicopter. Statistics of the maximum values of each transient were calculated, for the sequences measured under changing firing conditions. Based on these statistics and on the knowledge of the total number of rounds, the number of transients of different maxima during the entire life cycle could be evaluated. Also a life-cycle representative sequence that could be simulated by a shaker was generated. A second method for the gun firing sequence simulation was also tested. In this case, the shot with maximal peak value was used and the number of equivalent shocks representing the entire lifecycle calculated, using a fatigue accumulation model. The weakness of the method is in the fatigue model.

Digital Photoelasticity: Advanced Techniques and Applications

Digital Photoelasticity: Advanced Techniques and Applications

Digital Photoelasticity

The stated objectives of this book are: to provide an introduction to the basic concepts and methodologies for digital photoelasticity; to develop a foundation on which the future researcher and student can develop his own new and fruitful ideas; to promote research in the formulation of digital photoelasticity and to promote the application of these techniques to industries. It succeeds in its first objective but falls short of the others; in particular, almost no illustrative industrial applications are included. Each chapter has a comprehensive reference list which will allow the reader to pursue the topics in more detail. The fundamentals of transmission and reflection photoelasticity are described in the opening two chapters. There is a long section on fringe ordering, which is surprising given that many digital photoelastic techniques eliminate the need for it. Digital image processing is introduced at a level that will be useful for stress engineers who have not been formally trained in the subject. Topics including image sensing, display, perception, and storage and software operations including image acquisition, enhancement, segmentation and filtering are discussed. This chapter will be useful to researchers, students and practicioners, but the hardware section is likely to be out of date quite quickly. Early techniques of automated photoelasticity are dealt with together and comprehensively in a chapter entitled: Fringe Multiplication, Fringe Thinning and Fringe Clustering, and the subject of fringe ordering is discussed again. The major techniques of modern digital photoelasticity, phase shifting, polarization and Fourier transform methods are dealt with in a single chapter. In this arrangement the phase-stepping method received the majority of the attention including detailed discussion of its variants and the sources and treatment of errors. Polarization stepping and Fourier methods are treated as poor relations, which might be justified in terms of current popularity but seems inappropriate for a textbook on digital photoelasticity. The process of phase unwrapping is described in a separate chapter, together with optical tiling, but the issues of demodulation of the isoclinic and isochromatic data are not discussed. This emission leaves the student unequipped to construct their own algorithms. Colour image processing, focused principally on the red, green and blue (RGB) method, and spectral content analysis are explained, together with the associated hardware and its limitations. The chapter describing the evaluation of contact and fracture mechanics is narrow in its coverage of the field. The application of the techniques to a spur gear provides an opportunity, which is missed, to deal with the issues associated with interacting contact and crack-tip singularities. The treatment of stress separation is similarly poor, with historical issues dealt with comprehensively but modern developments ignored or only given brief attention. The inclusion of rapid prototyping might at first seem out of place, but this is an enabling and digital technology for model making which is an integral part of photoelasticity. This chapter provides a valuable insight for those new to the subject. Readers looking for a vision of the future in the closing chapter on recent developments and future trends will be disappointed; instead it appears more like a collection of loose ends or after-thoughts. Integrated photoelasticity and dynamic photelasticity are reviewed and identified as fertiles areas of future research, but several research groups are already making progress here so the chapter will be out of date very quickly. Photoelasticians brought up on Frocht's books will find the quality of the photographs in this book poor. This reviewer found the CD-ROM supplied with the book of little value because only two executable programs are supplied and the remainder of the programmes require compiling by the reader using a turbo C or turbo C++ compiler that is not supplied. One of the executable programs did not work, and the other simply showed a set of images for different phase-stepping algorithms, which could be readily included with the book. Each chapter contains exercises at the end, which will be useful for students and teachers. Some of the exercises are linked to the uncompiled programs on the CD-ROM. This is the first monograph in its field and forms a useful contribution despite falling short of its objectives in some areas. E A Patterson

A Novel Instrument for Automated Principal Strain Separation in Reflection Photoelasticity

Abstract A novel instrument is presented capable of measuring the principal strains at a point on a photoelastic coating. The oblique incidence method of principal strain separation is employed, using three fringe order measurements, which are made by the automated method known as spectral contents analysis. The oblique incidence measurements are made after rotation of the polariscope about any two orthogonal axes. Three polariscopes are built into the instrument, as is a CCD camera, to facilitate accurate positioning of the instrument. The instrument was used to analyze the principal strain distributions in a disk under diametrical compression, a plate with a central hole under tension, and a stepped plate in tension. The strains in the disk and the plate with a hole were compared with theoretical results, and the strains in the stepped plate were compared with a finite element analysis of the problem. The correlation between the experimental and theoretical or numerical strains was better than 0.95 in most cases.

Dispersion of Birefringence and its Influence on On-Line Measurement of Stresses in Polymer Films

Birefringence distribution in biaxially oriented polymer films is often used to assess non-uniformities in processing and in physical properties. Off-line characterization of this property can be performed by direct measurement of anisotropy in the refractive index, a process that is slow and not suitable for on-line use. By comparison, the spectral contents analysis (SCA) method is fast and easily adaptable for on-line process monitoring and real-time process control. However, implementation of this method requires accurate knowledge of the chromatic dispersion of birefringence. This paper discusses the contribution of birefringence dispersion to the precision of birefringence measurements by the SCA method, and it describes a new spectrometric procedure for accurate measurement of this material property.

Integration of spectral and phase-stepping methods in photoelasticity

The theoretical description of spectral contents analysis is extended to red-green-blue (RGB) photoelasticity. The integration of the resultant methodology for spectral analysis with phase stepping is used to achieve automatic analysis of photoelastic fringe patterns containing a minimum fringe order of no greater than four, and without the requirement for a zero-order fringe order to be present in the pattern.

A review of automated methods for the collection and analysis of photoelastic data

Photoelasticity is one of the most widely used full-field methods for experimental stress analysis. However, the collection of photoelastic parameters can be a long and tedious process. The advent of automated photoelastic systems has allowed the experimentalists to speed up the rate of analysis and to perform more complex investigations. This paper provides a survey of recent methods of automated photoelasticity developed in the last 20 years, i.e. methods of the fringe centres, half-fringe photoelasticity, phase-stepping photoelasticity, methods based on the Fourier transform, spectral content analysis (SCA) and RGB (red, green, blue) photoelasticity.

On image analysis for Birefringence measurements in photoelasticity

A number of techniques have been proposed during the past twenty years that have the common purpose of automating photoelastic fringe analysis. These techniques, which are reviewed, include spectral contents analysis, phase-shifting or stepping and Fourier transform procedures. It is shown that all of the techniques are based on a common theory and, consequently, all of the analytical methods suffer similar disadvantages. There is some illustrated discussion of the sources of errors when making full-field measurements.

Automated photoelasticity in white light: Influence of quarter-wave plates

This paper deals with the influence of optical retarders on the isochromatic fringes obtained by automated white light photoelasticity using methods such as those based on spectral content analysis (SCA) and on primary colour (red, green and blue) analysis. In the following the light intensity equations of dark- and light-field polariscopes with both crossed and parallel optical retarders are reviewed. In particular, it is shown that the retardance error of the quarter-wave plates produces an attenuation of the maximum intensity in dark field and an increase of minimum intensity in light field. Experimental evidence of the influence of optical retarders is also shown.

Completely automated photoelastic fringe analysis

The integration of two automated systems of photoelastic analysis has been performed. This combines the results obtained with a full-field polariscope based on phase-stepping techniques with those from spectral contents analysis. The latter technique identifies the absolute value of the isochromatic parameter at a point. This information is used to calibrate maps of relative retardation produced by the phase-stepping method. Thus, completely automatic determination of calibrated isochromatic and isoclinic patterns in photoelastic specimens is achieved. The evaluation of the whole system, establishing limits of validity and influences of parameters, is performed. Results show that the analysis carried out with the combined system, incurred errors no longer than those of each system working independently.

Digital birefringence measurement using the cross-correlation method

A digital system for measuring a small amount of birefringence or small changes in existing birefringences is presented. The apparatus is based on a data-acquisition system for a spectral-contents analysis. The use of cross-correlation techniques significantly reduces the data-processing time and permits measurement of the dispersion of the stress-optical coefficient of materials. The accuracy is of the same order as in the modulation techniques. The sensitivity of this method could be changed by changing the order of interference. A comparison is made with other existing polarization methods.

Photoelastic analysis of frozen stressed specimens using spectral-contents analysis

The complete birefringence, or isochromatic fringe order, in stress-frozen photoelastic models has been found by measuring the spectral contents of idividual points. A calibration procedure is porposed. The effects of nonuniform birefringence and dispersion of birefringence are considered. The results are presented from the analysis of models of a plate with a central hole and a disk subject to three radial loads.