Power Spectrum Plots Research Articles

Multitaper spectral analysis of atmospheric radar signals

Abstract. Multitaper spectral analysis using sinusoidal taper has been carried out on the backscattered signals received from the troposphere and lower stratosphere by the Gadanki Mesosphere-Stratosphere-Troposphere (MST) radar under various conditions of the signal-to-noise ratio. Comparison of study is made with sinusoidal taper of the order of three and single tapers of Hanning and rectangular tapers, to understand the relative merits of processing under the scheme. Power spectra plots show that echoes are better identified in the case of multitaper estimation, especially in the region of a weak signal-to-noise ratio. Further analysis is carried out to obtain three lower order moments from three estimation techniques. The results show that multitaper analysis gives a better signal-to-noise ratio or higher detectability. The spectral analysis through multitaper and single tapers is subjected to study of consistency in measurements. Results show that the multitaper estimate is better consistent in Doppler measurements compared to single taper estimates. Doppler width measurements with different approaches were studied and the results show that the estimation was better in the multitaper technique in terms of temporal resolution and estimation accuracy.

Low- and high-frequency spectral behavior of cosmic-ray intensity for the period 1953–1996

Abstract. A study of the cosmic-ray intensity power spectrum using the Climax Neutron Monitor data in the frequency range from 10-9 Hz to 10-7 Hz (which corresponds to periodicities from 11 years to a few months) during the period 1953–1996, was carried out by means of the successive approximations method of analysis and was compared against the power spectrum and the maximum entropy methods. The contributions of the time evolution of several peaks to the global one were obtained. Except for the well-known 11-year and the 1-year variations, peaks at 7.7, 5.5, 2 and 1.7 years are found. Several peaks with periods less than 10 months have appeared in our analysis, while the occurrence of 5.1 months is obtained in all the examined solar cycles with a strong signature in cycle 21. Transitions of these quasi-periodicities are seen in power spectra plots. Some of them can be attributed to the modulation of the cosmic ray intensity by solar activity. Others are sporadic and have been previously attributed to the interplanetary magnetic field. The results obtained support once again the argument regarding the difference in the solar activity between odd and even solar cycles.Key words. Interplanetary physics (Cosmic rays, Interplanetary magnetic fields)

Time Evolution of low-Frequency Periodicities in Cosmic ray Intensity

The long-time series of daily means of cosmic-ray intensity observed by four neutron monitors at different cutoff rigidities (Calgary, Climax, Lomnický Stit and Huancayo/Haleakala) were analyzed by means of the wavelet transform method in the period range ∼ 60 to ∼ 1000 days. The contributions of the time evolution of three quasi-periodic cosmic-ray signals (∼ 150 d, ∼ 1.3 yr and ∼ 1.7 yr) to the global one are obtained. While the ∼ 1.7-yr quasi-periodicity, the most remarkable one in the studied interval, strongly contributes to the cosmic ray intensity profile of solar cycle 21 (particularly in 1982), the ∼ 1.3-yr one, which is better correlated with the same periodicity of the interplanetary magnetic field strength, is present as a characteristic feature for the decreasing phases of the cycles 20 and 22. Transitions between these quasi-periodicities are seen in the wavelet power spectra plots. Obtained results support the claimed difference in the solar activity evolution during odd and even solar activity cycles.

Calculation of two‐dimensional shear‐driven cavity flows at high reynolds numbers

AbstractThe time‐dependent Navier–Stokes equations are numerically integrated for two‐dimensional incompressible viscous flow in a shear‐driven square cavity. Using a time‐splitting method and finite differences on a staggered mesh, the momentum and pressure equations are directly solved by a tensor product method where one finite difference direction is diagonalized by eigenvalue decomposition. The effects of increasing Reynolds number are studied and the developing boundary layer is captured by using a finely clustered mesh. At Re = 30000 the flow is in a continuously developing unsteady regime. Power spectrum plots indicate that the unsteady flow oscillates with one fundamental frequency and exhibits some characteristics of transition between laminar and turbulent states.

Airfoil boundary-layer response to an unsteady turbulent flowfield

The time-dependant boundary-layer response to a periodic turbulent flowfield has been studied for attached flow over an airfoil at laminar, transitional, and turbulent conditions.Hot-wire velocity measurements were made for nonstationary mean velocity and turbulence intensity profiles and for power spectra and cumulative power spectra plots.

An experimental investigation into the comparison of the performance characteristics of TiN and ZrN coatings on split point drills using the static and stochastic models of the force system as a signature

This paper presents a comparison of the performance characteristics of TiN (Titanium nitride) and ZrN (Zirconium nitride) coatings on split point drills. The objective of this work was to choose the better coating for machining tough materials like INCONEL. This leads to an increased productivity of drilling holes in certain components of environmental control systems and fuel control systems (in aerospace industry) made of tough materials like INCONEL. The comparison of the performance characteristics was based on the measurement of the mean values and dynamic fluctuations of the cutting force and the number of holes drilled under the same optimum machining conditions. The measurements were carried out using two specially designed piezoelectric dynamometers. The dynamometer was calibrated from static and dynamic outputs and techniques were employed for increasing the measuring accuracy and reducing the cross interference by obtaining the elements of the tranfer function. Power spectrum plots of the drift force, axial force and torque were obtained so that these plots may be used as a signature. Results show that the ZrN coating is better than the TiN coating because (a) the mean values of the axialforce, drift force and torque are smaller, thus improving the roundness, (b) the dynamic flutuations of the forces and torque about the mean are smaller thus improving the surface quality of the holes produced, (c) the number of holes before the failure of the drill is about three times more for a ZrN coated split point drill as compared to a TiN coated drill.

Effect of exercise in healthy men on QRS power spectrum.

The effect of exercise on the QRS power spectrum has not been evaluated. We hypothesized that increased conduction velocity with exercise might selectively increase high-frequency QRS potentials. Using a digital electrocardiograph (ECG) cart, a single bipolar chest lead was recorded in 21 healthy adult males at rest and then immediately after treadmill exercise. Fast Fourier transform analysis of the ECGs was performed using a 128-ms sampling window, including the QRS but not the P wave. Step discontinuities between PR and ST segments were minimized using a linear correction function to obviate artifactual increases in high-frequency content after exercise. The power spectrum plots of rest and exercise data showed an increase in the amplitude of high frequencies that could not be accounted for on the basis of noise contamination. For the frequency band between 187.5 and 250 Hz, the root-mean-square (RSM) voltage of exercise QRS complexes (10.0 +/- 3.6 micro V) was significantly (P less than 0.02) greater than that of rest QRS complexes (7.8 +/- 1.2). However, base-line noise level (computed over a 32-ms interval during the ST segment) was not different between rest and exercise over this high-frequency band. The same exercise protocol was also applied in 11 normal male subjects using an analog ECG cart. For the 187.5- to 250-Hz band, the RMS voltage of the QRS after exercise (13.0 +/- 1.8 micro V) was also significantly (P less than 0.01) greater than at rest (10.2 +/- 2.8) without any difference in ST segment noise level over the same frequency range. These data indicate that exercise causes an increase in the amplitude of high-frequency QRS potentials in healthy men.

A Stable Tunable Microwave Source

This paper describes a prototype model of a phase-stabilized backward wave oscillator (BWO). The BWO is locked to a harmonic of a crystal oscillator by employing an automatic phase control loop. Tuning is accomplished by pulling the crystal oscillator and switching different crystals into the oscillator circuit. Power spectrum plots are taken in open-and closed-loop conditions, and methods of analyzing these plots are described. With the proper loop bandwidth, the BWO will assume the frequency stability of the reference oscillator. The phase-locked BWO is analyzed by a conventional feedback control technique. Each element in the loop is assigned a transfer function. From this the open-loop transfer function is determined. It is then shown that the improvement in short-term frequency stability is directly proportional to the open-loop transfer function and, hence, the open-loop gain. Utilizing plots of the absolute magnitude and phase angle of the open-loop gain vs. frequency, conclusions are drawn as to the maximum amount of gain which is possible while still maintaining system stability. Possible implementation of a series equalizer is considered in conjunction with this. Comments are also made on the relationship of the closed-loop bandwidth to the system lock-in range and system response to incremental changes in frequency.

Study of Ambient Noise in the Deep Ocean

An understanding of the characteristics and nature of the ambient noise is fundamental to the design of long-range submarine detection systems. During the summer of 1959 a series of deep-ocean ambient noise measurements were made with a 10-element vertical hydrophone array suspended to a depth of 500 fathoms in 2000-fathom water. Measurements were made over the frequency range of 5 to 25 cps. All data were recorded on magnetic tape and returned to the laboratory for analysis. The objectives of the measurements were to determine the spectral nature of the ambient noise and any special properties such as directionality that the ambient noise might have. By use of a magnet drum delay line, the array was steered in a vertical sense about the center of the array. The intensity and frequency spectrum of the beams was recorded. Broadband spectrum analysis was made, and both frequency and power spectrum plots were obtained of the ambient. A considerable amount of biological noise was recorded, and some classification of these signals was made. This paper describes the method and instrumentation used in recording the raw data and a description of the data analysis techniques. The results of the data analysis program are then discussed in detail.