Cross-correlation technique

Cross correlation methods are more complicated, both to conduct and interpret, than the three methods previously described. In essence, a random tracer signal is allowed to disturb a system and the response is recorded. If 

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McLeman<7, ) used a method which was really a pre-cursor of the cross-correlation technique. He injected a pulse of air into the conveying line as a result of which there was a very short period during which the walls at any particular point were not subject to... 

The radial velocities have been computed with the low resolution set-up (more spectral lines, no telluric line), using a cross-correlation technique. When excluding the seven outliers, the peak in centered at 83.0 0.4kms 1 with a dispersion of 1.9 0.2kms 1. Lithium abundance is being determined using Li i 6707.8 A. We used the B — V index to determined the ([3]), and the curve of growth from [7] to derive AT(Li). 

The dynamics of the system under study can, in fact, be recovered from a variety of stimulus response tests. These include impulse and step response experiments, and frequency response and cross-correlation techniques. Descriptions of these methods and the interrelationships between them are discussed in many references, see, for instance, refs. 22—25 and Sects. 3.2.1—3.2.4 of this chapter. 

Several methods have been proposed to overcome multiple scattering. One simple solution proposes the use of thin samples, but flare effects and wall interactions complicate data interpretation [II], Alternative solutions involve cross correlation [13,36,126,1271 and two color cross correlation techniques [ 11,13,36) employing simultaneous illumination of the sample by two laser beams with differing wavelengths. 

Photon cross-correlation spectroscopy (PCCS) uses a novel three-dimensional cross-correlation technique which completely suppresses the multiple scattered fractions in a special scattering geometry. In this setup two lasers A and B are focused to the same sample volume, creating two sets of scattering patterns, as shown in Fig. 21-14. Two intensities are measured at different positions but with identical scattering vectors. 

Concentrated Dispersions Photon Cross-Correlation Techniques, Fiber-Optics DLS, and Diffusing Wave Spectroscopy... 

Mathur, M. P. and G. E. Klinzing, Measurement of Particle Velocity in Pneumatic Transport of Coal Using Cross-Correlation Technique, Particulate Sci. and Tech., 2, 1984, pp. 223-235. 

The cross-correlation technique measures the time of flight of an inherent flow tag passing through two sensors separated by a known distance. The technique has been used successfully to monitor single-phase fluid flows in which turbulent eddies modulate the interrogating ultrasonic beams. This type of correlation flowmeter has also been developed for solid/liquid and gas/liquid flows, in which the density fluctuation, caused by clusters of solids and by gas bubbles, is the prime inherent flow tag. 

Before we present the test results, we describe some general features of the cross-correlation technique. First, the velocity data obtained by this technique are typically derived from the maximum cross-correlation peak, which evidently differs from the actual average velocity over the pipe cross section because of the finite acoustic beam width. Thus there appears to be a meter factor that is due to the velocity profile. Second, in the cross-correlation system, we use two parallel sets of electronics any phase difference caused by the electronics will lead to an error in measurement. 

In principle, the ultrasonic techniques described for solid-liquid flow measurement can be applied to measure air flow rate and particle velocity. Direct measurement of air flow rate by measuring upstream and downstream transit times has been demonstrated. But, the Doppler and cross-correlation techniques have never been applied to solid/gas flow because the attenuation of ultrasound in the air is high. Recent developments have shown that high-frequency (0.5-MHz) air-coupled transducers can be built and 0.5-MI Iz ultrasound can be transmitted through air for a distance of at least 1 in. Thus, the cross-correlation technique should be applicable to monitoring of solid/gas flow. Here, we present a new cross-correlation technique that does not require transmission of ultrasonic waves through the solid/gas flow. The new technique detects chiefly the noise that interacts with the acoustic field established within the pipe wall. Because noise may be related to particle concentration, as we discussed earlier, the noise-modulated sound field in the pipe wall may contain flow information that is related to the variation in particle concentration. Therefore, crosscorrelation of the noise modulation may yield a velocity-dependent correlation function. 

Sheen, S. H. and Raptis, A. C. Active Acoustic Cross-Correlation Technique Applied to Flow Velocity Measurement in a Coal/Liquid Slurry, Proc. of Sonics/Ultrasonic Symp., 1983, 591. 

Sheen, S. H., Raptis, A. C., Bobis, J. P., Lee, S., and Simpson, T., Evaluation of Active Ultrasonic Cross-Correlation Technique in Coal/Liquid Pipe Flow Measurements, Argonne National Laboratory Report, 1985 ANL/FE-85-12. 

The cross-correlation technique is used in the ANL capacitive instrument to measure particle velocity. The outputs from the velocity-sensing electrodes were amplitude-modulated capacitor currents. If one assumes that the output capacitance signals vary sinusoidally (Aasincoat), and the applied voltage to the... 

Particle velocity was determined by the cross-correlation technique. Figure 6.34 shows typical cross-correlation functions that were obtained from the velocity electrodes. For a solids feed rate of 3.2 lb/s, the cross-correlation functions show well-defined peaks, from which particle velocities can be... 

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