Abstract
In this paper the extended autocorrelation velocity estimator is evaluated and compared using a nondestructive ultrasonic device. For this purpose, three velocity estimators are evaluated and compared. The autocorrelation method (ACM) is the most used and well established in current ultrasonic velocity profiler technology, however, the technique suffers with phase aliasing (also known as the Nyquist limit) at higher velocities. The cross-correlation method (CCM) is also well known and does not suffer with phase aliasing as it relies on time shift measurements between emissions. The problem of this method is the large computational burden due to several required mathematical operations. Recently, an extended autocorrelation method (EAM) which combines both ACM and CCM was developed. The technique is not well known within the fluid engineering community, but it can measure velocities beyond the Nyquist limit without the ACM phase aliasing issues and with a lower computational cost than CCM. In this work, all three velocity estimation methods are used to measure a uniform flow of the liquid inside a controlled rotating cylinder. The root-mean-square deviation variation coefficient (CVRMSD) of the velocity estimate and the reference cylinder velocity was used to evaluate the three different methods. Results show that EAM correctly measures velocities below the Nyquist limit with less than 2% CVRMSD. Velocities beyond the Nyquist limit are only measured well by EAM and CCM, with the advantage of the former of being computationally 15 times faster. Furthermore, the maximum value of measurable velocity is also investigated considering the number of times the velocity surpasses the Nyquist limit. The combination of number of pulses and number of samples, which highly affects the results, are also studied in this work. Velocities up to six times the Nyquist limit could be measurable with CCM and EAM using a set of parameters as suggested in this work. The results validate the use of the NDT tool to measure velocities even beyond Nyquist limit by using EAM.
Highlights
Interest in knowing the instantaneous velocity profile in fluid dynamics has grown in recent years as new flow visualization techniques are improving
An existing nondestructive testing (NDT) system could be shared with NDT testers, and enable flow researchers to have a first contact with the ultrasonic velocity profiler technique
The results indicate that cross-correlation method (CCM) velocities gave CVRMSD less than 2% up to over four times NL
Summary
Interest in knowing the instantaneous velocity profile in fluid dynamics has grown in recent years as new flow visualization techniques are improving. An existing NDT system could be shared with NDT testers, and enable flow researchers to have a first contact with the ultrasonic velocity profiler technique Such systems are designed to generate shorter pulses (wideband) which does not favor the phase measurement. That is Besides the spike pulser, there are square wave type pulsers in NDT [21] Those devices a desirable characteristic for ultrasonic velocity measurement as most of applications suffer with generate single cycle square waves that can be tuned to the resonant frequency of the transducer. Thatthe is ause of squaredesirable wave pulser for velocity profile measurement has a great in the field.suffer with characteristic for ultrasonic velocity measurement as potential most of applications Inenergy this context, of EAM with NDT square waveand pulsers of great interest as use it isofa good loss dueatostudy liquid-solid interfaces, particle scattering depthisattenuation. A recommended set of parameters are generated maximum velocity according to the maximum velocity desired
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