Measurement of fluid turbulence based on pulsed ultrasound techniques. Part 1. Analysis

Abstract

The pulsed ultrasonic Doppler velocimeter has been used extensively in transcutaneous measurement of the velocity of blood in the human body. It would be useful to evaluate turbulent flow with this device in both medical and non-medical applications. However, the complex behaviour and limitations of the pulsed Doppler velocimeter when applied to random flow have not yet been fully investigated.In this study a three-dimensional stochastic model of the pulsed ultrasonic Doppler velocimeter for the case of a highly focused and damped transducer and isotropic turbulence is presented. The analysis predicts the correlation and spectral functions of the Doppler signal and the detected velocity signal. The analysis addresses specifically the considerations and limitations of measuring turbulent intensities and one-dimensional velocity spectra.Results show that the turbulent intensity can be deduced from the broadening of the spectrum of the Doppler signal and a mathematical description of the effective sample-volume directivity.In the measurement of one-dimensional velocity spectra at least two major complicacations are identified and quantified. First, the presence of a time-varying, broad-band random process (the Doppler ambiguity process) obscures the spectrum of the random velocity. This phenomenon is similar to that occurring in laser anemometry, but the ratio of the level of the ambiguity spectrum to the largest detected velocity spectral component can be typically two to three orders of magnitude greater for ultrasonic technique owing to the much greater wavelength.Secondly, the spatial averaging of the velocity field in the sample volume causes attenuation in the measured velocity spectrum. For the ultrasonic velocimeter, this effect is very significant.The influence of the Doppler ambiguity process can be reduced by the use of two sample volumes on the same acoustic beam. The signals from the two sample volumes are cross-correlated, removing the Doppler ambiguity process, while retaining the random velocity. The effects of this technique on the detected velocity spectrum are quantified explicitly in the analysis for the case of a three-dimensional Gaussianshaped sample-volume directivity.

Turbulent transport in the outer region of rough-wall open-channel flows: the contribution of large coherent shear stress structures (LC3S)

“…High-resolution three-dimensional acoustic Doppler velocity profiler (3D ADVP) Three-dimensional turbulent velocity profiles were measured along a vertical line over the range 0.02 < z/δ < 0.95 using an acoustic Doppler velocity profiler (ADVP). This type of instrument and its use in measuring turbulent flows are discussed by Garbini, Forster & Jorgensen (1982), Lhermitte & Lemmin (1994), Zedel et al (1996), Hurther & Lemmin (1998 and Blanckaert & deVriend (2004, 2005. A schematic of the instrument is shown in figure 1.…”

The influence of acoustic impedance mismatch on poststenotic pulsed-doppler ultrasound measurements in a coronary artery model

“…The high pass filter thus filters out a higher percentage of this reduced frequency range. However, the reduced velocities in the 7% mismatch case should not affect the percent of spectral broadening caused by ambiguity since this broadening is proportional to velocity (Garbini et al 1982).…”

Doppler Ultrasound‐Based Rheology

“…Outside the medical field, ultrasonic velocimetry was applied to measurements of liquid metal flow by Fowlis (1973), using continuous wave ultrasound, while Pinkel (1979) utilised pulsed ultrasound at low frequencies to measure ocean currents. Garbini et al (1982aGarbini et al ( , 1982b characterised turbulence using the spectral broadening of the echo signal. Takeda (1986) investigated pipe and Taylor vortex flows.…”