Research On The Applicability Of Acoustic Doppler Velocimeter

Acoustic Doppler Velocimeter (ADV) offers reliable instantaneousthree-dimensional velocity measurement in both laboratory and field applications. Whenused to investigate concentration fluctuations, however, ADV encounters practicalissues concerning parameter optimization and accuracy estimation. This study presentsan experimental test of ADV’s applicability in measurement of flow turbulence andsediment concentration. The experiments are conducted in a closed vertical flumesystem.The experiments involve both clear water and sediment-laden flows. The clearwater experiments focus on turbulent characteristics of the flow, and the sediment-ladenflow experiments aim to investigate fluctuation of sediment concentration. In theclear-water experiments, Particle Image Velocimetry (PIV) provides benchmarkvelocity data against which the results from ADV measurement are examined. Spectrumanalysis of the ADV data identifies various error sources. In the sediment-ladenexperiments, a relationship is established between sediment concentration and signalintensity. Theoretical analysis reveals the physical meaning of signal fluctuation.Accordingly, a relation between signal intensity and concentration fluctuation isproposed. The applicability of ADV in concentration measurement is discussed.Clear water experiments show that the accuracy of ADV is closely related tosampling frequency and noise signal. Low frequency fails to yield accuratemeasurement of high turbulent intensity. Experimental noise constitutes a major errorsource at appropriate measuring frequency. At low turbulence intensity (less than0.45cm/s in the present study), the presence of noise severely distorts ADV signals andthus the measurement of turbulence becomes meaningless. It seems that the skewnessand kurtosis are not affected by the sampling frequency (the errors of skewness andkurtosis are±0.3and±1, respectively). The sediment-laden experiments show that thesampling frequency and turbulence intensity exert limited influence on signal intensity.The signal intensity increases rapidly at low sediment, peaks at1.0g/L and thendecreases gradually. Though the signal intensity is affected by scattering of particles inthe sampling volume and light attenuation along the way, its fluctuation related only to the influence of scattering. Thus the fluctuation in signal intensity reflects actual changeof sediment concentration in the sampling volume. A combination of lower sedimentconcentration and higher turbulence intensity yields a higher level of sedimentconcentration fluctuation. The spatial scale also plays an important role in concentrationfluctuation.