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D. Hurther
and
U. Lemmin

Abstract

A novel noise reduction method and corresponding technique are presented for improving turbulence measurements with acoustic Doppler velocimeters (ADVs) commonly used in field studies of coastal and nearshore regions, rivers, lakes, and estuaries. This bifrequency method is based on the decorrelation of the random and statistically independent Doppler noise terms contained in the Doppler signals at two frequencies. It is shown through experiments in an oscillating grid turbulence (OGT) tank producing diffusive isotropic turbulence that a shift in carrier frequency of less than 10% is sufficient to increase the resolved frequency range by a decade in the turbulent velocity spectra. Over this spectral range, the slope of the velocity spectra agrees well with the universal inertial range value of −5/3. The limit due to spatial averaging effects over the sample volume can be determined from the abrupt deviation of the spectral slope from the −5/3 value. As a result, the relative error of the turbulent intensity estimate and the turbulent kinetic energy (TKE) dissipation rate, measured by two different methods, does not exceed 10% in the case of isotropic turbulence. Furthermore, the bifrequency method allows accurate estimates of the turbulent microscales as shown by the good agreement of the ratio between the Taylor and Kolmogorov microscales and an Re1/4 t power law. Compared to previous Doppler noise reduction methods (Garbini et al.), an increase in time resolution by a factor of 4 is achieved. The proposed method also avoids the loss of TKE energy contained in isotropic flow structures of size equal to and smaller than the sample volume. Different from Doppler noise methods proposed by Hurther and Lemmin and Blanckaert and Lemmin, this method does not require additional hardware components, electronic circuitry, or sensors because the redundant instantaneous velocity field information is captured with the same transducer. The required shift in carrier frequency is small enough for the bifrequency method to be easily implemented in commercial ADVs.

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R. Lhermitte
and
U. Lemmin

Abstract

Measurements of water velocity and turbulence in a water flume using a Doppler sonar operating at 1 MHz are presented. Analysis of the results shows that the instrument qualifies as a very useful tool for nonintrusive and accurate measurement of vertical profiles of water horizontal velocity u and vertical velocity w as well as water turbulence parameters including the ¯u′w′ Reynold's stress component.

The water circulating in the flume is well filtered, so that the backscattering signal relates primarily to temperature microstructure occurring at λ/2, with λ=0.15 cm being the sonar wavelength. Quantitative measurements of backscattering intensity show that the backscattering signal disappears if the water turbulence intensity is below a certain value. This relates to the fact that the high-wavenumber end of the temperature turbulence spectrum no longer reaches the λ/2 scale. Application of the method to measurement of oceanic turbulence is discussed.

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D. Hurther
and
U. Lemmin

Abstract

A method is proposed to reduce the noise contribution to mean turbulence parameters obtained by 3D acoustic Doppler velocity profiler measurements. It is based on a noise spectrum reconstruction from cross-spectra evaluations of two independent and simultaneous measurements of the same vertical velocity component over the whole water depth. The noise spectra and the noise variances are calculated and removed for the three fluctuating velocity components measured in turbulent, open-channel flow. The corrected turbulence spectra show a −5/3 slope over the whole inertial subrange delimited by the frequency band of the device, while the uncorrected turbulence spectra have flat high-frequency regions typical for noise effects. This method does not require any hypothesis on the flow characteristics nor does it depend on device-dependent parameters. The corrected profiles of turbulence intensities, turbulent kinetic energy, shear stress, and turbulent energy balance equation terms, such as production, transport, and dissipation, are in better agreement with different semitheoretical formulas and other measurements from the literature than those from the uncorrected data. Combined with the use of a phase array emitter, the proposed correction method allows measurements with a relative error under 10% in the outer flow region. The corrected inner flow region measurements are still affected by errors that may originate from spatial averaging effects within the sample volume due to the high local velocity gradient or the lack of validity of the universal laws concerning turbulence quantities over a rough bed.

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B. Ozen
,
S. A. Thorpe
,
U. Lemmin
, and
T. R. Osborn

Abstract

Measurements of temperature, velocity, and microscale velocity shear were made from the research submarine F. A. Forel in the near-surface mixed layer of Lake Geneva under conditions of moderate winds of 6–8 m s−1 and of net heating at the water surface. The submarine carried arrays of thermistors and a turbulence package, including airfoil shear probes. The rate of dissipation of turbulent kinetic energy per unit mass, estimated from the variance of the shear, is found to be lognormally distributed and to vary with depth roughly in accordance with the law of the wall at the measurement depths, 15–20 times the significant wave height. Measurements revealed large-scale structures, coherent over the 2.38-m vertical extent sampled by a vertical array of thermistors, consisting of filaments tilted in the wind direction. They are typically about 1.5 m wide, decreasing in width in the upward direction, and are horizontally separated by about 25 m in the downwind direction. Originating in the upper thermocline, they are characterized in the mixed layer by their relatively low temperature and low rates of dissipation of turbulent kinetic energy and by an upward vertical velocity of a few centimeters per second.

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