Ulrich Lemmin

A Correction Method for Turbulence Measurements with a 3D Acoustic Doppler Velocity Profiler

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, t...

Means of noise reduction in acoustic turbulence measurements

Although three-receiver acoustic Doppler velocimeters (ADV) can accurately measure the three-dimensional mean flowfield, their turbulence measurements suffer from parasitical noise contributions. By adding a fourth receiver and optimizing the transducer configuration, the turbulence results can be considerably improved. Redundant information is obtained for all velocity components, which theoretically allows to achieve noise-free turbulence measurements. Experiments show that the parasitical noise contribution is not completely eliminated but reduced by an order of magnitude. At the same time, the useful low-noise frequency range is extended by one order of magnitude. Furthermore, the noise levels of the different components can be directly estimated from the redundant information, which allows to (i) check the quality of the measurements and the system; (ii) estimate the accuracy of the turbulence measurements; and (iii) optimally choose the measuring frequency. Good turbulence results with a four-receiver ADV require a sufficiently high acoustic scattering level of the fluid.A simple, low cost and non-polluting technique to enhance the acoustic scattering level by generating micro hydrogen bubbles in the flow is presented and its efficiency is demonstrated. The principles presented have general validity for 3DADV instruments, but are illustrated and validated by means of measurements with an acoustic Doppler velocity profiler (ADVP) developed at the EPF Lausanne

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

Author Posting.

A constant-beam-width transducer for 3D acoustic Doppler profile measurements in open-channel flows

An ultrasonic constant-beam-width transducer system which is capable of generating an extended focal zone by electronically focusing the beam over the desired water-depth range is described. Beam directivity measurements show that the higher beam directivity and the reduction in side-lobe level lead to an increase of the signal-to-noise ratio by up to 15 dB compared with that of a plane disc transducer. It also allows one to attain a significant reduction of undesirable spectral broadening effects which in the case of plane disc transducers lower the final resolution and interfere with the correct interpretation of data. Using the focalized transducer, simultaneous 3D velocity component profile measurements over the whole water depth are carried out in uniform, open-channel flow and reveal the presence of coherent structures. In the transversal direction, a stationary secondary current composed of two stationary vortices is observed. Compared with a plane disc transducer system, the focalized system increases the resolution by up to 50%.

Evolution of superficial lake water temperature profile under diurnal radiative forcing

In lentic water bodies, such as lakes, the water temperature near the surface typicallyincreases during the day, and decreases during the night as a consequence of the diurnalradiative forcing (solar and infrared radiation). These temperature variations penetratevertically into the water, transported mainly by heat conduction enhanced by eddy diffusion,which may vary due to atmospheric conditions, surface wave breaking, and internaldynamics of the water body. These two processes can be described in terms of an effectivethermal diffusivity, which can be experimentally estimated. However, the transparency of thewater (depending on turbidity) also allows solar radiation to penetrate below the surface intothe water body, where it is locally absorbed (either by the water or by the deployed sensors).This process makes the estimation of effective thermal diffusivity from experimental watertemperature profiles more difficult. In this study, we analyze water temperature profiles in alake with the aim of showing that assessment of the role played by radiative forcing isnecessary to estimate the effective thermal diffusivity. To this end we investigate diurnalwater temperature fluctuations with depth. We try to quantify the effect of locally absorbedradiation and assess the impact of atmospheric conditions (wind speed, net radiation) on theestimation of the thermal diffusivity. The whole analysis is based on the results of fiber opticdistributed temperature sensing, which allows unprecedented high spatial resolutionmeasurements ( 4 mm) of the temperature profile in the water and near the water surface.

Winter cascading of cold water in Lake Geneva

[1] During the winters of 1998 and 1999, observations were made of the cascading of cold water from the nearshore, shallow shelf"" zones and down the sloping sides of Lake Geneva. Cascading starts on the average 10 hours after the onset of surface cooling. The draining cold water descends like a gravity current, and the downslope speed of the head of these slugs of cold water, U, has a mean value of 5.2 cm s(-1), with slugs persisting, on the average, for 8 hours. When the Monin-Obukov length scale at the water surface, L, is negative, implying convection occurs, and (d) over bar/\Lg 1, where d is the mean shelf depth, the nondimensionalized speed of the front of slugs,"" U/b(1/3) is found to be 1.3 +/- 0.4, where b is the surface buoyancy flux integrated over the time period from one slug to the next. Each slug is unsteady, the head being followed by several fronts in which the temperature of the current decreases and its thickness increases. These fronts travel faster than the mean flow by a factor of r = 1.38 +/- 0.3. Dynamical similarities are found with roll waves observed in turbulent open channel flows. The circulation induced by the cascade is found to give a positive skewness to the time derivatives of near-surface temperature in shallow waters, in contrast with negative values close to the slope. The volume of cold water carried by a slug increases with downslope distance as a consequence of turbulent entrainment and the contribution of convectively unstable plumes from the surface. The average volume carried by the slug across the 21 m depth contour is about 1.9 times the volume of water in shallower water (i.e., that on the shelf between shore and a depth of 21 m), implying that cascading is an efficient means of flushing shelf water. Integrated around the lake the mean total volume flux amounts to 11.5 the average winter river inflow.

A two-component acoustic velocity profiler for use in turbulent open-channel flow

The characteristics of an ultrasonic instrument, the bistatic acoustic Doppler velocity profiler (ADVP) are described. The bislatic system can measure profiles of two components of the instantaneous velocity vector with no restrictions on the flow conditions. Results of the bistatic ADVP measurements of water velocity and turbulence in uniform flow in laboratory open channels, were found to coincide well with the laws for mean velocity, variance and Reynolds stress. This indicates that the turbulence scales are sufficiently resolved in the ADVP measurements. Two-dimensional flow patterns derived from the ADVP measurements demonstrate the presence of coherent structures. The instrument was found to be a useful research tool for non-intrusive and accurate instantaneous profile measurements under turbulent flow conditions unaffected by water quality.

Application of an acoustic particle flux profiler in particleladen open-channel flow

An acoustic particle flux profiler (APFP) has been used to non-intrusively and simultaneously profile the instantaneous concentration and the 2-D velocity vector in particle-laden open-channel flow. A temporal resolution of 0.04s and a spatial resolution of φ14.2x4.5mm3 have been achieved by carefully choosing the APFP parameters. The performance of the APFP has been investigated in two flows under capacity charge conditions. The measured results for the mean profiles of velocity, velocity variance, Reynolds stress, concentration, particle flux, moment flux, diffusion coefficients, etc., are compared with the theoretical or semi-empirical formulas. Good agreement has been found. The fluctuation fields of velocity and concentration are highly correlated. Two kinds of coherent structures, the ejection and the inrush, are found to be the dominant structures in the outer region which are responsible for particle entrainment, resuspension and deposition in a particle-laden flow. It is concluded that the recent...

Improved Turbulence Profiling with Field-Adapted Acoustic Doppler Velocimeters Using a Bifrequency Doppler Noise Suppression Method

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 turbulen...

Eliminating velocity aliasing in acoustic Doppler velocity profiler data

We present an efficient dealiasing method for acoustic Doppler velocity measurements which is directly carried out on the Doppler phase shift signal. Corrections are applied to the Doppler frequency measured by the acoustic Doppler velocity profiler receivers, before any transformation into the velocity domain is made. The method does not require knowledge of the flow direction. It consists of a first step in which a threshold analysis determines the need for dealiasing, and a second one in which the Doppler information is corrected. For the second step, an algorithm is developed which follows the complex signal history. It determines the correction to be applied to the instantaneous Doppler frequency in order to make it coherent with the neighbouring space/time points. The efficiency and improvement obtained with this algorithm are demonstrated using field measurement data with our ADVP.