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Acoustic doppler technique is widely used in both fields and laboratory facilities to compute the mean water velocity and to characterize the turbulence of a flow. In general they provide the three dimensional components of flow velocity in a measurement volume in the water body with fairly good spatial and temporal resolution for engineering applications. The most sophisticated devices can even gauge a velocity profile measuring the water velocity in several measurement volumes along a line. However, these devices are semi int rusive which might have, depending on the experimental setup, substantial consequences in the measurements obtained due the flow perturbation created by the probe. The goal of this paper is to explore experimentally and numerically the wake effect of the p robe on the measurement volume in order to validate the measurements provided by this kind of instruments or incorporate some corrections if needed. A computational fluid dynamic (CFD) model is used to simulate an open channel flow where the model was validated with previous experimental results. In the other hand, the laboratory measurements were conducted in an open channel flume located in the Ven Te Chow Hydrosystems Laboratory of the University of Illinois. The measurements were done using particle image velocimetry technique (PIV) producing two dimensional velocity fields around the acoustic probe measurement volume with and without the presence of the probe. The numerical and experimental ranges of Reynolds numbers (Re) tested were 3x10 6 to 1x10 7 and 1x10 4 to 5x10 4 respectively. Non dimensional contour plots showing the difference between the flow velocity and turbulent quantities with and without the probe are built. Both results show that the errors are less than 10 percent around the probe. This methodology is still under development, however it provides more insight for experimental setups and it could be applied to other acoustic doppler instruments such as the ADV (Acoustic Doppler Velocimeter) and ADCP (Acoustic Doppler Current Profiler) among others.

Sand ripples develop spontaneously in an oscillatory flow over a movable sediment bed. The mechanics of sediment suspension in the presence of ripples is complex and its direct observation is complicated unless a technique can be used to capture the flow field and particle motion above and around the ripples. This work presents the results of an exploratory study conducted in a U-tube. The goal of the study was to use Particle-Image-Velocimetry (PIV) to capture the interaction between turbulent flow structures, suspended particles, and ripples and other bed instabilities. The PIV technique was chosen because of its potential to simultaneously determine the fluid velocity and the solid-particle velocity fields. Exploratory measurements indicate that the mechanisms responsible for the resuspension of bed particles in the presence of ripples are more complicated than anticipated. As water flows past each ripple, separation takes place at the crest of the ripple. Vortical structures generated downstream of the crest entrain sediment particles into the flow. When the flow reverses direction, each structure, along with the sediment it has suspended, is carried away from the ripples and into the outer flow field. As the structure dissipates, the sediment suspended in it is released. The PIV techniques implemented in this study provide information about velocity, vorticity, and turbulence distributions, helping to explain this complex flow field.

Non-intrusive velocity measurements in turbulent, stratified flows are hampered by index of refraction fluctuations. These fluctuations can reduce the data rate and quality in laser Doppler velocimetry by displacing the beams. In an experiment with particle image velocimetry (PIV), the displacement of light reflected from particles can produce spurious particle positions and velocities. Although matching the indices of refraction of the stratifying agents can eliminate these problems, the costs associated with stratifying a large facility can be high. Thus, we seek to quantify the errors incurred in PIV measurements as a function of the intensity of the turbulence. We present results of a laboratory experiment in which images of a stationary target mimicking typical PIV images are acquired under different conditions. Because the target is fixed, any displacement registered by the PIV system is an error. The experimental facility consists of a salt-stratified tank stirred with arrays of vertical rods oscillating horizontally. Direct force measurements allow the work done by the rods on the water, and thus the average dissipation ea, to be estimated. Concurrent density measurements during the stirring are used to compute the length and time scales of the largest eddies. PIV errors are then expressed as a function of the turbulence intensity and the path length LP. The former quantity is measured by ea/νN 2 , where N is the buoyancy frequency.