Means of noise reduction in acoustic turbulence measurements

Although three-receiver acoustic Doppler velocimeters (ADV) can accurately measure the three-dimensional mean flow field. 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 3D ADV instruments. but are illustrated and validated by means of measurements with an acoustic Doppler velocity profiler (ADVP) developed at the EPF Lausanne.