Session 8: Doppler technology — Part I

Acoustic Doppler measurements of flow benefit from scatterers and from suppression of noise. The introduction of micro bubbles into laboratory flows provide excellent targets but also contribute enough buoyancy to produce a rising plume. Zedel and Butt have adapted the dissolved air floatation method from mining to generate right-sized bubbles. Bubble size can be controlled to some degree by injecting air-saturated water at some selected overpressure into the tow tank where the air comes out of solution to create targets for Doppler scattering. Noise can be reduced with multiple frequencies and with multiple transducers in a system. Multiple frequency uncorrelated samples from a common volume can be used as independent measurements of a single velocity to lower the noise floor of turbulence spectra from acoustic Doppler sonars. This can be from monostatic or bistatic geometries. Maximum A Posteriori (MAP) estimation (a Bayesian technique) performs better than alternative treatments such as the product of measurement probability density functions. Smoothing parameters are the only variable left to adjust and an automatic tuning function has been determined that demonstrates the spectral shape of velocities calculated by this method compares well to that derived by PIV measurements from the same experimental jet. Although pulse to pulse coherent sonars with multiple frequency and multiple transducer designs have been experimented with for several years, this is now available commercially in Aanderaa´s Seaguard^® RCM with two frequencies in a single point current sensor. This has been deployed now near the surface, in the deep-sea, and in strong estuarine flows where in all cases it performed well. Profiling with a bistatic sensor is now available commercially with Nortek´s Vectrino-II so that boundary layers can be examined with fine resolution. With user friendly display software to allow the operator to optimize settings, profiles of velocity can- - be made as close as 5mm from a boundary and agree well with PIV measurements made simultaneously in a research flume.