2C bistatic velocity measurement
The bistatic technique provides an instantaneous 2C velocity vectors along a 1D profile (measurement along a single beam) at a high spatial and temporal resolution. This allows to observe the velocity gradient, the shear stress, the flow direction as a function of depth, the effect of an obstacle on the velocity field, etc.
UB-Lab from Ubertone allows to do bistatic measurements. It consists in measuring along a narrow beam the projection of the velocity vector over two observation axes and then calculate the component of the velocity profile in an orthonormal coordinate system (x, y).
The figure above presents an example of installation adapted to the 2D velocity profile measurement in a flow. Two transducers (Tr1 in emission-reception with a diameter of 20 mm and Tr2 in reception with a large angle opening) are oriented crossing the flow. The transducers are plugged into the UB-Lab laboratory profiler. The measurement cells are distributed along the narrow beam constituting the profile. The yellow cell in the figure above represents one of those cells. The fluid's velocity V in this cell, with an angle α to the main flow direction, can be observed through its two projections V1 et V2 along the acoustic beam axes. The norm V and the angle α of the velocity vector can be then calculated the trigonometric way.
The UB-Lab is also available with a symmetric bistatic configuration. In this case, one transducer (Tr0) is used as an emitter and a pair of transducers (Tr1 + Tr2) located on both sides are used as receivers. This configuration simplifies the processing of the data obtained simultaneously on both transducers. By using a second pair of transducers (Tr3 + Tr4) it is possible to obtain the 3rd component of the velocity vectors along the profile in average.
As the measurements are done simultaneously on the pair of transducers, the two components of the velocity are obtained instantaneously. This allows to study the flow at high frequency and extract informations like the Reynolds stress, the Kolmogorov spectra of turbulence, etc.