The interfaces
reflect and modify the acoustic field. The intensity of the acoustic field
received in a point inside the liquid depends on the material, the shape and
the number of these interfaces. This intensity is most of the time very difficult
to evaluate. This lack of knowledge does not allow a precise determination
of the size of the measuring volume.
These interfaces may generate, in certain situations, artifacts and induce
modifications in the velocity profiles as presented in the figures 1 and 2
below.
Figure 1
The ultrasonic
beam BC reflected by the far interface of the figure 1 transforms this interface
in a transmitter. The same particles contained in the liquid will backscatter
a second time energy in the direction to the transducer. The depth associated
to the path ABC is located outside the flowing liquid. Imaginary velocity
components are added to the real velocity profile. The measurement of velocities
near the far interface is affected by this phenomena. The size of the ultrasonic
beam determines mainly the level of this artifact.
Figure 2
The figure 2 displays another situation. The reflected ultrasonic waves inside a wall enlarge the ultrasonic beam inside the liquid and modify its shape. These reflections disturb the determination of the size and the shape of the measuring volume. The thickness, the acoustical impedance and the attenuation coefficient of the interface determine the level of this phenomena. This situation appears when measuring for instance through steel walls.
Moving interfaces
The interfaces often give strong reflections. Despite of the many reflections which are necessary to reach the transducer, the energy reflected by these interfaces is often stronger than the energy coming from the particles flowing with the liquid. Most of the algorithms used to compute the Doppler frequency shift do not allowed stationary components. The elimination of these stationary components by high-pass filtering implies an increase in the dynamic of the analyzed echoes and a reduction in the sensitivity in the measurement of low velocities. When some interfaces are in movement the correct estimation of all the velocity field is very difficult. The echoes generated by such interfaces may affect the velocity profile in many places due to the combination of many reflections. The Doppler frequency shift induced by these movable interfaces can not be removed if their values have the same values as the flowing particles.