Comprehensive Manual54
© 2018 Nortek AS
2.5.2 Low Flow Measurements
An important benefit of the pulse-coherent processing technique is that the instrument can be
configured to measure velocities as small as 1 mm/s, or even smaller. This is quite useful in
applications where the water motions are minimal, such as in settling tanks, bore holes and in some
lakes.
Measuring velocities of a few mm/s or smaller has a few inherent problems that should be
considered while planning the measurements:
All Velocimeters are designed to output as much power as possible. This is done to ensure a
strong echo (high SNR) and good quality data in a broad range of applications. However, the high
output power actually generates a non-linear pressure wave that accelerates the water below the
probe. If the horizontal water velocity is small (<10 mm/s), the pressure generates a (negative)
vertical velocity of 1-15 mm/s in the sampling volume. This problem can be minimized by using the
lowest velocity setting, and the lower power setting while measuring low flows. This is referred to
as Acoustic streaming: If ambient flow velocities are of the order of potential acoustic streaming
velocities, acoustic streaming may represent a significant source of bias.
Low flow means low turbulence levels, which may mean clear water. In general, all Velocimeters
have problems measuring in clear water because of lack of scatterers (there is no echo).
Experience shows that unpumped boreholes are difficult while settling tanks usually have enough
scatterers in suspension.
Thermal flows can superimposed on the net transport flow. In some case, the parameter of interest
is not really the point velocity but the net transport rates. If the liquid is not thermally balanced (e.
g, daytime heating) thermal convection cells can be generated. This velocity can be quite strong
(several mm/s) and it can be difficult to detect the lower, net background transport velocity.
2.5.3 Turbulence Measurements
Turbulent flows vary widely, both with distance and with time. Because turbulent flows exist in both
large and small scales, acoustic Doppler systems also come in large- and small-scale versions.
Systems designed for large-scale measurements have divergent acoustic beams that send sound
waves outward, away from the instrument. These systems are not described further in this section,
but is covered by the Signature Manuals. Systems designed for small scales tend to have
converging acoustic beams that focus their sound waves inwardly towards a small sampling volume,
just like the Velocimeters.
The ability to describe a turbulent flow in three dimensions (two horizontal directions and one vertical)
is important. The vertical velocity component of turbulent flows is a key contributor to the mixing of
different water masses and the exchange of energy, nutrients and particulates. Therefore, the ability
to directly measure the rapid velocity fluctuations in the vertical direction is critical for high-quality
turbulent-flow measurements.
There are two things to take into account when selecting an instrument to measure turbulence;
1) Sample quickly enough; In order to measure the rapid velocity fluctuations within these eddies,
the systems must sample quickly enough to resolve the variations. The importance of fast sampling
cannot be overstated. Without sufficient individual (and independent) measurements of the flow, the
resulting data will not contain enough detail to describe the phenomenon.
2) Measurement volume; the scale of your measurement volume must match the scale of the
turbulence that you are studying. The smaller the turbulence spatial scale, the smaller the
measurement volume must be in order to properly describe it. If the scales do not match, you will
only capture a portion of the energy within the flow.
2.5.4 Laboratory environments
Follow these guidelines when you mount your instrument in a lab:
Make sure that there are no obstructions between the sensor and the focal point (sampling
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