^ Back to Top ^

Measurement Description

The ASFM measures the flow velocity simultaneously at one measurement path in each measurement section.  The acoustic transducers (transmitters and receivers) are placed opposite to each other so that the paths are at the same nominal elevation in each intake bay, and sampling occurs at one discrete measurement path simultaneously in each bay.  After sampling at the first measurement path is complete, the ASFM computes the flow velocity for that path and then switches to the next measurement path.  The process is repeated until all measurement paths in the measurement section have been sampled.  The typical sequence is from the floor to the roof. When sampling and individual velocity calculations are completed, the discharge is computed for each bay. Discharges in individual bays are then added arithmetically to produce the total discharge for the operating condition being measured.  The preliminary results (discharges and the flow velocities and inclinations) are written to an electronic file, and the ASFM equipment is ready for a new measurement.

As a statistical measurement, ASFM requires a minimum sample size to produce a meaningful measurement.  The sampling period should be of sufficient duration to average out turbulent fluctuations.  At least three or four data collection cycles at each measurement path are recommended to permit calculation of an average discharge and standard deviation.

Fig. 2 ASFM Typical arrangement- fixed frame

There are two basic approaches to measurement paths placement: a number of measurement paths at fixed elevations on a fixed-frame spanning the full height of the measurement section (Fig 2), or one or more paths mounted on a smaller profiling frame that is moved through a series of elevations to sample the full height of the measurement section (Fig. 3). In a fixed-frame installation, the individual measurement paths are placed to optimally sample the vertical structure of the flow. It is recommended that uniform 1.5 m spacing of the measurement paths be used, except in regions where large variations in velocity are expected, where closer spacing will be beneficial. The integration algorithm used for computations does not require regular spacing of the sampling paths.

Fig. 3 ASFM Typical arrangement- moving frame

Profiling frames may be either stopped at each measurement position for the duration of the measurement or, if using a very slow (not more than 25 mm/sec) and constant travel speed, swept continuously over the total vertical dimension of the measurement section. If a continuous sweep is used, the position of the frame as a function of time must be recorded electronically. In general, profiling frames allow more flexibility in measurement path locations, but at the inconvenience of having cross-members in the flow.

It is also possible to attach the transducers directly to the intake side walls, but that would normally only be done if a permanent installation is intended. 

The measurement paths must not be placed too close to boundaries, such as the floor and roof, because of interfering echoes.  The limit of approach depends on the width of the measurement section. To ensure proper sampling near these boundaries, the uppermost and lowermost measurement paths should be positioned at the distances recommended in Table 1.   The limit also applies to the distance between the measurement paths and the mounting frame cross-members.

Table 1

Uncertainties can be reduced when the number of measurement paths is increased. As many as 30 paths can be used in one measurement section when the intake characteristics and the desired accuracy so dictate.

Measurement Limitations

A) General
The ASFM has been developed specifically for the short, rectangular converging intakes typically found in plants with heads of 35 meters or less. This makes unstable inflow velocities or divergent approach angles unavoidable in most measurement locations. Both the magnitude and the direction of the velocities measured by the ASFM must be therefore considered in the calculation of discharge.
A critical assumption of the ASFM measurement technology is that the intensity and spectral characteristics of the turbulence in the moving fluid are uniformly distributed along the acoustic transmission paths. This is heavily dependent on the following site-specific intake characteristics:

• Size and shape of the trashrack structural supports upstream of the ASFM measurement plane, and their distance from the measurement plane
• Non-uniformity and non-directionality of the inflow
• Convergence and shape irregularities upstream and downstream of the measurement plane

B) Intakes for which accurate results can be obtained directly
Experience has shown that the ASFM produces accurate and repeatable results when the following conditions are complied with:

• Trash rack vertical structural supports are less than 100 mm in width or, more than 6.0 m from the measurement plane and the trash rack have been cleaned prior to the testing
• The horizontal angle between the inflow velocity vector and the axis of the intake does not exceed 5 degrees, and the operation of the neighbouring units and the spillway, if applicable, is controlled to the degree necessary to remain within this limitation during the period required to perform the measurements.
• Flow velocities are between 0.5 and 8.5 m/s and measurement sections are wider
  than 1.5 m.
• There are no significant intake conduit shape or convergence irregularities. Any such irregularities must be investigated before starting the measurement. Fish diversion screens upstream of the measurement plane will typically require such investigations. There are no excessive air bubbles and/or acoustic noise. If their presence results in missed samples, enough valid samples must be obtained to be compatible with the assumptions used in the error analysis.

C) Intakes for which bias corrections will be required
For some intakes where the above requirements are not fulfilled, the systematic uncertainty of the measurement may be predicted before the measurement is started, and thus removed from the measurement results, as follows:

In a normal ASFM installation, the measurement paths are horizontal, and thus large vertical trash rack supports can introduce bias into the ASFM measurements. As illustrated in Fig. 4, the magnitude of the measurement bias is strongly dependent on the contrast between the turbulence of the intersecting vertical wakes and the remainder of the acoustic path, the distance between the measurement plane and the trash rack support and the width of the trash rack supports.  If the wakes from the major horizontal trash rack supports (parallel to the acoustic paths) have merged before they reach the measurement plane, then it is very likely that the bias due to the wakes from the vertical support members will be reduced to a negligible amount.  The distance downstream of the trash rack, Xmerge required for the wakes from the horizontal members to merge may be estimated as

 

where
H is the vertical separation between the major horizontal trash rack supports,
D is their width in the vertical and
X is the distance between the trash rack and the measurement plane (all dimensions in meters).

If the distance to the measurement section is less than Xmerge, an upper bound for the bias error due to wakes from the vertical members may be estimated using numerical calculations (Bouhadji et al. 2003) and comparisons with the available experimental data on similar structures (Wygnanski at al 1986, and Stewart and Townsend 1951). 

Fig. 4: Illustration of the relation between wake merging and ASFM bias

Experience has shown that with these corrections the resulting systematic uncertainty of measurements with the ASFM can be close to ±2% for many difficult intakes which do not comply with the requirements of paragraph B above. However, experience has also shown that there will be some exceptionally difficult intakes where no acceptable measurement uncertainties will be achievable.

.