Negative Bias in ASFM Discharge Measurements in Short Intakes -Transducer Spacing

1. Introduction The Acoustic Scintillation Flow Meter (ASFM) measures flow velocity in directions perpendicular to pairs of parallel acoustic beams. In its simplest hydroelectric power plant applications, the ASFM's acoustic beams originate from horizontally-aligned pairs of adjacent transducers mounted on intake walls or on the vertical members of frames designed to fit into intake gate slots. The generated horizontal acoustic beam signals are received on the other side of the intake by matching and directly opposing pairs of receiving transducers. Time delays, τ, corresponding to optimal cross-correlations of the fluctuating signal amplitudes at the adjacent receivers are assumed to be related to the component of the fluid velocity, v, which parallels the vector separation, d, of the parallel beams by the relationship: