Fish Passage in Large Culverts With Low Flow

APPENDIX B. DATA COLLECTION TECHNIQUES

This appendix provides additional details regarding data collection techniques and instrumentation.

ADV

ADV is an intrusive, remote-sensing technique originally developed for hydrodynamic investigations at U.S. Army Engineer Waterways Experiment Station. The theory is based on the shift in received frequency, that is, the so-called Doppler effect. The device sends out a beam of acoustic waves at a fixed frequency from a transmitter probe. These waves bounce off moving particulate matter in the water, and three acoustic receivers sense the shift in the frequency.

Figure 105 depicts the operation principle of the Doppler measurement technique.⁽³³⁾ The transmit transducer produces periodic short acoustic pulses. As the pulses travel along the beam, microbubbles, suspended sediment, or seeding material scatter a tiny fraction of the acoustic energy. These acoustic echoes are detected by the receive transducers if they originate at the sampling volume defined by the intersection of the transmit and receive beams. The frequency of the echo is Doppler shifted according to the relative motion of the scatters, assumed to be traveling with the velocity of the fluid flow. Orthogonal components can be computed by knowledge of the geometry of the beams. The quality of the measurement is dependent on the presence of scatterers and their behavior within the sampling volume. To ensure that ADV measurements provide an accurate representation of the flow velocity, one should evaluate two additional parameters, the signal to noise ratio and the correlation. Filtering the data using one or both of these parameters can improve the quality of measurement.⁽³⁴⁾ For more information, the reader is referred to SonTek™ and Precht, et al.^(21,35)

Figure 105. Illustration. Measurement probe

PIV

PIV is a non-intrusive and whole flow field measuring technique for assessment of the mean and instantaneous velocity vectors within a single plane of interest. PIV in its simplest form consists of a double-pulsed laser with a synchronized camera equipped with a CCD employed to capture particle displacements in successive video frames.^(22,36,37) Subsequently, image-processing algorithms are used to arrive at a final velocity distribution with an exceptional spatial resolution. Adoption of a density close to that of a testing medium and an appropriate size of particles are of great importance because they ensure that seeding particles are faithfully following the flow as well as scattering enough incident laser light even with low laser energy.⁽³⁸⁾

Sophisticated real-world problems raised the interest among scholars to direct the studies toward 3D PIV with increased temporal and spatial resolution. Reconstructing out-of-plane velocity vectors for highly 3D flows and accounting for traditional PIV perspective errors caused by the imbedded velocity component were major dilemmas that needed to be addressed. Scientists focused on two technical categories: 1) those in which the 3D velocity is calculated from 3D domain (e.g., holographic PIV, tomographic PIV, and scanning PIV), or 2) those in which the 3D velocity is reconstructed from a 2D domain. Because of its long calculation times and costly apparatus, the first category of techniques does not apply to many practical needs. For the latter one, SPIV, dual-plane SPIV, and off-axis SPIV have been successfully developed.⁽³⁷⁾ In SPIV, two coupled cameras capture the same plane at the same time, but with different off-axis view angles. Both cameras should focus on the same spot in the testing medium and be calibrated properly.⁽³⁹⁾ Velocity components that are obtained from cross-correlated dewarped images are sufficient to retrieve the third out-of-plane velocity component.^(40,41,42)

Human 3D perception of 2D views (binocular vision) is achieved with the coordinated use of both eyes. From a technical point of view, stereovision is the impression of the third spatial dimension (i.e., depth) from two dissimilar views of the same scene. Being inspired by this notion, SPIV was developed using two approaches. In the first approach, translational displacement configuration (figure 106), the disparity is accomplished by having CCD cameras with their optical axis parallel to each other and perpendicular to the object plane. (See references 43 through 46.) In the second approach, angular displacement (figure 107), the camera lens axis subtends an oblique angle to the laser sheet.^(41,42)

The translational method offers advantages of convenient mapping, is easy to apply, and produces well-focused images. However, reduced overlap field of view is one of the shortcomings.⁽⁴⁶⁾ The angular displacement approach is limited by an upper bound to the off-axis angle subtended by the center of the lens to the center of the region of interest (because of the lens design). Limitations in both approaches motivated the development of an alternative approach.

To address the upper bound off-axis angle restriction in angular displacement systems, the camera's optical axes are aligned neither parallel with each other nor orthogonal to the object plane in an alternative strategy.^(41,42) This configuration significantly reduces the out-of-plane velocity component relative error. The angular system similar to that of Willert has been employed in this project.⁽⁴¹⁾

Figure 106. Illustration. Translational configuration

Figure 107. Illustration. Angular configuration