Author(s): K. Koca; A. Lorke; C. Noss
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Abstract: Detailed flow structure measurements in natural flows can provide new insights into the understanding of environmental fluid mechanics processes. Inspired by the need for flow detailization in natural flows, a number of researchers (Liao et al., 2009; among others) employed the well-established Particle Image Velocimetry (PIV) technique, in field-based studies. PIV is advantageous over typical measurement techniques (e.g. Acoustic Doppler Velocimetry) because it provides direct measurements of instantaneous velocity field, its spatial derivatives and spatial covariances in two (or three) spatial dimensions. These features allow for the calculation of instantaneous vorticity, dissipation rates (Westerweel et al., 2013), and turbulent wave number spectra without relying on Taylor’s frozen turbulence approximation, as well as observation of coherent flow structures. The field-PIV systems developed so far, however, utilized high power, sophisticated laser systems, as well as sophisticated cameras, which makes them expensive and requires extensive deployment effort. We describe an alternative and inexpensive field-deployable PIV system based on a consumer-grade camera (Go Pro Hero 4, Go Pro Inc., USA) and a 225 m W (532 nm), continuous-wave laser module (Hercules, Laser Glow, Canada), which can be deployed in very shallow flows with a minimum water depth of 6 cm. To validate the developed system, simultaneous velocity measurements were performed in a flume using a Vectrino Profiler (Nortek, AS). The flow depth was constant at 30 cm, while the mean flow velocity, U, varied between 1.5 cm s -1 and 37.3 cm s -1 (four runs). Good agreement was found in a direct comparison of velocity time series (Figure 1). The velocities measured by the PIV were observed to be slightly less than those measured by Vectrino Profiler. The differences between the mean longitudinal velocities varied between 0.2%and 6.9%, with the biggest difference observed for U=37.3 cm s -1, whereas the differences were between 0.7%and 5.5%for the mean vertical velocities. The root-mean-square velocity fluctuations measured by both instruments were in a reasonable agreement, with differences between 5.6%and 8.2%for the longitudinal component, and between 5%and 13.9%for the vertical component. Despite the limitations due to the use of continuous illumination as opposed to the short duration, high intensity pulses in commercial PIV systems, small sensor with compressed video output, and lack of shutter speed control, we demonstrated that the proposed PIV system shows comparable performance to Vectrino Profiler, and can be practically used in field-based research, particularly in challenging shallow and slow flow applications. Figure 1. Comparison of velocity time series obtained by two instruments. a) U=1.5 cm s -1, b) U=37.3 cm s -1. Solid lines denote longitudinal velocity, u, while dashed lines denote vertical velocity, v.
Year: 2016