On the influence of gravel bed dynamics on velocity power spectra

Abstract

[1] A series of flume experiments were conducted to study the effect of bed form dynamics on the flow over a gravel bed comprising a wide distribution of grain sizes. Instantaneous high-frequency streamwise flow velocities were sampled using an acoustic Doppler velocimeter at a frequency of 200 Hz, while the simultaneous bed elevations were sampled using sonar transducers at a frequency of 0.2 Hz for a duration of 20 h. Spectral analysis of the measured velocity fluctuations reveals the existence of two distinct power law scaling regimes. At high frequencies, an inertial subrange of turbulence with ∼−5/3 Kolmogorov scaling is observed. At low frequencies, another scaling regime with spectral slope of about −1.1 is found. We interpret this range as the signature of the evolving multiscale bed topography on the near-bed velocity fluctuations. The two scaling ranges are separated by a spectral gap, i.e., a range of intermediate scales with no additional energy contribution. The high-frequency limit of the spectral gap corresponds to the integral scale of turbulence. The low-frequency end of the gap corresponds to the scale of the smallest bed forms identified by the velocity sensor, which depends on the position of the sensor. Our results also show that the temporal scales of the largest bed forms can be potentially identified from spectral analysis of low-resolution velocity measurements collected near the channel bed.

Landscape reorganization under changing climatic forcing: Results from an experimental landscape

“…The power spectral density (PSD) is an effective way of quantifying how the variance of a process is distributed across scales and has been extensively used for characterization of landscapes [e.g., Pelletier, 1999;Passalacqua et al, 2006;Perron et al, 2008;Singh et al, 2010;Guala et al, 2014;Keylock et al, 2014]. From the PSD of spatial elevation increments at transect AA 0 at SS c (Figure 13), three distinct scaling regimes are observed.…”

Turbulence characteristics of flow in an open channel with temporally varying mobile bedforms

“…They also found that turbulence intensities and Reynolds stresses are similar across transitions at individual flow depths, however, greatest in the outer layer. Singh et al (2010) used gravel bed of size = 7.7 mm and carried out spectral analysis of time series of velocities of flow. They found that at high frequencies, an inertial subrange with −5/3 Kolmogorov scaling was observed and at low frequencies, another scaling range with spectral slope of approximately −1.1 was found.…”

Time-Average Velocity and Turbulence Measurement Using Wireless Bend Sensors in an Open Channel with a Rough Bed

“…The ADV power spectral density exhibited a region with a −5=3 slope (in log-log scale) that shows the inertial subrange (see Fig. 2) (e.g., Singh et al 2010).…”