Assessing the turbulent kinetic energy budget in an energetic tidal flow from measurements of coupled ADCPs

Abstract

Two coupled four-beam acoustic Doppler current profilers were used to provide simultaneous and independent measurements of the turbulent kinetic energy (TKE) dissipation rate ε and the TKE production rate P over a 36 h long period at a highly energetic tidal energy site in the Alderney Race. The eight-beam arrangement enabled the evaluation of the six components of the Reynolds stress tensor which allows for an improved estimation of the TKE production rate. Depth-time series of ε, P and the Reynolds stresses are provided. The comparison between ε and P was performed by calculating individual ratios of ε corresponding to P . The depth-averaged ratio ε / P averaged over whole flood and ebb tide were found to be 2.2 and 2.8 respectively, indicating that TKE dissipation exceeds TKE production. It is shown that the term of diffusive transport of TKE is significant. As a result, non-local transport is important to the TKE budget and the common assumption of a local balance, i.e. a balance between production and dissipation, is not valid at the measurement site. This article is part of the theme issue ‘New insights on tidal dynamics and tidal energy harvesting in the Alderney Race’.

Characterization of the vertical evolution of the three-dimensional turbulence for fatigue design of tidal turbines

“…Similar results were documented by Gunawan et al [27]. Nonetheless, our spectra derived from time series of the along-beam velocities exhibit equivalent TKE levels within the inertial subrange, indicating a clear isotropy of the small-scale turbulence (see [13]). In view of this result, we are inclined to think that the anisotropy exhibited in the plot of the spectra associated with the streamwise, spanwise and vertical direction is an artefact of ADCP beam-spreading.…”

“…Two upward-looking RDI Workhorse 600 kHz four-beam ADCPs were deployed on the seafloor (mean and maximum water depth of 31 m and 35 m, respectively) approximately 4 km offshore, west of the port of Goury, in the eastern part of Alderney Race, France (figure 1) [13,14]. The instruments were mounted on a specifically designed frame having the following features: 3.3 m long, 2.5 m wide and 2 100 kg weight (figure 1).…”

“…where U = u 2 + v 2 + w 2 is the mean flow speed (σ 2 i ) is the variance associated with the streamwise, spanwise and vertical velocities and (σ 2 N i ) is the variance induced by Doppler noise. (σ 2 N i ) was estimated from the energy-spectra of the velocities [13]. The uncertainty, ( I i ), when calculating the mean turbulence intensity, was resolved through the computation of the standard error of the mean.…”

“…The present study is an extension of the work initiated by Thiébaut et al [13,14]. The following metrics are quantified: (i) the velocity, (ii) the turbulence intensity, (iii) the integral lengthscale and (iv) the six components of the Reynolds stresses.…”

On the scaling of turbulence in a high Reynolds number tidal flow

“…For instance, Rippeth et al (2003) reported values of in the northern Irish Sea with similar ratios near the bed shown by Guerra & Thomson (2017) for Puget Sound, USA. Thiébaut et al (2020) reported ratios ranging between 0.5 and 1.1 at within the Alderney Race in the English Channel. Notably, it was also shown that diffusive transport of turbulent kinetic energy was equal to , thus was considered as a likely source of the local imbalance between production and dissipation.…”