Hub-height time series measurements of velocity and dissipation of turbulence kinetic energy in a tidal channel

Abstract

The Nemo turbulence measurement system is an anchored, buoyant float designed to measure time-series of current velocity, velocity shear, and turbulent kinetic energy (TKE) dissipation rates in swift tidal channels. The system consists of a 4.5 m long streamlined float made from syntactic foam, with cutouts to house various instrument components: a 600 kHz downward-looking acoustic Doppler current profiler (ADCP); an acoustic Doppler velocimeter (ADV); and a turbulence module equipped with velocity shear probes and fast-response thermistors The system was successfully deployed over a two-week period in Islay Sound, a tidal channel between Islay and Jura Islands, Scotland, where flow speeds exceed 3 m/s. The depth of the channel was 53 m and the system was deployed such that it remained within the depth aperture of a proposed tidal energy extraction rotor. This environment represented significant challenges for the design of the mooring and the turbulence instrumentation. We describe the measurement system and its performance in terms of attitude and flight dynamics including a synopsis of time series of current velocities and shear probe turbulence data. Scientific interpretation will be presented in subsequent publications.