An Aquatic Eddy Covariance System for the In Situ Measurement of Turbulent Heat Flux

Aquatic eddy covariance systems have been used to make in situ measurements of turbulent transport, particularly of dissolved gasses, in freshwater and oceanic environments. Existing systems typically combine an Acoustic Doppler Velocimeter (ADV) and dissolved oxygen probes to quantify flux. Similar measurements of heat flux have been attempted using an ADV in combination with a thermistor. We report on the development of a PIV-based heat flux probe. This new system has several advantages; it allows direct measurements of turbulent heat flux with smaller separation distance (1-2mm), and allows simultaneous measurement of velocity gradients and turbulent dissipation rate giving better insight into physical processes.

The PIV system is comprised of a 2.5W, 420nm laser which expands to form a 1mm thick light sheet. Images are captured at a down-sampled resolution of 440x440px using an Alvium 1800 U-240m camera. Fast-response FP07 thermistors are used to measure temperature. A 24-bit ADC is used to digitise voltage outputs from custom-built temperature linearisation circuits to which the thermistors are connected. Images are acquired at a rate of 50Hz. Temperature data is recorded at a rate of 1000Hz.

The capabilities of the system were tested in an axisymmetric turbulent water plume (Re₀=3500 – 10000, Ri₀=10^-5 – 10^-4). Radial profiles of velocity, stress, dissipation, temperature, and heat flux show good agreement with published data. The eddy covariance system has been deployed in the Murray River in 2025.