Post-doc position

Experimental study of copepods in turbulence

Introduction

Copepods are small crustaceans belonging to zooplankton, with a size comparable to the Kolmogorov scale (on the order of millimeters). They are capable of swimming and jumping. While the relationship between turbulence and copepods is critical, it remains poorly understood. It is unclear whether copepods are attracted to turbulence or avoid it, nor is there a defined turbulence threshold that might trigger a transition between attraction and avoidance behaviors [1]. Understanding this dynamic is essential for interpreting the collective behavior of copepods in relation to their environment, including turbulent forcing.

Although significant progress in numerical simulation has been enabled over the past 15 years by understanding the dominant role played by small-scale interactions between water turbulence and the dispersed particulate phase, the validity of these models and identified processes remains limited by our inability to provide high-resolution velocity field measurements of both the fluid and the particles (organic or inorganic). In this context, pulsed ultrasound techniques in the MHz range outperform particle image velocimetry techniques due to acoustic attenuation being much lower than that encountered with laser-based optical systems. Using these acoustic techniques, the amplitude and phase of the measured echoes make it possible to estimate concentration and multi-component velocities [2,3]. These systems thus appear particularly well-suited to studying turbulence-particle interactions in aquatic environments.

In this framework, the aim of the project is to characterize the interactions between copepod behavior and turbulence using ultrasonic techniques in a controlled environment. The expected results will provide quantitative insights into whether copepods are attracted to, flee from, or remain indifferent to localized turbulence.

Laboratory Experimentation:

The study will be conducted using the Agiturb system, developed at LOG [4]. This system employs four counter-rotating agitators beneath a tank, generating a field which is approximately isotropic at the center, with turbulence levels controlled by the agitators' rotation speed, with Taylor-based Reynolds numbers R_λ from 130 to 350.

An ultrasonic Doppler sonar, developed under the RUPTURE project (UBLab 3C – Ubertone®), will be utilized. Its multi-bistatic geometry (one emitter, 4 receivers) enables collocated 3C velocity field measurements and concentration along a vertical profile with high spatial (mm) and temporal (ms) resolutions. Its capability to work at multiple carrier frequencies (MHz) is key to resolve both the dispersed and carrier phases of the flow and investigate turbulence-particles interactions in turbulent flows.

As such, in the context of the present post-doctoral research, this system is necessary to provide simultaneous time-resolved measurements of the fluid velocity as well as the copepod velocity and biovolume. High-speed cameras will also be available to reconstruct copepod trajectories.

In the frame of the french ANR RUPTURE and CPER IDEAL, the postdoctoral researcher will be expected to:

• Setting up the experiment using this equipment.

• Investigating the effect of the Reynolds number on the differential behavior of living copepods relative to the fluid.

1. Schmitt, F.G.. Turbulence et écologie marine, Ellipses, Paris, 2020, 246pp.

2. Fromant et al. (2018), JGR Earth Surface 123(10) 2546. https://doi.org/10.1029/2017JF004560

3. Fromant et al. (2024), J. Acoust. Soc. Am. 155(4) 2817. https://doi.org/10.1121/10.0025766

4. Le Quiniou et aL; (2022), Eur. Phys. J. Plus 137, 250. https://doi.org/10.1140/epjp/s13360-022-02455-7