Dynamics of Towed Particles in a Turbulent Flow

We present an experimental study exploring the dynamics of inertial spheres towed at a constant mean velocity in a turbulent flow. Our work focuses on finite size particles, in the sense that their diameter is significantly larger than the dissipation scale  and lies in the range of inertial scales of the background turbulence. Our experimental setup allows to register the Lagrangian velocity and acceleration of particles with different diameters, densities and free-stream velocities.The analysis of towed particles dynamics shows a clear trend of velocity and acceleration fluctuations to be reduced with increasing particle inertia, while acceleration fluctuations are found to be Gaussian, at odds of usual results for finite-size freely advected particles. Besides, the multi-scale spectral dynamics of the particle is shown to be related to that of the background turbulence by a simple linear second-order band-pass transfer function. We analyse this behaviour at the light of a simple model where the coupling between the particle and the flow is entirely attributed to the drag force, a usual approach for dense point-like particles freely-advected in a turbulent flow, which is known however to fail predicting the transport of freely advected finite-size particles. We show that this simple drag-dominated model quantitatively predicts the dynamics of finite size towed particles, simply requiring to tune the effective particle response time. This suggests that, contrary to the case freely advected finite size inertial particles, the towing configuration efficiently emphasizes the actual role of the drag force by imposing it as an unambiguous leading term in towed particle / turbulence interactions.