Helicoidal particles in turbulent flows with multi-scale helical injection

We present numerical and theoretical results concerning the properties of turbulent flowsunder strong multi-scale helical injection. We perform direct numerical simulations ofthe Navier-Stokes equations under a random helical stirring with power-law spectrumand with different intensities of energy and helicity injections. We show that there existsthree different regimes where the forward energy and helicity inertial transfers are: (i)both leading with respect to the external injections, (ii) energy transfer is leading andhelicity transfer is sub-leading, and (iii) both are sub-leading and helicity is maximal atall scales. As a result, the cases (ii-iii) give flows with Kolmogorov-like inertial energycascade and tunable helicity transfers/contents. We further explore regime (iii) in aLagrangian domain, by studying the kinetics of point-like isotropic helicoids, particleswhose dynamics is isotropic but breaks parity invariance. We investigate small-scalefractal clustering and preferential sampling of intense helical flow structures. Dependingon its structural parameters, the isotropic helicoids either preferentially sample co-chiralor anti-chiral flow structures. We explain these findings in limiting cases in terms of whatis known for spherical particles of different densities and inertia. Furthermore, we presenttheoretical and numerical results for a stochastic model where Lagrangian properties canbe calculated using analytical perturbation theory. Our study shows that a suitabletuning of the stirring mechanism can strongly modify the small-scale turbulent helicalproperties and demonstrates that isotropic helicoids are the simplest particles able topreferentially sense helical properties in turbulence