Wind effect on gyrotactic micro-organism surfacing in free-surface turbulence

Role of large-scale advection and small-scale turbulence on vertical migration of gyrotactic swimmers

“…Periodic boundary conditions are imposed on swimmers moving outside the computational domain in the horizontal (homogeneous) directions. In the wall-normal direction, the swimmers rebound elastically at both boundaries: For a detailed discussion on the assumption of fully elastic rebound to describe swimmer interaction with a free surface, the reader is referred to Mashayekpour et al [20].…”

“…The most evident macroscopic manifestation of floaters and motile particle dynamics in freesurface turbulence is the formation of highly concentrated clusters near the surface [13,20,24,29]. These originate from the interaction between individual cells and surface flow structures.…”

“…Among the most widely studied motile responses, one is gyrotaxis: Gyrotaxis originates from the competition between a stabilising gravitational torque, due to an asymmetric mass distribution of the swimmer that favors its orientation along the direction of gravity (gravitaxis), and a destabilizing viscous torque, produced by the local shear acting on the swimmer [5,11,12]. The role of gyrotaxis in determining the transport properties of self-propelled microorganisms has been investigated both in laminar [13][14][15] and in turbulent flows [7,[16][17][18][19][20][21][22]. Turbulence, in particular, leads to preferential sampling of downwelling and upwelling regions in the bulk of the flow [7,[21][22][23], small-scale fractal patchiness [16,17,24], and shear-induced trapping that may hinder vertical migration [18,20].…”

“…The role of gyrotaxis in determining the transport properties of self-propelled microorganisms has been investigated both in laminar [13][14][15] and in turbulent flows [7,[16][17][18][19][20][21][22]. Turbulence, in particular, leads to preferential sampling of downwelling and upwelling regions in the bulk of the flow [7,[21][22][23], small-scale fractal patchiness [16,17,24], and shear-induced trapping that may hinder vertical migration [18,20]. All these processes are significantly influenced by the multiscale structure of the flow, which encompasses large-scale advective motions generated by coherent structures as well as small-scale motions associated with the local fluid velocity gradients.…”

“…A convenient way to parametrize the relative importance of the flow scales on the swimmer dynamics is provided by the stability number, , defined as the inverse of the dimensionless time a perturbed gravitactic swimmer takes to return to the vertical orientation in still fluid [11,25]: High (respectively, low) stability number represents the case of a swimmer with small (respectively, large) reorientation time, which can hardly (respectively, easily) be destabilized by the background flow. Indicating with τ K the dimensionless Kolmogorov timescale of the flow, three different physical instances can be highlighted [18,20]: (i) τ K 1, representing the case in which the timescale of gravitaxis is large compared to the dissipative flow timescale, and swimmers dynamics are primarily controlled by the external action of turbulence; (ii) τ K 1, representing the case in which the timescale of gravitaxis is small compared to the timescale of the smallest turbulent eddies, and swimmers dynamics are controlled by their fast response to gravity; (iii) τ K ∼ O(1), representing the situation in which the two timescales are comparable, and the motion of the swimmers results from the competition between turbulence and gravitational bias.…”