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This work deals with the numerical investigation of polar active fluids. These systems, such as bacterial or algal suspensions, are intrinsically far from equilibrium since the internal constituents (swimmers) continuously interact and dissipate energy to the surroundings. We use a coarse-grained continuum model [1] where the swimmers have a macroscopic polarization whose evolution equation is coupled with the incompressible Navier-Stokes equations. The active force exerted by the swimmers is incorporated via a hydrodynamic stress and a hybrid lattice Boltzmann scheme is employed to solve the system of equations [1]. The contribution of this work is the use of a new phenomenological term which forces the alignment between polarization and velocity vector. We study both extensile and contractile fluids in a quasi-2-dimensional geometry. When periodic conditions are used on all the boundaries the increase of the alignment leads to a metastable ordered state. We also show that different initial conditions affect the steady state configuration. Finally, the influence of non-slip walls with different anchoring for the polarizations field and also size effects are addressed.
References [1] E. Tjhung, M. E. Cates and D. Marenduzzo, Nonequilibrium steady states in polar active fluids, Soft Matter, 2011, 7, 7453. |