Optimal control of scalar transport by an active suspension

An interesting aspect of active suspensions is that they can perform tasks, reflecting the biological suspension that in part motivates their study. This is considered for a suspension of non-interacting immobile agents advecting a passive scalar in a periodic domain, using the continuum model of Gao et al. (Gao et al., Phys. Rev. Fluids, 2017). Two classes of control are considered: one adjusts the local agent activity, and the other adjusts the initial nematic ordering. The adjoint governing equations are solved with spectral methods to provide a discrete-exact gradient to accelerate high-dimensional optimization. It is shown that mediating activity strength can induce sufficient order to manipulate the scalar field in multiple ways. Depending on the initial nematic alignments and nematic orders, activity-mediated mechanisms can develop transient pushing and pulling strategies through normal stress and persistent transport strategies with shearing stress. The initial structure for fixed activity strength is directly effective through combining multiple transport mechanisms. These different strategies for leveraging active suspensions for different objectives are discussed.