  Non-isothermal particles suspended in a fluid lead to complex interactions --
the particles respond to changes in the fluid flow, which in turn is modified
by their temperature anomaly. Here, we perform a novel proof-of-concept
numerical study based on tracer particles that are thermally coupled to the
fluid. We imagine that particles can adjust their internal temperature reacting
to some local fluid properties and follow simple, hard-wired active control
protocols. We study the case where instabilities are induced by switching the
particle temperature from hot to cold depending on whether it is ascending or
descending in the flow. A macroscopic transition from a stable to unstable
convective flow is achieved, depending on the number of active particles and
their excess negative/positive temperature. The stable state is characterized
by a flow with low turbulent kinetic energy, strongly stable temperature
gradient, and no large-scale features. The convective state is characterized by
higher turbulent kinetic energy, self-sustaining large-scale convection, and
weakly stable temperature gradients. The particles individually promote the
formation of stable temperature gradients, while their aggregated effect
induces large-scale convection. When the Lagrangian temperature scale is small,
a weakly convective laminar system forms. The Lagrangian approach is also
compared to a uniform Eulerian bulk heating with the same mean injection
profile and no such transition is observed. Our empirical approach shows that
thermal convection can be controlled by pure Lagrangian forcing and opens the
way for other data-driven particle-based protocols to enhance or deplete
large-scale motion in thermal flows.
