For the discovery of the QCD critical point it is crucial to develop dynamical models of the fluctuations of the net-baryon number that can be embedded in simulations of heavy-ion collisions.
In this talk we present the implementation of net-baryon number fluctuations near the critical point in an expanding system. We include the coupling to the fluctuations of energy density and momentum flow explicitly. We study the dynamical formation of the correlation length and its relation to the critical enhancement of higher-order cumulants for boost-invariant initial conditions. By adding second order fluid dynamical relaxation equations we distinguish between the propagative and the damped critical mode contributions.