  The reduction of dimensionality of physical systems, specially in fluid
dynamics, leads in many situations to nonlinear ordinary differential equations
which have global invariant manifolds with algebraic expressions containing
relevant physical information of the original system. We present a method to
identify such manifolds, and we apply it to a reduced model for the Lagrangian
evolution of field gradients in homogeneous and isotropic turbulence with a
passive scalar.
