  Constructing simpler models, either stochastic or deterministic, for
exploring the phenomenon of flow reversals in fluid systems is in vogue across
disciplines. Using direct numerical simulations and nonlinear time series
analysis, we illustrate that the basic nature of flow reversals in convecting
fluids can depend on the dimensionless parameters describing the system.
Specifically, we find evidence of low-dimensional determinism in flow reversals
occurring at zero Prandtl number, whereas we fail to find such signatures for
reversals at infinite Prandtl number. Thus, even in a single system, as one
varies the system parameters, one can encounter reversals that are
fundamentally different in nature. Consequently, we conclude that a single
general low-dimensional deterministic model cannot faithfully characterize flow
reversals for every set of parameter values.
