In a recent Rapid Communication [Phys. Rev. E 47, R2987 (1993)] H.-w. Xi, Gunton, and Vinals report on a numerical investigation of an extension of the Swift-Hohenberg equation including mean flow effects as well as non-Boussinesq effects. They claim that the basic mechanism for the formation of spirals are the non-Boussinesq effects. In contrast to H.-w. Xi et al., we had shown in a previous publication [M. Bestehorn et al, Z. Phys. B 88, 93 (1992)] that models describing a Boussinesq fluid may also spontaneously form spatially extended spirals if nonvariational extensions of the Swift-Hohenberg equation are used.

• Received 13 September 1993

DOI:https://doi.org/10.1103/PhysRevE.50.625