Published online by Cambridge University Press: 01 March 2004
Linear global modes in the Rayleigh–Bénard–Poiseuille system, for the case of two-dimensional non-uniform heating in the form of a single hot spot, are analysed in the framework of the envelope equation formalism. Global mode solutions are sought by means of WKBJ asymptotics. As for the one-dimensional case, an analytical selection criterion for the frequency may be derived from the breakdown of the WKBJ expansion at a two-dimensional double turning point located at the maximum of the local Rayleigh number. The analytical results, including the behaviour of the mode in the vicinity of the turning point, are compared with results obtained from numerical simulations of the envelope equation. Finally, the issue of the selection of the wavevector branches in the WKBJ expansion is discussed.