The instability of a plane compressible gas sheet in a quiescent viscous liquid medium of infinite expanse has been studied. It is found that there exist two unstable modes of disturbances, sinuous and varicose. For temporal instability, sinuous disturbance is stable if gas Weber number, defined as the ratio of aerodynamic to capillary forces, is less than unity, varicose mode controls the instability process except for large Weber numbers when both modes become equally important, and gas compressibility effect always enhances instability development and induces additional range of unstable wave numbers. For spatial-temporal evolution of disturbances, it is found that convective instability does not exist at all and the instability of plane gas sheets is always absolute in nature, which is strikingly opposite to the instability of plane liquid sheets. The absolutely unstable disturbance is found always temporally growing, although it may be spatially growing or decaying depending on flow conditions. Gas compressibility enhances and liquid viscosity damps out both temporal and -spatial part of absolute instability growth rate. Although Weber number promotes the temporal growth rate of absolute instability, it has a dual effect of enhancing and inhibiting the spatial growth rate.
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