Abstract
Abstract We have discussed the wave properties in a two-phase gas mixture, which is composed of a diffuse medium [phase I (PI)] and dense clouds [phase II (PII)]. Interestingly, there are two wave modes when PII has mobility relative to PI. One is the acoustic mode, and the other is the void mode (or pattern-propagating mode). In this paper, we present a more careful discussion concerning the restoring force of the void mode. After that, we estimate the damping rate of the void mode due to the thermal process, which acts as a friction effect. We call this thermal friction. For a typical interstellar medium (ISM), where the cold cloud phase is PII and the warm phase gas is PI, friction is not so significant within the galaxy evolution timescales. However, if the coronal phase is PI, PII of the warm clouds suffers thermal friction efficiently. Thus, when the evolution of ISM is described by the classical three-phase model, we can adopt a modified two-phase model within the context of multi-phase flow; in which PI corresponds to a mixture of the hot and warm phases and PII corresponds to the cold clouds.
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