The shock-induced combustion with periodic unsteadiness around a projectile fired into hypersonic flows is numerically studied. The mechanism of periodic unsteadiness is clarified using an x-t diagram of the flow variable on the stagnation streamline. The frequencies of the periodic unsteadiness obtained quantitatively agree with the experimental observations. The key parameters which are responsible in triggering the instability and in determining the frequency of the periodic unsteadiness are discussed by using the time integration of the species equations. The result indicates that the key parameters are induction time, heat release, and concentration of the heat release. The induction tune is a key parameter for the frequency of the unsteadiness. The concentration of heat release is important for the unsteadiness itself. Total energy creating the compression waves depends on the amount of heat release itself. All of these features are recognized in a simple zero-dimensional analysis.
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