It has been shown previously the relative location of 2D roughness element and the synchronization point is important in determining roughness effect on modal growth. In particular, it is found if roughness is placed at or downstream of synchronization point, perturbation can be damped by roughness. In this paper, the results of further parametric studies are presented. In addition, mean flows with and without roughness are examined by numerical simulation and linear stability theory (LST) to understand the mechanism of damping and amplifying effects on perturbation. It is found that roughness alters mean flow profiles which can possibly contribute to destabilization of first mode and stabilization of second mode. It is also found that LST is capable of predicting such stabilizing and destabilizing effects. Furthermore, numerical study on a supersonic Mach 3 flow in which only first mode exists is conducted. The results show roughness in this flow amplifies perturbation at all frequencies which confirms our hypothesis about the importance of synchronization location and roughness. The results can potentially lead to a new passive flow control method.
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