Unsteady fluid flows are ubiquitous in engineering applications. However, analysis and control of such flows is challenging owing to their time-varying base states. The influence of actuation on their time varying base states needs to be considered for development of effective flow control strategies. In this work, Vedasri will focus on the importance of timing of actuation and identify “when” to introduce perturbation for flow control. To this end, she will leverage phase reduction analysis, which expresses the high-dimensional nonlinear flow physics as a single variable phase dynamics. She will demonstrate this approach on laminar periodic flow over an airfoil at high angle of attack to identify the sensitive regions and the time to introduce actuation for maximum phase response. Further, she will identify the optimal waveform of actuation for transient flow modification to accelerate/delay the vortex shedding process over airfoil. Later, she will extend this approach to multi-modal flows, such as compressible flow over an open rectangular cavity, to reduce the pressure fluctuations.