A generalized memory effect in fluid/flame dynamics due to unsteady events

Though being an important mechanism in systems involving unsteady fluid flows (e.g., fluids interacting with moving structures, particles in turbulent flows, turbulent flame propagation), the memory effect is usually not considered and has not attracted much attention. The present study tries to highlight the importance of this memory effect based on a variety of examples. In this manner, the three key components characterizing the memory effect can be identified in a general manner. The central mechanism controlling the memory effect is then investigated by studying the flow over a shrinking particle. It is found that the Damköhler number comparing the timescales of the unsteady (trigger) event and of the feedback on the flow directly determines the intensity of the memory effect; a smaller Damköhler number results in a more intense effect. Finally, the flow induced by a flapping wing is considered to demonstrate that the memory effect could be also beneficial for practical applications, for instance for biomimetic locomotion.