RESONANT MODE INTERACTION IN A CANONICAL SEPARATED FLOW

1. Introduction The current study is directed towards understanding the flow physics of separated flows over airfoils with the ultimate goal of developing effective zero-net-mass-flux (ZNMF) jet based active separation control (ASC) strategies. The key control parameters in a ZNMF device are the jet frequency f and jet velocity V J. The former is usually non-dimensionalized as F + = f /f n where f n is some natural frequency in the uncontrolled flow. The latter is non-dimensionalized by U ∞. Note that V J is some characteristic measure of the jet velocity, such as the peak or an average velocity. As expected, control authority varies monotonically with V J /U ∞ (Seifert et al. 1996, Glezer & Amitay 2002; Mittal & Rampunggoon 2002) up to a point where a further increase would likely completely disrupt the boundary layer. Thus, there is little possibility of extracting an "optimal" value of this parameter. On the other hand, control authority has a highly non-monotonic variation with F + (Seifert & Pack 2000; Glezer et al. 2003) and this not only suggests the presence of rich flow physics and multiple flow mechanisms but also reveals the potential of optimizing the actuation scheme with respect to this parameter. Current strategies for ZNMF based separation control are explicitly or implicitly based on the proposition that the dynamics of a separated flow over an airfoil are dominated by the characteristic frequency of the separation region, f sep and that f sep ∼ U ∞ /L sep where L sep is the length of the separation region. However the situation is significantly more complex than this. Based on past studies (Chang 1976, Wu et al. 1998), one can consider the following three situations for flow past an airfoil. Case A represents attached flow at low angle-of-attack (AOA) where the boundary layer on the suction side develops under an adverse pressure gradient but does not separate. Such a flow