Effectiveness of micro synthetic jet actuator enhanced by flow instability in controlling laminar separation caused by adverse pressure gradient

This paper aims to understand how a micro synthetic jet actuator works effectively to prevent the boundary layer flow from laminar separation caused by adverse pressure gradient. Experimental results showed that synthetic jets were effective when the forcing frequency was a lower frequency of the Tollmien–Schlichting (T–S) wave, which received amplification from the base flow. They demonstrated the feasibility of controlling a large-scale structure feature, such as flow separation, using a micro actuator which requires very low power supply (±7.5 V) and generates inaudible noise in a boundary layer with the Reynolds number of 1.78 × 105–2.24 × 105. The interaction between the synthetic jet and the base flow is analyzed, and the significance of using the instability to make the synthetic jets effective is discussed. To enable a micro actuator to work effectively on preventing the boundary layer flow from laminar separation, the key issue is to use the synthetic jet actuator as an instability trigger to trigger T–S instability which resonates with the Kelvin–Helmortz (K–H) instability to accelerate the viscous transition. The triggered T–S waves are amplified and enhanced by the non-viscous K–H instability of the base flow and consequently become effective in controlling the K–H instability. As a trigger rather than a force generator, the synthetic jet actuator actuates independently to its size and weight.