Tong Zhao

Fluidic oscillators have emerged as a prominent topic of research in the field of flow control, owing to their broad sweep range and enhanced control efficiency. However, the underlying mechanisms governing the operation of fluidic oscillators remain poorly understood, and the effect of oscillation frequency on flow control performance has yet to be conclusively determined. In this study, a novel fluidic oscillator is proposed that achieves frequency decoupling by replacing the conventional feedback channel with synthetic jets, thereby enabling modulation of oscillation frequency at a constant momentum coefficient. When applied to a high-lift airfoil, results show that at a momentum coefficient of 14.1%, the lift coefficient increase achieved under F⁺ = 1 control outperforms that under F⁺ = 10 by more than 0.3. This finding suggests the presence of an optimal frequency for fluidic oscillators, which maximizes their flow control effectiveness. Notably, this optimal frequency is unaffected by variations in the momentum coefficient. A deeper analysis of the fluidic oscillator’s working principle reveals that periodic oscillations dominate the turbulent kinetic energy and Reynolds shear stress, driving enhanced chordwise momentum exchange. This increased energy transfer strengthens the boundary layer’s resistance to separation, effectively mitigating flow detachment and improving lift enhancement. Finally, the periodic flow field on the surface of the high-lift airfoil under fluidic oscillator control was examined. It was observed that, at low frequencies, the fluidic oscillator effectively controls the shedding of separation vortices, ensuring that the frequency of vortex shedding aligns with the oscillation frequency of the fluidic oscillator. This alignment likely contributes to the superior lift enhancement observed under low-frequency conditions.

Sweeping jet (SWJ) actuators have become a hot research topic in flow control due to their larger sweep range and higher control efficiency. However, the linear relationship between frequency and velocity ratio (VR = U_jet/U_∞) in the SWJ actuator makes it challenging to determine the dominant factor affecting the control effect. Decoupling the frequency and VR and determining the control mechanism of the SWJ actuator is, therefore, a difficult task. In this study, a novel type of SWJ actuator was designed using periodic synthetic jets instead of feedback channels. This achieved the implementation of different frequencies under the same VR, effectively decoupling frequency and VR. The SWJ actuator was then applied in flow separation control of a Hump airfoil, with F⁺ = f × c/U_∞ = 0.375, F⁺ = 1, and F⁺ = 10 being the three forcing frequencies studied. Numerical results demonstrated that all three forcing frequencies displayed a control effect on flow separation. At VR = 1.8, the control effectiveness is optimal for F⁺ = 1, and as VR continues to increase, F⁺ = 10 becomes the optimal control frequency.

Compliant cellular structures are good candidates for morphing applications. This paper proposes a novel chiral cellular structure composed of circular beams with great elastic properties and potential for morphing. The tensile and shear elastic properties of the structure are studied through theoretical derivations and then verified by finite element analysis. Results show that this novel chiral structure exhibits extremely low in-plane tensile and shear moduli, which are many orders of magnitude lower than that of the raw material. The structure also shows tensile–shear and shear–tensile coupling effects that cannot be ignored. The tensile and shear properties of the structure can provide a reference for employing this structure in engineering applications.