Chengyu Li

Wing kinematics measurement and aerodynamics of a dragonfly in turning flight.

This study integrates high-speed photogrammetry, 3D surface reconstruction, and computational fluid dynamics to explore a dragonfly (Erythemis Simplicicollis) in free flight. Asymmetric wing kinematics and the associated aerodynamic characteristics of a turning dragonfly are analyzed in detail. Quantitative measurements of wing kinematics show that compared to the outer wings, the inner wings sweep more slowly with a higher angle of attack during the downstroke, whereas they flap faster with a lower angle of attack during the upstroke. The inner-outer asymmetries of wing deviations result in an oval wingtip trajectory for the inner wings and a figure-eight wingtip trajectory for the outer wings. Unsteady aerodynamics calculations indicate significantly asymmetrical force production between the inner and outer wings, especially for the forewings. Specifically, the magnitude of the drag force on the inner forewing is approximately 2.8 times greater than that on the outer forewing during the downstroke. In the upstroke, the outer forewing generates approximately 1.9 times greater peak thrust than the inner forewing. To keep the body aloft, the forewings contribute approximately 64% of the total lift, whereas the hindwings provide 36%. The effect of forewing-hindwing interaction on the aerodynamic performance is also examined. It is found that the hindwings can benefit from this interaction by decreasing power consumption by 13% without sacrificing force generation.

Three-dimensional wake topology and propulsive performance of low-aspect-ratio pitching-rolling plates

The wake topology and propulsive performance of low-aspect-ratio plates undergoing a pitching-rolling motion in a uniform stream were numerically investigated by an in-house immersed-boundary-method-based incompressible Navier-Stokes equation solver. A detailed analysis of the vortical structures indicated that the pitching-rolling plate produced double-loop vortices with alternating signs from its trailing edge every half period. These vortices then shed and further evolved into interconnected “double-C”-shaped vortex rings, which eventually formed a bifurcating wake pattern in the downstream. As the wake convected downstream, there was a slight deflection in the spanwise direction to the plate tip, and the contained vortex ring size gradually increased. In addition, the analysis of the propulsive performance indicated that the shedding process of the double-loop vortices led to two peaks in the lift and thrust force production per half cycle. The observation of the double peaks in the force production i...

Vortex Formation and Aerodynamic Force of Low Aspect-Ratio Plate in Translation and Rotation

A low aspect-ratio plate under translational or rotational motion in a wide range of angles of attack is studied using Direct Numerical Simulation (DNS). Vortex formation and aerodynamic performance generated in the two motions are discussed. The aerodynamic forces obtained from two methods are calculated and compared. One method is the typical surface integration of pressure and shear stress tensor; another method is the time derivative of the volume integration of vorticity moment. The calculated forces from the two methods match reasonably well.

A Numerical Study of Flapping Plates Hinged with a Trailing-Edge Flap

In current work, effects of a controllable trailing-edge flap (TEF) on hovering flapping plates at low Reynolds number is computationally investigated. The leading-edge portion of flapping plate is driven by a prescribed kinematics in a horizontal stroke plane. The deflection of TEF follows a sinusoidal function with respect to the leading-edge and forms time-varying aft camber. Key parameters for determining plate kinematics, such as TEF deflection amplitude and phase shift, are studied to explore their effects on aerodynamic performance and flow modulation. This is done through an in-house immersed boundary method based Direct Numerical Simulation (DNS) solver. Results from current parametric studies will be used to analyze unsteady force productions due to dynamic TEF in flapping flight.