Numerical Investigation On The Insect Hovering Flight Lift Generation Mechanism And Energy Expenditure

The study of unsteady aerodynamics is one of the most important aspects of insect flight. Aerodynamics answers the questions of how the aerodynamic forces and moments, which support the weight of an insect, propel its motion in the air and control the motion, are produced, and what the energy expenditure is when producing these forces and moments. It is of great significance for the biologists who need to understand the effects of aerodynamic-force production, energy expenditure, and flight balance on the physiology, behavior, evolution, and other aspects of insects and engineers who desire to develop small robots with maneuverability comparable to that of flying insects.The finite-volume ALE Navier-Stokes solver is based on the artificial compressibility method (ACM) with a high-resolution method of characteristics-based scheme on unstructured grids. The unsteady flow is calculated with a matrix-free implicit dual time stepping scheme. A five-stage Runge-Kutta time integration algorithm is used between each physical time step to iterate the numerical solution in pseudo time until convergence is reached. ALE technique is adopted to keep the topological structure of the computational grid unchanged and exponential damping function is made for moving mesh, which makes the grid very rigid near the wall and soft far away from the wall. Two mesh modifications are adopted to deal with the mesh deformation problems when simulating clap-fling motion. Local time stepping and implicit residual smoothing are used to accelerate the convergence rate to steady state in pseudo time.Flow around oscillating cylinder is used to validate the ALE model. The comparison of the flow field, vortex, pressure, inline force, the drag coefficient and mass coefficient between the present model and published results is made, which shows accuracy of the model and the capability of dealing with moving mesh problems.2D rigid wing hovering motion is simulated. Delayed stall mechanism is the main reason for lift generation. Wake capture mechanism can enhance the lift generation. But for the delayed model, the interaction of two vortex who have the same turning direction could decrease the lift. Induced jet and rapid pitch mechanism lead to the decrease of lift. The optimal angular amplitude,45 degree, results in the biggest ratio of Cl to energy expenditure.At last, the clap-fling motion is investigated. Wake capture and delayed stall are two reasons for the lift generation of clap-fling motion. Compared to the single wing hovering motion, the lift generation increases by 159% when consuming the same amount of energy. As the increase of Re numbers, the intensity of the leading edge vortex is strengthened and the absolute value of pressure is amplified, which lead to the increase of lift. The frequency of the clap-fling motion is in proportion to the wing velocity. The larger the frequency, the bigger the lift.