Effects of confined boundaries on hovering flight using the unsteady vortex lattice method

Insect flight adjacent to single or multiple walls during hovering or approaching perching is common in nature. The aerodynamic forces of insects flying near multiple surfaces may deviate significantly from those experienced during normal hover over a single ground. This work investigates the effects of multiple walls on a three-dimensional hovering wing. The aerodynamics of the hovering wing is calculated by the unsteady vortex lattice method (UVLM). To handle the flow instabilities associated with strong wing-wake and wake-wake interactions in hover, a novel treatment of wake straining is incorporated with a vortex core growth model that stabilizes the numerical simulation and avoid numerical dispersion. The treatment significantly extends the capability of the UVLM and improves its accuracy in the modeling of wings during hovering flight. The ground effects are modeled by a lattice of bound vortices. To study the effects of multiple walls, two cases are considered: a pair of hovering wings in a U-shaped channel and between top and bottom walls. The lift coefficients and the flow fields are analyzed by varying the distance between the wing and the walls. Three typical regimes are identified due to the walls: force enhancement, force reduction and force recovery. The lift coefficient of the wing in the U-shaped channel is lower than the wing hovering over a single ground, while that of the wing between the top and the bottom walls is higher. These observations are closely related to the changes in wake and vortex structures causing changes in the downwash and effective angle of attack of the wing.