Active and passive pitch-controlled flapping wing propulsors; usage of the wake structure as a performance qualifier

Economic and ecological needs dictate for an ever growing need for increased efficiency, both in marine propulsion and energy saving systems. Biomimetic (flapping wing) systems, have already shown a serious potential as propulsors [1] and an even greater as a mechanism that converts energy from ship motions to thrust [2], [3]. In this paper, the problem of passively (spring loaded) or actively pitched controlled wing is formulated and solved using a free wake 3D Boundary Element Method [4]. For the spring loaded case, the unsteady BEM code is used to calculate the instantaneous forcing (i.e. pitching moment) entered in the nonlinear second order PDE in time, expressing equilibrium of moments including damping and inertia, around the pitch axis. Systematic simulations were conducted for a series of harmonically heaving wings of different aspect ratios, with the instantaneous pitch selected either passively via a spring-damper system or actively using a proper control algorithm. The results regarding developed mean thrust coefficient are presented in the form of systematic diagrams compatible with the design diagrams introduced in [1], allowing comparison of the different flapping wing propulsors. Results are also presented for the wake patterns of the different configurations, at similar propulsive conditions, revealing the connection between the propulsive effectiveness and 3D wake structure

Διερεύνηση της συμπεριφοράς συστήματος παλλώμενων πτερυγίων ως προωστήρα πλοίου

96 σ.The scope of this thesis is to produce a CFD program capable of simulating most of flapping foil systems, verify the capability to produce reliable results, run systematic simulations and visualize them, in order to look for the emergence of patterns that could lead to improvements. Additionally, there is also an attempt to start reverse engineering the fish and dolphin swimming. For this purpose a CFD program was made, using a boundary element time stepping method with free wake, which can take any given foil (or foils) and place it under any harmonic flapping motion given by the user. The program can calculate pressures, consequently forces and moments, nondimentionalize them and produce the wake geometry in visualizable form. From the simulations made, emerged results capable to produce improvements to the flapping foil systems, as well as recomendations for future work