A time-domain panel method for oscillating propulsors with both chordwise and spanwise flexibility

Liu, Pengfei (1996) A time-domain panel method for oscillating propulsors with both chordwise and spanwise flexibility. Doctoral (PhD) thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (29MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

A time domain panel method was formulated and a computer program package, OSFBEM, was developed to evaluate the propulsive performance of oscillating propulsors. -- This method was designed, and is able, to obtain hydrodynamic properties for an unsteady, 3-D flexible wing. A number of features were implemented including the geometry of both 2-D and arbitrary 3-D planforms; a non-zero thickness foil section and a section with a thickness as thin as 2% of the chord; large amplitude pitch and heave motions; non-zero trailing edge thickness; flexible motion and geometry parameters such as steady flow, unsteady motion, chordwise and spanwise flexibility; and prediction of skin friction and qualitative examination of sectional flow patterns in terms of boundary layer growth. Major limitations of this method include the inability to precisely predict separation, stall and hydrodynamic characteristics of a foil with a very small aspect ratio. -- A large amplitude theory was developed and used to analyze the propulsive efficiency and thrust. An instantaneous angle of attack of the oscillating foil and a large amplitude feathering parameter were defined for this study. As a result of this theoretical establishment, the thrust was identified to be directly related to the instantaneous angle of attack. Most importantly, the best efficiency was obtained at the maximum instantaneous angle of attack of about 10°, for any combination of motion parameters and any shape of planforms with and without flexibility that were examined in this research. -- Most previous numerical predictions on 3-D unsteady oscillating foils were based on the small amplitude theory. The present method, instead, is based on the finite amplitude theory and it also takes the sectional thickness distribution, planform shape and skin friction, etc., into account. Therefore, a parametric study was also conducted for rigid planforms to give results from a more realistic model. -- The chordwise and spanwise flexibility were implemented by using a positive approach, i.e., different amplitudes of deflexion and shape functions were predetermined, to simulate a fin whale's flukes. Two non-dimensional parameters, the spanwise and chordwise deflexion amplitude factors, together with another two parameters, the spanwise and chordwise deflexion phase angles relative to the pitch were defined. A parametric study was then conducted in terms of these parameters. -- A numerical procedure was also established to determine the angle of zero lift for a foil due to the chordwise flexibility and, this angle of angle of zero lift was then used to modify the instantaneous pitch angle to obtain the instantaneous angle of attack at each time step. A numerical scheme was also formulated for foils with spanwise flexibility in calculating the efficiency in which case the heave amplitude had a variation across the span. -- Major findings include the limitation and validity of the small amplitude theory obtained from a large amplitude analysis; determination of the instantaneous angle of attack of rigid and flexible oscillating foils; the relation between the maximum instantaneous angle of attack and the thrust; the instantaneous angle of attack for the best efficiency; sectional thickness ratio effect on efficiency and thrust; skin friction effect on the propulsive performance; pressure distribution and validity of the steady Kutta condition for an unsteady oscillating foil with both chordwise and spanwise flexibility; the chordwise and spanwise deflexion phase angles and their effects on the efficiency and thrust; and the effect of the spanwise deflexion amplitude on efficiency and thrust. -- Conclusions were drawn from these predicted results and suggestions on the geometry and motion parameters in oscillating foil design were also made.

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