Numerical modelling of hydrofoil fluid-structure interaction

Marine propellers operate in unsteady non-uniform wake regions generated by the hull and control surfaces subjecting the propeller to unsteady loading. Hydroelastic tailoring of propeller blades is a method to reduce unsteady loading as a propeller blade passes through a wake decit. This project seeks to gain greater insight into the effect of hydroelastic tailoring on a propeller by simplifying the problem into a single hydrofoil with a sinusoidal pitch oscillation. In this study, a hydrofoil with a NACA 0009 section and a trapezoidal planform area was used to investigate bending hydroelastic effects numerically using fluid-structure interaction modelling. The complexity of the numerical model was varied in a systematic manner, starting with a two-dimensional foil through to a three-dimensional two-way coupled fluid structure interaction simulation. The commercial package ANSYS was used with CFX for computational fluid dynamics and ANSYS mechanical for the structural simulation. In this study ANSYS was demonstrated to be a suitable tool to simulate fluid-structure interaction in the case of an oscillating hydrofoil in pure pitch. The computational fluid dynamics results were validated in two-dimensions using NACA 0012 and 0015 sections for both static and dynamic cases using published experimental results. In three-dimensions, stainless steel and aluminium, were investigated in addition to the rigid (uncoupled) case. This study varies independent parameters including Reynolds number, reduced frequency, amplitude of pitch oscillation and the mean incidence controlling the hydrofoil response. Comparison of static one-way and two-way coupled results shows that there are small but apparent differences between predicted bending deformations. However, bending deformations were shown to virtually have no effect on forces and moments, at least up to moderate incidences. Rigid three-dimensional lift and moment predictions show similar behaviour to both the two-dimensional unsteady viscous predictions and classical linear inviscid theory for cases of zero mean incidence. In particular, lift and moment vary linearly with amplitude of oscillation for all reduced frequencies. The lift and moment amplitude minima occur at reduced frequencies of about 0.6 and 0.7 respectively for both two and three-dimensional predictions; However, in the three-dimensional case the amplitudes, relative to the lift and moment at static incidence are reduced. For a four degrees mean incidence, the amplitudes of the lift and moment minima are signicantly reduced for two and three-dimensional predictions compared with the zero degree mean incidence case. Above a reduced frequency of one, for four degrees mean incidence, the rigid three-dimensional lift and moment amplitude predictions no longer vary linearly with incidence amplitude. The dynamic coupled analysis typically showed bending deformations to be similar to those for static predictions at a zero mean incidence but to be reduced for a four degrees mean incidence at maximum incidence. Lift and moment for the dynamic coupled cases are only slightly influenced for reduced frequencies less than one depending on material properties and Reynolds number. For a reduced frequency greater than one the lift and moment show a slight increase and vary non-linearly with the incidence amplitude.