Experimental study of the steady fluid-structure interaction of flexible hydrofoils

This paper presents results from an experimental study of the hydrodynamic and hydroelastic performance of six different flexible hydrofoils of similar geometry; four metal hydrofoils of stainless steel (SS) and aluminum (AL), and two composite hydrofoils of carbon-fiber reinforced plastic (CFRP). The two CFRP hydrofoils had differing layups, one with fibers at 0 degree and the other at 30 degree relative to the spanwise axis of the hydrofoil. All hydrofoil models have the same unswept trapezoidal planform of aspect ratio 3.33. Two section profiles were chosen, a standard NACA0009 (Type I) and a modified NACA0009 (Type II) with a thicker trailing edge for improved manufacture of CFRP hydrofoils. Hydrofoils were tested in a water tunnel mounted from a six-component force balance. Forces and deformations were measured at several chord-based Reynolds numbers up to Rec = 1.0 x 10⁶ and incidences beyond stall. Hysteresis, force fluctuations, and the natural frequency of the hydrofoils in air and in water were also investigated. Pre-stall forces on the metal hydrofoils were observed to be Reynolds number dependent for low values but became independent at 0.8 x 10⁶ and greater. Forces on the CFRP hydrofoils presented an increasing or decreasing lift slope for all Rec depending on the orientation of the carbon unidirectional layers. The change in loading pattern is due to the coupled bend-twist deformation experienced by the hydrofoils under hydrodynamic loading. Forces and deflections in the Type I hydrofoils were observed to be stable up to stall and non-dimensional tip deflections of were found to be independent of incidence and Rec. Type II metal hydrofoils had a mild Rec dependence, attributed to the blunt trailing edge, and Type II CFRP hydrofoils had a stronger incidence and Rec dependence. The natural frequency under stall conditions of all but one of the CFRP hydrofoils were in agreement with added mass and finite element analysis estimates. The disagreement was observed in the CFRP hydrofoil with layers aligned at 30◦ and is attributed to the complex behavior of the carbon layers and to the coupled bend-twist deformation experienced under hydrodynamic loading of the hydrofoil.