Material bend-twist coupling effects on cavitating response of composite hydrofoils

The objective of this work is to investigate the effects of material bend-twist coupling on the cavitating response of adaptive composite hydrofoils. Experimental results are shown for two composite hydrofoils and one stainless steel (SS) hydrofoil. All three hydrofoils have identical unloaded geometry and are tested in the same cantilevered configuration at the Cavitation Research Laboratory variable pressure tunnel at the University of Tasmania. The results show that material bend-twist coupling that leads to nose-up twist (N30 hydrofoil) will increase the mean hydrodynamic load, accelerate cavitation inception, increase the maximum cavity length, and lower the Type II cavity shedding frequency compared to the SS hydrofoil. The opposites are true for material bend-twist coupling that leads to nose-down twist (P30 hydrofoil). For all three hydrofoils, Type I shock-wave driven cavity shedding is observed when the maximum cavity length normalized by the chord is between 0.75 and 1.7, while Type II re-entrant jet driven cavity shedding is observed for the full range of cavitating flow. In addition, significant load amplification is observed when the Type II cavity shedding frequency is near the first wetted natural frequency of the P30 hydrofoil. To complement the experimental studies, semi-empirical relations have been developed to predict the hydroelastic response of the hydrofoils in cavitating flow, and good agreement is observed between predictions and measurements. The results show that by accounting for the flow-induced twist in the effective angle of attack (α_eff), and using ψ = σ/2α_eff as the effective cavitation parameter (with σ as the cavitation number), the hydroelastic response for all three hydrofoils can be collapsed onto the same trend lines.