Wingtip devices are common in aeronautical applications and are increasingly used on wind turbines. However, their use in hydrokinetic energy conversion applications such as tidal turbines to date is minimal, due to the concern for increased bio-fouling and also the fact that there is little or no data publically available describing their cavitation characteristics. In this study, three wingtip designs were considered for hydrokinetic turbine applications: a plain foil with a rounded tip (considered the reference case), a generic wingtip device (a winglet), and a novel “split-tip” device. The tips were studied numerically and experimentally at different angles of attack. The numerical simulations were performed in OpenFOAM using the k-omega SST model to predict the lift and drag characteristics of a “base” foil with each of the three wingtip devices. Additionally the pressure and vorticity were observed. Experiments were conducted in the University of New Hampshire High-Speed Cavitation Tunnel – HiCaT. A modular experimental test bed with an elliptical foil section was developed specifically for the study. The test bed extends to the centerline of the tunnel where wingtips are attached, and has four small-diameter tube openings to accommodate pressure measurements and/or mass injection studies. Water tunnel data were obtained for lift, and cavitation inception, and compared to the numerical simulations. The numerical results show decreased vorticity with presence of the wingtip devices, however, the advantage of using wingtips for decreasing drag and increasing lift forces is not conclusively exhibited. The experimental measurements suggest that there is a significant suppression of tip vortex cavitation with the use of wingtip devices at high angles of attack (around 10 degrees), but the advantage of using the wingtip devices diminishes at lower angles of attack.
It was shown by Arndt [1] that tip-vortex cavitation on hydrofoils can be related to the lift coefficient and the Reynolds number, where the cavitation index at inception is proportional to the square of the section lift coefficient and the Reynolds number based on hydrofoil chord, taken to the power m. The power exponent m has been generally accepted to be approximately 0.4. This relation is made into an equation via a coefficient of proportionality K, which depends on the wingtip and foil section geometry, and has been empirically determined to have values between 0.025 and 0.056 for previously investigated wings. While the value of the coefficient K for the reference wing tip remained comparatively constant for the range of conditions investigated (angles of attack, Reynolds numbers), it varied significantly for the foil terminated by the winglet. This may be due to the non-elliptical load distribution in the span-wise direction, but also raises the question whether the standard tip-vortex cavitation correlation for hydrofoils is applicable for general wingtip devices.
