Unsteady free-surface wave-induced boundary-layer separation for a surface-piercing NACA 0024 foil: Towing tank experiments

Abstract

Unsteady free-surface, wave-induced separation is documented using towing tank experiments for a surface-piercing NACA 0024 foil, which has insignificant separation, no wave effects, and 2-D flow in the deep. The test conditions are for minimal, reattaching, and nonreattaching separation: Froude numbers (Fr)=0.19, 0.37, and 0.55 and Reynolds numbers=0.822, 1.52, and 2.26×106. The measurements include mean far-field wave elevations for all three Fr, unsteady near-field wave elevations for medium and low Fr, mean foil-surface pressures for all three Fr, and unsteady foil-surface pressures for medium Fr. Unsteady measurements are statistically analyzed for mean, r.m.s., FFT, PSD, and phased-averaged organized oscillations and random fluctuation components. For Fr=0.19, there is no separation and the expected Kelvin waves are present. For Fr=0.37, the separation on the free and foil surfaces is demarcated by large r.m.s. (15–20% of the mean value dynamic range) and 1.94, 0.82, and 0.30 Hz dominant FFT frequency regions corresponding to shear layer, Karman shedding, and flapping instabilities, respectively. A large bow wave resembles a spilling breaker with low r.m.s. (2–5% of the mean value dynamic range) and 8.5 Hz dominant FFT frequency on the free and foil surfaces. Mean wave elevations in the separation region are relatively constant with intense free-surface oscillations, turbulence, and breaking. Outside the separation and wake region, the Kelvin waves are evident. The foil-surface pressure is high in the bow wave, low in the wave trough, gradually rises towards the trailing edge, and recovers 2-D distributions at large depths. For Fr=0.55, the bow wave is enormous. The separation region moves towards the trailing edge with increased splashing and bubbles. The Kelvin waves are no longer distinguishable due to increased free-surface turbulence. The mean foil-surface pressure, free-surface, and bottom effects interact with each other. The combination of the present albeit relatively sparse experimental data with complementary computational fluid dynamics studies provides a credible description of the flow physics.