The internal structure and the dynamics of two-dimensional (2D) sheet cavitation on the suction side of a 2D foil section were investigated experimentally. Experiments were conducted in a cavitation tunnel and situations ranging from steady sheet cavitation to unsteady cloud cavitation were obtained by varying the foil incidence and the cavitation number. Using a novel endoscopic technique, coupled with x-ray attenuation measurements, the two-phase morphology and the void fraction within the sheet cavitation were investigated. Supplemental information on the instantaneous shape of the sheet cavity and its instability frequency were also obtained by visualization and pressure measurements, respectively. The investigations focused on (a) the void fraction distribution and (b) the frequency of the cavity oscillations. It was found that the void ratio can reach as much as 50% depending on the conditions of operation, and the Strouhal numbers are around 0.25 in the case of partial cavity instability and 0.12 in the case of transitional sheet cavitation. Finally, the visualization of the vapor structures within the sheet cavity for various stations along the chord gives a qualitative understanding of the process of vapor production and condensation.

The present paper is devoted to the experimental study of unsteady cavitation on the suction side of a two-dimensional foil section positioned in a cavitation tunnel with a small incidence angle. When the pressure is decreased in the tunnel, a sheet of cavitation characterized by large amplitude fluctuations is obtained on the foil. The present study focuses on the effects of the foil wall roughness on the cavity unsteady behavior. Four different sizes d of irregularities have been tested, from the smooth surface to a 400μm grain size. The characteristic frequency of the flow unsteadiness is investigated by analyzing the data measured by a pressure transducer mounted flush on one vertical wall of the test section, whereas the mean cavity length is obtained by visual measurements on the foil side. Several types of cloud cavitation are identified in the case of the smooth surface. The effect of roughness is a significant decrease of the cavity length and a large increase of the oscillation frequency. It results in Strouhal numbers higher than the classical values obtained for partial cavity fluctuations. Moreover, the cavitation cycle is disorganized by the increase of the roughness, as it can be detected by the fast fourier transform analysis of the pressure signal. The general effect is a reduction of the pressure fluctuation intensity.

Instantaneous values of lift in the locust Schistocerca gregaria, flying in a wind tunnel, were measured by means of a piezo-electric probe. The airflow velocity was adjusted to be equal to that which would have been experienced by the freely flying insect. In order to obtain the true aerodynamic forces, the inertial forces must be subtracted from the gross forces measured. These inertial forces were calculated from the movement of the centre of gravity and from the mass of the different wing sections. The lift curves obtained have a shape similar to the curves deduced by Jensen from measurements on isolated wings in steady flows, but an approximately doubled amplitude. These amplitude differences appear to be caused by the existence of significant unsteady effects.

The unsteady cavitating flow field on a 2D hydrofoil is investigated by experimental means and numerical calculations. The self-oscillatory behavior of the cavitation sheet observed experimentally is correctly simulated by the numerical model. A particularly good agreement is obtained concerning the frequency of the phenomenon in various flow configurations. To confirm the low void ratio predicted by the model in the main part of the liquid/vapor cavity a new experimental device is applied: it is based on an endoscopic visualization of the interior of the mixture. Recent developments of the method are presented, and some first results are analysed.

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