The Dynamics of Ventilated Partial Cavities over a Wide Range of Reynolds Numbers and Quantitative 2D X-ray Densitometry for Multiphase Flow.

Abstract

Ventilated partial cavity drag reduction is a technique that could potentially enable reduction of a ship's frictional drag, leading to a 5 to 20% net fuel savings, and thus providing economic and environmental benefits. Ventilated partial cavity drag reduction experiments were conducted using two geometrically similar experimental setups. First, experiments were performed at the world's largest re-circulating water channel, the U.S. Navy's Large Cavitation Channel (LCC), at Reynolds numbers to 80 million. For these experiments the LCC was adapted to allow free surface testing, which in itself was a major effort. The effect of the cavity closure geometry, and the cavity's robustness in the presence of global flow perturbations mimicking the effect of ambient waves were studied. Next, the experiments were reproduced at 1:14th size scale at Reynolds numbers of the order of one million, and in these small scale experiments the effect of Weber number was also investigated by reducing the surface tension by a factor of two. Results from these two sets of experiments were compared, and a potential scaling of required ventilation gas flux discussed. In addition the energy economics of the partial cavity drag reduction technique were analyzed. We can note that for partial cavities, the air entrainment is dominated by the cavity closure dynamics. To gain a better understanding of these dynamics, knowing the void fraction distribution, both spatially and temporally, would be very useful. In the cavity's closure region, as well as in most cavitating flows, any intrusive probe would perturb the flow greatly. X-ray densitometry offers a way to obtain a two dimensional time-resolved projection of the void fraction distribution, and a quantitative measure of the void fraction along the beam paths. An x-ray densitometry system was developed for use with a pre-existing cavitation tunnel. The limitations of the x-ray system were investigated, methods to contend with the imaging artifacts found, and the measured void fraction profiles compared against those obtained employing dual fiber optical probes and high speed video.