On the scaling of air entrainment from a ventilated partial cavity

Abstract

The behaviour of a nominally two-dimensional ventilated partial cavity was examined over a wide range of size scales and flow speeds to determine the influence of Froude, Reynolds, and Weber number on the cavity shape, dynamics, and gas entrainment rate. Two geometrically similar experiments were conducted with a 14:1 length scale ratio. The results were compared to a two-dimensional semi-analytical model of the cavity flow, and Froude scaling was found to be sufficient to match basic cavity shapes. However, the air flux required to maintain a stable cavity did not scale with Froude number alone, as the dynamics of the cavity closure changed with increasing Reynolds number. The required air flux differed over one order of magnitude between the lowest and highest Reynolds number flows. But, for sufficiently high Reynolds numbers, the rate of scaled entrainment appeared to approach Reynolds number independence. Modest changes in surface tension of the small-scale experiment suggested that the Weber number was important only at the lowest speeds and smaller length scale. Otherwise, the Weber numbers of the flows were sufficiently high to make the effects of interfacial tension negligible. We also observed that modest unsteadiness in the inflow to the large-scale cavity led to a significant increase in the required air flux needed to maintain a stable cavity, with the required excess gas flux nominally proportional to the flow’s perturbation amplitude. Finally, discussion is provided on how these results relate to model testing of partial cavity drag reduction (PCDR) systems for surface ships.

Characterization of Bubble Size Distributions within a Bubble Column

“…This method is most common when the measurements are acquired with electrical impedance/resistivity [10][11][12], wire mesh [13][14][15] or optical [16] point probes, which can only provide a single length scale but a relatively large sample size. These measurements are sensitive to the bubble size, velocity, shape and orientation as well as the sensor design (e.g., response from optical/impedance probes are unique to the sensor design and fluid properties).…”

Internal flows of ventilated partial cavitation

“…Makiharju et al (2010) also used imaging techniques on cavity closure to determine the cavity shape, growth rate and collapse rate, closure oscillation, and closure type and observed the effect of each flow condition on the ventilation cost. To ensure the industrial applicability of PCDR, Mäkiharju et al (2013) later conducted both large-and small-scale experiments to examine the scaling factors of relevant flow parameters to the required ventilation rate for a partial cavity to be sustained under various flow parameters.…”

Experiment and simulation on air layer drag reduction of high-speed underwater axisymmetric projectile

“…The order of magnitude and trends of the experimental skin-friction reduction are reproduced quite well by this simple model. Elbing et al [1,[6][7][8] did a set of experiments to investigate the phenomena of skin-friction drag reduction in a turbulent boundary layer (TBL) at large scales and high Reynolds numbers. Two distinct dragreduction phenomena were investigated: bubble drag reduction (BDR) and air-layer drag reduction (ALDR).…”