Partial cavitation in the separated region forming from the apex of a wedge is examined to reveal the flow mechanism responsible for the transition from stable sheet cavity to periodically shedding cloud cavitation. High-speed visualization and time-resolved X-ray densitometry measurements are used to examine the cavity dynamics, including the time-resolved void-fraction fields within the cavity. The experimentally observed time-averaged void-fraction profiles are compared to an analytical model employing free-streamline theory. From the instantaneous void-fraction flow fields, two distinct shedding mechanisms are identified. The classically described re-entrant flow in the cavity closure is confirmed as a mechanism for vapour entrainment and detachment that leads to intermittent shedding of smaller-scale cavities. But, with a sufficient reduction in cavitation number, large-scale periodic cloud shedding is associated with the formation and propagation of a bubbly shock within the high void-fraction bubbly mixture in the separated cavity flow. When the shock front impinges on flow at the wedge apex, a large cloud is pinched off. For periodic shedding, the speed of the front in the laboratory frame is of the order of half the free-stream speed. The features of the observed condensation shocks are related to the average and dynamic pressure and void fraction using classical one-dimensional jump conditions. The sound speed of the bubbly mixture is estimated to determine the Mach number of the cavity flow. The transition from intermittent to transitional to strongly periodic shedding occurs when the average Mach number of the cavity flow exceeds that required for the generation of strong shocks.

To measure the void fraction distribution in gas-liquid flows, a two-dimensional x-ray densitometry system was developed. This system is capable of acquiring a two-dimensional projection with a 225 cm 2 area of measurement through 21 cm of water. The images can be acquired at rates on the order of 1 kHz. Common sources of error in x-ray imaging, such as x-ray scatter, image distortion, veiling glare, and beam hardening were considered, and mitigated. The measured average void fraction was compared successfully to that of a phantom target and found to be within 1%. To evaluate the performance of the new system, the flow in and downstream of a ventilated nominally two-dimensional partial cavity was investigated and compared to measurements from dual tip fiber optical probes and high speed video. The measurements were found to have satisfactory agreement for void fractions above 5% of the selected void fraction measurement range.

Air-induced drag reduction was investigated on a 12.9 m long flat plate test model at a free stream speed of 6.3 m s −1 . Measurements of the local skin friction, phase velocity profiles (liquid and gas) and void fraction profiles were acquired at downstream distances to 11.5 m, which yielded downstream-distance-based Reynolds numbers above 80 million. Air was injected within the boundary layer behind a 13 mm backward facing step (BFS) while the incoming boundary layer was perturbed with vortex generators in various configurations immediately upstream of the BFS. Measurements confirmed that air layer drag reduction (ALDR) is sensitive to upstream disturbances, but a clean boundary layer separation line (i.e. the BFS) reduces such sensitivity. Empirical scaling of the experimental data was investigated for: (a) the critical air flux required to establish ALDR; (b) void fraction profiles; and (c) the interfacial velocity profiles. A scaling of the critical air flux for ALDR was developed from balancing shear-induced lift forces and buoyancy forces on a single bubble within a shear flow. The resulting scaling successfully collapses ALDR results from the current and past studies over a range of flow conditions and test model configurations. The interfacial velocity and void fraction profiles were acquired and scaled within the bubble drag reduction (BDR), ALDR and transitional ALDR regimes. The BDR interfacial velocity profile revealed that there was slip between phases. The ALDR results showed that the air layer thickness was nominally three-quarters of the total volumetric flux (per unit span) of air injected divided by the free stream speed. Furthermore, the air layer had an average void fraction of 0.75 and a velocity of approximately 0.2 times the free stream speed. Beyond the air layer was a bubbly mixture that scaled in a similar fashion to the BDR results. Transitional ALDR results indicate that this regime was comprised of intermittent generation and subsequent fragmentation of an air layer, with the resulting drag reduction determined by the fraction of time that an air layer was present.

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In the present study, the experimental set-up of Ganesh et al. (J. Fluid Mech., vol. 802, 2016, pp. 37–78) is used to examine the dynamics of a shedding cavity by examining the vapour production rate of the natural cavity and determining how minimal injection of non-condensable gas can substantially alter the vapour production rate, the resulting cavity flow and the cavity shedding process. The influence of the dissolved gas content on the shedding natural cavity flow is also examined. High-speed visual imaging and cinemagraphic X-ray densitometry were used to observe the void fraction dynamics of the cavity flow. Non-condensable gas is injected across the span of the cavity flow at two locations: immediately downstream of the cavity detachment location at the apex of the wedge or further downstream into mid-cavity. The gas injected near the apex is found to increase the pressure near the suction peak, which resulted in the suppression of vapour formation. Hence, the injection of gas could result in a substantial net reduction in the overall cavity void fraction. Injection at the mid-cavity did less to suppress the vapour production and resulted in less significant modification of both the mean cavity pressure and net volume fraction. Changes in the cavity void fraction, in turn, altered the dynamics of the bubbly shock formation. Variation of the dissolved gas content alone (i.e. without injection) did not significantly change the cavity dynamics.