Ventilated cavity flow over a wall mounted fence is experimentally investigated in a cavitation tunnel. The flow over a 2-D fence, attached to the tunnel test section ceiling, is examined for a range of free-stream conditions. The dependence of cavity topology, cavitation number, upstream wall pressure distribution and drag on several parameters, including ventilation rate, fence height based Froude number (Fr), vapour pressure based cavitation number (σv ) and degree of fence immersion in the oncoming wall boundary layer, is investigated. Three different flow regimes are identified throughout the range of cavitation numbers for a particular set of free-stream conditions: shear layer cavitation, fully developed cavity and ‘blocked’ flow. The cavity exhibits a typical re-entrant jet closure and the re-entrant jet intensity is found to be a function of Fr. The high intensity re-entrant jet, present at high Fr, leads to an increase in drag. Drag decreases significantly with an increase in fence immersion in the oncoming boundary layer. Complementary measurements for a naturally cavitating flow are obtained for comparison.
A more detailed examination of the topology and unsteady behaviour of ventilated and natural cavity flows over a 2-D wall-mounted fence was undertaken for fixed length cavities with varying free-stream velocity using high-speed and still imaging, X-ray densitometry and dynamic surface pressure measurements in two experimental facilities. Two main unsteady features are observed, the irregular small-scale shedding of structures at the cavity closure and a larger-scale re-entrant jet oscillation. Small-scale cavity break-up was associated with a high-frequency broad-band peak in the wall pressure spectra, found to be governed by the overlying turbulent boundary layer characteristics, similar to observations from single-phase flow over a forward-facing step. A low-frequency peak reflecting the oscillations in size of re-entrant jet, analogous to the ‘flapping’ motion in single-phase flow, was found to be modulated by gravity effects (i.e. a Froude number dependency). Likewise, a significant change in cavity behaviour was observed as the flow underwent transition analogous to the transition from sub- to super- critical regime in open-channel flow.
A companion numerical study is undertaken to provide additional insight into particular flow features such as the separated flow region upstream of the fence and to assess the influence of blockage. An implicit unsteady compressible solver is used with a RANS k − ω SST turbulence model and VOF approach to capture the cavity interface. The numerical results are found to compare reasonably with the experimental data, additionally showing a significant influence of blockage on the studied flow.
Along with the 2-D fence, a 3-D wall mounted fence, spanning nominally a quarter of the tunnel test section, is investigated. The impact that 3-D effects have on the cavity topology and the relations between the parameters characterizing the flow is observed. The most notable effect of 3-D flow is a change in the closure mechanism observed for low Fr. Following a decrease in Fr the closure topology transforms from a well defined single re-entrant jet regime, through a phase of gradual re-entrant jet widening to a completely split re-entrant jet separated into two branches. Generally, the 2-D and 3-D flows exhibited similar trends with any significant difference attributable to differing levels of flow confinement due to lesser width of 3-D fence
