Plunging cavities (original) (raw)
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Evolution of the air cavity during a depressurized wave impact. I. The kinematic flow field
Physics of Fluids, 2010
The present paper on wave-impact events in depressurized environments completes the analysis of Part I by focusing on the dynamical features of the impacts and on the influence of the ambient pressure. Connection is made between the impact regimes typically described in the literature and the stages described in Part I ͓C. Lugni, M. Miozzi, M. Brocchini, and O. M. Faltinsen, "Evolution of the air cavity during a depressurized wave impact. I. The kinematic flow field," Phys. Fluids 22, 056101 ͑2010͔͒. The stages of isotropic/anisotropic compression and expansion of the air cavity are of particular interest. The impact duration at the wall is almost independent of its height above the undisturbed surface level, but its intensity rapidly decreases in the body of the fluid ͑the peak pressure halves within the first two compression/expansion cycles͒. The time evolution of the pressure loads on the wall is analyzed by means of the Hilbert transform and an empirical mode decomposition of the signals. This enables identification of the intrinsic mode functions which best fit the original signal during its evolution and quantification of the frequency downshifting which characterize the whole process. The pressure decay, largely governed by air leakage out of the cavity, is found to be very intense during the air cavity closure and the isotropic compression/ expansion cycle ͓stages ͑1͒ and ͑2͔͒; the decay observed during stage ͑3͒, i.e., during the anisotropic compression/expansion cycles, is weaker and independent of the vertical location down the wall. Differences between the observed decay rates and those of a three-dimensional bubble in an infinite fluid are mainly due to the bubble being two-dimensional, being close to the free surface and loosing air. The role of both ullage and vapor pressures on the impact is described, respectively, by means of the Euler and cavitation numbers. The frequency of the bubble oscillation depends on these numbers in a way that is closely similar to that displayed by the bubble area, see Part I. Physics 22, 056102-1 056102-2 Lugni, Brocchini, and Faltinsen Phys. Fluids 22, 056102 ͑2010͒ FIG. 3. ͑Color online͒ Time evolution of the acceleration of the tank as measured by the accelerometer ͑left panel͒ and its power spectrum ͑right panel͒. The recordings of two different runs are shown. Note that the time instant t = 0 coincides with the start of the motion of the tank.
Characteristics of Plunging Breaking Waves
Research Report No. CE150, Dept. of Civil Engineering, University of Queensland, Australia, 54 pages (ISBN 0 86776 6298), 1995
With plunging breaking waves, air bubble entrainment is caused by the top of the wave forming a plunging jet and entraining air when it impacts the water in front of the wave. The potential for air bubble entrainment is related directly to the plunging jet impact characteristics .New experiments were performed in a two-dimensional wave flume. The breaking process was investigated with a high-speed video camera. The results provide new information on the breaking point characteristics, the jet impact conditions and the energy dissipation process. The entrainment of air bubbles is detailed. And the rate of energy dissipation by plunging breakers is estimated.
Collapse and pinch-off of a non-axisymmetric impact-created air cavity in water
Journal of Fluid Mechanics, 2012
The axisymmetric collapse of a cylindrical air cavity in water follows a universal power law with logarithmic corrections. Nonetheless, it has been suggested that the introduction of a small azimuthal disturbance induces a long term memory effect, reflecting in oscillations which are no longer universal but remember the initial condition. In this work, we show that the walls of a non-axisymmetric, impact-created cavity indeed oscillate with a nearly constant amplitude and increasing frequency as they collapse. The cavities are characterized by azimuthal harmonic disturbances with a single mode number m and amplitude a m . Our experimental setup allows us to investigate the influence of these two parameters in greater depth than any previous studies. For small initial distortion amplitude (1 or 2 % of the mean disc radius), the cavity walls oscillate linearly with nearly constant amplitude. Non-linear effects, if at all, are observed only at the very end of the collapse. As the amplitude is increased, non-linear behaviour takes over earlier in the process, giving rise to spectacular collapses. For small amplitude disturbances we compare our experimental results with the model for the amplitude of the oscillations by and the model for the collapse of an axisymmetric, impact-created cavity previously proposed by . By combining these two models we can reconstruct the three-dimensional shape of the cavity at any time before pinch-off. arXiv:1109.5823v1 [physics.flu-dyn]
Experimental study on the interaction of non-buoyant jets and waves
Journal of Hydraulic Research, 2004
This paper presents experimental results of a turbulent non-buoyant jet vertically discharged in a stagnant ambient and of the same jet discharged in a flow field of regular waves. The study was carried out in a wave channel and jet velocities were measured with a backscatter four-beam two-component fiber-optic LDA system. The signal of the two velocity components was sampled simultaneously to that of a resistance probe of the water surface profile in wave field configurations. This transducer was placed in the transversal section of the channel crossing the measurement point of the LDA system. The present study also includes a comparative investigation of the main fluid mechanics characteristics in cases of jet discharged in a stagnant ambient, and the same jet discharged in flow fields of regular waves characterized by different periods. Analysis of these characteristics underlines the substantial differences between the two cases under study.
Collapse of a non-axisymmetric, impact-created air cavity in water
2011
The axisymmetric collapse of a cylindrical air cavity in water follows a universal power law with logarithmic corrections. Nonetheless, it has been suggested that the introduction of a small azimuthal disturbance induces a long term memory effect, reflecting in oscillations which are no longer universal but remember the initial condition. In this work, we create non-axisymmetric air cavities by driving a metal disc through an initially-quiescent water surface and observe their subsequent gravity-induced collapse. The cavities are characterized by azimuthal harmonic disturbances with a single mode number mmm and amplitude ama_mam. For small initial distortion amplitude (1 or 2% of the mean disc radius), the cavity walls oscillate linearly during collapse, with nearly constant amplitude and increasing frequency. As the amplitude is increased, higher harmonics are triggered in the oscillations and we observe more complex pinch-off modes. For small amplitude disturbances we compare our experimental results with the model for the amplitude of the oscillations by Schmidt et al. (2009) and the model for the collapse of an axisymmetric impact-created cavity previously proposed by Bergmann et al. (2009b). By combining these two models we can reconstruct the three-dimensional shape of the cavity at any time before pinch-off.
Journal of Fluids Engineering, Transactions ASME, Vol. 119, No. 3, pp. 603-608, 1997
When a water jet impinges a pool of water at rest, air bubbles may be. ent:ained a:"d carried away below the pool free suiface: this process. is call~d plungt'!g jet entramment. The study presents new experimental data obtamed wzth a vertlcal supponed jet. Distributions of air concentratiOn and mean air-water velocity, and bubble chord length distributions measured in the developing shear layer are presen.ted. The ~esults indicate that the distributions of void fraction follow closely analytzcal solutwn of the diffusion equi:ztion. Further, the momentum shear.layer and the ~ir bubble ~iffusion layer do not coincide. Chord length data show a wzde range of atr bubble szzes .and overall the experimental results suggest strong interac;tions between the entramed air bubbles and the momentum transfer mechanisms.
New Flow Features in a Cavity During Shock Wave Impact
Complex flows are induced in a cavity impacted by a shock wave. Early work concentrated on shallow parabolic cavities with the main interest being the focussing properties. Work on cylindrical surfaces has concentrated on wave reflection regimes. The advent of high framing rate, multi-frame, digital cameras has allowed much more detailed studies to be undertaken, which have identified a number of new features, as well as clarifying and modifying previous findings. Weak perturbations are introduced into the flow as identifiers of regions of influence from particular points on the wall. Particular emphasis is placed on the study of some new reflection patterns, the manner in which the waves focus in different cavity shapes, and complex shear layer flows with jetting and the development of Kelvin-Helmholtz instabilities.
Air Entrainment in the Developing Flow Region of Plunging Jets-Part 1: Theoretical Development
Journal of Fluids Engineering, Transactions ASME, Vol. 119, No. 3, pp. 597-602, 1997
Air-water bubbly flows are encountered in many engineering applications. One type of air-water shear flows is the developing flow region of a plunging jet. The mechanisms of air entrainment by plunging liquid jets are discussed in the light of new experimental evidence. Then the air bubble diffusion is analyzed analytically in the near-flow field of both circular and two-dimensional plunging jets. The theoretical developments are Compared with existing circular plunging jet data and new experi~ ments peifonned with a two-dimension(ll vertical supported jet. The study highlights two mechanisms of air entrainment at the plunge point depending upon the jet impact velocity and results suggest that the dispersion of air bubbles within the shear layer is primarily an advective diffusion process.