ORCID Profile
0000-0003-2189-5598
Current Organisation
University of Tasmania, Australian Maritime College
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Publisher: Springer Science and Business Media LLC
Date: 26-03-2018
Publisher: Elsevier BV
Date: 04-2021
Publisher: Cambridge University Press (CUP)
Date: 22-12-2017
DOI: 10.1017/JFM.2016.819
Abstract: The physics and spectral content of cloud cavitation about a sphere are investigated in a variable-pressure water tunnel using dynamic surface pressure measurement and high-speed imaging. Experiments are conducted using a polyvinyl chloride sphere at a Reynolds number of $1.5\\times 10^{6}$ with cavitation numbers, $\\unicode[STIX]{x1D70E}$ , ranging from inception to supercavitation. Three distinct shedding regimes are identified: a uni-modal regime for $\\unicode[STIX]{x1D70E} .9$ and two bi-modal regimes for $0.9 \\unicode[STIX]{x1D70E} .675$ and $0.675 \\unicode[STIX]{x1D70E} .3$ . For small cavity lengths ( $\\unicode[STIX]{x1D70E} .9$ ), Kelvin–Helmholtz instability and transition to turbulence in the overlying separated boundary layer form the basis for cavity breakup and coherent vortex formation. At greater lengths ( $\\unicode[STIX]{x1D70E} .9$ ), larger-scale shedding ensues, driven by coupled re-entrant jet formation and shockwave propagation. Strong adverse pressure gradients about the sphere lead to accumulation and radial growth of re-entrant flow, initiating breakup, from which, in every case, a condensation shockwave propagates upstream causing cavity collapse. When the shedding is most energetic, shockwave propagation upstream may cause large-scale leading edge extinction. The bi-modal response is due to cavity shedding being either axisymmetric or asymmetric. The two bi-modal regimes correspond to $\\unicode[STIX]{x1D70E}$ ranges where the cavity and re-entrant jet either remain attached or become detached from the sphere. There is a distinct frequency offset at transition between regimes in both shedding modes. Despite the greater cavity lengths at lower $\\unicode[STIX]{x1D70E}$ values, the second bi-modal regime initially exhibits shorter shedding periods due to increased cavity growth rates. The second regime persists until supercavitation develops for $\\unicode[STIX]{x1D70E} .3$ .
Publisher: ASME Press
Date: 2018
Publisher: ASME International
Date: 08-01-2019
DOI: 10.1115/1.4042067
Abstract: Despite recent extensive research into fluid–structure interaction (FSI) of cavitating hydrofoils, there remain insufficient experimental data to explain many of the observed phenomena. The cloud cavitation behavior around a hydrofoil due to the effect of FSI is investigated, utilizing rigid and compliant three-dimensional (3D) hydrofoils held in a cantilevered configuration in a cavitation tunnel. The hydrofoils have identical undeformed geometry of tapered planform with a constant modified NACA0009 profile. The rigid model is made of stainless steel and the compliant model of a carbon and glass fiber-reinforced epoxy resin with the structural fibers aligned along the spanwise direction to avoid material bend-twist coupling. Tests were conducted at an incidence of 6 deg, a mean chord-based Reynolds number of 0.7 × 106 and cavitation number of 0.8. Force measurements were simultaneously acquired with high-speed imaging to enable correlation of forces with tip bending deformations and cavity physics. Hydrofoil compliance was seen to d en the higher frequency force fluctuations while showing strong correlation between normal force and tip deflection. The 3D nature of the flow field was seen to cause complex cavitation behavior with two shedding modes observed on both models.
Publisher: IOP Publishing
Date: 03-12-2015
Publisher: IOP Publishing
Date: 03-12-2015
Publisher: Springer Science and Business Media LLC
Date: 14-01-2020
Publisher: AIP Publishing
Date: 2023
DOI: 10.1063/5.0132054
Abstract: The influence of nucleation on cavitation inception in a high Reynolds number shear layer in the wake of a backward-facing step was experimentally investigated in a water tunnel. The flow was investigated for two nuclei populations: the one naturally occurring in the water and for the water artificially seeded with monodisperse nuclei. Incipient events were observed to form in stretched quasi-streamwise vortices. The collapse of an incipient cavity resulted in a microbubble cloud dispersed into the shear layer and the step re-circulation zone. These microbubbles, generally larger than those naturally occurring in the water, act as preferential sites for re-nucleation, triggering the formation of developed cavitation. This phenomenon rendered statistical characterization of cavitation inception impractical for the natural nuclei population. The re-nucleation issue was addressed by seeding the flow with a population of large monodisperse nuclei, with a critical pressure higher than that of cavitation products. Spatial distribution of the nuclei within the seeded plume was characterized using a volumetric measurement based on Mie-scattering imaging. The ability to discern in idual incipient events enabled examination of the effect of cavitation number and the nuclei injection rate on the inception event rate. The event rate was found to follow a power law with cavitation number and vary linearly with the injection rate. Mapping of spatial distribution of cavitation susceptibility was obtained by combining the spatial distributions of incipient events and nuclei concentration. The current work provides a valuable dataset for the development of computational tools for modeling of cavitation inception in nucleated flows.
Publisher: Cambridge University Press (CUP)
Date: 17-06-2020
DOI: 10.1017/JFM.2020.323
Publisher: Cambridge University Press (CUP)
Date: 26-05-2020
DOI: 10.1017/JFM.2020.321
Publisher: ASME Press
Date: 2018
Publisher: No publisher found
Date: 2022
DOI: 10.1017/JFM.2022.511
Publisher: Elsevier BV
Date: 11-2016
Publisher: IOP Publishing
Date: 03-12-2015
Publisher: Cambridge University Press (CUP)
Date: 15-08-2022
DOI: 10.1017/JFM.2022.535
Abstract: The dynamics of cloud cavitation about a three-dimensional hydrofoil are investigated experimentally in a cavitation tunnel with depleted, sparse and abundant free-stream nuclei populations. A rectangular planform, NACA 0015 hydrofoil was tested at a Reynolds number of $1.4\\times 10^{6}$ , an incidence of $6^{\\circ }$ and a range of cavitation numbers from single-phase flow to supercavitation. High-speed photographs of cavitation shedding phenomena were acquired simultaneously with unsteady force measurement to enable identification of cavity shedding modes corresponding to force spectral peaks. The shedding modes were analysed through spectral decomposition of the high-speed movies, revealing different shedding instabilities according to the nuclei content. With no active nuclei, the fundamental shedding mode occurs at a Strouhal number of 0.28 and is defined by large-scale re-entrant jet formation during the growth phase, but shockwave propagation for the collapse phase of the cycle. Harmonic and subharmonic modes also occur due to local tip shedding. For the abundant case, the fundamental shedding is again large-scale but with a much slower growth phase resulting in a frequency of $St=0.15$ . A harmonic mode forms in this case due to the propagation of two shockwaves an initial slow propagating wave followed by a second faster wave. The passage of the first wave causes partial condensation leading to lower void fraction and consequent increase in the speed of the second wave along with larger-scale condensation. For a sparsely seeded flow, coherent fluctuations are reduced due to intermittent, disperse nuclei activation and cavity breakup resulting in an optimal condition with maximum reduction in unsteady lift.
Publisher: AIP Publishing
Date: 2023
DOI: 10.1063/5.0132034
Abstract: Cavitation in a tip leakage flow is experimentally investigated in a cavitation tunnel using a stationary hydrofoil analogy. The experiments were performed for different tip clearances (τ=gap height/maximum profile thickness) and hydrofoil incidences (α). The chord-based Reynolds number remained fixed at Re=3×106. The influence of nucleation on both inception and developed cavitation is evaluated by performing tests with two populations of freestream nuclei: a low concentration with strong critical tensions for activation and a high concentration with weak critical tensions. These populations represent the extremes that would be expected in practical tip leakage flows. Cavitation was characterized using high-speed imaging and acoustic measurements. Following a survey of developed cavitation topology for a range τ and α values, α=6° was selected for further investigation of cavitation inception as it demonstrated a rich variety of physical processes. From the acoustic measurements, the worst performance in terms of cavitation inception was observed at an intermediate gap height of around τ=0.6–0.8 for the “strong water” case. Broadly, cavitation and inception is intermittent when nuclei are sparse, becoming continuous as additional nuclei are introduced. While a continuous cavity in the seeded flow resulted in a higher baseline acoustic signature, sparse populations allow the leakage vortex to sustain tension, which can result in extremely loud incipient events. Optimization of gap height will, therefore, depend on the expected nuclei population during operation.
Publisher: Cambridge University Press (CUP)
Date: 04-03-2015
DOI: 10.1017/JFM.2015.73
Abstract: Cavitation occurrence about a jet in crossflow is investigated experimentally in a variable-pressure water tunnel using still and high-speed photography. The 0.012 m diameter jet is injected on the centreplane of a 0.6 m square test section at jet to freestream velocity ratios ranging from 0.2 to 1.6, corresponding to jet-velocity-based Reynolds numbers of $25\\times 10^{3}$ to $160\\times 10^{3}$ respectively. Measurements were made at a fixed freestream-based Reynolds number, for which the ratio of the undisturbed boundary layer thickness to jet diameter is 1.18. The cavitation number was varied from inception (up to about 10) down to 0.1. Inception is investigated acoustically for bounding cases of high and low susceptibility to phase change. The influence of velocity ratio and cavitation number on cavity topology and geometry are quantified from the photography. High-speed photographic recordings made at 6 kHz provide insight into cavity dynamics, and derived time series of spatially averaged pixel intensities enable frequency analysis of coherent phenomena. Cavitation inception was found to occur in the high-shear regions either side of the exiting jet and to be of an intermittent nature, increasing in occurrence and duration from 0 to 100 % probability with decreasing cavitation number or increasing jet to freestream velocity ratio. The frequency and duration of in idual events strongly depends on the cavitation nuclei supply within the approaching boundary layer. Macroscopic cavitation develops downstream of the jet with reduction of the cavitation number beyond inception, the length of which has a power-law dependence on the cavitation number and a linear dependence on the jet to freestream velocity ratio. The cavity closure develops a re-entrant jet with increase in length forming a standing wave within the cavity. For sufficiently low cavitation numbers the projection of the re-entrant jet fluid no longer reaches the cavity leading edge, analogous to supercavitation forming about solid cavitators. Hairpin-shaped vortices are coherently shed from the cavity closure via mechanisms of shear-layer roll-up similar to those shed from protuberances and jets in crossflow in single-phase flows. These vortices are shed at an apparently constant frequency, independent of the jet to freestream velocity ratio but decreasing in frequency with reducing cavitation number and cavity volume growth. Highly coherent cavitating vortices form along the leading part of the cavity due to instability of the jet upstream shear layer for jet to freestream velocity ratios greater than about 0.8. These vortices are cancelled and condense as they approach the trailing edge in the shear layer of opposing vorticity associated with the cavity closure and the hairpin vortex formation. For lower velocity ratios, where there is decreased jet penetration, the jet upstream shear velocity gradient reverses and vortices of the opposite sense form, randomly modulated by boundary layer turbulence.
Publisher: Elsevier BV
Date: 11-2014
Publisher: ASME Press
Date: 2018
Publisher: Cambridge University Press (CUP)
Date: 11-01-2022
Abstract: Experimental studies of the influence of fluid–structure interaction on cloud cavitation about a stiff stainless steel (SS) and a flexible composite (CF) hydrofoil have been presented in Parts I (Smith et al. , J. Fluid Mech. , vol. 896, 2020 a , p. A1) and II (Smith et al. , J. Fluid Mech. , vol. 897, 2020 b , p. A28). This work further analyses the data and complements the measurements with reduced-order model predictions to explain the complex response. A two degrees-of-freedom steady-state model is used to explain why the tip bending and twisting deformations are much higher for the CF hydrofoil, while the hydrodynamic load coefficients are very similar. A one degree-of-freedom dynamic model, which considers the spanwise bending deflection only, is used to capture the dynamic response of both hydrofoils. Peaks in the frequency response spectrum are observed at the re-entrant jet-driven and shock-wave-driven cavity shedding frequencies, system bending frequency and heterodyne frequencies caused by the mixing of the two cavity shedding frequencies. The predictions capture the increase of the mean system bending frequency and wider bandwidth of frequency modulation with decreasing cavitation number. The results show that, in general, the litude of the deformation fluctuation is higher, but the litude of the load fluctuation is lower for the CF hydrofoil compared with the SS hydrofoil. Significant dynamic load lification is observed at subharmonic lock-in when the shock-wave-driven cavity shedding frequency matches with the nearest subharmonic of the system bending frequency of the CF hydrofoil. Both measurements and predictions show an absence of dynamic load lification at primary lock-in because of the low intensity of cavity load fluctuations with high cavitation number.
Publisher: Elsevier BV
Date: 04-2014
Publisher: ASME Press
Date: 2018
Publisher: ASME Press
Date: 2018
Publisher: Elsevier BV
Date: 08-2018
Publisher: SAGE Publications
Date: 27-01-2015
Abstract: A numerical analysis of the inviscid flow over base-ventilated intercepted hydrofoils is presented. The low-order, non-linear boundary element formulation used is described along with the significant issues concerning the modelling of supercavities with this method. The use of transom-mounted interceptors is well established for the manoeuvring and trim control of high-speed vessels. The flow field over a forward-facing step at the trailing edge of a blunt-based hydrofoil section, with consequent cavity detachment from the outer edge of the step, is similar to that of the transom-mounted interceptor operating at high speed with free surface detachment from the outer edge. Due to this similarity, the term ‘intercepted’ hydrofoil is used to describe this arrangement. The results presented show that a number of geometric parameters, in particular thickness, leading-edge radius and trailing-edge slope, have a significant effect on the hydrodynamic performance of base-ventilated intercepted hydrofoils.
Publisher: The Royal Society
Date: 09-07-2018
Abstract: Cavitating and bubbly flows involve a host of physical phenomena and processes ranging from nucleation, surface and interfacial effects, mass transfer via diffusion and phase change to macroscopic flow physics involving bubble dynamics, turbulent flow interactions and two-phase compressible effects. The complex physics that result from these phenomena and their interactions make for flows that are difficult to investigate and analyse. From an experimental perspective, evolving sensing technology and data processing provide opportunities for gaining new insight and understanding of these complex flows, and the continuous wavelet transform (CWT) is a powerful tool to aid in their elucidation. Five case studies are presented involving many of these phenomena in which the CWT was key to data analysis and interpretation. A erse set of experiments are presented involving a range of physical and temporal scales and experimental techniques. Bubble turbulent break-up is investigated using hydroacoustics, bubble dynamics and high-speed imaging microbubbles are sized using light scattering and ultrasonic sensing, and large-scale coherent shedding driven by various mechanisms are analysed using simultaneous high-speed imaging and physical measurement techniques. The experimental set-up, aspect of cavitation being addressed, how the wavelets were applied, their advantages over other techniques and key findings are presented for each case study. This paper is part of the theme issue ‘Redundancy rules: the continuous wavelet transform comes of age’.
Publisher: ASME Press
Date: 2018
Publisher: Springer Science and Business Media LLC
Date: 09-03-2020
Publisher: Elsevier BV
Date: 08-2015
Publisher: Springer Science and Business Media LLC
Date: 06-2020
Publisher: Springer Science and Business Media LLC
Date: 09-01-2021
Publisher: Cambridge University Press (CUP)
Date: 10-07-2019
DOI: 10.1017/JFM.2019.455
Abstract: The topology and unsteady behaviour of ventilated and natural cavity flows over a two-dimensional (2-D) wall-mounted fence are investigated for fixed length cavities with varying free-stream velocity using high-speed and still imaging, X-ray densitometry and dynamic surface pressure measurement in two experimental facilities. Cavities in both ventilated and natural flows were found to have a re-entrant jet closure, but not to exhibit large-scale oscillations, yet the irregular small-scale shedding at the cavity closure. Small-scale cavity break-up was associated with a high-frequency broadband 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 the re-entrant jet region, analogous to ‘flapping’ motion in single-phase flow, was found to be modulated by gravity effects (i.e. a Froude number dependence). 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. Differences in wake topology were examined using shadowgraphy and proper orthogonal decomposition, from which it was found that the size and number of shed structures increased with an increase in free-stream velocity for the ventilated case, while remaining nominally constant in naturally cavitating flow due to condensation of vaporous structures.
Publisher: Elsevier BV
Date: 05-2022
Publisher: Research Publishing Services
Date: 2012
Publisher: Elsevier BV
Date: 09-2017
Publisher: Springer Science and Business Media LLC
Date: 22-02-2020
Publisher: Elsevier BV
Date: 12-2017
Publisher: Elsevier BV
Date: 04-2018
Publisher: Elsevier BV
Date: 2019
Location: Australia
Start Date: 2015
End Date: 2016
Funder: Office of Naval Research Global
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Funder: Defence Science and Technology Group
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Funder: Defence Science and Technology Group
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Funder: Defence Science and Technology Group
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Funder: Defence Science and Technology Group
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