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 proce… Show more
“…Although a third frequency is also evident in spectra from surface pressure and high-speed imaging that only occurs at a cavitation number of about 0.8. The two dominant frequencies are attributed to axisymmetric and asymmetric global modes and the third to due local flow effects [21,22]. Recently similar experiments have been made with a nuclei abundant flow.…”
Section: Nucleation Effects On Cavitation Physicsmentioning
Microbubbles are systemic to cavitation physics. They are present in all practical liquid flows, they provide nuclei for inception, their dynamics play a role in many of the phenomena and processes involved in cavitation and ultimately they are generated by cavitation collapse and condensation. Various techniques for the generation, measurement and dispersion of microbubbles for experimental modelling of cavitation nucleation and dynamics in hydrodynamics test facilities are presented. Controlled generation of microbubbles are also required for development and calibration of optical measurement techniques and for flow diagnostics. Several techniques for generation of mono-, and poly-disperse microbubble populations in the size range 1 to 100 microns are discussed. Microbubble concentrations generated by devices or from cavitation itself vary from 1000 bubbles per cubic cm down to 0.01, in addition to the size range, necessitating the use of several methods for measurement of both size range and concentration. Experimental results demonstrating the influence of nuclei populations on stable and unstable cavitation in canonical flows about spheres and hydrofoils are presented. Nuclei populations affect shedding mechanisms and modes, spectral content, amplitudes of forces and microbubble content in downstream wakes.
“…Although a third frequency is also evident in spectra from surface pressure and high-speed imaging that only occurs at a cavitation number of about 0.8. The two dominant frequencies are attributed to axisymmetric and asymmetric global modes and the third to due local flow effects [21,22]. Recently similar experiments have been made with a nuclei abundant flow.…”
Section: Nucleation Effects On Cavitation Physicsmentioning
Microbubbles are systemic to cavitation physics. They are present in all practical liquid flows, they provide nuclei for inception, their dynamics play a role in many of the phenomena and processes involved in cavitation and ultimately they are generated by cavitation collapse and condensation. Various techniques for the generation, measurement and dispersion of microbubbles for experimental modelling of cavitation nucleation and dynamics in hydrodynamics test facilities are presented. Controlled generation of microbubbles are also required for development and calibration of optical measurement techniques and for flow diagnostics. Several techniques for generation of mono-, and poly-disperse microbubble populations in the size range 1 to 100 microns are discussed. Microbubble concentrations generated by devices or from cavitation itself vary from 1000 bubbles per cubic cm down to 0.01, in addition to the size range, necessitating the use of several methods for measurement of both size range and concentration. Experimental results demonstrating the influence of nuclei populations on stable and unstable cavitation in canonical flows about spheres and hydrofoils are presented. Nuclei populations affect shedding mechanisms and modes, spectral content, amplitudes of forces and microbubble content in downstream wakes.
“…The CRL tunnel has ancillary systems for rapid degassing and the circuit architecture enables continuous injection and removal of cavitation nuclei and large volumes of incondensable gas. Further description of the facility is given in Brandner, Venning & Pearce (2018). Figure 1.Tunnel schematic showing the experimental layout including the microbubble nuclei injection, contraction, test section and diffuser.…”
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.
“…2020 a , b ), which utilises off-focus imaging of the spherical microbubbles illuminated with a coherent, polarised light source to determine the size of each bubble. The images were processed using a continuous wavelet transform (Brandner, Venning & Pearce 2018) to give the non-cumulative distribution in diameter for typical test pressures ranging between 50 and 30 kPa, as shown in the upper right plot in figure 4. The injected population may also be approximated with a power law for all but the larger sized, but low concentration, bubbles.…”
The influence of nucleation on cavitation about a sphere from inception through to supercavitation at a transcritical Reynolds number of
$1.5\times 10^{6}$
is investigated experimentally. Two extreme free-stream nuclei populations, deplete and abundant, were investigated. Unsteady surface pressures from two sensors on opposing sides of the sphere were acquired simultaneous with high-resolution high-speed photography at cavitation numbers between 1.0 and 0.3. High-resolution spectrograms derived from these measurements reveal principally bi-modal shedding in attached and detached regimes. Correlations between unsteady pressure measurements show the high modes to be axisymmetric and low modes asymmetric. Modal topology is also discerned from the high-speed imaging. The bi-modal shedding for lower cavitation numbers is driven by coupled re-entrant jet formation and upstream shockwave propagation. The attached regime is shown to have two sub-regimes. For the abundant case, the continuous supply of activated nuclei around the sphere periphery in the first bi-modal regime has the effect of driving the high symmetric mode preferentially over the asymmetric low mode compared with the deplete case. For the first bi-modal regime, frequencies were unaffected by nucleation changes although peak responses were centred at a cavitation number of about 0.8 for the deplete and 0.825 for the abundant. For the second attached regime, where cavity lengths are of the order of the sphere size, changes in nucleation altered frequencies and amplitudes of peak unsteady pressures. For the abundant case, the continuous nuclei supply significantly reduced coherence with modal peak amplitudes reduced by an order compared with the deplete case. Continuous nuclei activation increased the probability of the high mode over the low compared with the deplete case but to a lesser extent than the first regime. Nuclei activations also significantly reduced inter-cavity and cavity durations, but not growth and collapse phases, which increased modal frequencies compared with the deplete case. The second regime asymmetric low mode topology, for both nucleation cases, is shown to be alternate shedding of oblique vortices from diametrically opposing sides of the sphere similar to low Reynolds number shedding about spheres and other axisymmetric bluff bodies in single-phase flows.
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