 Award ID(s):
 1844932
 Publication Date:
 NSFPAR ID:
 10308644
 Journal Name:
 Journal of Fluid Mechanics
 Volume:
 924
 ISSN:
 00221120
 Sponsoring Org:
 National Science Foundation
More Like this

We experimentally investigate the rise velocity of finitesized bubbles in turbulence with a high energy dissipation rate of $\unicode[STIX]{x1D716}\gtrsim 0.5~\text{m}^{2}~\text{s}^{3}$ . In contrast to a 30–40 % reduction in rise velocity previously reported in weak turbulence (the Weber number ( $We$ ) is much smaller than the Eötvös number ( $Eo$ ); $We\ll 1

We present experiments on large air cavities spanning a wide range of sizes relative to the Hinze scale $d_{H}$ , the scale at which turbulent stresses are balanced by surface tension, disintegrating in turbulence. For cavities with initial sizes $d_0$ much larger than $d_{H}$ (probing up to $d_0/d_{H} = 8.3$ ), the size distribution of bubbles smaller than $d_{H}$ follows $N(d) \propto d^{3/2}$ , with $d$ the bubble diameter. The capillary instability of ligaments involved in the deformation of the large bubbles is shown visually to be responsible for the creation of the small bubbles. Turning to dynamical, threedimensional measurements of individual breakup events, we describe the breakup child size distribution and the number of child bubbles formed as a function of $d_0/d_{H}$ . Then, to model the evolution of a population of bubbles produced by turbulent bubble breakup, we propose a population balance framework in which breakup involves two physical processes: an inertial deformation to the parent bubble that sets the size of large child bubbles, and a capillary instability that sets the size of small child bubbles. A Monte Carlo approach is used to construct the child size distribution, with simulated stochastic breakups constrained by our experimental measurementsmore »

We experimentally investigate the breakup mechanisms and probability of Hinzescale bubbles in turbulence. The Hinze scale is defined as the critical bubble size based on the critical mean Weber number, across which the bubble breakup probability was believed to have an abrupt transition from being dominated by turbulence stresses to being suppressed completely by the surface tension. In this work, to quantify the breakup probability of bubbles with sizes close to the Hinze scale and to examine different breakup mechanisms, both bubbles and their surrounding tracer particles were simultaneously tracked. From the experimental results, two Weber numbers, one calculated from the slip velocity between the two phases and the other acquired from local velocity gradients, are separated and fitted with models that can be linked back to turbulence characteristics. Moreover, we also provide an empirical model to link bubble deformation to the two Weber numbers by extending the relationship obtained from potential flow theory. The proposed relationship between bubble aspect ratio and the Weber numbers seems to work consistently well for a range of bubble sizes. Furthermore, the time traces of bubble aspect ratio and the two Weber numbers are connected using the linear forced oscillator model. Finally, having accessmore »

Abstract White dwarf photospheric parameters are usually obtained by means of spectroscopic or photometric analysis. These results are not always consistent with each other, with the published values often including just the statistical uncertainties. The differences are more dramatic for white dwarfs with heliumdominated photospheres, so to obtain realistic uncertainties we have analysed a sample of 13 of these white dwarfs, applying both techniques to up to three different spectroscopic and photometric data sets for each star. We found mean standard deviations of $\left\langle \sigma {T_{\mathrm{eff}}}\right\rangle = 524$ K, $\left\langle \sigma {\log g}\right\rangle = 0.27$ dex and $\left\langle \sigma {\log (\mathrm{H/He})}\right\rangle = 0.31$ dex for the effective temperature, surface gravity, and relative hydrogen abundance, respectively, when modelling diverse spectroscopic data. The photometric fits provided mean standard deviations up to $\left\langle \sigma {T_{\mathrm{eff}}}\right\rangle = 1210$ K and $\left\langle \sigma {\log g}\right\rangle = 0.13$ dex. We suggest these values to be adopted as realistic lower limits to the published uncertainties in parameters derived from spectroscopic and photometric fits for white dwarfs with similar characteristics. In addition, we investigate the effect of fitting the observational data adopting three different photospheric chemical compositions. In general, pure helium model spectra result in larger Teff compared to those derived from modelsmore »

The dynamics of air bubbles in turbulent Rayleigh–Bénard (RB) convection is described for the first time using laboratory experiments and complementary numerical simulations. We performed experiments at $Ra=5.5\times 10^{9}$ and $1.1\times 10^{10}$ , where streams of 1 mm bubbles were released at various locations from the bottom of the tank along the path of the roll structure. Using threedimensional particle tracking velocimetry, we simultaneously tracked a large number of bubbles to inspect the pair dispersion, $R^{2}(t)$ , for a range of initial separations, $r$ , spanning one order of magnitude, namely $25\unicode[STIX]{x1D702}\leqslant r\leqslant 225\unicode[STIX]{x1D702}$ ; here $\unicode[STIX]{x1D702}$ is the local Kolmogorov length scale. Pair dispersion, $R^{2}(t)$ , of the bubbles within a quiescent medium was also determined to assess the effect of inhomogeneity and anisotropy induced by the RB convection. Results show that $R^{2}(t)$ underwent a transition phase similar to the ballistictodiffusive ( $t^{2}$ to $t^{1}$ ) regime in the vicinity of the cell centre; it approached a bulk behavior $t^{3/2}$ in the diffusive regime as the distance away from the cell centre increased. At small $r$ , $R^{2}(t)\propto t^{1}$ is shown in the diffusive regime with a lower magnitude compared to the quiescent case, indicating that the convective turbulence reducedmore »