A new measuring technique dedicated to bubble velocity and size measurements in complex bubbly flows such as those occurring in bubble columns is proposed. This sensor combines the phase detection capability of a conical optical fiber, with velocity
measurements from the Doppler signal induced by an interface approaching the extremity of a single-mode fiber. The analysis of the probe functioning and of its response in controlled situations, have shown that the Doppler probe provides the translation velocity of bubbles projected along the probe axis. A reliable signal processing routine has been developed that exploits the Doppler signal arising at the gas-to-liquid transition: the resulting uncertainty on velocity is at most 14%. Such a Doppler probe provides statistics on velocity and on size of gas inclusions, as well as local variables including void fraction, gas volumetric flux, number density and its flux. That sensor has been successfully exploited in an air-tap water bubble column 0.4m in diameter for global gas hold-up from 2.5 to 30%. In the heterogeneous regime, the transverse profiles of the mean bubble velocity scaled by the value on the axis happen to be self-similar in the quasi fully developed region of the column. A fit is proposed for these profiles. In addition, on the axis, the standard deviation of bubble velocity scaled by the mean velocity increases with Vsg in the homogeneous regime, and it remains stable, close to 0.55, in the heterogeneous regime.
We present an experimental study on the settling velocity of dense sub-Kolmogorov particles in active-grid-generated turbulence in a wind tunnel. Using phase Doppler interferometry, we observe that the modifications of the settling velocity of inerti
al particles, under homogeneous isotropic turbulence and dilute conditions $phi_vleq O(10)^{-5}$, is controlled by the Taylor-based Reynolds number $Re_lambda$ of the carrier flow. On the contrary, we did not find a strong influence of the ratio between the fluid and gravity accelerations (i.e., $gammasim(eta/tau_eta^2)/g$) on the particle settling behavior. Remarkably, our results suggest that the hindering of the settling velocity (i.e. the measured particle settling velocity is smaller than its respective one in still fluid conditions) experienced by the particles increases with the value of $Re_lambda$, reversing settling enhancement found under intermediate $Re_lambda$ conditions. This observation applies to all particle sizes investigated, and it is consistent with previous experimental data in the literature. At the highest $Re_lambda$ studied, $Re_lambda>600$, the particle enhancement regime ceases to exist. Our data also show that for moderate Rouse numbers, the difference between the measured particle settling velocity and its velocity in still fluid conditions scales linearly with Rouse, when this difference is normalized by the carrier phase rms fluctuations, i.e., $(V_p-V_T)/usim -Ro$.
The integral length scale ($mathcal{L}$) is considered to be characteristic of the largest motions of a turbulent flow, and as such, it is an input parameter in modern and classical approaches of turbulence theory and numerical simulations. Its exper
imental estimation, however, could be difficult in certain conditions, for instance, when the experimental calibration required to measure $mathcal{L}$ is hard to achieve (hot-wire anemometry on large scale wind-tunnels, and field measurements), or in standard facilities using active grids due to the behaviour of their velocity autocorrelation function $rho(r)$, which does not in general cross zero. In this work, we provide two alternative methods to estimate $mathcal{L}$ using the variance of the distance between successive zero crossings of the streamwise velocity fluctuations, thereby reducing the uncertainty of estimating $mathcal{L}$ under similar experimental conditions. These methods are applicable to variety of situations such as active grids flows, field measurements, and large scale wind tunnels.
We present a sweep-stick mechanism for heavy particles transported by a turbulent flow under the action of gravity. Direct numerical simulations show that these particles preferentially explore regions of the flow with close to zero Lagrangian accele
ration. However, the actual Lagrangian acceleration of the fluid elements where particles accumulate is not zero, and has a dependence on the Stokes number, the gravity acceleration, and the settling velocity of the particles.
