arXiv subjects

Enrico Calzavarini

Publications and source records attributed to Enrico Calzavarini.

At least 55 records · Page 3Linked to original sources

arXiv2012

How gravity and size affect the acceleration statistics of bubbles in turbulence

We report results from the first systematic Lagrangian experimental investigation in the regime of very light (air bubbles in water) and large particles (D/η >> 1) in turbulence. Using a traversing camera setup and particle tracking, we study the Lagrangian acceleration statistics of ~3 mm diameter (D) bubbles in a water tunnel with nearly homogeneous and isotropic turbulence generated by an active-grid. The Reynolds number (Re_λ) is varied from 145 to 230, resulting in size ratios, D/η in the range of 7.3--12.5, where η is the Kolmogorov length scale. The experiments reveal that gravity increases the acceleration variance and reduces the intermittency of the PDF in the vertical direction. Once the gravity offset is subtracted, the variances of both the horizontal and vertical acceleration components are about 5\pm2 times larger than the one measured in the same flow for fluid tracers. Moreover, for these light particles, the experimental acceleration PDF shows a substantial reduction in intermittency at growing size ratios, in contrast to neutrally buoyant or heavy particles. All these results are closely matched by numerical simulations of finite-size bubbles with the Faxén corrections.

physics.flu-dyn↗

arXiv2012

Rotation Rate of Rods in Turbulent Fluid Flow

The rotational dynamics of anisotropic particles advected in a turbulent fluid flow are important in many industrial and natural setting. Particle rotations are controlled by small scale properties of turbulence that are nearly universal, and so provide a rich system where experiments can be directly compared with theory and simulations. Here we report the first three-dimensional experimental measurements of the orientation dynamics of rod-like particles as they are advected in a turbulent fluid flow. We also present numerical simulations that show good agreement with the experiments and allow extension to a wide range of particle shapes. Anisotropic tracer particles preferentially sample the flow since their orientations become correlated with the velocity gradient tensor. The rotation rate is heavily influenced by this preferential alignment, and the alignment depends strongly on particle shape.

physics.flu-dyn↗

arXiv2012

Three-dimensional Lagrangian Voronoi analysis for clustering of particles and bubbles in turbulence

Three-dimensional Voronoi analysis is used to quantify the clustering of inertial particles in homogeneous isotropic turbulence using data from numerics and experiments. We study the clustering behavior at different density ratios and particle response times (i.e. Stokes numbers St). The Probability Density Functions (PDFs) of the Voronoi cell volumes of light and heavy particles show a different behavior from that of randomly distributed particles -i.e. fluid tracers-implying that clustering is present. The standard deviation of the PDF normalized by that of randomly distributed particles is used to quantify the clustering. Light particles show maximum clustering for St around 1-2. The results are consistent with previous investigations employing other approaches to quantify the clustering. We also present the joint PDFs of enstrophy and Voronoi volumes and their Lagrangian autocorrelations. The small Voronoi volumes of light particles correspond to regions of higher enstrophy than those of heavy particles, indicating that light particles cluster in higher vorticity regions. The Lagrangian temporal autocorrelation function of Voronoi volumes shows that the clustering of light particles lasts much longer than that of heavy or neutrally buoyant particles. Due to inertial effects, the Lagrangian autocorrelation time-scale of clustered light particles is even longer than that of the enstrophy of the flow itself.

physics.flu-dyn↗

arXiv2011

Axially-homogeneous Rayleigh-Benard convection in a cylindrical cell

Previous numerical studies have shown that the "ultimate regime of thermal convection" can be attained in a Rayleigh-Benard cell when the kinetic and thermal boundary layers are eliminated by replacing the walls with periodic boundary conditions (homogeneous Rayleigh-Benard convection). Then, the heat transfer scales like Nu ~ Ra^{1/2} and turbulence intensity as Re ~ Ra^{1/2}, where the Rayleigh number Ra indicates the strength of the driving force. However, experiments never operate in unbounded domains and it is important to understand how confinement might alter the approach to this ultimate regime. Here we consider homogeneous Rayleigh-Benard convection in a laterally confined geometry - a small aspect-ratio vertical cylindrical cell - and show evidence of the ultimate regime as Ra is increased: In spite of the confinement and the resulting kinetic boundary layers, we still find Nu ~ Re ~ Ra^{1/2}. The system supports exact solutions composed of modes of exponentially growing vertical velocity and temperature fields, with Ra as the critical parameter determining the properties of these modes. Counterintuitively, in the low Ra regime, or for very narrow cylinders, the numerical simulations are susceptible to these solutions which can dominate the dynamics and lead to very high and unsteady heat transfer. As Ra is increased, interaction between modes stabilizes the system, evidenced by the increasing homogeneity and reduced fluctuations in the r.m.s. velocity and temperature fields. We also test that physical results become independent of the periodicity length of the cylinder, a purely numerical parameter, as the aspect ratio is increased.

physics.flu-dyn↗

arXiv2011

Multi-Time Multi-Scale Correlation Functions in Hydrodynamic Turbulence

High Reynolds numbers Navier-Stokes equations are believed to break self-similarity concerning both spatial and temporal properties: correlation functions of different orders exhibit distinct decorrelation times and anomalous spatial scaling properties. Here, we present a systematic attempt to measure multi-time and multi-scale correlations functions, by using high Reynolds numbers numerical simulations of fully homogeneous and isotropic turbulent flow. The main idea is to set-up an ensemble of probing stations riding the flow, i.e. measuring correlations in a reference frame centered on the trajectory of distinct fluid particles (the quasi-Lagrangian reference frame introduced by Belinicher & L'vov, Sov. Phys. JETP 66, 303 (1987)). In this way we reduce the large-scale sweeping and measure the non-trivial temporal dynamics governing the turbulent energy transfer from large to small scales. We present evidences of the existence of dynamic multiscaling: multi-time correlation functions are characterized by an infinite set of characteristic times.

physics.flu-dyn↗

arXiv2010

Impact of trailing wake drag on the statistical properties and dynamics of finite-sized particle in turbulence

We study by means of an Eulerian-Lagrangian model the statistical properties of velocity and acceleration of a neutrally-buoyant finite-sized particle in a turbulent flow statistically homogeneous and isotropic. The particle equation of motion, beside added mass and steady Stokes drag, keeps into account the unsteady Stokes drag force - known as Basset-Boussinesq history force - and the non-Stokesian drag based on Schiller-Naumann parametrization, together with the finite-size Faxen corrections. We focus on the case of flow at low Taylor-Reynolds number, Re_lambda ~ 31, for which fully resolved numerical data which can be taken as a reference are available (Homann & Bec 651 81-91 J. Fluid Mech. (2010)). Remarkably, we show that while drag forces have always minor effects on the acceleration statistics, their role is important on the velocity behavior. We propose also that the scaling relations for the particle velocity variance as a function of its size, which have been first detected in fully resolved simulations, does not originate from inertial-scale properties of the background turbulent flow but it is likely to arise from the non-Stokesian component of the drag produced by the wake behind the particle. Furthermore, by means of comparison with fully resolved simulations, we show that the Faxen correction to the added mass has a dominant role in the particle acceleration statistics even for particle with size in the inertial range.

physics.flu-dyn↗

arXiv2010

Dynamics of inertial particles in a turbulent von Karman flow

We study the dynamics of neutrally buoyant particles with diameters varying in the range [1, 45] in Kolmogorov scale units (eta) and Reynolds numbers based on Taylor scale between 580 and 1050. One component of the particles' velocity is measured using extended Laser Doppler Velocimetry at the center of a Von-Karman (VK) flow, acceleration is derived by differentiation. We find that although the particle acceleration variance decreases with increasing their diameter with scaling close to (D/eta)^-2/3, in agreement with previous observations, the characteristic time of acceleration autocorrelation increases much strongly than previously reported, and linearly with D/eta. A new analysis of the probability density functions of the acceleration shows smaller wings for larger particles; the flatness indeed decreases as also expected from the behavior of Eulerian pressure increments in the VK flow. We contrast our measurements with former observations in wind-tunnel turbulent flows and numerical simulations, and discuss if the observed differences arise from inherent properties of the VK flow.

physics.flu-dyn↗

arXiv2009

Velocity gradients statistics along particle trajectories in turbulent flows: the refined similarity hypothesis in the Lagrangian frame

We present an investigation of the statistics of velocity gradient related quantities, in particluar energy dissipation rate and enstrophy, along the trajectories of fluid tracers and of heavy/light particles advected by a homogeneous and isotropic turbulent flow. The Refined Similarity Hypothesis (RSH) proposed by Kolmogorov and Oboukhov in 1962 is rephrased in the Lagrangian context and then tested along the particle trajectories. The study is performed on state-of-the-art numerical data resulting from numerical simulations up to Re~400 with 2048^3 collocation points. When particles have small inertia, we show that the Lagrangian formulation of the RSH is well verified for time lags larger than the typical response time of the particle. In contrast, in the large inertia limit when the particle response time approaches the integral-time-scale of the flow, particles behave nearly ballistic, and the Eulerian formulation of RSH holds in the inertial-range.

physics.flu-dyn↗

arXiv2008

Flow organization in non-Oberbeck-Boussinesq Rayleigh-Benard convection in water

Non-Oberbeck-Boussinesq (NOB) effects on the flow organization in two-dimensional Rayleigh-Benard turbulence are numerically analyzed. The working fluid is water. We focus on the temperature profiles, the center temperature, the Nusselt number, and on the analysis of the velocity field. Several velocity amplitudes (or Reynolds numbers) and several kinetic profiles are introduced and studied; these together describe the various features of the rather complex flow organization. The results are presented both as functions of the Rayleigh number Ra (with Ra up to 10^8) for fixed temperature difference (Delta) between top and bottom plates and as functions of Delta ("non-Oberbeck-Boussinesqness") for fixed Ra with Delta up to 60 K. All results are consistent with the available experimental NOB data for the center temperature Tc and the Nusselt number ratio Nu_{NOB}/Nu_{OB} (the label OB meaning that the Oberbeck-Boussinesq conditions are valid). Beyond Ra ~ 10^6 the flow consists of a large diagonal center convection roll and two smaller rolls in the upper and lower corners. In the NOB case the center convection roll is still characterized by only one velocity scale.

physics.flu-dyn↗

arXiv2008

Acceleration statistics of finite-sized particles in turbulent flow: the role of Faxen forces

The dynamics of particles in turbulence when the particle-size is larger than the dissipative scale of the carrier flow is studied. Recent experiments have highlighted signatures of particles finiteness on their statistical properties, namely a decrease of their acceleration variance, an increase of correlation times -at increasing the particles size- and an independence of the probability density function of the acceleration once normalized to their variance. These effects are not captured by point particle models. By means of a detailed comparison between numerical simulations and experimental data, we show that a more accurate model is obtained once Faxen corrections are included.

physics.flu-dyn↗

arXiv2008

Quantifying turbulence induced segregation of inertial particles

Particles with density different from that of the advecting turbulent fluid cluster due to the different response of light/heavy particles to turbulent fluctuations. This study focuses on the quantitative characterization of the segregation of dilute poly-disperse inertial particles evolving in turbulent flow, as obtained from Direct Numerical Simulation of the Navier-Stokes equations. We introduce an indicator of segregation amongst particles of different inertia and/or size, from which a length scale r_{seg} quantifying the degree of segregation of two different types of particles is deduced.

nlin.CD↗

arXiv2008

Non-Oberbeck-Boussinesq effects in turbulent thermal convection in ethane close to the critical point

As shown in earlier work (Ahlers et al., J. Fluid Mech. 569, p.409 (2006)), non-Oberbeck Boussinesq (NOB) corrections to the center temperature in turbulent Rayleigh-Benard convection in water and also in glycerol are governed by the temperature dependences of the kinematic viscosity and the thermal diffusion coefficient. If the working fluid is ethane close to the critical point the origin of non-Oberbeck-Boussinesq corrections is very different, as will be shown in the present paper. Namely, the main origin of NOB corrections then lies in the strong temperature dependence of the isobaric thermal expansion coefficient β(T). More precisely, it is the nonlinear T-dependence of the density ρ(T) in the buoyancy force which causes another type of NOB effect. We demonstrate that through a combination of experimental, numerical, and theoretical work, the latter in the framework of the extended Prandtl-Blasius boundary layer theory developed in Ahlers et al., J. Fluid Mech. 569, p.409 (2006). The latter comes to its limits, if the temperature dependence of the thermal expension coefficient β(T) is significant.

physics.flu-dyn↗

arXiv2008

Dimensionality and morphology of particle and bubble clusters in turbulent flow

We conduct numerical experiments to investigate the spatial clustering of particles and bubbles in simulations of homogeneous and isotropic turbulence. Varying the Stokes parameter and the densities, striking differences in the clustering of the particles can be observed. To quantify these visual findings we use the Kaplan--Yorke dimension. This local scaling analysis shows a dimension of approximately 1.4 for the light bubble distribution, whereas the distribution of very heavy particles shows a dimension of approximately 2.4. However, clearly separate parameter combinations yield the same dimensions. To overcome this degeneracy and to further develop the understanding of clustering, we perform a morphological (geometrical and topological) analysis of the particle distribution. For such an analysis, Minkowski functionals have been successfully employed in cosmology, in order to quantify the global geometry and topology of the large-scale distribution of galaxies. In the context of dispersed multiphase flow, these Minkowski functionals -- being morphological order parameters -- allow us to discern the filamentary structure of the light particle distribution from the wall-like distribution of heavy particles around empty interconnected tunnels.

nlin.CD↗

arXiv2008

Quantifying microbubble clustering in turbulent flow from single-point measurements

Single-point hot-wire measurements in the bulk of a turbulent channel have been performed in order to detect and quantify the phenomenon of preferential bubble accumulation. We show that statistical analysis of the bubble-probe colliding-times series can give a robust method for investigation of clustering in the bulk regions of a turbulent flow where, due to the opacity of the flow, no imaging technique can be employed. We demonstrate that micro-bubbles (radius R_0 ~ 0.1 mm) in a developed turbulent flow, where the Kolmogorov length-scale is, eta ~ R_0, display preferential concentration in small scale structures with a typical statistical signature ranging from the dissipative range, O(eta), up to the low inertial range, O(100 eta). A comparison with Eulerian-Lagrangian numeri- cal simulations is also presented to further support our proposed way to characterize clustering from temporal time series at a fixed position.

physics.flu-dyn↗

arXiv2007

Non-Oberbeck-Boussinesq effects in two-dimensional Rayleigh-Benard convection in glycerol

We numerically analyze Non-Oberbeck-Boussinesq (NOB) effects in two-dimensional Rayleigh-Benard flow in glycerol, which shows a dramatic change in the viscosity with temperature. The results are presented both as functions of the Rayleigh number (Ra) up to $10^8$ (for fixed temperature difference between the top and bottom plates) and as functions of "non-Oberbeck-Boussinesqness'' or "NOBness'' ($Δ$) up to 50 K (for fixed Ra). For this large NOBness the center temperature $T_c$ is more than 5 K larger than the arithmetic mean temperature $T_m$ between top and bottom plate and only weakly depends on Ra. To physically account for the NOB deviations of the Nusselt numbers from its Oberbeck-Boussinesq values, we apply the decomposition of $Nu_{NOB}/Nu_{OB}$ into the product of two effects, namely first the change in the sum of the top and bottom thermal BL thicknesses, and second the shift of the center temperature $T_c$ as compared to $T_m$. While for water the origin of the $Nu$ deviation is totally dominated by the second effect (cf. Ahlers et al., J. Fluid Mech. 569, pp. 409 (2006)) for glycerol the first effect is dominating, in spite of the large increase of $T_c$ as compared to $T_m$.

physics.flu-dyn↗

arXiv2007

Microbubbly drag reduction in Taylor-Couette flow in the wavy vortex regime

We investigate the effect of microbubbles on Taylor-Couette flow by means of direct numerical simulations. We employ an Eulerian-Lagrangian approach with a gas-fluid coupling based on the point-force approximation. Added mass, drag, lift, and gravity are taken into account in the modeling of the motion of the individual bubble. We find that very dilute suspensions of small non-deformable bubbles (volume void fraction below 1%, zero Weber number and bubble Reynolds number <10) induce a robust statistically steady drag reduction (up to 20%) in the so called wavy vortex flow regime (Re = 600-2500). The Reynolds number dependence of the normalized torque (the so-called Torque Reduction Ratio (TRR) which corresponds to the drag reduction) is consistent with a recent series of experimental measurements performed by Murai et al. (J. Phys. 14, 143 (2005)). Our analysis suggests that the physical mechanism for the torque reduction in this regime is due to the local axial forcing, induced by rising bubbles, that is able to break the highly dissipative Taylor wavy vortices in the system. We finally show that the lift force acting on the bubble is crucial in this process. When neglecting it, the bubbles preferentially accumulate near the inner cylinder and the bulk flow is less efficiently modified.

physics.flu-dyn↗

arXiv2004

Rayleigh and Prandtl number scaling in the bulk of Rayleigh-Benard turbulence

The Rayleigh (Ra) and Prandtl (Pr) number scaling of the Nusselt number Nu, the Reynolds number Re, the temperature fluctuations, and the kinetic and thermal dissipation rates is studied for (numerical) homogeneous Rayleigh-Benard turbulence, i.e., Rayleigh-Benard turbulence with periodic boundary conditions in all directions and a volume forcing of the temperature field by a mean gradient. This system serves as model system for the bulk of Rayleigh-Benard flow and therefore as model for the so called ``ultimate regime of thermal convection''. With respect to the Ra dependence of Nu and Re we confirm our earlier results \cite{loh03} which are consistent with the Kraichnan theory \cite{kra62} and the Grossmann-Lohse (GL) theory \cite{gro00,gro01,gro02,gro04}, which both predict $Nu \sim Ra^{1/2}$ and $Re \sim Ra^{1/2}$. However the Pr dependence within these two theories is different. Here we show that the numerical data are consistent with the GL theory $Nu \sim Pr^{1/2}$, $Re \sim Pr^{-1/2}$. For the thermal and kinetic dissipation rates we find $\eps_θ/(κΔ^{2}L^{-2}) \sim (Re Pr)^{0.87}$ and $\eps_u/(ν^3 L^{-4}) \sim Re^{2.77}$, also both consistent with the GL theory, whereas the temperature fluctuations do not depend on Ra and Pr. Finally, the dynamics of the heat transport is studied and put into the context of a recent theoretical finding by Doering et al. \cite{doe05}.

nlin.CD↗

arXiv2002

Matched filters for coalescing binaries detection on massively parallel computers

We discuss some computational problems associated to matched filtering of experimental signals from gravitational wave interferometric detectors in a parallel-processing environment. We then specialize our discussion to the use of the APEmille and apeNEXT processors for this task. Finally, we accurately estimate the performance of an APEmille system on a computational load appropriate for the LIGO and VIRGO experiments, and extrapolate our results to apeNEXT.

gr-qc↗