Valentina Lavezzo

On the role of gravity and shear on inertial particle accelerations in near-wall turbulence

Recent experiments in a turbulent boundary layer by Gerashchenko et al . ( J. Fluid Mech. , vol. 617, 2008, pp. 255–281) showed that the variance of inertial particle accelerations in the near-wall region increased with increasing particle inertia, contrary to the trend found in homogeneous and isotropic turbulence. This behaviour was attributed to the non-trivial interaction of the inertial particles with both the mean shear and gravity. To investigate this issue, we perform direct numerical simulations of channel flow with suspended inertial particles that are tracked in the Lagrangian frame of reference. Three simulations have been carried out considering (i) fluid particles, (ii) inertial particles with gravity and (iii) inertial particles without gravity. For each set of simulations, three particle response times were examined, corresponding to particle Stokes numbers (in wall units) of 0.9, 1.8 and 11.8. Mean and r.m.s. profiles of particle acceleration computed in the simulation are in qualitative (and in several cases quantitative) agreement with the experimental results, supporting the assumptions made in the simulations. Furthermore, by comparing results from simulations with and without gravity, we are able to isolate and quantify the significant effect of gravitational settling on the phenomenon.

Anisotropy in pair dispersion of inertial particles in turbulent channel flow

The rate at which two particles separate in turbulent flows is of central importance to predict the inhomogeneities of particle spatial distribution and to characterize mixing. Pair separation is analyzed for the specific case of small, inertial particles in turbulent channel flow to examine the role of mean shear and small-scale turbulent velocity fluctuations. To this aim an Eulerian-Lagrangian approach based on pseudo-spectral direct numerical simulation (DNS) of fully developed gas-solid flow at shear Reynolds number Reτ = 150 is used. Pair separation statistics have been computed for particles with different inertia (and for inertialess tracers) released from different regions of the channel. Results confirm that shear-induced effects predominate when the pair separation distance becomes comparable to the largest scale of the flow. Results also reveal the fundamental role played by particles-turbulence interaction at the small scales in triggering separation during the initial stages of pair dispersi...

On the Error Estimate in Sub-Grid Models for Particles in Turbulent Flows

The use of Large Eddy Simulation (LES) has emerged in recent years as a powerful simulation technique with the specific goal of achieving a good statistical accuracy while retaining a computational cost lower than Direct Numerical Simulations (DNS) (Sagaut, 2006). In LES, only large-scale motions are directly computed (resolved on the computational grid) while small scale motions are not computed explicitly but modeled via Sub-Grid Scale (SGS) models. Due to the complex statistical properties of turbulence, many models and methodologies have been proposed in the past. Although none of the proposed models can be considered a perfect substitute to DNS, their performance can be sometimes considered fairly accurate for what concerns the most common Eulerian turbulent flow statistics. The problem of particle transport in turbulence demands much more to LES than just reproducing low order Eulerian statistics (e.g. spectra, average profiles etc) (Salazar and Collins, 2009; Toschi and Bodenschatz, 2009). Here we propose a way to quantify the effect of (the error due to) sub-grid modeling on particle properties.

Direct Numerical Simulation and Lagrangian Particle Tracking in turbulent Rayleigh Bénard convection

Over the past years, turbulent convection has been the subject of extensive studies (see e.g. Ahlers et al., 2009; Kunnen et al., 2008; Lohse and Xia, 2010), which attempted to determine the main flow features and the contribution of different parameters to the heat transfer in various geometries, but only few of them focused on Lagrangian statistics. Lagrangian tracking can put some light on the local properties of the flow by gathering information on the temperature and velocity fields along the particle trajectory (Schumacher, 2009; van Aartrijk and Clercx 2008). This, in particular, has direct relevance for many industrial and environmental applications where the fluid heat transfer is modified by the presence and deposition of particles on the walls (e.g. nuclear power plants, petrochemical multiphase reactors, cooling systems for electronic devices, pollutant dispersion in the atmospheric boundary layer, aerosol deposition etc.).

Direct Numerical Simulation of inertial particle accelerations in near-wall turbulence: effect of gravity

Particle acceleration in turbulent flows can be considered a key issue for many environmental and industrial applications e.g. cloud formation, atmospheric transport, combustion systems etc. It is thus, important to understand the nature of the acceleration since it affects the collision rate, the dispersion of droplets or particles in the carrier fluid.