Maria Vittoria Salvetti

A priori tests of a new dynamic subgrid‐scale model for finite‐difference large‐eddy simulations

This work focuses on subgrid‐scale (SGS) modeling for finite‐difference large‐eddy simulations, employing filters in physical space. When a filter in physical space is used, an overlap is allowed between the unresolved and the resolved scales. For such a filter, all the three terms in the classical decomposition of the SGS stress tensor are present: the Leonard and cross‐terms, due to the overlap between scales, and the true SGS Reynolds tensor, expressing the pure effect of the small scales. A dynamic subgrid‐scale stress model is proposed, for finite‐difference large‐eddy simulation of incompressible and compressible flows in which the Leonard and cross‐parts of the SGS stress tensor are assumed to be proportional to the resolved part (the ‘‘modified Leonard term’’), which is computed explicity. The SGS Reynolds stress is modeled by the eddy‐viscosity Smagorinsky model. The two unknown parameters in this model are computed dynamically, as in Germano et al. [Phys. Fluids A 3, 1790 (1991)], but using a le...

Direct numerical simulation of particle wall transfer and deposition in upward turbulent pipe flow

Abstract Transfer and deposition of inertial particles or droplets in turbulent pipe flow are crucial processes in a number of industrial and environmental applications. In this work, we use direct numerical simulation (DNS) and Lagrangian tracking to study turbulent transfer and deposition of inertial particles in vertical upward circular pipe flow. Our objects are: (i) to quantify turbulent transfer of heavy particles to the wall and away from the wall; (ii) to examine the connection between particle transfer mechanisms and turbulence structure in the boundary layer. We use a finite difference DNS to compute the three-dimensional time dependent turbulent flow field (Reτ=337) and Lagrangian tracking of a dilute dispersion of heavy particles––flyashes in air––to simulate the dynamics of particles. Drag, lift and gravity are used in the equation of motion for the particles, which are assumed to have no influence on the flow field. Particle interaction with the wall is fully elastic. Results on preferential distribution of particles in the boundary layer, particle fluxes to and off the wall and particle deposition mechanisms are shown. Our findings confirm: (i) the specific tendency of particles to segregate in the near-wall region; (ii) the crucial role of the instantaneous realizations of the Reynolds stresses in determining particle fluxes toward and away from the wall; (iii) the relative importance of free-flight and diffusion deposition mechanisms.

Automatic evaluation of arterial diameter variation from vascular echographic images

An automatic procedure for accurate arterial diameter evaluation from B-mode images obtained by diagnostic ultrasound systems is presented. It is used for measuring brachial artery dilation following reactive hyperemia induced by forearm ischemia, which is an appropriate parameter to study endothelial function in humans. B-mode images obtained from the diagnostic ultrasound system are acquired on a personal computer as grey intensity fields (pixels). A completely automatic algorithm is then applied and the artery walls are identified by two discrete sets of points. Artery diameter is evaluated by parabolic least-square approximation. The accuracy and extensive range of applicability of the diameter evaluation procedure were demonstrated both by preliminary analytic test cases and in vivo analyses. Reproducibility of the diameter estimate was assessed by in vivo measurements. The proposed procedure permits fast and accurate analysis of large amounts of data, because it requires no action by the operator. It thus represents a valuable tool for assessment of endothelium-dependent vasodilation, especially in large, multicentric clinical trials.

A parallel multiphase flow code for the 3D simulation of explosive volcanic eruptions

A new parallel code for the simulation of the transient, 3D dispersal of volcanic particles in the atmosphere is presented. The model equations, describing the multiphase flow dynamics of gas and solid pyroclasts ejected from the volcanic vent during explosive eruptions, are solved by a finite-volume discretization scheme and a pressure-based iterative non-linear solver suited to compressible multiphase flows. The solution of the multiphase equation set is computationally so demanding that the simulation of the transient 3D dynamics of eruptive columns would not be cost-effective on a single workstation. The new code has been parallelized by adopting an ad hoc domain partitioning scheme that enforces the load balancing in the presence of a large number of topographic blocking-cells. An optimized communication layer has been built over the Message-Passing Interface. It is shown that the present code has a remarkable efficiency on several high-performance platforms and makes it possible, for the first time, to simulate fully 3D eruptive scenarios on realistic volcano topography.

Some issues concerning large-eddy simulation of inertial particle dispersion in turbulent bounded flows

The problem of accurate Eulerian-Lagrangian modeling of inertial particle dispersion in large-eddy simulation (LES) of turbulent wall-bounded flows is addressed. We run direct numerical simulation (DNS) of turbulent channel flow at shear Reynolds number Re-tau=150 and corresponding a priori and a posteriori LES on two coarser grids. For each flow field, we tracked swarms of particles with different inertia to examine the behavior of particle statistics, specifically focusing on particle preferential segregation and accumulation at the wall. Our object is to discuss the necessity of a closure model for the particle equations when using LES and we verify if the influence of the subgrid turbulence filtered by LES is an important effect on particle motion according to particle size. The results show that well-resolved LES gives particle velocity statistics in satisfactory agreement with DNS. However, independent of the grid, quantitatively inaccurate predictions are obtained for local particle preferential segregation, particularly in the near-wall region. Inaccuracies are observed for the entire range of particle size considered in this study, even when the particle response time is much larger than the flow time scales not resolved in LES. The satisfactory behavior of LES in reproducing particle velocity statistics is thus counterbalanced by the inaccurate representation of local segregation phenomena, indicating that closure models supplying the particle motion equation with an adequate rendering of the flow field might be needed. Finally, we remark that recovering the level of fluid and particle velocity fluctuations in the particle equations does not ensure a quantitative replica of the subgrid turbulence effects, thus implying that accurate subgrid closure models for particles may require information also proportional to the higher-order moments of the velocity fluctuations. (c) 2008 American Institute of Physics.

Low-dimensional modelling of a confined three-dimensional wake flow

The laminar flow past a square cylinder symmetrically placed between two parallel walls is considered. A classical vortex wake is shed from the cylinder, but three-dimensional instabilities are present and they develop in complicated flow patterns. The possibility of extracting an accurate low-order model of this flow is explored.

Mechanisms for deposition and resuspension of heavy particles in turbulent flow over wavy interfaces

It has been long recognized that turbulent flow over steep waves can produce coherent flow structures of different temporal and spatial scales. In particular, quasistreamwise vortices grow up on the upslope side of the wave and interact with geometry-dependent vortical structures, aligned spanwise and located within the recirculation bubble in the wave trough, thus creating the conditions for the development of a three-dimensional highly turbulent flow field. In this work, we have analyzed the trajectories of O(105) small dense particles (either in solid form or in the form of liquid droplets) released into a turbulent air flow over waves precisely to clarify the role of coherent vortical structures in controlling particle deposition and resuspension. The three-dimensional time-dependent flow field at Reτ=170 is calculated using large-eddy simulation, and the dynamics of individual different-sized particles is described using a Lagrangian approach. Drag, gravity, and lift are used in the equation of motio...

Large-eddy simulation of free-surface decaying turbulence with dynamic subgrid-scale models

This paper describes large-eddy simulations of decaying turbulence in an open channel, using different dynamic subgrade-scale models, viz. the dynamic model of Germano et al. [Phys. Fluids A 3, 1790 (1991)] (DSM), the dynamic mixed model in Zang et al. [Phys. Fluids A 5, 3186 (1993)] (DMM), and the dynamic two-parameter model of Salvetti and Banerjee [Phys. Fluids 7, 2831 (1995)] (DTM). These models are incorporated in a finite-volume solver of the Navier–Stokes equations. A direct numerical simulation of this flow conducted by Pan and Banerjee [Phys. Fluids 7, 1649 (1995)] showed that near the free surface turbulence has a quasi-two-dimensional behavior. Moreover, the quasi-two-dimensional region increases in thickness with the decay time, although the structure remains three-dimensional in the central regions of the flow. The results of the large-eddy simulations show that both the DMM and the DTM are able to reproduce the features of the decay process observed in the direct simulation and to handle the...

Intrinsic filtering errors of Lagrangian particle tracking in LES flow fields

Large-eddy simulation (LES) of two-phase turbulent flows exhibits quantitative differences in particle statistics if compared to direct numerical simulation (DNS) which, in the context of the present study, is considered the exact reference case. Differences are primarily due to filtering, a fundamental intrinsic feature of LES. Filtering the fluid velocity field yields approximate computation of the forces acting on particles and, in turn, trajectories that are inaccurate when compared to those of DNS. In this paper, we focus precisely on the filtering error for which we quantify a lower bound. To this aim, we use a DNS database of inertial particle dispersion in turbulent channel flow and we perform a priori tests in which the error purely due to filtering is singled out removing error accumulation effects, which would otherwise lead to progressive divergence between DNS and LES particle trajectories. By applying filters of different type and width at varying particle inertia, we characterize the statis...

Investigation of the steady engulfment regime in a three-dimensional T-mixer

The steady engulfment regime in a fully three-dimensional micro T-mixer is investigated. This regime is of significant interest for applications since it implies high mixing between the flow streams entering the device. Direct numerical simulations are first used to characterize this regime. In particular, the main vortical structures typical of the engulfment regime and their effects on mixing are investigated. Three-dimensional linear stability analysis is successively applied to the characterization of the instability leading to the engulfment regime. The critical Reynolds number and the global unstable mode are first computed for a configuration characterized by fully-developed inlet velocity conditions. The sensitivity of this instability to a generic modification of the base flow is then investigated, thanks to the computation of the mode adjoint to the direct unstable one. Finally, this kind of analysis is specialized to investigate the effect of a perturbation of the velocity distribution at the i...