Giuliano De Stefano | Università della Campania Luigi Vanvitelli (original) (raw)

Papers by Giuliano De Stefano

Lecture notes in computer science, 2024

Since the inception of Computational Fluid Dynamics, turbulence model-ing and numerical methods e... more Since the inception of Computational Fluid Dynamics, turbulence model-ing and numerical methods evolved as two separate fields of research with the perception that once a turbulence model is developed, any suitable computa-tional approach can be used for the numerical simulations of the model. Over the last decade, our group has pursued research with cardinally different philos-ophy in its belief that in order to increase the computational efficiency of turbu-lent flow simulations and substantially improve the accuracy of predictions of flow characteristics, both the numerics and physics-based modeling need to be tightly integrated to ensure better capturing of the flow physics on a near opti-mal adaptive computational grid, ultimately leading to substantial reduction in the computational cost, while resolving dynamically dominant flow structures. Turbulence is difficult to approximate mathematically, and to calculate numerically, because it is active over a large and continuous range of length scales (e.g. from less than a millimeter to hundreds of kilometers in the atmos-phere). The range of active scales increases with Reynolds number (like

Computation, Apr 3, 2024

This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY

Lecture Notes in Computer Science, 2023

Physics of Fluids, Mar 1, 2023

This work investigates the effect of different thermal wall boundary conditions on the wavelet-ba... more This work investigates the effect of different thermal wall boundary conditions on the wavelet-based adaptive large-eddy simulation of supersonic turbulent channel flow. The compressible flow governing equations are expressed in terms of wavelet-based Favrefiltered variables, and are supplied with the anisotropic minimum dissipation closure model. Various computations are performed, where the resolved temperature field is constrained by either Dirichlet (isothermal) or Neumann (adiabatic) boundary conditions at the walls. The turbulence diagnostics include mean flow features and turbulent fluctuations statistics. The successful comparison with reference direct numerical simulations demonstrates the validity and the efficiency of the wavelet-based adaptive approach for wall-bounded turbulent compressible flow, regardless of the thermal boundary conditions that are imposed.

ERCOFTAC series, 2011

Stochastic Coherent Adaptive Large-Eddy Simulation is a novel approach to the numerical simulatio... more Stochastic Coherent Adaptive Large-Eddy Simulation is a novel approach to the numerical simulation of turbulence, based upon the wavelet thresholding filter, where the coherent energetic eddies are solved while modelling the influence of the less energetic background flow. In this study, in order to examine the quality and reliability of the method, additional explicit wavelet filtering is introduced by considering two different filtering levels: the physical level, which controls the turbulence model, and the numerical level that is responsible for the accuracy of the numerical simulations. The theoretical basis for explicit filtering and consistent dynamic modelling is given, and some preliminary numerical experiments are presented.

Bulletin of the American Physical Society, Nov 18, 2007

Direct and Large-Eddy Simulation IX, 2015

The direct numerical simulation (DNS) of unsteady flow past a square-cylinder has a very high com... more The direct numerical simulation (DNS) of unsteady flow past a square-cylinder has a very high computational cost, even at moderately.

Bulletin of the American Physical Society, Nov 20, 2006

This is the first of two talks, which describe ongoing localized SGS model development for the St... more This is the first of two talks, which describe ongoing localized SGS model development for the Stochastic Coherent Adaptive Large Eddy Simulation (SCALES) methodology. The SCALES approach has the potential for significant improvement over regular grid LES methods with its ability to resolve and dynamically track the most energetic coherent structures in a turbulent flow through dynamic grid adaptation based on wavelet threshold filtering. In this talk we propose a new local Lagrangian pathline/tube dynamic model, as an extension of the original formulation by Meneveau et al. (J. Fluid Mech., 1996). The new procedure involves the definition of the following filtered averages over the trajectory of a fluid particle:

Ocean Modelling, 2007

A large-eddy simulation method based upon the finite volume approach is developed and evaluated f... more A large-eddy simulation method based upon the finite volume approach is developed and evaluated for buoyancy driven turbulence in the absence of rotational effects. The effect of the sub-grid scale motions is modelled by using the dynamic scaling formulation, without introducing any ad hoc assumption for the vertical stratification. This way, the eddy coefficients for momentum and temperature equations are independently computed, while avoiding to introduce unjustified buoyancy production terms in the constitutive equations. As a result, the computational cost is considerably reduced with respect to the classical stratification formulation. The method is presented in detail, by stressing the particular features of the finite volume large-eddy simulation approach. The resulting numerical code is validated against a direct numerical simulation. Numerical experiments are conducted by simulating the thermal buoyancy driven turbulence near the water surface generated in a finite-depth stably stratified horizontal layer with an isothermal bottom surface. Diagnostics include time evolution of kinetic and thermal energy as well as energy spectral distribution. Interesting results are obtained by making a comparison with a reference spectral solution. The method is further validated for a turbulent ocean test case, that is the deepening of the mixed layer in a stable stratified fluid.

Physics of Fluids, 2008

Stochastic coherent adaptive large eddy simulation ͑SCALES͒ is an extension of the large eddy sim... more Stochastic coherent adaptive large eddy simulation ͑SCALES͒ is an extension of the large eddy simulation approach in which a wavelet filter-based dynamic grid adaptation strategy is employed to solve for the most "energetic" coherent structures in a turbulent field while modeling the effect of the less energetic background flow. In order to take full advantage of the ability of the method in simulating complex flows, the use of localized subgrid-scale models is required. In this paper, new local dynamic one-equation subgrid-scale models based on both eddy-viscosity and non-eddy-viscosity assumptions are proposed for SCALES. The models involve the definition of an additional field variable that represents the kinetic energy associated with the unresolved motions. This way, the energy transfer between resolved and residual flow structures is explicitly taken into account by the modeling procedure without an equilibrium assumption, as in the classical Smagorinsky approach. The wavelet-filtered incompressible Navier-Stokes equations for the velocity field, along with the additional evolution equation for the subgrid-scale kinetic energy variable, are numerically solved by means of the dynamically adaptive wavelet collocation solver. The proposed models are tested for freely decaying homogeneous turbulence at Re = 72. It is shown that the SCALES results, obtained with less than 0.5% of the total nonadaptive computational nodes, closely match reference data from direct numerical simulation. In contrast to classical large eddy simulation, where the energetic small scales are poorly simulated, the agreement holds not only in terms of global statistical quantities but also in terms of spectral distribution of energy and, more importantly, enstrophy all the way down to the dissipative scales.

Stochastic Coherent Adaptive Large Eddy Simulation (SCALES) is an extension of Large Eddy Simulat... more Stochastic Coherent Adaptive Large Eddy Simulation (SCALES) is an extension of Large Eddy Simulation that uses a wavelet filter-based dynamic grid adaptation strategy to solve for the most energetic coherent structures in a turbulent flow field, while modelling the effect of the less energetic ones. A localized dynamic subgrid scale model is needed to fully exploit the ability of the method to track coherent structures. In this paper, new local Lagrangian models based on a modified Germano dynamic procedure, redefined in terms of wavelet thresholding filters, are proposed. These models extend the original path-line formulation of Meneveau et al [J. Fluid Mech., 319, 1996] in two ways: as Lagrangian path-line diffusive and Lagrangian path-tube averaging procedures. The proposed models are tested for freely decaying homogeneous turbulence with initial Re λ = 72. It is shown that the SCALES results, obtained with fewer than 0.4% of the total non-adaptive nodes required for a DNS with the same wavelet solver, closely match reference DNS data. In contrast to classical LES, this agreement holds not only for large scale global statistical quantities, but also for energy and, more importantly, enstrophy spectra up to the dissipative wavenumber range.

Journal of Fluid Mechanics, 2010

The stochastic coherent adaptive large eddy simulation (SCALES) methodology is a novel approach t... more The stochastic coherent adaptive large eddy simulation (SCALES) methodology is a novel approach to the numerical simulation of turbulence, where a dynamic grid adaptation strategy based on wavelet threshold filtering is utilized to solve for the most ‘energetic’ eddies. The effect of the less energetic unresolved motions is simulated by a model. Previous studies have demonstrated excellent predictive properties of the SCALES approach for decaying homogeneous turbulence. In this paper the applicability of the method is further explored for statistically steady turbulent flows by considering linearly forced homogeneous turbulence at moderate Reynolds number. A local dynamic subgrid-scale eddy viscosity model based on the definition of the kinetic energy associated with the unresolved motions is used as closure model. The governing equations for the wavelet filtered velocity field, along with the additional evolution equation for the subgrid-scale kinetic energy, are numerically solved...

A perfect modeling framework for the systematic study of the effect of filter shape on the resolv... more A perfect modeling framework for the systematic study of the effect of filter shape on the resolved scales of motion in large eddy simulation is developed. The effects of the explicit and implicit filtering approaches in large eddy simulation are considered. A simple model for smooth filtering is proposed and the related effects are analyzed. The proposed approach provides an effective research tool for assessing the behavior of sub-grid scale models in a dynamic fashion. The performances of various classical models are examined by using the perfect modeling formalism for simulating the large and/or the small residual scales effect. Numerical experiments are performed for decaying isotropic turbulence. The consistency of the sub-grid scale models with the effective composite filter employed in real simulations is discussed. The necessity of using mixed models when solving doubly-filtered Navier-Stokes equations is verified. It is found that time evolution of large scale velocity field is more sensitive to sub-grid large scale models like Bardina model, while the grid-filtered sub-filter scale model is necessary to ensure the proper energy dissipation.

Physics of Fluids, 2002

The large eddy simulation ͑LES͒ equations of turbulent flows are formally derived by applying a l... more The large eddy simulation ͑LES͒ equations of turbulent flows are formally derived by applying a low-pass filter to the Navier-Stokes equations. As a result the subgrid-scale ͑SGS͒ stress tensor strongly depends on the assumed filter shape, which causes a SGS model to be filter dependent. In particular, depending on the choice of the filter the corresponding SGS model should satisfy very different requirements in terms of large scale dynamics and kinetic energy budget. This paper is an attempt to systematically study the effect of the filter shape on the subgrid scale model and its subsequent effect on LES. For the sake of simplicity, we consider numerical simulation of a one-dimensional homogeneous flow, governed by the viscous Burgers equation. Large eddy simulations of the solution of the Burgers problem are performed using subgrid scale models obtained by filtering data from direct numerical simulations. Diagnostics include temporal evolution of energy and dissipation as well as energy spectra. It is demonstrated both theoretically and numerically that the assumed filter shape can have a significant effect on LES in terms of spectral content and physical interpretation of the solution. The results are generalized for LES of three-dimensional turbulent flows and specific recommendations for the use of filters and corresponding SGS models are made.

The Stochastic Coherent Adaptive Large Eddy Simulation (SCALES) is a novel wavelet-based approach... more The Stochastic Coherent Adaptive Large Eddy Simulation (SCALES) is a novel wavelet-based approach that resolves energy containing turbulent motions using wavelet multiresolution decomposition and self-adaptivity. The extraction of the most energetic structures is achieved using wavelet thresholding filter with a priori prescribed threshold level. This strategy, although successful, has a major drawback: the thresholding criterion is global and does not fully utilize the spatial/temporal intermittency of the turbulent flow. In the current numerical effort, for the first time (to the best of our knowledge), the concept of physics-based spatially variable thresholding in the context of wavelet-based numerical techniques for solving PDEs is introduced. The procedure consists of tracking the wavelet thresholding-factor within a Lagrangian frame by exploiting a Lagrangian Path-Line Diffusive Averaging approach that uses linear averaging along characteristics. The results for incompressibl...

International Journal for Numerical Methods in Fluids, 2001

A general methodology is presented in order to obtain a hierarchy of high-order filter functions,... more A general methodology is presented in order to obtain a hierarchy of high-order filter functions, starting from the standard top-hat filter, naturally linked to control volumes flow simulations. The goal is to have a new filtered variable better represented in its high resolved wavenumber components by using a suitable deconvolution. The proposed formulation is applied to the integral momentum equation, that is the evolution equation for the top-hat filtered variable, by performing a spatial reconstruction based on the approximate inversion of the averaging operator. A theoretical analysis for the Burgers' model equation is presented, demonstrating that the local de-averaging is an effective tool to obtain a higher-order accuracy. It is also shown that the subgrid-scale term, to be modeled in the deconvolved balance equation, has a smaller absolute importance in the resolved wavenumber range for increasing deconvolution order. A numerical analysis of the procedure is presented, based on high-order upwind and central fluxes reconstruction, leading to congruent control volume schemes. Finally, the features of the present high-order conservative formulation are tested in the numerical simulation of a sample turbulent flow: the flow behind a backward-facing step.

Applied Sciences

A hybrid VOF–Lagrangian method for simulating the aerodynamic breakup of liquid droplets induced ... more A hybrid VOF–Lagrangian method for simulating the aerodynamic breakup of liquid droplets induced by a traveling shock wave is proposed and tested. The droplet deformation and fragmentation, together with the subsequent mist development, are predicted by using a fully three-dimensional computational fluid dynamics model following the unsteady Reynolds-averaged Navier–Stokes approach. The main characteristics of the aerobreakup process under the shear-induced entrainment regime are effectively reproduced by employing the scale-adaptive simulation method for unsteady turbulent flows. The hybrid two-phase method combines the volume-of-fluid technique for tracking the transient gas–liquid interface on the finite volume grid and the discrete phase model for following the dynamics of the smallest liquid fragments. The proposed computational approach for fluids engineering applications is demonstrated by making a comparison with reference experiments and high-fidelity numerical simulations,...