Carlo Benocci

Finite-difference computations of high reynolds number flows using the dynamic subgrid-scale model

The dynamic subgrid-scale model is used in finite-difference computations of turbulent flow in a plane channel, for a range of Reynolds numbers (based on friction velocity and channel half-width) between 200 and 5000. Adoption of approximate wall boundary conditions allows the use of very coarse grids in all directions. The comparison of first- and second-order moments with the reference data is satisfactory, despite the mesh coarseness. Turbulent kinetic energy budgets also compare well with DNS data. Near the wall, the dynamic formulation gives improved results over the Smagorinsky model, as observed in previous simulation. In the core of the flow where, at high Reynolds number, the turbulent eddies obey inertial-range dynamics, the Smagorinsky and dynamic models give similar results. The behavior of the model, its implementation when approximate wall boundary conditions are used, and the effect of numerical resolution are discussed.

Approximate Wall Boundary Conditions for Large Eddy Simulations

A set of new, approximate solid wall boundary conditions for large eddy simulation (LES) is proposed, which models the wall shear stress in function of the resolved flowfield outside the viscous sublayer. The condition is derived assuming that the unsteady turbulent boundary layer equations hold for the region between the wall and the first grid point. Applied in the test case of the plane periodic channel, the new formulation shows improved results compared to existing models.

Parallel Multi-Domain Large-Eddy Simulation of the Flow Over a Backward-Facing Step at RE=5100

The present article describes the recent developments carried out at the von Karman Institute concerning the use of domain decomposition techniques as applied to the space filtered incompressible Navier-Stokes equations for the simulation of turbulent flows in complex geometries. Issues related tothe numerical method, based on a grid staggered velocity pressure representations, are addressed, with special emphasis on thesolution strategy of the elliptic kernel. Numerical results are presented for the flow over abackward facing step for which extensive and accurate DNS data are available in literature.

Principal Component Analysis on a LES of a Squared Ribbed Channel

The present paper reports on the application of Principal Component Analysis (PCA) on the flow and thermal fields generated by the large-eddy simulation (LES) of a square ribbed duct heated by a constant heat flux applied over the bottom surface of the duct. PCA allows to understand the complexity of the resulting turbulent heat transfer process, identifying the flow and thermal quantities which are most relevant to the process. Different algorithms have been employed to perform this analysis, showing high correlation between turbulent coherent structures, identified by Q − criterion, and the heat transfer quantified by the non-dimensional magnitude Enhancement Factor (EF), both identified as Principal Variables (PV) of the process.

Use of Immersed Boundary Technique in a Cartesian LES solver to study wake flows

In the present contribution a pre-existing Large Eddy Simulation code (MiOma) based on finite differences and Cartesian structured grids has been extended through the use of the Immersed Boundary Technique. The latter allows the simulation of complex geometries far beyond the ones originally allowed by the multi-domain technique present in MiOma and opens the way to study the wake behind complex bluff bodies, using a starting point the circular cylinder at Re = 300.

Conditional Averaging of the Fully Developed Stationary Ribbed Duct Flow Using Q Criteria

The fully developed flow in square section ribbed duct was simulated (see Figure 1 (left)). The computation can be characterized by the Reynolds number of 40000 defined by the bulk velocity (Ub) and the hydraulic diameter (Dh). The already experimentally investigated by [1] configuration of high blockage (rib to hydraulic diameter ratio h/Dh = 0.3) was computed. The pitch distance compared to the rib height (p/h = 10) was used presenting a practical situation used in internal cooling channels of turbine blades. In the computation constant mass flow was enforced in the streamwise direction with periodic condition approximating the fully developed flow.

Large-Eddy Simulation of Turbulent Flows over a Cylinder of Square Cross-Section

LES is applied to the study of incompressible flow around a square cylinder in a unperturbed free stream at Reynolds number 22.000. The importance and effects of the spatial discretization and of the subgrid model are highlighted; results are compared with existing experimental data and good agreement is found.

An Efficient Iterative Solution Method for the Chebyshev Collocation of Advection-Dominated Transport Problems

A new Chebyshev collocation algorithm is proposed for the iterative solution of advection-diffusion problems. The main features of the method lie in the original way in which a finite-difference preconditioner is built and in the fact that the solution is collocated on a set of nodes matching the standard Gauss--Lobatto--Chebyshev set only in the case of pure diffusion problems. The key point of the algorithm is the capability of the preconditioner to represent the high-frequency modes when dealing with advection-dominated problems. The basic idea is developed for a one-dimensional case and is extended to two-dimensional problems. A series of numerical experiments is carried out to demonstrate the efficiency of the algorithm. The proposed algorithm can also be used in the context of the incompressible Navier--Stokes equations.

Large Eddy Simulation of a Planar Co-Flowing Jet

Turbulent jet flows are of considerable interest both for fundamental research and practical applications. However, in spite of extensive previous research, major deficiencies in the knowledge and understanding of this flow field still exist. Time-accurate computations, t hrough direct or large eddy simulation (LES), have become increasingly viable as a means of investigating turbulent flow problems. As a preliminary step towards the application of these techniques to jet flows, the LES of a co-flowing jet is proposed at Re = 30000 (based on the uniform jet velocity and the nozzle width h) and a ratio of free-stream velocity U 1 versus jet velocity U j of 0.16, reproducing the conditions of the experimental study by Bradbury 1965.