Stefano Zaghi

Experimental and numerical investigations on fast catamarans interference effects

Experimental and numerical analysis of the interference effect for a fast catamaran is carried out. This work presents the status of an ongoing NICOP project, the focus is on the effect of the separation distance between the demihull on the performances as well as on the interference. To this aim, experiments and numerical simulations are performed for five different separation lengths (and for the monohull configuration) and for a wide range of Froude number (from 0.2 to 0.8).

Application of dynamic overlapping grids to the simulation of the flow around a fully-appended submarine

The hydrodynamic characterization of control appendages for ship hulls is of paramount importance for the assessment of maneuverability characteristics. However, the accurate numerical simulation of turbulent flow around a fully appended maneuvering vessel is a challenging task, because of the geometrical complexity of the appendages and of the complications connected to their movement during the computation. In addition, the accurate description of the flow within the boundary layer is important in order to estimate correctly the forces acting on each portion of the hull.To this aim, the use of overlapping multi-block body fitted grids can be very useful to obtain both a proper description of each particular region in the computational domain and an accurate prediction of the boundary layer, retaining, at the same time, a good mesh quality. Moreover, block-structured grids with partial overlapping can be fruitfully exploited to control grid spacing close to solid walls, without propagation of undesired clustering of grid cells in the interior of the domain. This approach proved to be also very useful in reducing grid generation time.In the present paper, some details of the flow simulation around a fully appended submarine is reported, with emphasis on the issues related to the complexities of the geometry to be used in the simulations and to the need to move the appendages in order to change the configuration of the various appendages.

Enabling hydrodynamics solver for efficient parallel simulations

In this paper we present the parallel solver χnavis, a general purpose solver for Computational Fluid Dynamics (CFD). The solver is based on the finite volume discretization of the unsteady incompressible Navier-Stokes equations; main features include a level set approach to handle free surface flows and a dynamical overlapping grids approach, which allows to deal with bodies in relative motion. The baseline code features a hybrid MPI/OpenMP parallelization, proven to scale when running on order of hundreds of cores (i.e. Tier-1 platforms). This paper deals with latest developments aimed to extend the capabilities of the χnavis software to exploit modern parallel architectures. Scalability properties will be demonstrated for different cases. As example of application, the computation of the flow fields around a submarine in prescribed oscillatory motion and a surface flow around a catamaran in steady drift advancement are presented.

3D Simulations of Pre-Ignition Transient of P80 SRM

The first three stages of the new small European launcher Vega are solid rocket motors characterized by a star-shaped finocyl region (close to the propulsive nozzle). Zefiro 16 (a prototype developed as a precursor of the three stages of Vega) static fire tests have evidenced pressure oscillations during ignition startup. This phenomenon has been analyzed by means of quasi-1D and 3D numerical models. The numerical results of both models have very good agreement with experimental data and they have allowed a complete explanation of pressure oscillation phenomenology. These studies have suggested that the use helium instead nitrogen as pressurizing gas could eliminate pressure oscillations. Instead the subsequent static fire tests of P80, Zefiro 23 and Zefiro 9 have been done with helium showing no pressure oscillations during ignition transient as numerical simulations have predicted. On the contrary a second firing of P80, made with nitrogen as pressurizing gas, has exhibited pressure oscillations [ref.]. Therefore we have done numerical simulations of P80 ignition transient using nitrogen: because of their differences the comparison between Zefiro 16 and P80 is interesting.

Comparison Between Different Pressurant Gases for Ignition Transient of P80 SRM

In the framework of the qualifying process of the new small European launcher Vega several experimental tests and numerical studies were done. Among them the qualification of P80 is particularly interesting: P80 is the only motor of the Zefiro-like family that was tested with different pressurizing gas both with nitrogen and helium. The static firing tests of P80 have shown that the pressure oscillations during ignition transient are more intense using nitrogen rather than helium. Several previous numerical studies 8 have pointed out that the pressurizing gas is fundamental for the behavior of the interaction between the hot igniter jets and combustion chamber geometry variations: it can favor or limit the generation of pressure oscillations depending on its compressibility. The present paper reports the numerical analysis of the P80 ignition transient both for nitrogen and helium and a comparison with the experimental date provided by the P80 static firing tests.

Application of WENO-Positivity-Preserving Schemes to Highly Under-Expanded Jets

The starting transient of highly under-expanded supersonic jets is studied by means of very high resolution weighted essentially non oscillatory finite volume schemes, coupled with a positivity-preserving scheme in order to ensure positivity of pressure and density for high compression/expansion ratio. Numerical behaviour of the schemes is investigated in terms of grid resolution, formal accuracy and different approximated Riemann solvers. The transient flow field is also discussed.