Riccardo Broglia

A Second Order Godunov-Type Scheme for Naval Hydrodynamics

A second order Godunov-type scheme for the simulation of free surface turbulent incompressible flows is presented. The scheme is applied to the RANSE written in pseudo-compressible formulation, whose asymptotic solution is computed by means of a Runge-Kutta time integration coupled with a multigrid algorithm. Examples of application of this scheme to the computation of the flow in a driven-cavity and past a surface-piercing hull are reported and, for the latter, compared with towing tank experiments carried out at INSEAN. Convergence properties when halving the grid size are also shown.

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.

Vortex Suppression Efficiency of Discontinuous Helicoidal Fins

Vortex-Induced Vibration (VIV) is one of the most demanding areas in the offshore industry, and detailed investigation of the fluid-structure interaction is becoming fundamental for designing new structures able to reduce VIV phenomenon. To carry on such analysis, and get reliable results in term of global coefficients, the correct modelling of turbulence, boundary layer, and separated flows is required. Nonetheless, the more accurate is the simulation, the more costly is the computation. Unsteady RANS simulations provide a good trade-off between numerical accuracy and computational time. This paper presents the analysis of the flow past a cylinder with several three-dimensional helical fins at high Reynolds number. Flow field, vortical structures, and response frequency patterns are analysed. Spectral analysis of data is performed to identify carrier frequencies, deemed to be critical due to the induced vibration of the whole structure. Finally, helical strakes efficiency in reducing the riser vibrations is also addressed, through direct consideration on the carrier shedding frequency.Copyright

Analysis of the Flow Around a Manoeuvring VLCC

This work describes the numerical simulation of a turning circle manoeuvre performed by the MOERI KVLCC2 induced by the rotation of the rudder. To this purpose, the Navier-Stokes equations are integrated, the hydrodynamical forces acting on the hull are computed and the hull is moved at each time step according to the rigid body equations. Because of the scarceness of experimental results for this kind of simulations, the validation of the proposed method is postponed to the oral presentation when the data from the SIMMAN 2008 Workshop (http://www.simman2008.dk/ ) will be available.Copyright

Numerical Simulation of the Flow around an Array of Free-Surface Piercing Cylinders in Waves

Abstract The flow around four free-surface piercing cylinders in waves is simulated by an Euler solver. Three different angles of the incoming wave front are considered in order to investigate the variation of the resulting global and local loads on the four bodies. The discretization of the fluid domain has been made easy and efficient by the use of a chimera method for the generation of the computational mesh. The free-surface has been simulated by a single-phase level set approach.

Numerical Investigation of the Unsteady Flow at High Reynolds Number Over a Marine Riser With Helical Strakes

Prediction of Vortex-Induced Vibrations (VIV) is one of the main topics in the design of deepwater risers. The understanding and modelling of the complex fluid-structure interaction requires advanced analysis techniques coupling, in a correct manner, both structural and fluid dynamics aspects. This study aims to develop, optimise and calibrate a numerical code to provide reliable results within a reasonable analysis timeframe and without, or very limited, need of experimental verification. For this purpose, the unsteady Reynolds Average Navier-Stokes (RANS) code χnavis is applied to solve a typical riser VIV problem and compute the three-dimensional riser-fluid dynamics interaction. During a preliminary analysis phase, the two-dimensional (2-D) flow past (i) a bare circular cylinder and (ii) a straked riser at high Reynolds numbers is simulated (different incidences flow/strake vanes are analysed). Numerical results are validated and calibrated against published test data. The core analysis phase is then focused on the numerical investigation of the unsteady flow over a three-dimensional (3-D) helical strake. In this phase, the three-dimensional flow field, turbulent structures and response frequency patterns are analysed. Spectral analysis of data is performed to identify carrier frequencies deemed to be critical due to the induced vibration of the whole structure, and helical strakes efficiency in reducing the riser vibrations is also addressed. Finally, comparison between numerical and experimental results shows that the complexity of a three-dimensional model is indeed compensated by a significantly improved accuracy of the obtained results.Copyright

A Residual Theorem Approach Applied to Stokes’ Problems with Generally Periodic Boundary Conditions including a Pressure Gradient Term

The differential problem given by a parabolic equation describing the purely viscous flow generated by a constant or an oscillating motion of a boundary is the well-known Stokes’ problem. The one-dimensional equation is generally solved for unbounded or bounded domains; for the latter, either free slip (i.e., zero normal gradient) or no-slip (i.e., zero velocity) conditions are enforced on one boundary. Generally, the analytical strategy to solve these problems is based on finding the solutions of the Laplace-transformed (in time) equation and on inverting these solutions. In the present paper this problem is solved by making use of the residuals theorem; as it will be shown, this strategy allows achieving the solutions of First and Second Stokes’ problems in both infinite and finite depth. The extension to generally periodic boundaries with the presence of a periodic pressure gradient is also presented. This approach allows getting closed form solutions in the time domain in a rather fast and simple way. An ad hoc numerical algorithm, based on a finite difference approximation of the differential equation, has been developed to check the correctness of the analytical solutions.

Analysis of the Roll Decay Motion for a Patrol Boat by URANS Simulations

The simulations of the flow around a vessel of the Italian Navy in free roll decay have been carried out by the numerical solution of the Reynolds Averaged Navier-Stokes equations. The focus is on the analysis of the roll motion coefficients (damping and period of oscillations) at different Froude and Reynolds numbers. To this aim, numerical simulations were carried out at three different speeds, with corresponding Froude numbers equal to 0.160, 0.227 and 0.337, and Reynolds numbers ranging from 4.073 106 to 1.300 107 at model scale. Computations were carried out by means of an in-house unsteady RANS solver; the scheme is based on a finite volume discretization, and it is globally second order accurate. The free surface is handled by means of a suitable single phase level set algorithm; moreover, Chimera overlapping grid capabilities have been implemented in the code, which has been also efficiently parallelized. An analysis of the roll motion, longitudinal and lateral forces and roll moment is carried out for the different speeds considered. A preliminarily grid convergence analysis is also performed.Copyright