Andrea Di Mascio

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).

RANS simulations of a junction flow

The ERCOFTAC junction flow is numerically simulated with both a structured and an unstructured RANS solver for incompressible flows. The structured code adopts a finite volume, cell-centered formulation while the unstructured code uses residual distribution schemes and a vertex centered storage of the unknowns. Two differential eddy viscosity models, based on local quantities, are considered in the computations: the one-equation Spalart–Allmaras model and the two equations model proposed by Lam and Bremhorst. The grid dependence of the numerical solutions is evaluated by means of a convergence analysis based on computation of the GCI and a code-to-code comparison. The numerical results provided by both turbulence models are compared with the experimental measurements of the pressure and velocity fields.

Analysis of free-surface flows through energy considerations: Single-phase versus two-phase modeling.

The study of energetic free-surface flows is challenging because of the large range of interface scales involved due to multiple fragmentations and reconnections of the air-water interface with the formation of drops and bubbles. Because of their complexity the investigation of such phenomena through numerical simulation largely increased during recent years. Actually, in the last decades different numerical models have been developed to study these flows, especially in the context of particle methods. In the latter a single-phase approximation is usually adopted to reduce the computational costs and the model complexity. While it is well known that the role of air largely affects the local flow evolution, it is still not clear whether this single-phase approximation is able to predict global flow features like the evolution of the global mechanical energy dissipation. The present work is dedicated to this topic through the study of a selected problem simulated with both single-phase and two-phase models. It is shown that, interestingly, even though flow evolutions are different, energy evolutions can be similar when including or not the presence of air. This is remarkable since, in the problem considered, with the two-phase model about half of the energy is lost in the air phase while in the one-phase model the energy is mainly dissipated by cavity collapses.

Marine propellers performance and flow-field prediction by a free-wake panel method

A Boundary Element Method (BEM) hydrodynamics combined with a flow-alignment technique to evaluate blades shed vorticity is presented and applied to a marine propeller in open water. Potentialities and drawbacks of this approach in capturing propeller performance, slipstream velocities, blade pressure distribution and pressure disturbance in the flow-field are highlighted by comparisons with available experiments and RANSE results. In particular, correlations between the shape of the convected vortex-sheet and the accuracy of BEM results are discussed throughout the paper. To this aim, the analysis of propeller thrust and torque is the starting point towards a detailed discussion on the capability of a 3-D free-wake BEM hydrodynamic approach to describe the local features of the flow-field behind the propeller disk, in view of applications to propulsive configurations where the shed wake plays a dominant role.

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

The role of mesh generation, adaptation, and refinement on the computation of flows featuring strong shocks

Within a continuum framework, flows featuring shock waves can be modelled by means of either shock capturing or shock fitting. Shock-capturing codes are algorithmically simple, but are plagued by a number of numerical troubles, particularly evident when shocks are strong and the grids unstructured. On the other hand, shock-fitting algorithms on structured grids allow to accurately compute solutions on coarse meshes, but tend to be algorithmically complex. We show how recent advances in computational mesh generation allow to relieve some of the difficulties encountered by shock capturing and contribute towards making shock fitting on unstructured meshes a versatile technique.

Numerical Simulation of 3D Unsteady Flowfield in Aft-Finocyl Solid Rocket Motor

The flowfield inside the second solid stage of the European launch vehicle VEGA is simulated with a full 3D unsteady flow solver in order to characterize the unsteadinesses, aero-acoustics and dynamics loads resulting from the growth of complex vorticity patterns in the internal flow of the aft-finocyl solid rocket motor. The analysis considers different configurations during the firing of the solid stage: the first one corresponds to an advanced stage of combustion, when the aft-part of the propellant grain is almost completely burnt out and the aft-end cavity is almost empty of propellant; the second one corresponds to an early stage of combustion, with a large burning surface and a small internal volume available for the flow. The first configuration is computed for different motor geometrical conditions: a symmetric case, a case with an imposed small angle of nozzle gimbaling and a case with an imposed propellant grain offset with respect to motor assembly. The second configuration is analyzed considering a small angle of nozzle gimbaling. The rotation of the gimbaled nozzle, as well as the propellant grain offset are imposed in such a way to remove any symmetry in the motor geometry. The motor configuration advanced into the firing shows the presence of low-level limit cycle oscillations of the flowfield at high frequency, resulting into the generation of lateral force oscillations of the order of one percentage of the generated thrust. This peculiar behavior is not altered by the perturbation of the motor geometry due to the imposed nozzle gimbaling; whereas it is slightly modified in terms of lower characteristic frequencies and lower amplitude of the oscillations, for the case with the imposed propellant grain offset. The presence of such flowfield fluctuations is caused by the onset of unstable sound generating vortical structures along the motor axis, that produce pressure fluctuations and that are coupled with acoustic waves. The motor configuration early into the firing, instead, does not show any significant of flow instability.

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