Off-Design Propulsion Power Plant Investigations by Means of Free Running Manoeuvring Ship Model Test and Simulation Techniques

Twin screw vessels\u27 propulsion system experiences strong off design conditions during tight manoeuvres due to the propellers inflow asymmetry arising from the coupled yaw-drift motion. Unfortunately, simplified mathematical models based upon statistical data or ad hoc executed captive model test (PMM or CMT) do not provide such a detailed information. Indeed, free running model tests are the best mean in order to get ship\u27s trajectory and kinematics parameters data and propulsion behaviour by recording the loads (thrust and torque) on the shafts. More insight into this complex aspect is desired in order to improve and generalize the application of existing manoeuvring mathematical models for the preliminary design of unconventional propulsive configuration control system

Turning Ability Characteristics Study of a Twin Screw Vessel by CFD

The turning circle manoeuvre of a self-propelled tanker like ship model is numerically simulated through the integration of the Unsteady Reynolds Averaged Navier-Stokes (URANS) equations coupled with the equations of the motion of a rigid body. The solution is achieved by means of the unsteady RANS solver Xnavis developed at CNR-INSEAN. The ship model is in its fully appended conguration, and it is characterized by the presence of two propellers and one rudder. Each propeller is taken into account by a model based on the actuator disk concept. It is shown that, in order to accurately predict the trajectory, the side force developed by the propeller should be taken into account; several models are tested. Comparison with experimental data from free running tests is provided. The main features of the ow eld, with particular attention to the vortical structures detached for the hull is presented as well

Prediction of Manoeuvring Properties for a Tanker Model by Computational Fluid Dynamics

The turning circle manoeuvre of a self-propelled tanker like ship model is numerically simulated through the integration of the unsteady Reynolds Averaged Navier-Stokes (URANS) equations coupled with the equations of the motion of a rigid body. The solution is achieved by means of the unsteady RANS solver developed at CNR-INSEAN. The model is considered with two different stern appendages configurations (each one providing a different dynamic behaviour): twin screw with a single rudder and twin screw, twin rudder with a central skeg. Each propeller is taken into account by a model based on the actuator disk concept; anyhow, in order to correctly capture the turning manoeuvring behaviour of the model, a suitable description of the propeller performance in oblique flow operation has be considered. Comparison with experimental data from free running tests will demonstrate the feasibility of the CFD computations. The main features of the flow field, with particular attention to the vortical structures detached from the hull is presented as well

Analysis of twin screw ships\u27 asymmetric propeller behaviour by means of free running model tests

Twin screw ships may experience considerably asymmetric propeller functioning during manoeuvres. This phenomenon may result in large power fluctuations during tight manoeuvres, with increases of shaft torque up to and over 100% of the steady values in straight course and considerable unbalances; this, in its turn, may be potentially dangerous, especially in case of particularly complex propulsion plant configurations, such as those with coupled shaftlines. A joint research project supported by the Italian Navy has been set up in order to deeply investigate the phenomenon, by means of large scale model testing and related numerical simulations. In the present work, the extensive experimental campaign results on a free running model of a twin-screw ship are presented, allowing to obtain a deeper insight of the problem. In particular, tests have been carried out simulating different simplified control schemes, starting from the most common constant rate of revolution tests and including different control strategies (constant torque and power). Usual standard manoeuvres (turning circle, zigzag and spiral) have been carried out, providing results for asymmetric shaft functioning and ship manoeuvrability behaviour. Results from the present analysis allow to obtain the complete model for the time domain simulation of asymmetric shaft functioning

Investigation of asymmetrical shaft power increase during ship maneuvers by means of simulation techniques

Marine propulsion plants can experience large power fluctuations during tight maneuvers, with increases of shaft torque up to and over 100% of the steady values in straight course and considerable asymmetry between internal and external shafts during turning circle. This phenomenon (studied in Viviani et al 2007a and 2007b can be of particular interest for twin screw ships propulsion systems with coupled shaftlines, in which asymmetrical loads can represent a challenge for the whole propulsion system (e.g. unique reduction gear, shaftlines, automation). A joint research has been set up in order to deeply investigate the phenomenon, by means of large scale model testing and related numerical simulations. In the present work, preliminary simulation results with different simplified automation systems and with an automation system more similar to the real one are reported, allowing to get a better insight into this complex problem

Experimental investigation of single blade loads by captive model tests in pure oblique flow

Maneuvering motion is a critical off-design condition experienced by the propeller during realistic operations. Failures of the propulsive system, loss of efficiency and modification of the propeller side effects (propeller-hull induced pressure and noise) are the undesired consequences of these working conditions. Free running model tests, still representing the primary approach for a reliable performance assessment, requires facilities and devices that are not commonly affordable; alternatively, rectilinear towing tank can be used for maneuvering investigations by static or dynamic tests and can be a valid alternative to investigate propeller performance in offdesign. On these basis, in this paper the propeller performance in maneuvering conditions is investigated by means of oblique towing tests in case of a twin screw model equipped with a novel set-up for single blade loads measurements.In the experiments, the drift angle and the advance speed of the model varied systematically, to focus on the relation between propeller operating conditions and loads. Moreover, the averaged and periodic blade loads are compared, in terms of the equivalent drift angle, to the measurements obtained by free running model tests, in order to demonstrate the reliability of pure oblique flow tests for the preliminary quantification of the off-design loads developed by the propeller

Skin friction on a NACA0015 profile in the wake of a marine propeller

We report about the feasibility to experimentally characterize the interaction between a propeller and a hydrofoil model at incidence in its wake by using a temperature-sensitive paint. The experiments were conducted in a cavitation water tunnel at a freestream speed of 3.4 m/s and at a propeller rotational speed of 17 rps for two model angles of attack (4° and 8°). Time-resolved maps of the temperature evolution on both suction and pressure sides of the model obtained by means of the temperature-sensitive paint allow to take advantage of the direct link between friction velocity and celerity of propagation of temperature disturbances. By minimizing the dissimilarity between the observed propagation and that of the ideal wave suggested by the Taylor hypothesis, a quantitative estimation of the friction velocity vector field is gained. Detached eddy simulations are also conducted for the examined problem in the case of a model angle of attack of 4°, providing numerical data that are compared with the experimental results. The analysis of time-averaged results unveil the existence of laminar separation bubbles, stretched and folded according to the constructive / destructive coupling of the pressure fields induced by the hydrofoil incidence and by the propeller streamtube

Hydrodynamic Characterization of USV Vessels with Innovative SWATH Configuration for Coastal Monitoring and Low Environmental Impact

AbstractThe high costs associated with the use of research oceanographic vessels and the maturity of the unmanned surface vehicles (USV) makes now possible to develop systems for monitoring coastal areas based on networks of independent USVs. This type of vessels is a valid alternative to conventional vessels, which have a limited mission profile due to their high environmental impact (conventional propulsion systems based on polluting fossil fuels) inhibiting their access to protected coastal regions. Moreover, conventional vessels have high hydrodynamic resistance (limiting the autonomy) producing high levels of noise that can dramatically influence the monitoring equipment shipped: beside the environmental impact reduction, there is also the necessity of low-resistance/low-noise hydrodynamic specification. Consequently, the coastal monitoring (of also protected regions) needs unconventional vessels able to address both the issues related to the environmental impact and the hydrodynamic performance.In this framework, this work aims to characterize the hydrodynamic performance of a system based on USV units able to launch and recover autonomous vehicles of different nature (gliders, AUVs, motor-gliders, wire-guided ROVs), and able to acquire environmental data (in the column water from free-surface to the sea floor), in order to meet the requirements of civil and military applications. The cutting-edge aspects that characterize the USV studied are the hull SWATH type (Small Waterplane Area Twin Hulls) non-conventional, optimized so as to ensure a unique seakeeping and a reduced resistance, along with the propulsion system with propellers in mantle, developed to combine propulsive efficiency and low noise. In the present paper, a SWATH-shaped USV designed for monitoring of protected coastal regions is numerically studied solving the Navier-Stokes equations on the fully appended vessels with several environmental conditions. An accurate hydrodynamic characterization will presented in order to investigate its performances and eventual maneuverability issues

Phase-resolved evolution of transition and critical loci on a lifting hydrofoil in a propeller wake

This contribution proposes details about the time and phase-averaged locations of laminar-turbulent transition, separation, and reattachment on a lifting NACA 0015 hydrofoil placed in a propeller wake following a domain decoupling approach, where pieces of information at the hydrofoil’s surface, supplied by experimental investigations, are supported and justified by numerical data in the flowfield, forced by the interaction with the upstream propeller. Measurements rely on time-resolved temperature maps at the wall provided by the luminescence of a Temperature Sensitive Paint applied on the hydrofoil. The maps of standard deviation and skewness obtained as averages in the time and phase domains illustrate the spatial distribution and the temporal evolution of the transition and critical loci. Numerical data about the flow field, coming from Detached Eddy Simulation, support the connections between the critical loci dynamics on the hydrofoil’s surface and the phase evolution of the pressure gradients imposed by the propeller to the incoming flow. The close connection between the time-resolved temperature maps and the skin friction exerted by the fluid on the surface allows for the detailed description of the boundary layer around the hydrofoil. Statistics of temperature’s time series supply a neat, non-redundant, and valuable information on laminar-turbulent transition, separation, and reattachment within and out of the propeller’s stream tube. This information appears as the natural complement of the time-resolved 3D flow maps obtained with the Detached Eddy Simulation numerical approach that, relieved of the difficulties inherent in the near-wall calculus, provides a fast and accurate description of the propeller’s wake