Numerical Estimation of Hull Hydrodynamic Derivatives in Ship Maneuvering Prediction

Prediction of the maneuvering characteristics of a ship at the design stage can be done by means of model tests, computational simulations or a combination of both. The model tests can be realized as a direct simulation of the standard maneuvers with the free running model, which gives the most accurate results but is also the least affordable, as it requires a very large tank or natural lake, as well as the complex equipment of the model. Alternatively, a captive model test can be used to identify the hydrodynamic characteristics of the hull, which can be used to simulate the standard maneuvers with the use of dedicated software. Two types of captive model tests are distinguished: circular motion tests (CMT) and planar motion mechanism tests (PMM). The paper presents an attempt to develop a computational method for ship maneuverability prediction in which the hydrodynamic characteristics of the hull are identified by means of computational fluid dynamics (CFD). The CFD analyses presented here directly simulate the circular motion test. The resulting hull characteristics are verified against the available literature data, and the results of the simulations are verified against the results of free running model tests. Reasonable agreement shows the large potential of the proposed method.

EXPERIMENTAL AND NUMERICAL INVESTIGATION ON MANEUVERING PERFORMANCE OF SMALL WATERPLANE AREA TWIN HULL

Free running model tests and a system-based method are employed to evaluate maneuvering performance for a Small Waterplane Area Twin Hull (SWATH) ship in this paper. A 3 degrees of freedom Maneuvering Modeling Group (MMG) model is implemented to numerically simulate the maneuvering motions in calm water. Virtual captive model tests are performed by using a Reynolds-averaged Navier-Stokes (RANS) method to acquire hydrodynamic derivatives, after a convergence study to check the numerical accuracy. The turning and zigzag maneuvers are simulated by solving the maneuvering motion model and the predicted results agree well with the experimental data. Moreover, free running model tests are carried out for three lateral separations and the influence of the lateral separations on maneuvering performance is investigated. The research results of this paper will be helpful for the maneuvering prediction of the small waterplane area twin hull ship.

Captive Model Tests for Assessing Maneuverability of a Damaged Surface Combatant with Initial Heel Angle

The present study is about the application of a four-degree-of-freedom (4DOF) maneuvering mathematical model based on Abkowitz’s model for assessing damaged ship maneuverability with initial asymmetry. A scaled model of the Office of Naval Research Tumblehome hull with a damaged compartment was used as the test model. Based on the survivability regulations for naval vessels, the damaged compartment was designed and located near the bow, such that it had an initial heel and trim. Static and dynamic captive model tests were performed on the damaged ship model to determine the maneuvering coefficients for the maneuvering mathematical model. Maneuvering simulations were carried out with the captive model test data and 4DOF maneuvering mathematical model. The advance speed in the maneuver reduced more in the damaged condition than in the intact condition, and maneuverability was severely degraded during starboard turning.

Simulating the Acceleration to Take-Off Phase of a WIG-Craft Using Results of a Constrained Experiment

The acceleration to take-off (in calm water and rough seas) is a short duration but very important motion regime of a WIG-craft. It determines the transport efficiency of a WIG-craft as a viable alternative to high speed marine vehicles or low speed aircrafts used in conveying workers to and offshore oil and gas fields. The development of a simulation model based on results from constant speed captive model tests for a WIG boat is imperative, when in the absence of appropriate experimental test rig, there is the need to investigate the attitude of the WIG-craft during its acceleration phase. Theoretical tools for investigating the characteristics of the acceleration phase of a WIG boat are uncommon and where they exist, they are almost unreliable, not been experimentally validated. Moreover, the cost associated with conducting acceleration tests is huge. The test facilities are not readily available in most maritime engineering research institutions. This study is concerned with the development of a simulation model with input from results of captive model tests to investigate the running attitude, forces and moments acting on a WIG-boat accelerating to take-off in calm water. A constrained model tests at constant speed levels were conducted for a range of model draughts and trim angles. Multivariate multiple regression method was used to develop model equations that fits the measured aero-hydro dynamic lift, drag and moment data as a function of draught, speed and trim angle. The hydrostatic and aero-hydrodynamic steady state forces and moments where combined into a state-space form which are solved in MATLAB. The state variables and the first and second derivatives of the states of the boat as well as the forces and moments acting on it are generated as output from the simulation model. Though the simulation model proved successful in predicting the attitude of the WIG-boat including crashing and excessive acceleration and power requirement during take-off, the results from the model still need to be verified with CFD analysis, a full WIG-boat trial tests or with the expensive high speed towing tank capable of carrying out acceleration runs. The method has the potential to be improved to account for the unsteady forces and moments that exists when the WIG-boat accelerates from offshore environment.

Manoeuvring Prediction of KVLCC2 with Hydrodynamic Derivatives Generated by a Virtual Captive Model Test

This paper describes the application of computational fluid dynamics rather than a towing tank test for the prediction of hydrodynamic derivatives using a RANS-based solver. Virtual captive model tests are conducted, including an oblique towing test and circular motion test for a bare model scale KVLCC2 hull, to obtain linear and nonlinear hydrodynamic derivatives in the 3rd-order MMG model. A static drift test is used in a convergence study to verify the numerical accuracy. The computed hydrodynamic forces and derivatives are compared with the available captive model test data, showing good agreement overall. Simulations of standard turning and zigzag manoeuvres are carried out with the computed hydrodynamic derivatives and are compared with available experimental data. The results show an acceptable level of prediction accuracy, indicating that the proposed method is capable of predicting manoeuvring motions.

THE EFFECT OF LEEWAY ANGLE ON THE PROPELLER PERFORMANCE

One of the aspects influencing the performance of wind assisted vessels is the effect of leeway (drift) angle on the propeller performance. It is known that due to leeway the delivered propeller power and propulsive efficiency can vary significantly from the straight sailing condition. This effect of leeway angle is studied using a combination of viscous flow calculations and captive model tests for one twin screw and three single screw vessels. It is observed that the changes in mean axial velocity and pre-swirl rotation in the wakefield due to a combination of leeway angle and propeller suction are sufficient to describe the trends observed in captive model tests. This knowledge is used in a proposed prediction method to model the changes in propeller thrust and torque due to leeway angle at the design stage. The prediction model combined with a fit of the average wake parameters for the studied vessel types is finally used the show the trends in propulsive efficiency and delivered propeller power at constant propeller rotation rate and ship speed for small leeway angles.