Oblique Towing Test Research Articles

Computational Study on Influence Factors and Vortical Structures in Static Drift Tests

This paper conducted a computational study on the KCS benchmark model at static drift conditions. At the first instance, the roles played by the grid size, turbulence model, and time step are qualitatively and quantitatively analyzed with the orthogonal experimental method (OEM). After the verification of simulated results compared with experimental data in a Static Oblique Towing Test (OTT), hydrodynamic performance is obtained with the employment of the SST κ-ω turbulence model. The grid size is set as 0.07 m while the time step as 0.01 s. The characteristics of the wake field are illustrated in different forms, such as contours of the free surface, distribution of pressure and hydrodynamic forces, variation of turbulent kinetic energy (TKE), and so on. For a deep insight into the physical mechanisms of the asymmetrical flow field, the Detached Eddy Simulation (DES) method is also utilized to capture vortical structures occurring around the hull, in comparison with results obtained through the Reynolds Averaged Navier Stokes (RANS) model. With the aim of a hydrodynamic derivative estimation or detailed flow characteristics analysis, corresponding selections of the computational method are disparate.

Numerical Assessment of the Hydrodynamic Loads for a Manta Type Unmanned Underwater Vehicle: Oblique Towing Test

Numerical Assessment of the Hydrodynamic Loads for a Manta Type Unmanned Underwater Vehicle: Oblique Towing Test

A Method for Forcing a Number of Motions or Rotations in 6 Degrees of Freedom Ship Simulators

In this paper, a method is introduced which enables the forcing of any degrees of freedom in 6 DoFs ship simulator. It is based on the introduction of an extra force in the equation of motion of the ship and on the forcing of the second derivatives of the forced degrees of freedom rather than the forced degrees of freedom themselves. The method is explicit which makes it easy to implement in existing software. Examples of its application to oblique towing tests and forced heading in wind and waves are presented.

Dynamic Oblique Towing Test (DOTT) to calculate the ship's hull maneuvering derivatives

An accurate and time-efficient numerical method is developed to find the hydrodynamic maneuvering derivatives with only a single simulation by computational fluid dynamics. The method is based on the oblique towing test by introducing an oscillatory variation to the model ship drift angle as time progress. The fluid flow velocity and direction are fixed during the simulation but the orientation of the model ship is changing sinusoidally with respect to the fluid flow with a certain amplitude and frequency. The velocity and acceleration of the model ship are calculated using the kinematic relationships and the forces and moment on the model ship are computed in a time period while the model ship drift angle is changing. The method is named dynamic oblique towing test (DOTT) and is applied to a container model ship and a surface combat model. The hydrodynamic maneuvering derivatives are obtained by an optimized curve fitting using Genetic Algorithms. The results of the computation in coefficient derivation and maneuver simulation indicate the accuracy and time efficiency of the method.

Effects of strut cover and lower vertical fin in podded propulsion units on ship maneuverability

To investigate the effects of a strut cover and lower vertical fin in podded propulsion units (POD units) on the maneuverability of a POD-driven ship, free-running model tests (turning test, spiral/revers spiral test, and zig-zag maneuver test) were conducted for three different profiles of the POD units: only a vertical strut without the strut cover or fin (F0), smaller strut cover and smaller fin (FS), and larger strut cover and larger fin (FL). The profile area increases in the order of F0, FS, and FL. The tests were conducted using two propeller positions: one with a propeller equipped in front of the POD units (PULL position) and another with the propeller equipped behind the POD units (PUSH position). The steady turning performance at a steering angle of 35° in the PULL position deteriorated in the order of F0, FS, and FL. In contrast, in the case of the PUSH position, the turning performance improved in the order of F0, FS, and FL. Thus, the PULL and PUSH positions demonstrated different tendencies. The non-dimensional steady yaw rate exceeded 1.0 in all cases, which indicates that the present ship has an excellent turning performance. Furthermore, captive model tests (the POD steering force test in straight towing, oblique towing test, and circular motion test) were conducted to determine the effects of the strut cover and lower vertical fin on the lateral force and yaw moment acting on the ship model. It was found that the large strut cover and fin increase the steering force and improves the course stability of the ship in both the PULL and PUSH positions.

Study of Influencing Factors on CFD Method and Separated Flow by the Virtual Static Captive Test

In this paper, Computational Fluid Dynamic (CFD) methods are adopted to predict the manoeuvring behaviour of the KCS benchmark model. At first instance, the model is calibrated on the basis of three factors namely (a) turbulence model, (b) grid size and (c) time step. Secondly, a Static Oblique Towing Test (OTT) that utilises the SST K-ω turbulence model with a grid number of 1.64 million and a time step of 0.01 s is conducted. The maximum difference of the lateral force between the port and starboard ship sides is observed at x/Lpp = 0.055. Finally, application of Reynold Averaged Navier Stokes (RANS) and Dettached Eddy Simulation (DES) CFD models indicates that the latter may be preferable in terms of capturing the oblique flow field.

Ship maneuverability modeling and numerical prediction using CFD with body force propeller

To obtain an accurate ship maneuverability model for motion prediction and autonomous navigation, a ship maneuverability modeling and numerical prediction method is proposed based on Computational Fluid Dynamics (CFD). A Mathematical Model Group (MMG) model is utilized to describe the ship maneuverability with data from virtual captive model and free-running tests. Static Oblique Towing Test (OTT) and dynamic Circular Motion Test (CMT) are both performed to acquire necessary data for the MMG model identification. The forces and moments acting on ship hull are predicted by solving unsteady Reynolds-Averaged Navier–Stokes (RANS) equations. The MMG hydrodynamic derivatives are obtained with the results of captive model tests. In the free-running simulations, to reduce the computational cost of considerable propeller grids, a body force propeller method is introduced to provide thrust forces directly, and the ship motion is handled by overset grids of ship hull and rudder. A 3-degree of freedom MMG model is identified with numerical simulation results, i.e., the self-propulsion test, rudder angle 35°turning test and 15/1 zig-zag test, for a standard KRISO Container Ship (KCS). These tests show that the motion prediction and free-running maneuvering results can agree with the standard experimental data well.

A Quasi-Static Motion Prediction Model of a Multi-Hull Navigation Vessel in Dynamic Positioning Mode

A Quadmaran vessel is a maritime navigation vessel with four hulls, and is expected to be utilized for high-density water quality observation in aquaculture farms. The performance of the vessel during dynamic positioning (DP) is integral to the accuracy of collected data. However, there are no models available to understand the motion characteristics of the multi-hull vessels in DP mode. Therefore, a computational model was developed to predict the DP motion of the Quadmaran vessel based on quasi-static assumptions. The fluid forces on the hulls, the thrust force of the propellers, and the external wind forces are modelled systematically, assuming the average speed while in DP does not exceed 0.25 m/s. A series of oblique towing tests and static turning tests were conducted on a 1/3 scale model to determine the hydrodynamic coefficients, while the Propeller Open Test was conducted to survey the propeller performance. The wind load was also incorporated into the computational model. Experimental validation tests were conducted in calm water and actual sea areas, and the correlation between experiment and simulation results was good. We concluded that the computational model can predict the DP motion of the Quadmaran vessel with an accuracy of roughly 2 m.

Experimental investigation of single blade loads by captive model tests in pure oblique flow. Part II: Propeller in-plane loads and preliminary comparison of single blade loads during transient phases

Off–design and realistic operative conditions of the propulsion system have been progressively considered a frontier topic for the development of novel and successful design procedures. The availability of high accurate data on propeller performance, both experimental or numerical, is pivotal for the development of reliable tools and the enhancement of traditional ones. In this context, this paper continues the investigation described in (Ortolani et al., 2020), that was dedicated to analyze the single blade loads obtained by oblique towing tests and assess their consistency with those measured during the steady turning phase, reproduced by free running model tests. In this paper, the same analysis is broadened to two aspects. At first, the in–plane forces and moments that are obtained by a conversion from the rotating frame of the measure to the fixed frame are discussed. These loads need quantification, because they are the primary cause of damages of the shafting structure, vibratory loads and also contribute to dynamic response of the ship. Then, the study is switched to the comparison of single blade loads during transient motions of the turning maneuvers, at weak and tight rudder angles, performed at the same reference speed as the captive model test. The analysis lays the basis for the enhancement of ship system (comprehensive) mathematical models, already used in ship design, to real–time analysis of vibratory loads and emitted noise.

Experimental study on the maneuvering derivatives of a half-scale SUBOFF model in the vertical plane

The maneuverability of a submersible in the vertical plane is investigated. For this purpose, extensive experiments are performed on a half-scale SUBOFF model in the circulating water channel (CWC) with the planar motion mechanism (PMM). Oblique towing tests containing high attack angles, pure heave tests and pure pitch tests are involved. The validity of the experiments is identified by the accordance of the present hydrodynamic coefficients and the ones of Defense Advanced Research Projects Agency (DARPA) . In order to fit the measured data for cases with high attack angles, this paper develops the expression of dimensionless pitch moment M′ by the piecewise function. Besides, the blockage effects are corrected by Tamura equation. In addition, the uncertainty analysis (UA) is conducted referring to the recommended procedure 7.5-02-06-04 by 27th ITTC (2014b). The results show that, the developed equation turns out to be superior to the standard one. And the total uncertainties of the hydrodynamics are less than 2%, which reveals the accuracy and credibility of the experimental measurement.

A velocity decomposition method combining potential and viscous flow for the estimation of submarine's hydrodynamic coefficients

Estimating hydrodynamic coefficients is important in the design of submarines. To enhance the efficiency of the computational fluid dynamics (CFD) method for evaluating the hydrodynamic performance of submarines, we have developed a velocity decomposition solver for the simulation of a moving body based on the open-source CFD platform OpenFOAM. The total velocity is decomposed into potential flow velocity and non-potential flow velocity. The potential flow velocity is further decomposed into unit components, which can be computed by the proposed method with less interpolation cost. Further, the original potential flow velocity can be recovered by linear summation of these unit velocity components. This avoids solving Laplace equation at each time step. The non-potential flow velocity is solved by using a complementary Navier–Stokes equation in a smaller domain as compared to that used in the conventional CFD method. In addition, a buffer source term is introduced for the transition of non-potential flow velocity to potential flow velocity. Oblique towing test and planar motion mechanism test are numerically simulated using the proposed method, which reveal good consistency between the experimental data and simulation results. This validates the efficiency and reliability of the proposed method in estimating the hydrodynamic coefficients of submarines.

Determination of Hydrodynamic Coefficients of Oblique Towing Test of a High-Speed Boat by Computational Fluid Dynamics

Determination of Hydrodynamic Coefficients of Oblique Towing Test of a High-Speed Boat by Computational Fluid Dynamics

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 off-design. 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.

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

Abstract 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.

Experimental and numerical investigations of advancing speed effects on hydrodynamic derivatives in MMG model, part I: Xvv,Yv,Nv

Experimental and numerical studies on the hydrodynamic forces and moments of the ONR Tumblehome (ONRT) hull during oblique towing tests are performed. The experiments are carried out in the towing tank of Marine Design and Research Institute of China (MARIC) and the numerical simulations are based on the open-source CFD library OpenFOAM. Validations of the numerical methods are provided by comparison with experimental measurements from tests at two advancing speed. And then the investigations of advancing speed effects on hydrodynamic derivatives used in MMG model are performed numerically. Specifically, Xvv,Yv,Nv of the ONRT hull at four advancing speed are yielded via linear regression of the hydrodynamic forces and moments predicted by CFD calculations. Finally, the relationship between hydrodynamic derivatives and advancing speed are concluded and the expressions of hydrodynamic derivatives that can be used in MMG model are given.

回流水槽における操縦流体力の計測精度について

In this paper it is reported the possibility of the maneuvering test of the model ship in Circulating Water Channel (CWC). The wall effect on the model ship with the drift angle in CWC is researched by using model test and Computational Fluid Dynamics (CFD) analysis. Then a condition/criterion is studied for drift angle of the model and the distance between ship model and the side wall of CWC. Hydrodynamic force acting on the turning hull is measured in Yawing test by Planar Motion Mechanism (PMM)．The frequency dependency is shown in the test data result from the setting condition (period and r’). To avoid the frequency dependency, reasonable period range is proposed. Oblique towing test and Yawing test are carried out with keeping the above criterion. The analyzed data agrees with the results by towing tank. Then the maneuvering test in CWC has enough accuracy for course stability criterion of the ship practically.

Virtual maneuvering test in CFD media in presence of free surface

Maneuvering oblique towing test is simulated in a Computational Fluid Dynamic (CFD) environment to obtain the linear and nonlinear velocity dependent damping coefficients for a DTMB 5512 model ship. The simulations are carried out in freely accessible OpenFOAM library with three different solvers, rasInterFoam, LTSInterFoam and interDyMFoam, and two turbulence models, k-ε and SST k-ω in presence of free surface. Turning and zig-zag maneuvers are simulated for the DTMB 5512 model ship using the calculated damping coefficients with CFD. The comparison of simulated results with the available experimental shows a very good agreement among them.

Numerical Simulation of Oblique Towing Tests and Rotating Arm Tests for a Submarine Model in Same Grid Topology

The numerical simulation of oblique towing tests (OTT) and rotating arm tests (RAT) for a submarine model was undertaken with the same grid topology. As for the OTT simulation, to improve the efficiency and accuracy of the computation, the drift sweep procedure was adopted. In contrast with the steady drift case, rotating motion for a submarine is more complex. The rotating reference frame was adopted to deal with the rotation problem. The Coriolis force and centrifugal force due to the computation in a body-fixed rotating coordinate system were treated explicitly and added to the momentum equations as source terms. The user-defined boundary conditions were coded to give the correct magnitude and direction of entrance velocity. Lastly, the computed forces and moments as a function of drift angle during the two kinds of simulations were compared with experimental results, as well as literature values. They always show the correct trend. The flow field and streamline patterns in rotating motion were depicted. It indicates that the method used in the paper is reliable and the same grid topology can be used for the simulation of two simplified maneuvers.

Roll-Coupling Effect on Ship Maneuverability

Roll-coupling effect on ship maneuvering motions is not negligible for high speed vessels, large container ships, passenger ships etc. To perceive the roll-coupling effect on the ship maneuverability, this paper proposes new simplified formulae for course stability criterion and steady turning index in the framework of the linear motion theory. Then, an approximation is employed where the depth wise acting point of lateral force related to roll moment is constant in any oblique motion mode. The course stability criterion including the roll-coupling effect is written aswhere C′ denotes the usual course stability criterion, and the linear hydrodynamic derivatives and l′ the depth wise acting point of the lateral force. The formula is simple and useful for checking the course stability taking the roll-coupling effect into account. To obtain the hydrodynamic derivatives, oblique towing test and circular motion test with various heel angles were conducted for models of a container ship, a pure car carrier and a ferry. In general, course stability deteriorates with increasing Froude number Fn or decreasing of metacentric height GM′, whereas turning performance improves and the time until response reaches steady turning state increases.

Estimation of the hydrodynamic coefficients from captive model test results by using support vector machines

A novel method of artificial intelligence technology, support vector machines (SVM) is proposed for estimation of the hydrodynamic coefficients in the mathematical models of ship manoeuvring motion from captive model test results, including oblique towing test and pure sway test results. By analyzing the captive model test results, SVM is used to identify the hydrodynamic coefficients, which are obtained from the expansion of the inner product of a linear kernel function. To get rid of the influence of parameter drift in the estimation of the hydrodynamic coefficients from pure sway test results, an equivalent transformation is made for the hydrodynamic mathematical model. The hydrodynamic forces under different operating conditions are predicted by using the identified hydrodynamic mathematical model. Comparison between the predicted hydrodynamic forces and the test results shows that the identified hydrodynamic mathematical model has good generalization performance and SVM is an effective method for estimating the hydrodynamic coefficients from captive model test results.