Surge-varying LOS based path following of under actuated surface vehicles

1048-1055Subject to harsh ocean environment, a novel path following control scheme with accurate guidance and high anti-disturbance ability for under actuated surface vehicles is proposed. The innovative work is as follow: (1) Based on the traditional line-of-sight (LOS), a surge-varying LOS (SVLOS) guidance law is designed to achieve double guidance of speed and heading, which enhances the flexibility and precision of the previous LOS; (2) Unknown disturbances are exactly estimated by an exact disturbance observer (EDO), wherein the limitations of bounded and asymptotic observations can be avoided; (3) The EDO-based robust tracking controllers enable accurate disturbance compensation and guided signal tracking in harsh ocean environment. Rigorous theoretical analysis and significant simulation comparison have been done to demonstrate superiority of the EDO-SVLOS scheme

Numerical study on hydrodynamics of ships with forward speed based on nonlinear steady wave

In this paper, an improved potential flow model is proposed for the hydrodynamic analysis of ships advancing in waves. A desingularized Rankine panel method, which has been improved with the added effect of nonlinear steady wave-making (NSWM) flow in frequency domain, is employed for 3D diffraction and radiation problems. Non-uniform rational B-splines (NURBS) are used to describe the body and free surfaces. The NSWM potential is computed by linear superposition of the first-order and second-order steady wave-making potentials which are determined by solving the corresponding boundary value problems (BVPs). The so-called mj terms in the body boundary condition of the radiation problem are evaluated with nonlinear steady flow. The free surface boundary conditions in the diffraction and radiation problems are also derived by considering nonlinear steady flow. To verify the improved model and the numerical method adopted in the present study, the nonlinear wave-making problem of a submerged moving sphere is first studied, and the computed results are compared with the analytical results of linear steady flow. Subsequently, the diffraction and radiation problems of a submerged moving sphere and a modified Wigley hull are solved. The numerical results of the wave exciting forces, added masses, and damping coefficients are compared with those obtained by using Neumann–Kelvin (NK) flow and double-body (DB) flow. A comparison of the results indicates that the improved model using the NSWM flow can generally give results in better agreement with the test data and other published results than those by using NK and DB flows, especially for the hydrodynamic coefficients in relatively low frequency ranges

Hybrid method for predicting ship manoeuvrability in regular waves

The ship's manoeuvring behaviour in waves is significantly different from that in calm water. In this context, the present work uses a hybrid method combining potential flow theory and Computational Fluid Dynamics (CFD) techniques for the prediction of ship manoeuvrability in regular waves. The mean wave-induced drift forces are calculated by adopting a time domain 3D higher-order Rankine panel method, which includes the effect of the lateral speed and forward speed. The hull-related hydrodynamic derivatives are determined based on a RANS solver using the double body flow model. The two-time scale method is applied to integrate the improved seakeeping model in a 3-DOF modular type Manoeuvring Modelling Group (MMG model) to investigate the ship's manoeuvrability in regular waves. Numerical simulations are carried out to predict the turning circle in regular waves for the 5175 container carrier. The turning circle's main characteristics as well as the wave-induced motions are evaluated. A good agreement is obtained by comparing the numerical results with experimental data obtained from existing literature. This demonstrates that combining potential flow theory with CFD techniques can be used efficiently for predicting the manoeuvring behaviour in waves. This is even more true when the manoeuvring derivatives cannot be obtained from model tests when there is lack of such experimental data

Hydrodynamische Kräfte am schrägfahrenden Schiffsrumpf bei höheren Geschwindigkeiten im tiefen und flachen Wasser

Hydrodynamische Kräfte am schrägfahrenden Schiffsrumpf bei höheren Geschwindigkeiten im tiefen und flachen Wasser Es wird ein drei-dimensionales numerisches Verfahren zur Berechnung der stationären Potentialströmung und der hydrodynamischen Kräfte am einen schrägfahrenden Schiffsrumpf bei höheren Geschwindigkeiten im tiefen und flachen Wasser vorgestellt. Das Störpotential wird durch Verteilungen von Rankine-Quellen und eine halb-unendliche Dipolschicht dargestellt. Unter der Voraussetzung kleiner Driftwinkel wird die Umströmung in einen symmetrischen Anteil infolge der Längsbewegung und einen antimetrischen Anteil infolge der seitlichen Bewegung aufgeteilt; die symmetrische und die antimetrische Umströmung werden dann getrennt voneinander bestimmt.Rechnungen wurden für ein "Wigley-Modell" von 10 m Länge im tiefen und flachen Wasser durchgeführt. Die numerischen Ergebnisse für den Wellenwiderstand, die stationäre Absenkung und Vertrimmung, die Querkraft und das Giermoment sowie die Wellenerhebung wurden präsentiert. Das vorgestellte Verfahren wurde durch die Testrechnungen als funktionsfähig bewiesen. Weitere Verifikation und Verbesserungen des Verfahrens sind aber noch notwendig.This paper describes a three-dimensional numerical approach for computing the steady potential flow and the hydrodynamical forces acting on ships of small angles of yaw at high speed in deep and shallow water. The disturbed velo city potential is expressed by a distribution of Rankine sources and a semi-infinite dipole sheet. Based on the assumption that the angles of yaw are small, the flow is splitted into asymmetrie one due to the longitudinal motion of the ship and an antisymmetrie one due to the lateral motion; the symmetrie flow and the antisymmetrie flow are then determined respectively. Calculations were performed for a Wigley model 10 m long. Numerical results, i.e.,the wave-making resistance, the steady sinkage and trim, the lateral hydrodynamical force and the yawing moment were presented. The proposed method was justified by the numerical investigation. However, it is necessary to verify and to improve this method further

Berechnung der Potentialströmung um einen schräg fahrenden Schiffsrumpf auf tiefem und flachem Wasser

Berechnung der Potentialströmung um einen schräg fahrenden Schiffsrumpf auf tiefem und flachem Wasser A Rankine panel method for calculating lifting potential flow about three-dimensional surface-piercing bodies in deep and/or shallow water is developed and applied to ships in steady motion at small drift angles. The disturbance velocity potential is represented by a source distribution on the hull surface and on a horizontal plane above the free surface, and by a normal dipole distribution on the centerplane of the ship and downstream of the trailing edge of the hull. The singularity strengths are determined so that the boundary conditions on the hull surface and free surface and the Kutta condition at the trailing edge are satisfied at the corresponding collocation points. Moreover, the radiation condition is satisfied by using the numerical technique of "staggered grids" suggested by Jensen, while the shallow water effect is taken into account by the method of images

Hydrodynamische Kräfte am manövrierenden Schiff auf flachem Wasser bei endlicher Froudezahl

Hydrodynamische Kräfte am manövrierenden Schiff auf flachem Wasser bei endlicher Froudezahl Hydrodynamische Kräfte am manövrierenden Schiff auf flachem Wasser bei endlicher Froudezahl: Die Einflüsse des Flachwassers auf das Manövrierverhalten von Schiffen sind vom Blickpunkt der Kollisions- und Bodenberührungsgefahr gesehen besonders wichtig. Deswegen ist eine genaue Untersuchung zur Erfassung dieser Einflüsse erforderlich. Fast alle bisherigen theoretischen Untersuchungen beruhen auf der Theorie von Newman; die Wasseroberfläche wird als unverformt angenommen. Die Ergebnisse haben aber gezeigt, dass bei sehr flachem Wasser die Verformung der Wasseroberfläche selbst für übliche Fahrgeschwindigkeiten nicht mehr vernachlässigt werden darf. Aus diesem Grunde wird in der vorliegenden Arbeit versucht, die hydrodynamischen Kräfte am manövrierenden Schiff auf flachem Wasser unter Berücksichtigung der Einflüsse endlicher Froudezahl zu berechnen. Durch Anwendung der Methode der angepaßten asymptotischen Entwicklungen wird die Integro-Differentialgleichung für die Queranströmgeschwindigkeit unter Berücksichtigung der Verformung der Wasseroberfläche abgeleitet. Nach diesem theoretischen Modell werden dann die Absenkung und Vertrimmung sowie die dementsprechenden hydrodynamischen Koeffizienten berechnet. Die Ergebnisse sind zufriedenstellend und zeigen im gewissen Sinne Hinweise auf die Weiterarbeit. Das von dieser Arbeit vorgestellte Verfahren basiert auf der Theorie des schlanken Körpers

Assessment of CFD-Based Ship Maneuvering Predictions Using Different Propeller Modeling Methods

Propeller modeling in virtual captive model tests is crucial to the prediction accuracy of ship maneuvering motion. In the present study, the Computational Fluid Dynamics (CFD) method with two propeller modeling methods, Sliding Mesh (SM) and Multiple Reference Frames (MRF), was used to simulate the captive model tests for a KVLCC2 tanker model. The virtual captive model tests, including for resistance, self-propulsion, rudder force, oblique towing, circular motion, oblique towing and steady turning tests with rudder angle, were conducted by solving the Reynolds-averaged Navier–Stokes (RANS) equations. The computed hydrodynamic forces, hydrodynamic derivatives, and hull-propeller-rudder interaction coefficients were validated against the available captive model test data and the CFD results obtained by a Body Force (BF) method in the literature. Then the standard turning circle and zig-zag maneuvers were simulated by using the MMG (Maneuvering Modeling Group) model with the computed hydrodynamic derivatives and hull-propeller-rudder interaction coefficients, and the results were validated against available free-running model test data. The most satisfactory agreement in terms of the ship hydrodynamic forces and maneuvering parameters and the most accurate rudder normal force were obtained by the SM method rather than by the MRF or the BF methods, while the lateral forces and yaw moments obtained by the SM and the MRF methods were all in good agreement with the model test data

Predictions of ship maneuverability based on virtual captive model tests

Maneuverability is an important hydrodynamic performance of a ship, and should be taken into account during the ship design stage. The present study of Computational Fluid Dynamic (CFD) calculations aims to offer a numerical tool for maneuvering prediction with high accuracy. The virtual captive model tests for a model scale KCS container ship are conducted using unsteady Reynolds-averaged Navier–Stokes (RANS) computation to obtain the full set of linear and nonlinear hydrodynamic derivatives in the 3rd-order Abkowitz model. The numerical uncertainty analysis is carried out for the pure sway and yaw–drift tests to verify the numerical accuracy. It is concluded that the lower order Fourier coefficients are preferred in the computation of the hydrodynamic derivatives. Moreover, part of the computed hydrodynamic forces and moments are compared with the available captive model test data, and good agreement is obtained. By substituting the computed hydrodynamic derivatives into the mathematical model, the standard turning and zigzag maneuvers are predicted. By comparing the predicted maneuvering results with the available experimental data and the prediction results by others, it is demonstrated that acceptable prediction accuracy can be achieved with the present method, which shows the effectiveness of the present method in predicting ship maneuverability

Numerical Simulation of the Flow around NACA0018 Airfoil at High Incidences by Using RANS and DES Methods

In this work, the flow around the NACA0018 airfoil with a wide range of attack angles was investigated based on the open-source computational fluid dynamics (CFD) platform OpenFOAM. Two numerical methods, Reynolds-averaged Navier–Stokes (RANS) and the detached eddy simulation (DES), were employed. Under the premise of a grid convergence analysis, the computed lift and drag coefficients were validated by the available experimental data. The pressure distribution, the complex flow mechanisms of the airfoil under the attached flow regime, the mild separation flow regime, and the post-stall flow regime, combined with the shedding vortex structures, streamlines, and vorticity distributions, are discussed. From the numerical results, it can be seen that the DES computation presents a better accuracy in the prediction of the lift and drag coefficients, with a deviation less than 10% at the largest angle of attack. Meanwhile, it also presents remarkable improvements in capturing the local flow field details, such as the unsteady separated flow and the shedding vortex structures