Trim Moment Research Articles

Influence of the hull shape of a submerged body on its movements in a near-surface water environment

ABSTRACT Forces and moments affecting a submerged body by liquid can have a significant effect on the nature of its movement under low submergence conditions. The study considers the movement of submerged bodies with various cross-sectional shapes near the free water surface, using the methods of experimental and numerical simulation. We numerically analysed the influence of the hydrodynamic pressure field formed around the body on the wave pattern when moving at different velocities, and determined the dependency of the wave resistance coefficient for models with given parameters. We determined the nature of the change in the theoretical dependencies of the lift force and trimming moment on the Froude number. The use of the towing tank allowed us to obtain the dependencies of the relative vertical displacement of submerged bodies of various shapes under the influence of the lift force and trim angles during their movement.

Experimental and numerical study on the high-speed ship hydrodynamics influenced by an interceptor with varied angle of attack

Efforts to improve the hydrodynamic performance of high-speed ships have been underway for a long time. There are different approaches, one of which is to take advantage of an interceptor. Conventionally, the interceptor blades are mounted vertically on the ship's bottom transom, oriented at a zero-degree angle of attack (AoA). This study comprehensively explores high-speed ships' hulls with and without interceptor configurations, encompassing both negative and positive AoA of the interceptor, conducted through experimental and numerical methods using a fully captive model. The interceptors are strategically positioned and configured. Each configuration was examined under varying AoA settings, with uniform interceptor depths and systematic trim angle adjustments. The Computational Fluid Dynamics (CFD) approach simulates the local flow dynamics around the hull, thoroughly analyzing resistance, pressure distribution, lift force, wave profile, and trim moment. The results indicate that interceptor placement near the keel with AoA adjustments significantly reduces hydrodynamic resistance, while AoA changes have limited impact in other positions. Lift force analysis shows interceptors improve lift compared to the bare hull, but this improvement is not linear across positions. Furthermore, it is observed that adjustments in AoA influence lift, with a negative AoA generally being considered favorable. In summary, carefully considering placement, AoA, and height-to-length ratio is necessary to maximize interceptor advantages.

Motion of a submerged body under a free surface and an ice cover in finite water depth conditions

In this study, we consider the motion of a submerged body near the free surface of a liquid and a liquid covered with an ice cover in finite water depth conditions. The influence of water depth on the nature of motion of a submerged body has been experimentally and theoretically analysed. The experiments were performed in an ice tank. The use of a cable towing system allowed the researchers to measure the magnitude of the relative vertical displacement of the submerged body resulting from the action of the lift force. The influence of ice cover on the relationship between the vertical displacement, trim angle, criterion for ice breaking, and speed of movement of the submerged body model was established. For theoretical research, the ice cover was modelled by a viscoelastic floating plate, and the submarine was modelled by a source–sinks system in a fluid flow. The influence of water depth, the presence of an ice cover, changes in the submergence depth, and the speed of movement of the submerged body on the wave resistance, lift force, and trimming moment were analysed. Theoretical results are supported by experimental data.

Design and dry wind tunnel test of progressive flexible variable bending wing

AbstractThe deformable wing structure can change its aerodynamic shape according to the change of flight mission and flight environment, so as to obtain better lift-drag, stability and control characteristics, which is considered as one of the future research directions of aviation technology. Considering the current technology maturity and reliability, a gradient corrugated fin is designed to realise the bending deformation of the wing. The structure of the skin is optimised to keep the skin smooth during deformation. In addition, a progressive push and pull rod is proposed to drive the wing deformation, and the fluid-structure interaction simulation is carried out for the wing deformation. At the same time, the changes of wing aerodynamic characteristics under different angles of leading and trailing edges and different push rod action schemes are analysed. Finally, a dry wind tunnel simulation test of the designed progressive flexible variable bending wing is carried out. The results of fluid-structure interaction simulation and dry wind tunnel test show that the progressive flexible variable bending wing proposed in this paper has a simple and reliable structure and remarkable deformation effect. It has advantages in increasing lift and reducing drag, ensuring high lift-drag ratio and providing wing trim moment. The deformable wing dry wind tunnel test platform designed by this method is structurally reliable, easy to operate, and can accurately reflect the influence of wing deformation on its aerodynamic force, which provides a verification means for the development of the design method and the design of practical aircraft in the future.

The Effect of an Ice Cover on the Trimming Moment of Submarines

The Effect of an Ice Cover on the Trimming Moment of Submarines

Unsteady simulation of AUVs approaching seafloor by self-propulsion using multi-block hybrid dynamic grid method

Autonomous underwater vehicles (AUVs) have been widely applied in the vicinity of the seafloor for various purposes, including pipeline, cable, and structure inspection; bottom detection and following; and bottom-mounted docking systems. Squatting is a crucial problem that occurs when cruising in the vicinity of the seafloor, which affects the maneuverability and safety of an AUV. Therefore, it is necessary to have a high-accuracy prediction of the AUV hydrodynamic performance to obtain an automatic guidance controller for safety. In this study, the hydrodynamic performance was investigated by a physics-based unsteady simulation of a self-propelled AUV approaching seafloor using multi-block hybrid dynamic grid method, combined with six degrees of freedom (6DOF), user-defined functions (UDFs), and Arbitrary Lagrangian–Eulerian (ALE) approach. This simulation was used to explore the principle of the complex unsteady motions experienced by an AUV operating at the vertical plane in the vicinity of the seafloor. The results indicated that, for an AUV speed higher than the critical speed (0.5 m/s at H/D= 1.832, where Re=5.49 × 105), the AUV experienced a speed loss and increased resistance, thrust, and suck force. The speed loss was due to the larger increment in resistance than that in thrust. The magnitude and direction of the trim moment were highly sensitive to variations in AUV speed, resulting from the two low-pressure regions. One region was at the AUV bow neck induced by the bow vortex and the other was at the AUV stern neck induced by the rotating propeller. The unsteady interaction between the AUV and seafloor during AUV self-propulsion was elucidated by flow analysis.

Computational Investigation of Pitch Motion of a High-Speed Craft Incorporated with Trim-Tabs

An excessive trim by stern on a high-speed craft is vulnerable to decrease comfortable ride incorporated with the large pitch motion. To prevent such problem, the craft is then equipped with a device known as “trim-tabs” providing a lift force at the stern region, which leads to reduce pitch angle/trim of the craft. This paper presents computational modelling of trim-tabs on high-speed craft towards reducing the pitch motion in the calm water condition. Here, the commercial Computational Fluid Dynamic (CFD) software; called Numeca Fine Marine has been accordingly utilised to evaluate several effects of Froude numbers (Fr) and angle of the trim-tabs into characteristics of the lift force of the high-speed craft. The results revealed that the pitch motion of the high-speed craft equipped with the trim-tabs (5o) has significantly reduced up to 76% at Fr = 1.0. This can be explained that the installed trim-tabs has increased the negative trim moment, which inherently creates the restoring moment pushing the stern up along the pitch-axis. Meanwhile, this may cause involuntary the total ship’s resistance increased. In general, this simulation pointed out the advantage of the trim-tabs to increase the comfortability of the high-speed craft.

Optimization of the stages of a ship's cargo plan development for shippining of general cargoes

One of the main tasks in stability calculations is to provide the ship with the necessary (optimal) trim whose final value is influenced by the arrangement of cargo on the ship. Today, however, there are rules and requirements but there is no unified approach to developing a cargo plan for a vessel that simultaneously transports various types of general cargo. In order to improve the efficiency of the above calculations, a procedure has been proposed to optimize developing a cargo plan for a vessel carrying heterogeneous general cargoes at the same time, the main idea of which is to distribute consignments on the ship in two stages, taking into consideration the compensating trimming moment. The scheme to develop a cargo plan has been improved by introducing the developed procedure. The results of verification confirmed its effectiveness in practice. Possible deviations of the values for the trim required (optimal) for the voyage from the actual one calculated after the allocation of stocks and consignments of goods have been investigated using an example of the series of developed cargo plans. It should be noted that the value for the trim, required (optimal) and actual, for each individual cargo plan does not differ by more than 8 %. The results reported in this paper give grounds to assert the expediency of their application when developing cargo plans for tramp shipping vessels. The introduction of the procedure could make it possible to effectively load a vessel with the full utilization of both its carrying capacity and cargo capacity. The use of the proposed scheme for developing a cargo plan to transport heterogeneous cargoes would reduce the total time for calculating the stability and strength of the vessel in general

Application of time-domain methods for marine hydrodynamic and hydroelasticity analyses of floating systems

ABSTRACT The application of transient three-dimensional numerical code ITU-WAVE is presented for the prediction of hydrodynamic characteristics of floating bodies. The time histories of unsteady motions are directly presented with respect to Impulse Response Functions (IRFs). First-order steady forces of wave-resistance, sinkage force and trim moment are solved as the steady-state limit of surge radiation IRF. The numerical prediction of second-order mean force is presented using near-field method based on the direct pressure integration. The hydrodynamic and structural parts are fully coupled through modal analysis for the solution of hydroelastic analysis. A stiff structure is then studied assuming that contributions of rigid body modes are much bigger than elastic modes. A discrete control of latching is used to increase the bandwidth of the efficiency of Wave Energy Converters (WEC). ITU-WAVE numerical results for different floating bodies show satisfactory agreements compared to analytical, other numerical and experimental results.

Nonlinear System Identification for the Prediction of Unsteady Vertical Plane Hydrodynamic Forces on a Planing Hull

A method to derive a nonlinear model of the vertical plane hydrodynamic forces and moments on a planing hull using forced motion Unsteady RANSE CFD simulations and nonlinear system identification is investigated. The use of forced motion simulations on the fully three-dimensional hull form allows three-dimensional effects to be included in the nonlinear force model which have previously been neglected by strip theory or approximated by 2D+T theory. Nonlinear system identification allows the most significant dynamic terms in the hydrodynamic model to be included in the model without over regressing the response. The process of identifying the nonlinear model is investigated in detail to determine the best collection of forced motion simulations to derive an adequate model. This methodology is intended to provide higher fidelity predictions of vertical plane motions of planning hulls than previous methods while requiring a lower computational expense than a comprehensive CFD evaluation in waves. The final nonlinear model, developed for the Generic Prismatic Planing Hull (GPPH), is validated against static and dynamic simulations not included in the system identification and is found to produce a promisingly accurate model of the hydrodynamic forces and moments When used to predict the heave force and trimming moment of a forced coupled heave and trim motion, the selected nonlinear model is found to predict the heave force and trimming moment with an average absolute error of 3.9% of the maximum force and 5.5% of the maximum moment respectively.

Stability and Manoeuvrability Simulation of a Semi-Autonomous Submarine Free-Running Model SUBOFF with an Autopilot System

On the basis of a full-appendage DARPA SUBOFF model (DTRC model 5470), a scale (λ = 0.535) semi-autonomous submarine free-running model (SFRM) was designed for testing its manoeuvrability and stability in the constrained water. Prior to the experimental tests of the SFRM, a six-degree-of-freedom (6-DOF) manoeuvre model with an autopilot system was developed by using logic operations in MATLAB. The SFRM’s attitude and its trim polygon were presented by coping with the changes in mass and trimming moment. By adopting a series of manoeuvring tests in empty tanks, the performances of the SFRM were introduced in cases of three sailing speeds. In addition, the PD controller was established by considering the simulation results of these manoeuvring tests. The optimal control gains with respect to each manoeuvring test can be calculated by using the PID tuner in MATLAB. Two sets of control gains derived from the optimal characteristics parameters were compared in order to decide on the most appropriate PD controller with the line-of-sight (LOS) guidance algorithm for the SFRM in the autopilot simulation. Eventually, the simulated trajectories and course angles of the SFRM would be illustrated in the post-processor based on the Cinema 4D modelling.

Numerical Investigations on the Resistance and Longitudinal Motion Stability of a High-Speed Planing Trimaran

The planing trimaran is a novel kind of aerodynamic alleviated marine vehicle and possesses a unique hybrid hydrodynamic and aerodynamic characteristics. In this paper, to investigate the hull behavior of a planing trimaran under the effect of tunnel forces, the tunnel and demihull are treated as appendages mounted on the slender main hull. Numerical simulations were carried out for planing trimaran and a monohull which was built according to the main hull configuration. Mesh convergence studies were implemented based on the public experimental data of a similar planing trimaran. Calculated results show that the presence of tunnel and demihull could decrease the resistance of the main hull and improve the longitudinal motion stability at high speeds. Flow evolutions of waves, velocity vector, wetted length and lift distribution were performed to explain the variation of the tunnel lift and its moment and their influence on hull behavior. Parameter studies on demihull length were carried out as well. It was found that, as the demihull length is increased, the tunnel trimming moment would decrease especially in the final stage of ventilation, making resistance reduced but the motion stability weakened.

Technology of steel jackets load-out in deep-water offshore platforms from the barge

Currently, great attention is paid to the development issue of the shelf’s fuel and energy resources foremost in the oil-gas fields of the Caspian Sea. The problem solution requires the studying of great majority of scientific-technical issues. One of the significant problems is the lead-out of steel jacket from the offshore platform as a major element of oil-gas field hydro-technical facilities meant for the operation in the deep water. The calculations for the execution of operations with steel jacket of deep stationary platform from the block with the detailed chara- cteristics by the mass and gravity center coordinates alongside line data have been carried out with “SACS” and “STAAD.PRO” software programs. The steel jacket is pushed astern with the push-pull equipment on the barge. Due to the shift of gravity centre the jacket changes the trim in the stern. Through the elevation of trim angle brought in alignment with the friction ration between the jacket and barge, the jacket slides further itself. Herewith, the trim increases until the gravity centre of steel jacket on the barge is not in alignment with rotation centre of the large rocker arms. The studies helped to fix the position of the jacket’s gravity centre from the aft perpendicular, the trim moment, the trim of the barge, the draft with the bow and stern and the trim angle as well.

Experimental Analysis on Sinking Time of Littoral Submarine in Various Trim Angle

A submarine must conform to Archimedes’ Principle, which states that a body immersed in a fluid has an upward force on it (buoyancy) equal to the weight of the displaced fluid, (displacement). Submarines are ships capable of being submerged. The history of submarines and their operation have largely revolved around being able to alter the density of the vessel so that it may dive below the surface, maintain a depth, and return to the surface as needed. The way modern submarines accomplish this task is to bring in and remove water from tanks in the submarine called ballast tanks. Ballast tanks fit into two categories: those used for major adjustment of mass (main ballast tanks); and those used for minor adjustments (trim tanks). The effect of each tank is plotted and this is compared with the changes in mass and trimming moment possible during operations using a trim polygon to determine whether the ballast tanks are adequate. On the water surface, metacentric height (GM) is important, whereas below the surface it is the distance between the centre of buoyancy and the centre of gravity (BG) which governs the transverse stability of a submarine.

Interceptor and trim tab combination to prevent interceptor's unfit effects

Trim tabs, small surfaces connected to the end of the craft to control the trim by adjusting the angle of tab, relative to the larger surface, have been used to optimize the running trim of displacement, semi-planning, and planning vessels for many years. Interceptors are the same as trim tabs, but are vertically installed at the end of the craft to control the trim by changing the height. As demonstrated in this paper, the same size as the interceptor and trim tab (when span and chord of trim tab are respectively equal to span and height of interceptor), the interceptor shows better efficiency (better trim and resistance reduction). While efficiency of trim tab just depends on the trim tab angle, the effective factors on the interceptor effectiveness are little complicated. The interceptor efficiency highly depends on the interceptor height and boundary layer thickness at transom. Although the higher interceptor increases the amount of lift force, but that could create a very strong moment against trim moment and consequently negative trim angle. The results of this investigation prove that the combination of an interceptor with a trim tab shows better performance compared to an interceptor or a trim tab. Also instead of increasing the interceptor height to gain more lift, which could make intense negative trim, it is better to use integrated interceptor with trim tab. To do so, a comprehensive series of dynamic CFD simulations have been performed in the case of a simple planning boat model with three different trim control appendages. Unsteady Reynolds Average Navier-Stokes equations (URANS) are applied to model the flow around the considered model with interceptor, trim tab and combination of an interceptor and a trim tab at an equal span length. The model is analyzed based on finite volume method and SIMPLE algorithm using dynamic meshes in the Fluent computational code. For validation of the CFD results, the Savitsky planning boat calculations (only for model boat) and the grid convergence index (GCI) were used to estimate the uncertainties due to grid-spacing and time-step.

A havelock source panel method for near-surface submarines

A panel method is described for calculating potential flow around near-surface submarines. The method uses Havelock sources which automatically satisfy the linearized free-surface boundary condition. Outputs from the method include pressure field, pressure drag, wave resistance, vertical force, trim moment and wave pattern. Comparisons are made with model tests for wave resistance of Series 58 and DARPA SUBOFF hulls, as well as with wave resistance, lift force and trim moment of three length-to-diameter variants of the DSTO Joubert submarine hull. It is found that the Havelock source panel method is capable of determining with reasonable accuracy wave resistance, vertical force and trim moment for submarine hulls. Further experimental data are required in order to assess the accuracy of the method for pressure field and wave pattern prediction. The method is implemented in the computer code “HullWave” and offers potential advantages over RANS-CFD codes in terms of speed, simplicity and robustness.

The applicability of hydrofoils as a ship control device

Centrifugal forces are commonly created when ships turn, which may cause a ship to capsize in a critical situation. A mathematical model has been developed to optimize the stability coefficients for ship, with the aim to prevent capsizing and to increase ship maneuverability in high-speed water craft. This model can be used to develop algorithms for control system improvement. The mathematical model presented in this paper optimized the use of multipurpose hydrofoils to reduce heeling and the trimming moment, maintaining an upright ship’s position and lessening the resistance via transverse force. Conventionally, the trimming and heeling of a ship are controlled using ballast water; however, under variable sea conditions it is sometimes difficult to control a ship’s motion using ballast water. In this case, a hydrofoil would be more stable and maneuverable than a ballast tank controlled vessel. A movable hydrofoil could theoretically be adapted from moveable aerofoil technology. This study proves the merit of further investigation into this possibility.

Spin-Yaw Lock-In of a Rotationally Symmetric Missile

Spin―yaw lock-in has usually been associated with small aerodynamic asymmetries that produce a trim angle that rotates with the missile. Recent trajectory calculations using wind-tunnel measurements ofthe aerodynamic moment acting on a rotationally symmetric missile predicted spin―yaw lock-in. Thus, an asymmetry-induced trim moment is not necessary for lock-in. For a side moment that varies with the roll angle between the angle-of-attack plane and a fixed plane on the missile, this paper shows that lock-in of the coning motion and the rolling motion can occur. The conditions on the aerodynamic moments for occurrence of lock-in are derived and appropriate stability criteria are obtained. A simple case of a hypersonic finned missile is considered and the existence of lock-in is demonstrated.

トランサム船尾流場の解析法について

An analytical method in so-called Neumann-Kelvin problem is proposed to represent 2-D flow about semi-displacement type ship having blunt stem and transom stern. At first, the usual Neumann-Kelvin solution, complex potential, is deduced from Green's integral formula by using the boundary conditions on the linearized free surface and on the exact body surface and is represented by the velocity distribution along the body surface and by wave sources located at the fore and aft cross points. This new representation shows explicitly the non-uniqueness of Neumann-Kelvin solutions. Next, the transom wall is removed and the representation is reduced to the sum of integration along the body surface, the unknown being the velocity differences, and the wave source located at the fore cross point. The resulting boundary integral equation has two unkowns which are determined from planning conditions, Kutta velocity condition and the other wave height condition at the transom point. The numerically obtained solutions show good flows around the body with blunt stem and transom stern. It is reported to be able to assess the Froude number when planing starts by combining wave breaker indices, the values of resistance, lift and trim moment when the rise of center of gravity and the trim are varied and the Froude number when heave and pitch are unstable. It is also shown that a concave camber over the bottom of planing hull makes a profit for attitude and resistance. At last, a possibility is suggested to obtain a good evalution of resistance for body with transom stern by using high-speed approximation of inverse mirror solution.

Time Domain Computation of the Wave-Making Resistance of Ships

The Ship Stability Research Centre, Department of Naval Architecture and Marine Engineering, The Universities of Glasgow and Strathclyde, Scotland, UKA linearized three-dimensional potential flow formulation in time domain is applied to calculate wave-making resistance of ships in calm water. Steady-state perturbation potentials for resistance are obtained as the steady-state limit of the surge radiation impulse response function using the transient free surface source distribution over the body surface. Five different vessels are used to validate the present numerical approximation. The results, including steady-state wave-making resistance, sinkage force, trim moment, and wave profile along the waterline, are compared with other published numerical and experimental results.