Hydrodynamic Moments Research Articles

Numerical Study on the Water Takeoff Performance of Unmanned Seaplane

The research on the water takeoff performance of unmanned seaplane is of great significance to its overall optimization design and taxiing stability control system. This paper proposes a new numerical method for solving the water takeoff performance of unmanned seaplane. The aerodynamics and hydrodynamics are solved separately by decoupling solution principle. The influence of aerodynamic force and moment, engine thrust and moment on hydrodynamic force and taxiing attitude is considered synthetically by the vertical approximate balance hypothesis and instantaneous pitch balance hypothesis. The discrete velocity piecewise solution is used to solve the takeoff process. The results show that the taxi resistance peak appeared in the range of 0.3~0.35 times of takeoff velocity, and the greater is the trim angle, the smaller are the hydrodynamic resistance and hydrodynamic moment when the unmanned seaplane taxiing at medium or high speed. What's more, compared with the fixed trim angle hypothesis, the combined trim angle method is more consistent with the actual takeoff process.

RANS investigation of the hydrodynamic interactions between two approaching ships in shallow water considering the effects of drift motion and rudder deflection

There exists a high risk of collision between two approaching ships in the lightering operation. The investigations of the hydrodynamic interactions between such two ships are of great interest to ensure navigation safety. To this end, the RANS (Reynolds-Averaged Navier-Stokes) technique based on OpenFOAM is employed to simulate the flow around two approaching Wigleys in this work. The two Wigleys are identical. Due to the slow ship speed, the water-air flow is simplified as the double-body flow in the present consideration. The computed hydrodynamic side forces and yaw moments are compared with the available experimental data, and acceptable agreements are found. The effects of lateral distance between the two ships, water depth, rudder deflection, ship scale and drift motion on the hydrodynamic forces and moments on both ships are analyzed. This study shows that the suction forces and bow-out moments generally become larger as the water depth or lateral distance decreases, while the suction forces trends to become repulsion forces when both the water depth and lateral distance are very small. It is also found that the rudder deflection of one ship has a very little hydrodynamic disturbance on the other ship, rather than the drift motion which affects much the hydrodynamic forces and moments on the other ship especially in very shallow water.

An efficient and accurate approach for zero-frequency added mass for maneuvering simulations in deep and shallow water

An overall increase in ship sizes in conjunction with a general decrease in ship powering has increased the relevance of reliably predicting a ship’s maneuvering performance. These predictions rely often on mathematical models based on modified Taylor series expansion, where nonlinear multivariate algebraic polynomials that require hydrodynamic coefficients are determined from physical or numerical planar motion mechanism (PMM) tests. The added mass coefficients are needed for a fast and accurate simulation. For maneuvering simulations, these added mass coefficients should be obtained for zero frequency. Despite the advance of high-performance computing, numerical PMM tests based on solving the unsteady Reynolds-averaged Navier–Stokes equations is time-consuming and often not suitable for practical design-related investigations. Furthermore, hydrodynamic coefficients obtained from PMM tests are frequency-dependent. An alternative to numerical PMM tests consists of steady numerical tests. This kind of analysis requires much less computing time and effort for the users. However, the acceleration-dependent hydrodynamic forces and moments cannot be derived from these steady computations. Here we introduced a fast and accurate procedure to obtain water depth-dependent zero-frequency added mass and added moment of inertia. Using Euler Equations, we impulsively accelerated the ship hull over only one time step. We validated our method against an analytical solution for an ellipsoid. We performed systematic computations of added mass and added moment of inertia for two benchmark containerships, a benchmark tanker, and a cruise ship, thereby demonstrating the effects of water depth on added mass for different hull shapes.

Experimental study to investigate the effects of bridge geometry and flow condition on hydrodynamic forces

Stream-crossing bridges are designed to pass the design flow while maintaining a minimum freeboard; however, flood events are reported as the most frequent cause of bridge failure in the U.S. and around the world. Floodwater exerts significant forces on bridges, shearing and overturning their decks, which may cause them to fail. When a bridge fails, it loses its total or partial serviceability, causing fatalities, delays in emergency transportation and evacuation efforts, and economic losses. An accurate estimation of hydrodynamic forces on a bridge superstructure, therefore, is vital to assess its vulnerability to flooding. The purpose of this study is to investigate the effects of bridge geometry and flow conditions on hydrodynamic forces. A series of experiments on bridge scale models were performed in a laboratory flume to analyze the response of the drag, lift and moment coefficients to bridge geometry (deck width, and girders height and shape) and flow conditions (inundation ratio, proximity ratio, blockage ratio, and Froude number). The experimental results showed that hydrodynamic force and moment coefficients are dependent on the extent of the bridge submergence, Froude number, and proximity of the bridge to the streambed. The relationship between bridge geometry and force coefficients indicated that the geometric features of bridges have a large influence on flow field as well as drag, lift, and moment coefficients.

CFD database method for roll response of damaged ship during quasi-steady flooding in beam waves

A database method is proposed to predict the roll response of a damaged ship upon quasi-steady flooding in regular beam waves. To construct the hydrodynamic database, the hydrodynamic loads acting on the damaged ship are decomposed into wave excitation moment and motion-induced moment, which are calculated based on computational fluid dynamics (CFD). The wave excitation moments for different wave frequencies are formulated using the high-order trigonometric function. The ship hydrodynamic coefficients (i.e., added moment of inertia and damping coefficient) for different roll frequencies and amplitudes are regressed from the motion-induced moment and then used as samples for database construction by cubic spline interpolation. The Runge-Kutta method is employed to solve the ship's roll motion equation, in which the hydrodynamic coefficients are updated according to the roll amplitude in real time. To test the performance of the present method, the roll motion responses of a damaged frigate DTMB-5415 are computed. The results obtained by the database method are in good agreement with the CFD results and experimental data. The characteristics of motion-induced moment, roll hydrodynamic coefficients and wave excitation moment are analyzed. In addition, the influence of various factors (e.g., sensitivity of mesh and time step, order of regression method, interval of sample points and type of load decomposition model) on the computation accuracy are also discussed. Compared to directly performing CFD simulation of damaged ship motion in waves, the proposed database method can predict the ship's roll response with high efficiency and acceptable accuracy.

자유항주모형시험과 회귀분석을 통한 선체 동역학 모델의 개발

The present study suggests a procedure of establishing a ship dynamics modeling by regression of free-running model test results. The hydrodynamic force and moment of the whole model ship is derived from the low-pass filtered acceleration in the turning circle and zigzag maneuver tests. Force and moment of the propeller and rudder are separated from that of the whole ship to acquire the hull force and moment terms, based on the principles of the component model. The low-pass filter frequency is verified in prior to dynamics modeling, to find the threshold frequency of 2.5 Hz. The dynamics modeling of the hull is compared with the component modeling by captive model tests. Because of strong correlation between sway velocity, yaw angular velocity, and heel angle, each maneuvering coefficient is not able to be validated, but the whole modeling shows good agreement with the captive model tests.

Hydrodynamic Forces and Wake Distribution of Various Ship Shapes Calculated Using a Reynolds Stress Model

The Reynolds-averaged Navier–Stokes (RANS)-based computational fluid dynamics (CFD) calculation using a two-equation turbulence model, such as the k–omega shear-stress transport (SST) model, is a mainstream method with sufficient accuracy for the estimation of integral hydrodynamic forces and moment at both the model-scale and full scale. This paper confirmed that the Reynolds stress model (RSM) has sufficient estimation accuracy of viscous resistance and wake distribution at the hull design stage. Herein, the ability of RSMs to estimate the viscous resistance and wake distribution of a JBC ship is evaluated. Specifically, the verification and validation (V&V) method is employed to indicate the numerical and model uncertainties of each turbulence model used to estimate the viscous resistance. The RSMs showed higher numerical uncertainty than the k–omega SST. However, the uncertainty of the experimental measurements is generally smaller than the numerical uncertainty. Moreover, the linear pressure–strain (LPS) and the linear pressure–strain two-layer (LPST) models show less comparison error of the viscous resistance than the k–omega SST. Furthermore, the LPST and k–omega SST models are applied to twenty ships with various full and fine hull forms to calculate the viscous resistance and compare it with the experimental results. The viscous resistance of the LPST model showed a small difference when employed in experimental fluid dynamics (EFD) and CFD calculations. Using the LPST model, the viscous resistance can be estimated with high accuracy in our setting. For industrial use, this study could provide an important insight into the designing of various types of vessels.

Water Depth Effect to Ferry Ship Resistance with Computational Fluid Dynamic Method

Predicting ship resistance in shallow water conditions is very important for the operational considerations of a ship. Shallow waters greatly affect viscous resistance, wave resistance and also propulsion efficiency. This study aims to determine the effect of water depth on the resistance of the ferry. The CFD method was used to simulate this phenomenon and its effects on the hull starting from the coefficient of ship resistance, surge force, sway force and yaw (hydrodynamic moment). Input speed at 9 – 13 knots with depth ratio H/T = 4 (deep water), H/T = 3 (deep water), H/T = 2 (medium water), H/T = 1.3 (shallow water), and H/T = 1 (very shallow water). The simulation results show a significant increase in total resistance, surge force, sway force, and hydrodynamic moment in shallow water conditions in every speed variation. This condition is caused by the increase in pressure received by the ship's hull when operating in shallow water.

Sharp Profile for Icebreaking Propellers to Improve Their Ice and Hydrodynamic Characteristics

Stern-first operation under severe ice conditions (ridges) is one of the most effective modes to increase the operating efficiency of icebreakers and ice ships. However, when a ship overcomes the ridge astern, the propellers continuously interact with ice blocks, and ice moment affects the propeller and the main engine. This leads to propeller speed drop and propeller thrust reduction. Propeller stop is also possible. This is the reason why the propeller ice moment needs to be decreased. Blade profiles with a sharp leading edge are used for this purpose because their thickness is significantly less than that of a traditional icebreaking profile. The application of sharp profiling makes it possible to significantly reduce the ice moment (ice loads) on the propeller, reduce the drop in its speed, and increase the hydrodynamic thrust. The main task when installing blades with sharp profiles is to ensure the strength of their leading edges exposed to ice pressure. In this article, the authors tackle upon some methods of assigning integral and local ice loads on propellers. Solutions for ensuring the local strength of the blade edges were developed and presented. The influence of sharp profiling on the hydrodynamic and cavitation characteristics of ice propellers was considered. The article presents examples of calculating the hydrodynamic propeller thrust and moment, as well as ice loads on a propeller with a sharp and traditional profile, when an ice ship moves through a ridged ice isthmus with its stern first.

Pitch motion reduction of semisubmersible floating offshore wind turbine substructure using a tuned liquid multicolumn damper

A tuned liquid multicolumn damper (TLMCD) composed of three liquid columns and bottom connecting pipes is proposed to reduce the pitch motion of a semisubmersible substructure for wind turbine. The pitch damping of the semisubmersible floating offshore wind turbine (FOWT) substructure scale model with TLMCD in regular waves is studied experimentally. The TLMCD with the optimized mass ratio and tuning ratio provides effective pitch motion control especially near resonance period. The OpenFOAM is further developed to simulate dynamic response of the FOWT substructure under internal sloshing and external wave excitations, where the wave elevation and the FOWT substructure response have been validated against the experimental data. The design natural frequency of the TLMCD is equal to that of the FOWT substructure, and the analytical solutions and numerical results of liquid column decay are in good agreements. The motion response of the FOWT substructure, velocity distribution and the wall pressure are used to analyze the damping mechanism of the TLMCD. Based on the hydrodynamic moment, the energy dissipation characteristics of the TLMCD are analyzed quantitatively, where the damping and exciting moment are distinguished from each other. The analyses show that the TLMCD has the best damping effect near resonance frequency, where the 2.0% mass ratio of the liquid can reduce the maximum pitch motion by 10.84% to 18.53%.

Fluid structure interaction of a subaqueous pendulum: Analyzing the effect of wake correction via large eddy simulations

The dynamic behavior of a subaqueous cylindrical pendulum and corresponding flow dynamics are investigated. The objectives were twofold: (i) to examine whether the two-dimensional model equations sufficiently capture the three-dimensional dynamics and (ii) to investigate the emerging three-dimensional vortical flow structures. Large eddy simulations with two-way coupling fluid structure interaction were carried out using the immersed boundary method to simulate the motion of the pendulum and its interactions with the initially stagnant water. The resulting pendulum motion is compared against measured data obtained in a series of experimental tests to validate the simulation results and the model equations with and without wake corrections. An analysis of the flow vorticity revealed the development of a vortex ring during the first swing and the formation of tip vortices. The evolution of the vortex rings emerging from the motion of the subaqueous cylindrical pendulum was visualized using Q-criteria showing a reasonable agreement with vortical structures observed in the experiment using particle imaging velocimetry. The hydrodynamic moments acting on the simulated pendulum and the moments calculated from the model equations are analyzed. Using the insights from these numerical simulations, a modification of the wake correction is proposed to enhance the accuracy of the rate of decay and period. The transient effect of coherent flow on pendulum dynamics, especially the added mass effect, is discussed.

An experimental study of the water entry trajectories of truncated cone projectiles: The influence of nose parameters

We report on an experimental study of the trajectories of truncated cone projectiles on water entry. The water entry trajectory stability is of great significance to the motion control of projectile. In this paper, the truncated cone nose shape can be described by the area of the leading plane and the cone angle α. Two high-speed cameras are used to capture the trajectories of the projectiles and the initial stage of cavity dynamics. We reveal that the trajectory stability of a projectile is highly dependent on the wetted surface of the nose, which is determined by the location of the separation line between the surfaces of the cavity and body. The increase in the leading plane area is beneficial to the formation of a stable trajectory, in which only the leading plane is wetted. In an unstable trajectory case, the large hydrodynamic moment from the wetted surface on the side of the nose causes a significant rotation of the projectile. However, for the projectile with the cone angle α≳60°, though the side of the nose is fully wetted, the trajectory of the projectile turns into stable again. Results show that the attitude deflection of the projectile is determined by the cone angle of the nose. It is also found that the attitude deflection results in an irregular cavity, which further aggravates the rotation of the projectile. We quantify the relationship between the trajectory stability and two nose parameters systematically, and a phase diagram is obtained for a large parameter space. The findings in this work can be used as a reference for future designs to ensure stable trajectories on water entry.

Changes in the Hydrodynamic Characteristics of Ships During Port Maneuvers

To reach a port, a ship must pass through a shallow water zone where seabed effects alter the hydrodynamics acting on the ship. This study examined the maneuvering characteristics of an autonomous surface ship at 3-DOF (Degree of freedom) motion in deep water and shallow water based on the in-port speed of 1.54 m/s. The CFD (Computational fluid dynamics) method was used as a specialized tool in naval hydrodynamics based on the RANS (Reynolds-averaged Navier-Stoke) solver for maneuvering prediction. A virtual captive model test in CFD with various constrained motions, such as static drift, circular motion, and combined circular motion with drift, was performed to determine the hydrodynamic forces and moments of the ship. In addition, a model test was performed in a square tank for a static drift test in deep water to verify the accuracy of the CFD method by comparing the hydrodynamic forces and moments. The results showed changes in hydrodynamic forces and moments in deep and shallow water, with the latter increasing dramatically in very shallow water. The velocity fields demonstrated an increasing change in velocity as water became shallower. The least-squares method was applied to obtain the hydrodynamic coefficients by distinguishing a linear and non-linear model of the hydrodynamic force models. The course stability, maneuverability, and collision avoidance ability were evaluated from the estimated hydrodynamic coefficients. The hydrodynamic characteristics showed that the course stability improved in extremely shallow water. The maneuverability was satisfied with IMO (2002) except for extremely shallow water, and collision avoidance ability was a good performance in deep and shallow water.

Experimental study on trans-media hydrodynamics of a cylindrical hybrid unmanned aerial underwater vehicle

This paper presents the experimental investigation of the hybrid unmanned aerial underwater vehicle (HAUV) hydrodynamics during the water-air trans-media process. A novel experimental platform that can control the water-air transition profile of HAUV with different velocities, accelerations, and attitudes is firstly developed with the capability of measuring the precise hydrodynamic forces of the vehicle. This technique makes it possible to perform a series of constrained model experiments to obtain the hydrodynamic forces of HAUV during the water-air transition. The hydrodynamic forces on the HAUV changing with the velocity, acceleration, and attitude of the vehicle during the transition are measured. Analysis of the test data shows that when the velocity is more than 0.1 m/s, the suction force caused by the free surface effect increases first and then remains constant with the velocity increasing. The maximum value of the suction force is about 10% of gravity. At low accelerations, less than 0.15 m/s2, the vertical force is approximately proportional to the square of time. When the attitude is less than 15°, the hydrodynamic moment first decreases and then increases to a certain value. When the attitude increases, the hydrodynamic moment decreases directly to a certain value. The influence of the moment caused by hydrodynamic forces other than time-varying buoyancy cannot be ignored. These results contribute to the design, theoretical modeling, numerical simulation, and control of HAUV.

The controlled impact of elastic plates on a quiescent water surface

The impact of flexible rectangular aluminum plates on a quiescent water surface is studied experimentally. The plates are mounted via pinned supports at the leading and trailing edges to an instrument carriage that drives the plates at constant velocity and various angles relative to horizontal into the water surface. Time-resolved measurements of the hydrodynamic normal force ($F_n$) and transverse moment ($M_{to}$), the spray root position ($\xi _r$) and the plate deflection ($\delta$) are collected during plate impacts at 25 experimental conditions for each plate. These conditions comprise a matrix of impact Froude numbers${Fr} = V_n(gL)^{-0.5}$, plate stiffness ratios$R_D= \rho _w V_n^2 L^3D^{-1}$and submergence time ratios$R_T= T_sT_{1w}^{-1}$. It is found that$R_D$is the primary dimensionless ratio controlling the role of flexibility during the impact. At conditions with low$R_D$, maximum plate deflections on the order of$1$ mm occur and the records of the dimensionless form of$F_n$,$M_{to}$,$\xi _r$and$\delta _c$are nearly identical when plotted vs$tT_s^{-1}$. In these cases, the impact occurs over time scales substantially greater than the plate's natural period, and a quasi-static response ensues with the maximum deflection occurring approximately midway through the impact. For conditions with higher$R_D$($\gtrsim 1.0$), the above-mentioned dimensionless quantities depend strongly on$R_D$. These response features indicate a dynamic plate response and a two-way fluid–structure interaction in which the deformation of the plate causes significant changes in the hydrodynamic force and moment.

An investigation into the nonlinear effects in the roll motion of 2-D bodies by SPH method

Nonlinear effects in the roll motion of a 2–D body in the free surface are investigated by utilizing the Smoothed Particle Hydrodynamics (SPH) method. The continuity and Navier–Stokes equations are solved by employing the Weakly Compressible SPH (WCSPH) approach and our in-house code, which relies on the accumulated development process through the authors’ previous works including the fluid–solid coupling modeling by viscous penalty technique, (Tofighi et al. (2015)), and a hybrid Velocity-Variance Free Surface (VFS) and Artificial Particle Displacement (APD) algorithm (Ozbulut et al., 2014, 2018, 2020; Kolukisa et al., 2020). In this work, the GPU parallelization of the code is performed, to mitigate the computational burdens of the relatively high number of particles due to the long wavelengths generated by the roll motion of the cylinder. Presently, an alternative and exact approach in reducing the equation of roll motion with quadratic damping term is introduced. In parallel to this approach, a quadratic regression equation approximating the hydrodynamic roll moment is also employed. Aside from hydrodynamic coefficients, with linear and quadratic damping coefficients, vortex flow characteristics are also presented, qualitatively and quantitatively. It is understood from the results that, with the present capability, the WCSPH approach introduced is able to disclose nonlinearities inherently exist in the roll motion of oscillatory bodies in the free surface.

CFD Research on the Hydrodynamic Performance of Submarine Sailing near the Free Surface with Long-Crested Waves

The simulations of submarine sailing near the free surface with long-crested waves have been conducted in this study using an in-house viscous URANS solver with an overset grid approach. First, the verification and validation procedures were performed to evaluate the reliability, with the results showing that the generation of irregular waves is adequately accurate and the results of total resistance are in good agreement with EFD. Next, three different submerged depths ranging from 1.1D to 3.3D were selected and the corresponding conditions of submarine sailing near calm water were simulated, the results of which were then compared with each other to investigate the influence of irregular waves and submerged depths. The simulations of the model near calm water at different submerged depths demonstrated that the free surface will cause increasing resistance, lift, and bow-up moments of the model, and this influence decreases dramatically with greater submerged depths. The results of the irregular wave simulations showed that irregular waves cause considerable fluctuations of hydrodynamic force and moments, and that this influence remains even at a deeper submerged depth, which can complicate the control strategies of the submarine. The response spectrum of hydrodynamic forces and moments showed slight amplitudes in the high-frequency region, and the model showed less sensitivity to high-frequency excitations.

Identification of particular hydrodynamic parameters for a modular type 4 DOF underwater vehicle by means of CFD method

PurposeThe development of robust control algorithms for the position, velocity and trajectory control of unmanned underwater vehicles (UUVs) depends on the accuracy of their mathematical models. Accuracy of the model is determined by precise estimation of the UUV hydrodynamic parameters. The purpose of this study is to determine the hydrodynamic forces and moments acting on an underwater vehicle with complex body geometry and moving at low speeds and to achieve the accurate coefficients associated with them.Design/methodology/approachA three-dimensional (3D) computer-aided design (CAD) model of UUV is designed with one-to-one dimensions. 3D fluid flow simulations are conducted using computational fluid dynamics (CFD) software programme in the solution of Navier Stokes equations for laminar and turbulent flow analysis. The coefficients depending on the hydrodynamic forces and moments are determined by the external flow analysis using the CFD programme. The Flow Simulation k-ε turbulence model is used for the transition from laminar flow to turbulent flow. Hydrodynamic properties such as lift and drag coefficients and roll and yaw moment coefficients are calculated. The parameters are compared with the coefficient values found by experimental methods.FindingsAlthough the modular type UUV has a complex body geometry, the comparative results of the experiments and simulations confirm that the defined model parameters are accurate and close to the actual experimental values. In the proposed k-ε method, the percentage error in the estimation of drag and lifting coefficients is decreased to 4.2% and 8.39%, respectively.Practical implicationsThe model coefficients determined in this study can be used in high-level control simulations which leads to the development of robust real-time controllers for complex-shaped modular UUVs.Originality/valueThe Lucky Fin UUV with 4 degrees of freedom is a specific design and its CAD model is first extracted. Verification of simulation results by experiments is generally less referenced in studies. However, it provides more precise parameter identification of the model. Proposed study offers a simple and low-cost experimental measurement method for verification of the hydrodynamic parameters. The extracted model and coefficients are worthwhile references for the analysis of modular type UUVs.

Hydrodynamic Modeling and Parameter Identification of a Bionic Underwater Vehicle: RobDact.

In this paper, the hydrodynamic modeling and parameter identification of the RobDact, a bionic underwater vehicle inspired by Dactylopteridae, are carried out based on computational fluid dynamics (CFD) and force measurement experiment. Firstly, the paper briefly describes the RobDact, then establishes the kinematics model and rigid body dynamics model of the RobDact according to the hydrodynamic force and moment equations. Through CFD simulations, the hydrodynamic force of the RobDact at different speeds is obtained, and then, the hydrodynamic model parameters are identified. Furthermore, the measurement platform is developed to obtain the relationship between the thrust generated by the RobDact and the input fluctuation parameters. Finally, by combining the rigid body dynamics model and the fin thrust mapping model, the hydrodynamic model of the RobDact at different motion states is constructed.

THE IMPACT OF SELECTED PARAMETERS ON PRESSURE DISTRIBUTION IN THE FACE THROTTLE OF A CENTRIFUGAL PUMP AUTOMATIC BALANCING DEVICE

Face throttles are a necessary functional element of non-contact face seals and automatic balancing devices of centrifugal pumps of different constructions. To calculate the hydrodynamic forces and moments acting on the rotor and fluid flow through the automatic balancing device, it is necessary to know the pressure distribution in the cylindrical and face throttle when considering all important factors which predetermine fluid flow. The face throttle surfaces are moving, which leads to unsteady fluid flow. The movement of the walls of the face throttle causes an additional circumferential and radial flow, which subsequently leads to the additional hydrodynamic pressure components. The paper analyses viscous incompressible fluid flow in the face throttle of an automatic balancing device taking into account the axial and angular displacements of throttle’s surfaces and the inertia component of the fluid. The effect of local hydraulic losses as well as random changes in the coefficients of local hydraulic resistance at the inlet and outlet of the throttle is analysed.