Towing Tank Research Articles

Michell Investigation of the Significant Influence on the Hydrodynamic of a Warp-Chine Pentamaran

This study attempted to investigate the hydrodynamic performance of various pentamaran configurations with a focus on the interference flow around the component hulls. A computer simulation was conducted based on Michell’s thin ship theory alongside a commercial CFD computation as a comparison. Experiments in the towing tank were performed to validate the numerical calculations, resulting in some hydrodynamic characteristics on the far-field wave pattern, wave interference, wave resistance, and total resistance. Analyses on both transversal and divergent waves were performed to assess the magnitude of wave resistance occurring due to the placement of the side hull to the main hull. Analyses on both waves were also conducted to assess the magnitude of wave resistance due to the placement of outriggers. Looking at the results, numerical calculations based on Michell’s theory were in parallel with experimental data, particularly at Fn greater than .4. Michell’s theory was observed as doing a little preferable agreement with the results of experiments than CFD. Besides, flow patterns obtained numerically from Michell’s and CFD analyses appeared as identical to photographs observed in a towing tank. This investigation identified that a configuration with aligning placement of the main to side hull on the formation of arrow tri-hull, near the Kelvin angle, would cancel the wave formed by the leading hull and can be used as a practical setting to reduce the total wave resistance.

Application of Michell’s Integral to High-Speed Round-Bilge Hull Forms

Computational fluid dynamics (CFD) techniques are becoming increasingly popular in analyzing fluid flow problems in almost all branches of engineering, especially in resistance prediction of ships where complex fluid flow exists, although absolute accuracy is still limited. Application of CFD techniques in resistance prediction of ships is no longer new and is being enthusiastically embraced by researchers as can be seen from articles in journals and conference proceedings. Although the towing tank tests provide better absolute accuracy, modification to hull forms is limited, resulting from both practical and financial constraints. In this respect, CFD techniques and theoretical formulations have an added advantage in the sense that rapid modifications to hull forms can be carried out and results can be obtained, so that a comparative study could be made within a few hours. In this way, a naval architect is in a better position to select a hydrodynamically efficient design. Furthermore, robust and fast hydrodynamic computational methods are essential elements of advanced numerical optimization techniques. A good high-speed design should have favorable resistance qualities, so that operational capabilities are not degraded. Innumerable investigations available in the literature enumerate resistance prediction of systematic series of high-speed hull forms. In this article, the results of a comparative study on resistance of high-speed round-bilge hull forms using CFD techniques, theoretical analysis, and experimental results have been presented. The need for better hull forms and the increased interest in resistance performance call for better data and efficient algorithms to be available at the design and optimization stage to obtain the right balance between resistance and other conflicting requirements. This article provides a study of the following. The result of modeling in SHIPFLOW, which uses a combined potential-flow boundary-layer viscous-flow zonal approach. Application of a fast resistance estimation method with the wave-making resistance by Michell’s integral, the friction resistance by International Towing Tank Conference 1957. Application of Michell’s integral to determine the wave resistance of the systematic series models. Comparison between the total resistance coefficients obtained from experimental work, SHPFLOW, and the fast resistance estimation method.

Theoretical and experimental investigations of wave field around vessel in shallow water

Un-running vessel at the shallow-water road anchorage is under exposure to waves that come at arbitrary angle from the high sea. 3D waves from deep-sea area become practically 2D when entering shallow water. While mean periods are kept, waves become shorter and their crests become higher and sharpener than for deep-water ones. As a result of diffraction of waves that come from the deep-water sea at the vessel, a transformation zone appears where waves become 3D again. Dimensions of the waves’ transformation zone, character and height of waves in this zone specify safety of auxiliary crafts, e.g. tugboats, bunker vessels, pilot and road crafts, oil garbage collectors and boom crafts. In the complex 3D waves the trajectory of auxiliary vessel’s movement has to be safe, vessel’s motions have to be moderate. Besides waves’ height is one of the parameters that are used for forecast of movement of spilled oil. Last years the biggest part examination of waves’ problems was devoted to estimation of waves’ impact onto stationary or floating shelf facilities. For validity estimation, waves’ characteristics defined due to different theories, are compared with experimental ones. But characteristics of the waves around shelf facilities are hardly able to be compared to same ones of waves around bodies with vessel-type shape. At the experiments with vessels’ models, waves’ impact onto vessel was examined, but not the transformation of the waves themselves. So, comparing of waves area’s characteristics defined by both theoretical experimental ways is an actual problem. Aim of the paper is verification of results of wave area investigation; wave area is located around a vessel that is exposed of arbitrary angle waves at shallow water conditions. Description of experimental investigations of transformed waves in the towing tank is done; transformation zone appears around vessel’s model while running waves diffract on it. Distribution of waves’ amplitudes at the designated points was fixed by the special designed and manufactured unit. Experimental data is compared with computation results both of linear and non-linear theories. It was assumed that experimental results and theoretical data satisfactory meet each other; also that non-linear computations define the maximal values of waves’ amplitudes at all cases.

Modulation of the vibroacoustic behavior of a partially immersed cylindrical shell by regular waves

The effect of small regular waves on the vibroacoustic behavior of a partially immersed cylindrical shell under point-force excitation is studied using theoretical and experimental approaches. An approximate theoretical model of the circumferential vibroacoustic behavior of this system is established by assuming that the waves only change the draught of the shell. Vibroacoustic coupling equations are derived, and experimental verification is carried out in a towing tank. Both the theoretical and experimental results show modulation of the vibration and sound radiation of the shell by the water waves. These modulation effects are determined by the wave parameters, namely, (i) the fluctuation period of the resonant peaks in the radial velocity and sound pressure is determined by the wave period; (ii) the bandwidth of the modulation is determined by the wave amplitude. For a partially immersed cylindrical shell subjected to regular waves, the air–water demarcation points on the shell surface are the sources of the acoustic radiation. This radiation is modified by the water waves, and the propagation and radiation paths of the subsonic flexural waves and the resonant frequency of the shell are changed as a result. Based on this mechanism, a simple formula is derived to predict the modulation of the resonant frequency of the radial velocity and the sound pressure using the phase velocities of the subsonic flexural waves on the dry and wet parts of the shell. The predicted results are consistent with the theoretical and experimental results.

Experimental investigation on ventilated cavity flow of a model ship

A ventilated cavity model ship is investigated in a calm water towing tank under a moderate Froude number range. The gas ventilation rate required for cavity from early formation to complete coverage is explored, as well as the cavity morphology, closure modes, shedding features, and cavity growth characteristics. Several flow patterns of the cavity are classified, i.e., discrete foamy flow and initial coalescing cavity, corresponding to the formation stage; jet growth cavity(JGC), bifurcation growth cavity(BGC), meniscus growth cavity(MGC), and waterfall growth cavity(WGC), corresponding to the logarithmic growth stage; and bottle neck stable cavity, corresponding to the maturity stage. The shedding features of small periodic oscillation shedding, group rips evolution shedding, and wave pinch-off shedding are observed corresponding to JGC, BGC, and MGC and WGC, respectively. Three hypothetical mechanisms, rectangular jet dominant, pressure difference dominant, and wave pinch-off dominant, are proposed to be responsible for the formation of JGC, BGC and MGC, and WGC, respectively. A logarithmic mathematical model is related to cavity length and ventilation rate. The dependence of gas demand required to establish a complete coverage cavity on Froude number (0.37–1.10, ship length based) is more significant than Reynolds number (5.95 × 106–1.78 × 107, ship length based).

Machine learning post processing of underwater vehicle pressure sensor array for speed measurement

An array of pressure sensors can be used to correct the drift in inertial navigation systems for underwater vehicles (UVs) in absence of other navigation support systems such as acoustic positioning, GPS and Doppler velocity measurements. To date, multiple pressure sensor arrays have been designed, proposed, and tested to prove the concept. However, it has not been researched the inclusion of non-linearities is required in the post-processing. This paper focuses on the use of machine learning as a novel approach to improve the post-processing accuracy, including non-linearities caused by the vehicle acceleration on the estimated speed compared to the linear parametric equation methodology. A series of towing tank experiments have been conducted over an array of pressure sensors located on an UV platform. The results show that pressure measurement array requires the use of non-linear post-processing methodologies as linear methodologies are not able to accurately account for vehicle acceleration effects.

Hardware in the Loop Simulation and Control Design for Autonomous Free Running Ship Models

This paper presents an hardware-in-the-loop (HIL) simulation system tool to test and validate an autonomous free running model system for ship hydrodynamic studies with a view to verification of the code, the control logic and system peripherals. The computer simulation of the plant model in real-time computer does not require the actual physical system and reduces the development cost and time for control design and testing purposes. The HIL system includes: the actual programmable embedded controller along with peripherals and a plant model virtually simulated in a real-time computer. With regard to ship controller design for ship model testing, this study describes a plant model for surge and a Nomoto first order steering dynamics, both implemented using Simulink software suit. The surge model captures a quasi-steady state relationship between surge speed and the propeller rpms, obtained from simple forward speed towing tank tests or derived analytically. The Nomoto first order steering dynamics is obtained by performing the standard turning circle test at model scale. The control logic obtained is embedded in a NI-cRIO based controller. The surge and steering dynamics models are used to design a proportional-derivative controller and an LQR controller. The controller runs a Linux based real-time operating system programmed using LabVIEW software. The HIL simulation tool allows for the emulation of standard ship hydrodynamic tests consisting of straight line, turning circle and zigzag to validate the combined system performance, prior to actual for use in the autonomous free-running tests.

Spoiler plate effects on the suppression of vortex-induced motions of a single circular cylinder

Several methodologies have been proposed to mitigate vortex-induced motions (VIM) on offshore structures, such as the use of strakes, splitter plates, and spoiler plates. Among the mitigation devices, spoiler plates remain insufficiently explored. Given the situation, a parametric experimental investigation was undertaken to investigate the suppression of vortex-induced motions of a circular cylinder with a low aspect ratio using spoiler plates. The experiments were carried out in a towing tank. The circular cylinder tested had an aspect ratio (L/D) of 1.5, which is typically employed in monocolumns and columns of semi-submersible (SS) platforms. Several configurations were investigated by changing the spoiler plate aspect ratio (AR,S), the number of rings with spoiler plates along the vertical axis of the cylinder (nL), and the number of spoiler plates per ring (nD). The reduced velocities investigated varied from 1 to 12, and the Reynolds number from 12,500 to 95,000. The results demonstrated the occurrence of a desynchronization of the vortex shedding, which led to a significant reduction of the in-line and transverse amplitudes. Moreover, a decrease in drag forces of approximately 50% occurred for the most improved case, compared with a bare cylinder, demonstrating a potential solution for this mitigation device.

A simplified approach for voyage analysis of fouled hull in a tropical marine environment

ABSTRACT Tropical ocean environment teems with microorganisms, of which some adhere, either to the static ship hull or while en-route to the destination increasing roughness. A typical voyage analysis takes into account parameters like speed, power, propeller speed, displacement, fouling and weather conditions. Wind and wave data in the present study have been acquired from data buoys deployed along the voyage route from Chennai to Singapore. The differential equation of a ship undergoing acceleration is considered and solved to obtain voyage time and distance. Calculations to reach design speed were performed for various loading conditions. Limited experiments were performed in the towing tank using a ship model for various draft conditions. Granville's similarity law has been relied upon to arrive at the frictional resistance and is discussed in this paper. The loads due to wind and waves were specifically calculated for the ship with a fouled hull heading in its design speed.

Impact of trim on added resistance of KRISO container ship (KCS) in head waves: An experimental and numerical study

In this study, added resistance and motion responses of KRISO Container Ship (KCS) were evaluated experimentally and numerically in six different trim angles. A series of towing tank experiments were performed for six different trim angles at design speed in calm water and regular head waves. The ship motions and added resistance were measured for several wavelength conditions considering short and long wave ranges with wave steepness of 1/60. Next, computations of the towed model in calm water and waves were performed using Unsteady Reynolds-Averaged Navier-Stokes (URANS) CFD and 3-D potential methods. Effects of trim angles on added resistance were analysed and results concerning the performance of the vessel at different trim angles were plotted. Experimental and numerical results for the heave and pitch motions and the added resistance were compared and URANS CFD simulation results showed good agreement with the experimental data for the ship in head waves. Also, the results were compared to those from potential theory and range of trim and wave conditions were identified for the application of rapid linear potential flow method.

Planing craft control using pneumatically driven trim tab

Dynamic instabilities are quite common in planing crafts. This study deals with the elimination of one type of dynamic instabilities known as porpoising. The controller device is a pneumatically driven trim tab that applied to the transom of the boat. The governing dynamic equations of the planing craft and the pneumatically driven system are derived. The stability analysis of the system is carried out and porpoising instability is shown under some conditions. A dual control scheme consisting of the actuator control subsystem (inner control subsystem) and the planing craft (outer control subsystem) is proposed in order to control the planing craft from porpoising instability. Since the dynamic of the pneumatic actuator system is highly nonlinear, its governing equation is reduced to a linear equation through the feedback linearization method and then the linear–quadratic regulator (LQR) control method is used to control this system. The planing craft is also controlled based on an optimal control method with the help of the controlled actuator. Finally, the actuator controller performance is examined through tracking predefined trajectories, and then it is used to control the planing craft. The stability of the planing craft is also shown in the presence of the actuator under the conditions that cause the porpoising instability. The simulation results indicate that this model can effectively be implemented to counteract porpoising in the planing crafts. Eventually, a prototype of planing craft equipped with pneumatically actuated trim tabs is constructed and the effectiveness of the proposed control system is examined in a towing tank.

Determination of Drag and Lift Related Coefficients of an AUV Using Computational and Experimental Fluid Dynamics Methods

Determination of hydrodynamic coefficients is a vital part of predicting the dynamic behavior of an Autonomous Underwater Vehicle (AUV). The aim of the present study was to determine the drag and lift related hydrodynamic coefficients of a research AUV, using Computational and Experimental Fluid Dynamics methods. Experimental tests were carried out at AUV speed of 1.5 m s-1 for two general cases: I. AUV without control surfaces (Hull) at various angles of attack in order to calculate Hull related hydrodynamic coefficients and II. AUV with control surfaces at zero angle of attack but in different stern angles to calculate hydrodynamic coefficients related to control surfaces. All the experiments carried out in a towing tank were also simulated by a commercial computational fluid dynamics (CFD) code. The hydrodynamic coefficients obtained from the numerical simulations were in close agreement with those obtained from the experiments.

Unsteady RANS CFD Simulations of Sailboat’s Hull and Comparison with Full-Scale Test

The hydrodynamic investigation of a hull’s performance is a key aspect when designing a new prototype, especially when it comes to a competitive/racing environment. This paper purports to perform a fully nonlinear unsteady Reynolds Averaged Navier-Stokes (RANS) simulation to predict the motion and hydrodynamic resistance of a sailboat, thus creating a reliable tool for designing a new hull or refining the design of an existing one. A comprehensive range of speeds is explored, and results are validated with hydrodynamic full-scale tests, conducted in the towing tank facility at University of Naples Federico II, Italy. In particular, this work deals with numerical ventilation, which is a typical issue occurring when modeling a hull; a simple and effective solution is here proposed and investigated, based on the phase-interaction substitution procedure. Results of the computational fluid dynamic (CFD) campaign agree with the experimental fluid dynamic (EFD) within a 2% margin.

Resistance and wave characterizations of inland vessels in the fully-confined waterway

The resistance and ship-generated waves of two inland vessels in the fully-confined waterway are investigated based on numerical simulations. The simulation results are validated using towing tank experiments. The influences of the channel dimension, water depth, ship draught and velocity are studied. Specially, the ship resistance is characterized as a function of the blockage ratio and the theory of the linear ship wakes is corrected using the ship draught. These results are expected to predict the ship hydrodynamics in the restricted waterway and reveal the acting mechanism of the confinement effect on a certain level.

Effect of roughness in full-scale validation of a CFD model of self-propelled ships

This paper presents a comparison of full-scale computational fluid dynamics (CFD) simulations with speed trial measurements for a ro-ro vessel and a general cargo vessel. Significant work has been done on validating CFD simulation in model scale. However, in full-scale very few publicly available studies have been conducted due to limited access of validation data. The present study includes extensive validation and verification of both resistance, propeller open-water and self-propulsion simulations in both model and full-scale. The self-propulsion simulations include modelling of the free surface and rotation of the 3D propeller. Full-scale resistance and propeller open-water as well as model scale self-propulsion simulations show good agreement with towing tank measurements and predictions. However, the full-scale self-propulsion simulations using the traditional approach of including the roughness as a point force estimated by an empirical formula significantly underestimate the power from the speed trial measurements. By including the effect of hull and propeller roughness directly into the CFD model, by modifying the wall functions, the discrepancy between CFD and speed trial measurements decreases significantly. This indicates that inclusion of a roughness model directly into the CFD simulation could be a more accurate method than the traditional approach of using empirical formulas originally designed for towing tank extrapolation.

Marine hydrokinetic energy harvesting performance of diamond and square oscillators in tandem arrangements

The global demand for renewable energy sources has arisen significant research interests in marine hydrokinetic energy harvesting from flow induced vibrations. The current study deals with the hydrokinetic energy harvesting potentials of diamond and square oscillators at various inline configurations. Both the sharp edge sections and the wake induced vibrations in tandem arrangement can lead to galloping type of instability with high energy efficiency ratios. The present in-water towing tank experiments comprise 1DoF vibrating systems at Re ranging from 2 × 103 to 4.8 × 104.The results show that the upstream wake generally enhances the system efficiency and the mechanical work done by the oscillating square cylinder. With tandem diamond cylinders, the upstream vortices enrich both the overall oscillations and the harvested mechanical power by the downstream cylinder. The energy efficiency in the tandem configuration is 3–5 times higher than a single diamond cylinder. The TOPSIS algorithm is applied to sort the energy performance of different configurations. The study proves the extreme advantages of tandem diamond oscillators over square configuration in the extraction of the hydroelastic energy. High-amplitude, high-frequency vibrations of tandem diamond cylinders make them even surpass the typical circular cross-sections in generating the mechanical power.

The development, design and characterisation of a scale model Horizontal Axis Tidal Turbine for dynamic load quantification

The paper describes the development and characterisation of three 0.9 m diameter lab-scale Horizontal Axis Tidal Turbines. The blade development process has been outlined and was used to generate a design specification. Each turbine houses instrumentation to measure rotor thrust, torque and blade root bending moments on each blade, in both ‘flapwise’ and ‘edgewise’ directions. A permanent magnet synchronous machine and encoder are integrated to allow for servo-control of the turbine as well as to provide position and rotational velocity measurements, resulting in three turbines that can be individually controlled using speed or torque control. Analogue signals are captured via a real-time operating system and field programmable gate array hardware architecture facilitating sample rates of up to 2 kHz. Results from testing the pilot turbine at three differing facilities during the development process are presented. Here good agreement, less than 7% variation, was found when comparing the testing undertaken at various flume and tow tank facilities. Lastly, the findings of a test campaign to characterise the performance of each of the three turbines are presented. Very good agreement in non-dimensional values for each of the three manufactured turbines was found.

Fuzzy Control of Waves Generation in a Towing Tank

This paper presents the results of research related to the transformation of electrical energy into potential and kinetic energy of waves generated on the water surface. The waves are generated to model the environmental conditions for the needs of the model tests. The model tests are performed on model-scale objects to predict the features of full-scale maritime objects. It is done to improve human safety and the survivability of constructions. Electrical energy is transformed into the energy of the water waves using a wave maker. The wave maker considered is a facility with an electrohydraulic drive and an actuator submerged into the water. The actuator movement results in the waves being mechanically-generated in accordance with the wave maker theory. The study aimed to investigate the advantage of the newly implemented fuzzy-logic controller over the hitherto cascading proportional-integral controllers of the wave maker actuator. The research was focused on experimental investigation of the transformation process outcomes harvested under the fuzzy-logic controller, versus the cascading proportional-integral controllers. The waves were generated and measured in the real towing tank, located in the Maritime Advanced Research Centre (CTO S.A.). The investigation confirmed the advantage of the fuzzy-logic controller. It provides more accurate transformation of energy into the desired form of the water waves of specified parameters—frequency and amplitude—and more flat amplitude-frequency characteristic of the transformation process.

Investigation on the nonlinear effects of the vertical motions and vertical bending moment for a wave-piercing tumblehome vessel based on a hydro-elastic segmented model test

Although the nonlinear effects of the ship motions, wave-induced loads and structural responses of conventional vessels have been investigated experimentally and numerically in recent years, similar studies on the wave-piercing tumblehome (WPTH) vessels are rare, in spite of the urgent necessary due to their extensive applications. This paper experimentally investigated the nonlinear effects of the vertical motions and vertical bending moment (VBM) for a WPTH vessel based on a hydro-elastic segmented model test. The model test was carried out in head regular waves for three amplitudes, in the towing tank at Harbin Engineering University. Based on the band-pass filter technique, four kinds of decomposed harmonics are defined by mean offset, linear wave-frequency response, nonlinear wave-frequency response and nonlinear high-frequency vibration. Meanwhile, based on mean offset, three kinds of combined responses are defined by considering linear wave-frequency response, nonlinear wave-frequency response and nonlinear high-frequency vibrations step by step. The transfer functions of vertical motions and VBM are presented as function of the incident wave frequency, and the measured data and VBM were compared with numerical calculations for validation. With the method proposed above, the results of VBM are analyzed by focusing on the influences of decomposed harmonics on the amplitudes and asymmetry of combined responses. Strong nonlinear effects are observed in the VBM. The nonlinearity of VBM can be identified by the significant amplitudes of decomposed harmonics, variation of amplitude with the wave amplitude and remarkable asymmetry about the zero axis. The hogging VBM of WPTH vessel can be even larger than the sagging VBM, which is in contrast to the general experiences from conventional vessels. Combined the calculated relative motion with the decomposed and combined histories of VBM, the occurrence of the transient impact and its characteristics are discussed, and these are then used to analyze the unconventional asymmetry of VBM. Furthermore, the influences of the bow shape over the vertical motions and VBM are discussed. At the end of the paper, the uncertainties in the test are provided.

Assessing the applicability of flow measurement by using non-contact observation methods in open channels.

River discharge is one of the important hydraulic data to evaluate and manage the regional water resources. Estimating river discharge is generally based on field measurements. The measurement data are then applied to construct water level-discharge rating curves. However, it is sometimes difficult to obtain accurate discharge data due to the high uncertainty of flow. A commonly used technique is the propeller-type flowmeters (PTF), which average the results of 1-, 2-, or 3-point methods to obtain a vertical mean velocity. In this study, three types of flowmeters were employed to compare the accuracy of flow measured. The devices were calibrated using a tow tank testing: PTF, acoustic Doppler profiler (ADP), and radar surface velocimeter (RSV). To assess the applicability of the non-contact observation method, a series of 16 experiments in channels were conducted. Surface velocity measurement using the RSV was compared with the measurements obtained by PTF. The relationship between measured surface velocity of RSV and measured vertical mean velocity of PTF was established. The results show that the RSV can effectively estimate the river discharge in the open channel flow.