Towing Tank Research Articles

예인수조 LDV를 이용한 평판 경계층과 와이어 타입 난류촉진장치의 상호작용에 관한 연구

The present study aims to investigate the interaction of a wire-type turbulence stimulator and the laminar boundary layer on a flat plate by flow field measurement. For the towing tank tests, a one-dimensional Laser Doppler Velocimetry (LDV) attached on a two-axis traverse was used to measure the streamwise velocity component of the boundary layer flow in zero pressure gradient, disturbed by a turbulence stimulator. The wire diameter was 0.5 and 1.0 mm according to the recommended procedures and guidelines suggested by the International Towing Tank Conference. Turbulence development by the stimulator was identified by the skin friction coefficient, mean and Root Mean Square (RMS) of the streamwise velocity. The laminar boundary layer with the absence of the wire-type stimulator was similar to the Blasius solution and previous experimental results. By the stimulator, the mean and RMS of the streamwise velocity were increased near the wall, showing typical features of the fully developed turbulent boundary layer. The critical Reynolds number was reduced from 2.7×10<SUP>5</SUP> to 1.0×10<SUP>5</SUP> by the disturbances caused by the wire. As the wire diameter and the roughness Reynolds number (Rek) increased, the disturbances by the stimulator increased RMS of the streamwise velocity than turbulent boundary layer.

An Experimental Analysis of Active Pitch Control for an Assault Amphibious Vehicle Considering Waterjet-Hydrofoil Interaction Effect

The present study aims to reduce the pitch motion of an assault amphibious vehicle system in seaways by waterjet impeller revolution rate control. A series of seakeeping tests were performed in a towing tank with a 1/4.5-scale model. This vehicle is manufactured as a box-shaped hull, and since an appendage that generates lift force is attached, the amount of change in pitch motion is large according to the forward speed. For pitch motion reduction, the impeller revolution rate and resultant pitch moment were controlled through a proportional-integral-derivative controller. Improvements in seakeeping performance were examined in both regular and irregular conditions by the model tests in terms of root mean square of pitch motion. The tuned controller decreased pitch motion by more than 60%.

THE RESISTANCE ASPECT OF FISHING BOAT SKIPJACK POLE AND LINE

The operation of fishing vessels skipjack pole and line contributes in catching tuna and skipjack fishes particularly in Indonesian waters. A previous study conducted by the authors found that there was no suitable method provided for the resistance computation atearly ship design phase. Besides, there was aninitial trim existed on the vessel during the operation which contributes for the resistance. The purpose of the study is to find the difference of resistance between the model test and the existing methods. The study was executed also to find the effect of initial trim of the vessel. The study began with collecting the database of a parent ship then to develop and transform into a model-scale for testing purpose in the towing tank. The results of model test were converted to the full-scale vessel. The resistance of full-scale vessel was computed based on the Holtrop and Guldhammer methods. The result of full-of resistance obtained from the model test and the methods were collected, evaluated and compared. The results showed the difference of the resistance for all methods. The result of model test is greater 21 % than that of Holtrop method at the service speed of 10 knots. Meanwhile, the result of model test is lower 14 % than that of Gulhammer method at the same speed. In addition, at the speed of 10 knots the initial trim of 0.5O increase 5 % ofthe resistance, the initial trim of 1O increase 10 % of resistance and the initial trim of 2O increase 16 % of resistance compared to the vesselwithout initial trim. In conclusion, the existing resistance methods are not suitable to be applied for skipjack pole and line fishing vessels. In addition, the initial trim contributes to increase the resistance and should be avoided during the vessel operation.

Experimental tests and CFD simulations of a horizontal wave flow turbine under the joint waves and currents

This paper focus on the comparison of traditional vertical-axis turbines and a newly proposed horizontal wave flow turbine. Firstly, the major design parameters of this new concept are presented. Then a series of experimental tests of a prototype in the towing tank have been performed to investigate its hydrodynamic performance. It is found that the highest energy conversion rate is about 24.83% and the average impeller rotation power also reaches its maximum value of 352 W among all the six design cases. In addition, the Computational Fluid Dynamics (CFD) simulations have been performed to investigate its hydrodynamic characteristics and energy harvesting efficiency under the combined action of waves and currents. The results also show that the amplitude of the turbine's thrust and torque moment fluctuations increase with increasing motion amplitude. Furthermore, the wave dynamics cause instantaneous fluctuations in the flow field distribution for both types and the horizontal wave flow turbine has a more stable flow field. The comparison of both types of turbine based on the CFD simulations show that the torque moment, thrust force, and lateral force coefficients of the horizontal wave flow turbine are approximately 1.2–1.5 greater than those of the vertical axis turbine. Therefore, the horizontal wave flow turbine shows better energy harvesting efficiency if compared with the traditional vertical one.

Linear frequency modulation signal for channel impulse response measurement in shallow water environment

Understanding the characteristics of channel propagation plays a very important role in the design of underwater acoustic communication. Underwater acoustic channel (UAC) model is usually carried out by measuring directly at a specific location and time. This paper presents the use of Linear Frequency Modulation (LFM) signal for channel impulse response estimation and power delay profile (PDP) measurement in a shallow water environment. Measurements were made in a laboratory-scale towing tank with three variations of distance and depth. Furthermore, the measurement results are compared with the existing underwater acoustic channel model. The measurement results show that the response impulses at the three distances have almost the same pattern and the resulting amplitude is similar to the existing underwater acoustic channel model simulation. The RMS delay spread generated from the three distances has a value of about 0,2 msec. Meanwhile, the maximum excess delay obtained at a distance of 120 m has the highest value, which is 29 msec.

Experimental investigation into the improvement of self-starting capability of vertical-axis tidal current turbine

Indonesia has a lot of tidal current energy resources that can be harnessed using a vertical-axis turbine. However, one of the disadvantages of the vertical-axis turbine is its low self-starting capability. Under the actual condition, tidal current velocity is fluctuated and sometimes is very low, making the turbine unable to rotate. The purpose of this study is to investigate and improve the self-starting capability of a vertical-axis turbine. The study was conducted experimentally using a towing tank in Institut Teknologi Sepuluh Nopember (ITS) at Surabaya, Indonesia. The speed of towing carriage was set with the starting condition of 0 up to 0.4 m/s with an increment of 0.1 m/s. The stages of the test are as follows: first, the original vertical-axis turbine, a turbine with three straight-blades, was investigated; second, the modified vertical-axis turbine (inclined-blade) was proposed to increase the self-starting capability. The inclined-blade turbine has smooth self-start characteristics compared with the straight-blade turbine. It can start rotating at 0.2 m/s with a turbine rotation speed of 2.25 rpm. The original vertical-axis turbine with straight blades can start rotating at 0.3 m/s with a turbine rotation speed of 1.93 rpm. Therefore, a vertical-axis turbine modified with inclined-blades has better self-start capability that is suitable to implement in an area with low tidal current velocity.

Experimental Investigation of Vortex-Induced Vibrations on Yawed and Inclined Flexible Cylinders

Abstract An experimental setup was built to investigate the vortex-induced vibration (VIV) phenomenon on yawed and inclined flexible cylinders, in which five yaw angles θ = 0 deg, 10 deg, 20 deg, 30 deg, and 45 deg and five azimuth angles β = 0 deg, 45 deg, 90 deg, 135 deg, and 180 deg were combined. The experiments were carried out in a towing tank facility at Reynolds numbers from 1800 to 18,000, comprising vibrations up to the eighth natural mode. Time histories of displacements were recorded using a submerged optical system that tracks 17 reflective targets. A modal decomposition scheme based on Galerkin’s method was applied, aiming multimodal behavior investigations. Such an approach allowed the analysis of the modal amplitude throughout time, revealing interesting results for such a class of VIV tests. The flexible cylinder total response is generally a combination of two or more modes. Only for azimuths 0 deg, 90 deg, and 180 deg, a unimodal response was observed for the two first lock-in regimes. The frequency response showed that when the response was multimodal, non-dominant modes can follow the vibration frequency of the dominant one. Assuming a priori the independence principle (IP) valid to define the reduced velocities (Vr), it was observed that the resonance region was restricted to 3 ≤ Vr ≤ ~8 for the tested cases, indicating that the IP can be at least partially applied for flexible structures. As the literature scarcely explores the simultaneous yawed and inclined configurations, the present work may contribute to further code validation and improvements regarding the design of slender offshore structures.

A fine drag coefficient model for hull shape of underwater vehicles

The increasingly diverse mission requirements result in challenges to the rapid design and functional diversification of underwater vehicles (UVs). The genealogical design of UVs shape is one of the key steps to deal with these challenges. In the present work, based on the hydrodynamic characteristics with different shape features of UVs, the hull shape genealogy including the spherical/spheroidal shape, slender axisymmetric shape, slender flat shape and round-dish shape was established according to the genealogy classification theory. Influence of the hull shape parameters and Reynolds numbers on hydrodynamics of the spherical/spheroidal shape, slender axisymmetric shape and slender flat shape for wide application in hull shape genealogy was investigated by using computational fluid dynamics (CFD) simulation method. The mathematical models of the drag coefficients of the hull shape genealogy with different shape parameters and Reynolds numbers were established by fitting the data obtained from CFD calculation. The accuracy of the established drag coefficient models was verified by the tank tests of “Petrel-L″ model developed by Tianjin University and SUBOFF model. Furthermore, the established drag coefficient models show higher accuracy than the traditional drag coefficient models, namely G&J model, VT model and MIT model, by comparing with the towing tank tests of SUBOFF model. The present research provides an important reference for realizing the rapid and efficient design of UVs with the same hydrodynamic shapes.

Experimental investigation on effect of sloshing on ship added resistance in head waves

Engineering practice presents some impact of sloshing and ship motion interaction on added resistance in waves, and this paper is an attempt to figure out the effect of such impact through experiment while theoretical investigation is troubling scholars due to the complexity of this problem. A Liquefied Natural Gas (LNG) carrier scaled to 1/27.384, with one prismatic tank, is modeled for free running tests in regular head waves of 6 different wavelengths in the deepwater towing tank of China Scientific Research Center (CSSRC). For each wave case, the revolution rate of propeller, the ship speed, and corresponding torque and thrust are measured, and the added resistance is thereafter obtained by subtracting the still water resistance from the mean total resistance on assumption of a constant thrust deduction. The experimental results are comparatively discussed in conditions of sloshing and ballast weight. It is found that under current test conditions, internal sloshing affects ship motion by reducing its value, and the added resistance in waves decreases accordingly while the energy consumption decreases. The experimental results that liquid sloshing has an evident influence on the added resistance shall be a very important consideration for the development of green LNG cargos.

Experimental and numerical assessment of vertical accelerations during bow re-entry of a RIB in irregular waves

This paper presents the comparison of a self-conducted towing tank experiment with the simulation results of a calibrated state-of-the-art strip-theory method and a first-principles numerical method. The experiment concerns a Rigid Inflatable Boat (RIB) in moderate-to-high irregular waves. These waves result in bow emersion events of the RIB. Bow re-entry induces vertical accelerations which, in reality, can lead to severe injuries and structural damage. State-of-the-art methods for predicting the vertical acceleration levels are based on assumptions, require calibration and are often limited in application range. We demonstrate how the vertical acceleration as a function of time is found from a 3D numerical method based on the Navier–Stokes equations, employing the Volume of Fluid (VoF) method for the free surface, without any further assumptions or limitations. 2D+t strip theory methods like Fastship are based on the mechanics of wedges falling in water. The 3D numerical method that is part of the software ComFLOW is compared to previous research on falling wedges in 2D to investigate the effect of air and to find suitable grid distances for the 3D simulation of the RIB. The 3D RIB simulations are compared to Fastship and the experiment. With respect to the experiment, the ComFLOW simulations show a slight underestimation of the levels of heave and pitch. The underestimation of Fastship is larger. The prediction of acceleration in ComFLOW is hardly different from the experiment and a significant improvement with respect to Fastship. ComFLOW is demonstrated to predict acceleration levels better than before, which creates opportunities for using it in seakeeping optimization and for the improvement of methods like Fastship. The properties of the RIB and the experiment are available as open data at Wellens (2020).

Cavitator Design for Straight-Running Supercavitating Torpedoes

A practical cavitator design method for straight-running-type supercavitating torpedoes was developed in this paper. Design requirements were first drawn in terms of torpedo performance characteristics, such as maximum range and motion stability. This method determines the optimum cavitator satisfying the design requirements that not only minimize the total drag of the torpedo, extending the maximum range, but also provide hydrodynamic forces required for straight level flight. The design procedure includes determining a design cavitation number and cavitator type (disk or cone) for obtaining the optimal cavitator that minimizes the total drag of a torpedo in straight level flight. To determine such an optimal cavitator, the equations of force and moment equilibrium for straight level flight were iteratively solved by the existing mathematical models that determine the cavity shapes generated by disk- and cone-shaped cavitators and hydrodynamic forces acting on the vehicle. For validation, model experiments on a small-scale supercavitating vehicle were conducted in a towing tank, and the results agree well with those of the mathematical models used in this study. A preliminary design based on the newly proposed method was also implemented for a realistic supercavitating vehicle. More precise computations using CFD should be conducted to investigate the physics in more detail in the near future.

Approximate ship wake solution for fast computation

An approximate ship wake solution that accurately predicts both the wake patterns and the wake amplitudes in any water depth is proposed and validated. Derivation based on the small-amplitude wave theory is first revisited, treating a ship as a moving pressure field acting on the water surface. For ship shapes symmetric about the transverse (pitch) axis and away from the immediate vicinity of the ship, an approximate solution is sought. In contrast to the exact analytical solution consisting of a double integral, the approximate solution is simplified through analytical means to consist of a single integral. Singularities inside the integrand of the exact solution have also been removed. Hence, the computational effort is greatly reduced. Two extreme ship shape functions are considered to plot the solutions: a sharp rectangular shape and a smooth Gaussian shape. The newly derived approximate solution is validated using 37 satellite images of ship wakes, one towing tank experiment, and two field experiments. Then, water depth effects on wake patterns are discussed. Providing means to quickly relate the ship shape, weight, speed, and water depth to the wake characteristics, the new ship wake solution is a potentially powerful tool in the study of ship wake inversion.

Study on trim optimization for an oil tanker

The purpose of this paper is to study the influence of trim on resistance performance for an oil tanker. First, the three-dimensional ship model, calculation basin and structural mesh were obtained. Commercial code of computational fluid dynamics was utilized to calculate the resistance of target ship under various trim conditions and drafts. Then, we compared the results calculated by numerical simulation with the towing tank test results to validate effectiveness of CFD method. Finally, influence of trim on ship resistance performance was analysed. It is concluded that this method can find the optimal trim value and provide practical advice for actual operation of the real ship. Consequently, it can be used to improve the efficiency of energy-saving and emission reduction, which provides a new direction for the development of green ships.

Drag and inertia coefficients of live and surrogate shellfish dropper lines under steady and oscillatory flow

Against the background of a drastically increased demand of marine proteins, off-bottom, bivalve aquaculture, provides significant potential for production growth when moved into more energetic marine waters. Hence, research, industry and politics are currently proposing the development of new offshore sites. The highly energetic conditions at these sites present a challenging environment for bivalve aquaculture. In this work, physical experiments of suspended bivalves provide new knowledge on the commonly used design parameters: the drag and inertia coefficients. Live bivalves and manufactured surrogate models at a 1:1 scale were tested in a towing tank as well as under waves. The drag coefficient of live blue mussels was determined to be Cd = 1.6 for Reynolds numbers between 2.3 × 104 and 1.4 × 105. The inertia coefficient obtained from the wave tests was Cm = 2.1 for Keulegan Carpenter numbers KC < 10. In a pursuit to better understand the differences between live mussels and surrogates in laboratory conditions, the analysis revealed that appropriate surrogates can be identified. A method to determine the characteristic diameter of mussel dropper lines is suggested. The results facilitate the future design of aquaculture systems in high-energy environments and allow for an integration into numerical models.

Experimental investigations on the integrated bubbly wake strength of two different scale ship models

ABSTRACT The integrated bubbly wake strength of two ship models with different scales is investigated in this paper. The laser spot video recording technique is implemented to measure the light scattering properties of laser beams transmitted through the wake. The bubbly wake is generated by the movement of two geometrically similar models towed at constant velocities in a towing tank. The effect of the model’s velocity and scale are investigated using data provided by the multiple laser emitter and detectors. Results show the dominance of small bubbles on the persistence of the wake. Also, the effects of wetness of the stern on the vertical distribution of the wake are evident. Comparing the wake of two scale models shows a stronger wake for the bigger model in similar conditions. Moreover, the importance of Froude and Weber numbers on the scaling of major features of the bubbly wake of ship models is prominent.

Towards the Design and Implementation of an Image-Based Navigation System of an Autonomous Underwater Vehicle Combining a Color Recognition Technique and a Fuzzy Logic Controller.

This study proposes the development of an underwater object-tracking control system through an image-processing technique. It is used for the close-range recognition and dynamic tracking of autonomous underwater vehicles (AUVs) with an auxiliary light source for image processing. The image-processing technique includes color space conversion, target and background separation with binarization, noise removal with image filters, and image morphology. The image-recognition results become more complete through the aforementioned process. After the image information is obtained for the underwater object, the image area and coordinates are further adopted as the input values of the fuzzy logic controller (FLC) to calculate the rudder angle of the servomotor, and the propeller revolution speed is defined using the image information. The aforementioned experiments were all conducted in a stability water tank. Subsequently, the FLC was combined with an extended Kalman filter (EKF) for further dynamic experiments in a towing tank. Specifically, the EKF predicts new coordinates according to the original coordinates of an object to resolve data insufficiency. Consequently, several tests with moving speeds from 0.2 m/s to 0.8 m/s were analyzed to observe the changes in the rudder angles and the sensitivity of the propeller revolution speed.

Using Blade Element Momentum Theory to Predict the Effect of Wave-Current Interactions on the Performance of Tidal Stream Turbines

The non-uniformity and dynamics of the environment tidal stream turbines need to operate within will significantly influence the durability and reliability of tidal energy systems. The loadings on the turbine will increase substantially when the turbine is deployed in high magnitude waves with non-uniform tidal currents. The limitations of numerical solutions will be understood when the outcomes are verified with empirical data from system operations. In this paper, a Blade Element Momentum model is used to predict and compare the performance of a scaled turbine within a flume and a tow tank. Firstly, the numerical and experimental work is analysed for a turbine operating at flow speeds of 0.5m/s amd 1.0 m/s, wave heights of 0.2 m and 0.4 m and wave periods of 1.5 s and 1.7 s. Good agreement between the model and the experimental work was observed. However, in low TSRs the model tends to under predict the thrust, and the variation between the maximum and minimum values obtained within the experiments. Secondly, a turbine operating at flow speeds of 1.0 m/s and 4 different inflow profiles is analysed, where the wave heights for these cases were 0.09 m and 0.19 m and with wave periods of 2 s and 1.43 s. In this evaluation, the model tends to over predict the values of Ct and Cp when compared to those calculated from the experimental data. However, when investigating the values used to calculating both the thrust and torque coefficients, there is better agreement with these, which means the methodology used to determine these coefficients with inflow profiles should be revised.

Validation of added resistance in waves by tank tests and sea trial data

ABSTRACT With the start of the EEDI (energy efficiency design index for new ships) regulations by the International Maritime Organization, a review of the analysis method for speed/power trials was required. The International Towing Tank Conference (ITTC) Specialist Committee on Performance of Ships in Service has conducted a review since 2011. For wave correction in speed/power trials, various methods were validated to improve correction accuracy during the activities. In this paper, the process of validation and implementation of the wave correction method discussed through the committee is shown. The comparison based on the results of tank test in regular waves, tank test in long-crested irregular waves and speed/power trials of full-scale ships has been examined. From the studies, the features of each method of wave correction are understood, which is helpful for the implementation of the wave correction performed in speed/power trials.

Effects of water depth and speed on ship motion control from medium deep to very shallow water

To extend the knowledge on ship manoeuvring and predict ship's behaviour accurately from medium deep to very shallow water, this paper comprehensively investigates water depth and speed effects on ship's manoeuvrability, steering model and motion control through experimental studies. Free running model tests at four water depths and six speeds were conducted using a scaled ship model in the towing tank at Flanders Hydraulics Research (FHR). The influence of under keel clearance and speed on the acceleration tests and zigzag manoeuvres were firstly analysed. Secondly, the shallow water effect on ship steering model was discussed via theoretical analysis and experimental validation. Then, the investigation of the effects of speed and under keel clearance on the ship motion controller's parameters and performance was executed, and a novel adaptive controller was proposed. To improve controller's performance in shallow water, a scheme was proposed to optimize its parameters. The results indicate that both speed and water depth have considerable influence on ship's manoeuvrability and controllability. Especially, water depth restrictions change the hydrodynamic forces, reduce the propulsion efficiency and manoeuvreability, the ship becomes more difficult to manoeuvre and control in shallow water. These impacts on ship manoeuvring, modelling and motion control cannot be neglected.

Drag force and reconfiguration of cultivated Saccharina latissima in current

The design of aquaculture systems requires an understanding of the drag forces on cultivated kelp. This study measured the drag on line segments of cultivated Saccharina latissima in a towing tank. The drag on segments of farm line with full kelp bundles and with stipes alone (fronds removed) was measured at tow speeds of 0.10 to 0.50 m/s. The drag on individual fronds cut from the line was also measured. Video images were collected to evaluate the plant reconfiguration. Both kelp blades and stipes contributed to the total drag force on the line bundle. Within the velocity range of our experiments, the kelp blades were essentially horizontal. However, the pronation of kelp stipes increased as flow velocity increased. The reconfiguration of kelp stipes was observed to decrease the vertical extent of the kelp bundle. Due to this reconfiguration, the measured force, F, increased with velocity, U, at a rate slower than quadratic, and was consistent with scaling laws derived for reconfiguration. Specifically, F∼Uα with α=1.35±0.17.