Towing Tank Research Articles

Suppression Of Wall Reflection In PIV Images Over Rough Walls

Wall reflections in particle image velocimetry (PIV) measurements is one of the limiting factors in obtaining velocity information in the vicinity of rough walls. This work tests two different techniques to suppress wall reflections for time-resolved, two-dimensional particle tracking velocimetry (2D-PTV) and stereoscopic-PIV (stereo-PIV). The 2D-PTV was performed for a turbulent boundary layer developed on a rough wall in a water towing tank facility. To measure velocity in the vicinity of the rough wall, the near-wall region on the viewing window was covered with tinted papers to mitigate the wall reflection and maintain a visible reference plane of the towed wall. Using this technique allowed detection of particles at a wall-normal distance of approximately 300 μm (10 wall units) from the roughness valleys for an imaging system with 1 m standoff distance. To suppress the wall refection for the stereo-PIV over a rough wall performed in a wind tunnel, the roughness is coated with fluorescent paint to shift the reflected light to a higher wavelength. A narrow band-pass filter was then used on the imaging lenses, allowing only particle images to be recorded while extinguishing the majority of the reflected laser light from the surface. Reliable velocity measurements with this technique were obtained up to 500 μm (5.0 wall units) from the roughness valleys.

Experimental study on the performance of a hydrofoil using passive water-jet vortex generators

ABSTRACT This paper presents an experimental study of a passive version of the Water-jet vortex generator to reduce the drag force over a NACA 661-012 hydrofoil. The passive blowing used the overpressure zone at the leading edge of the hydrofoil’s lower surface as a source of water jets that create jets on the upper surface of the hydrofoil using the blowing tubes. The experimental tests were carried out in the towing tank in Universidad Politécnica de Madrid (UPM) with a hydrofoil model with a 250-mm long chord at Reynolds numbers of (1.4 105 < Re < 2.8 105). It was found that the passive water-jet vortex-generator reduces the drag force with a slight lift decrease.

Валидация технологии прогнозирования гидродинамических характеристик судовых движителей в пакете программ «ЛОГОС»

Object and purpose of research. The objects of research are models of propeller and impeller of a water jet. The purpose of the work is to validate the new calculation module LOGOS of domestic software complex for engineering analysis and the technology for its use in predicting the hydrodynamic performance of ship propulsion based on comparison with experimental research data and reference calculations in foreign commercial software. Materials and methods. Methods of computational fluid dynamics are used to determine the hydrodynamic performance of ship propulsion systems. The characteristics of the viscous fluid flow are obtained from the solution of non-stationary Reynolds equations (RANS (Reynolds averaged Navier-Stokes equations)) by control volume method. The Reynolds equations are closed by a two-parameter semi-empirical turbulence model. To validate the results of numerical modeling in the LOGOS software package, data from experimental studies and reference calculations performed at the Krylov Center are used. Main results. Validation of the new calculation module of LOGOS software complex, performed on the basis of comparison with the results of experimental studies and reference calculations, demonstrates the high accuracy of modern domestic software in the field of computational fluid dynamics in solving problems of determining the hydrodynamic performance of propellers. Conclusion. The paper shows that the determination of hydrodynamic performance of ship propulsion models in a uniform viscous fluid flow can be performed using the domestic software package LOGOS. The accuracy of predicting the hydrodynamic performance of propellers in the meantime compares favourably to leading foreign software products and is confirmed by the results of experimental studies carried out in the deepwater towing tank of Krylov Center. At the same time, further development of the LOGOS software package is required for the correct solution of more complex tasks related to ship propulsion.

Experimental and numerical investigation on the resistance characteristics of a high-speed planing catamaran in calm water

There has been an increasing interest in high-speed planing catamaran hulls for their exceptional resistance performance in military and maritime transportation fields. In this paper, experimental tests and numerical investigation on a planing catamaran under different displacements are performed to analyze the resistance characteristics and mechanism of the tunnel flow. Model tests are conducted in a towing tank for the Froude number ranging from 0.76 to 1.93. A computational fluid dynamics (CFD) code incorporated with dynamic overset mesh technique is employed to simulate the flow around the planing catamaran in calm water. The numerical results are validated with measured data and show good agreement. The mean error in the trim angle and the resistance took the whisker spray drag into account are 8.9% and 4.5%, respectively. Numerical results of pressure, comparisons of wave profile along transom stern centerline, lift force distribution on the tunnel, and components of tunnel lift are presented and compared for the analysis of tunnel flow under different displacements. The tunnel could contribute the maximum lift about 26% of ship weight in the case of M = 202.9 kg. This study would provide a better understanding of hydrodynamics and the aerodynamics of the tunnel for the planing catamaran.

Physics of gust response mitigation in open-loop pitching manoeuvres

This paper experimentally investigates the flow field development and unsteady loading of three force-mitigating pitch manoeuvres during a transverse gust encounter. The manoeuvres are constructed using varying levels of theoretical and simulation fidelity and implemented as open-loop kinematics in a water towing tank. It is found that pitch actuation during a gust encounter results in two important changes in flow topology: (i) early detachment of the leading-edge vortex (LEV) and (ii) formation of an LEV on the pressure side of the wing upon gust exit. Each of the pitch manoeuvres is found to mitigate a significant portion of the circulatory contribution of the lift force while only manoeuvres with accurate modelling of the added-mass force are found to adequately mitigate the total lift force. The penalty of aerodynamic lift mitigation using pitch manoeuvres was a twofold increase in the pitching moment transients experienced by the wing for all cases. By quantifying changes in the vertical gust momentum before and after the encounter, lift-mitigating manoeuvres were found to reduce the disturbance to the gust's flow field, thereby reducing the momentum exchange between the gust and the wing.

Validation of the resistance of a plate moving through mud: CFD modelling and towing tank experiments

When investigating the effect of muddy seabeds on marine vessels using Computational Fluid Dynamics (CFD) software, one challenge is to adequately describe the complex non-Newtonian fluid behaviour of mud. Although a number of rheological models have been proposed in the past, mud sediments are often simply regarded either as highly viscous Newtonian fluids or as Bingham fluids in many engineering applications. In this study, we investigate the accuracy of the Bingham model for numerical predictions of the viscous forces on a plate moving through fluid mud in laminar regime. In this context, a plate could be regarded as the flat bottom of a ship hull. The aim is to provide CFD practitioners with information about the accuracy of the Bingham model for the prediction of the frictional resistance of a ship sailing through fluid mud. This work presents a comparison of experimental and numerical data on the resistance of a plate moving through fluid mud from the Europoort area (Netherlands). Results suggest that the regularised Bingham model can be a reasonable compromise between simplicity and accuracy for CFD simulations to investigate the effect of muddy seabeds on marine vessels. A comparison between CFD data and analytical formulas is also presented.

대형 캐비테이션터널에서 펌프젯 추진기 단독성능 시험 및 해석 기법 연구

In order to study the open-water test and analysis techniques for pumpjet propulsors in the Large Cavitation Tunnel (LCT), at the Korea Research Institute of Ships and Ocean Engineering, a set of test equipment was designed and manufactured. The pumpjet propulsor is composed of rotor, stator and duct resulting in the strong interaction between the components. A ring-shaped sensor was developed to measure the thrust and torque for duct and stator. The test equipment including the pumpjet is installed on an existing POW dynamometer in the reverse direction. The results from the reverse POW test setup were validated against those from the conventional POW test setup in the Towing Tank (TT) as well as in the LCT. The pumpjet open-water test was conducted at the Reynolds number of around 1.0×106, at which the obtained experimental data became stable in the Reynolds number effect test. The open-water test for the rotor (rotor-only) was conducted to study whether the duct and stator should be considered as a part of the hull or the propulsor. On the basis of the test results, it was shown that the duct and stator could be included in the propulsor. The total thrust, combined thrust of rotor, duct, and stator was used for the pumpjet open-water test analysis. As the whole pumpjet is defined as a propulsor, it is thought that the self-propulsion test and analysis could be conducted in the same way as that of the conventional propeller.

Hydrodynamic Performances of Aeronautical Propeller for Drones

Currently, several countries already invest on the application of drones for different scopes. These purposes are mainly civilian and military and some of these include the presence of water as search and rescue, inspection of pipelines, getting a bird’s eyes view over oil spills or structural integrity monitoring of bridges. For this reason, an interesting feature for drones is their ability to maneuver in fluids with different density as air and water. This research topic, shows strong scientific potentiality but it also represents a great challenge from scientific and technical view point: i) aeronautical propeller for drones in water must generate reverse thrust or braking force through an off-design rotation in off-design condition (different fluid’s density); ii) the propeller rotational velocity in water is generally one order of magnitude lower than the one for air application requiring the motors to be properly desing to meet such a wide operational range. In this paper, an experimental campaign aimed at the performance quantification of a three-bladed propeller for drone propulsion in water has been carried out in a towing tank. In particular, the generated forces are acquired through a load cell for different advance ratios J provided by varying both rotational regime and free stream velocity. The results show expected losses in the performance for the off-design rotation in terms of both thrust and efficiency. At low rotational speeds, higher values of efficiency are presented for small advance ratio. The maximum efficiency increase for higher RPS and it’s slightly influenced by small variation of propeller disk angle.

Hydrodynamic Performance Analysis of a Submersible Surface Ship and Resistance Forecasting Based on BP Neural Networks

This paper investigated the resistance performance of a submersible surface ship (SSS) in different working cases and scales to analyze the hydrodynamic performance characteristics of an SSS at different speeds and diving depths for engineering applications. First, a hydrostatic resistance performance test of the SSS was carried out in a towing tank. Second, the scale effect of the hydrodynamic pressure coefficient and wave-making resistance was analyzed. The differences between the three-dimensional real-scale ship resistance prediction and numerical methods were explained. Finally, the advantages of genetic algorithm (GA) and neural network were combined to predict the resistance of SSS. Back propagation neural network (BPNN) and GA-BPNN were utilized to predict the SSS resistance. We also studied neural network parameter optimization, including connection weights and thresholds, using K-fold cross-validation. The results showed that when a SSS sails at low and medium speeds, the influence of various underwater cases on resistance is not obvious, while at high speeds, the resistance of water surface cases increases sharply with an increase in speed. After improving the weights and thresholds through K-fold cross-validation and GA, the prediction results of BPNN have high consistency with the actual values. The research results can provide a theoretical reference for the optimal design of the resistance of SSS in practical applications.

On the robust autorotation of a samara-inspired rotor in gusty environments

Autorotating samaras have evolved to propagate successfully to their germination sites with the help of wind. This wind, in turn, is inherently unsteady across an extensive range of scales in the atmospheric boundary layer. To generate lift, samaras rely on the formation of a stably-attached leading-edge vortex (LEV) on the suction side of their wings. The kinematics of autorotating samaras experiencing gusts were examined experimentally in order to provide insights into the aerodynamic mechanisms responsible for successful propagation. The gust response of seven mature Boxelder Maple (Acer negundo) samaras was investigated using a small unsteady wind tunnel able to create vertical gusts. Interestingly, the samaras were found to have a stable tip-speed ratio (λ) during the gust phase, thus suggesting that the LEV remained stably-attached. Inspired by samaras, we designed a three-bladed rotor that incorporates key aerodynamic and geometric properties of samaras so as to exhibit a stably-attached LEV. The gust response of the samara-inspired rotor was examined using a towing-tank facility. The gust was emulated in the towing tank by accelerating the rotor from an initial steady speed to a final steady speed. Different gust intensities were tested by varying the rotor’s normalized inertia number (I*) by systematically increasing the rotor moment of inertia (I). Similar to the natural samaras, the rotor exhibited a robust tip-speed ratio during all simulated gusts. The rotor’s tip-speed ratio increased by a maximum of 11% and 6% during the slowest and fastest simulated gusts, respectively. By maintaining a stable tip-speed ratio during the gust, the samara-inspired rotor is thought to maintain stable LEVs resulting in stable autorotation. Therefore, by learning from the samara-inspired rotor, we suggest that samaras propagate successfully from their parent trees in unsteady (realistic) environments in part due to their robust autorotation properties.

Experimental and Numerical Study of the Influence of Clumped Weights on a Scaled Mooring Line

Recently, several experimental and numerical studies have underlined the advantages of adding clumped weights at discrete positions of mooring lines. To confirm the influence of these weights, an experimental study was performed for a 1:30 scale model of a mooring line. In this study, the clumped weight is modeled as a scaled disc placed at different positions along the mooring line. The series of experiments has been carried out at the CEHIPAR towing tank using a submerged studless chain both with and without clumped weights. The experiments consist of the excitation of the suspension point with horizontal periodic motions using different amplitudes and periods, where the mooring line’s tension at the fairlead is measured using a load cell and a dynamometer, and the motion of a part of the line is recorded using low-cost submerged cameras. Similarly to previous experiments, the fairlead tensions increase with higher amplitudes and lower periods, and a clear pattern in the motions of the line at different depths is found. The dissipated energy and the fairlead tension is also increased by the addition of the clumped weight, and the variation of this energy with its position along the line is monitored. The presence of clumped weights is also implemented into a finite element numerical code, previously validated without clumped weights, where all the previous experiments with clumped weights are replicated with remarkable accuracy. This double experimental and computational approach to the problem provides an important dataset for numerical code validations and opens future discussions about the impact of clumped weights on floating platforms.

Numerical and Experimental Investigations on Hydrodynamic Performance of a Newly Designed Deep Bottom Trawl

In this paper, a new type of bottom trawl was designed for target fishing vessels to use in deep-water fishing grounds. The trawl’s hydrodynamic performance was investigated using numerical simulation and physical modeling methods, and a numerical model based on the finite element method was proposed for estimating hydrodynamic forces and predicting performances. A series of physical model tests based on Tauti’s law were carried out in a towing tank to explore the hydrodynamic performance of the trawl and to assess the applicability of the numerical simulation method. The results showed that the working towing speed of the trawl was 3.5 kn. The drag force and the height of net opening were 50 kN and 5.62 m, respectively, and the swept area was 128 m2 at that speed. The simulated result was close to the experimental result, with a maximum relative error less than 20%, and an average relative error of 10%. The net shape and tension distribution of the trawl were analyzed using the numerical simulation method, and the hanging ratio in T-direction of the mesh of the codend was 0.25 at the working towing speed. The newly designed deep bottom trawl had a superior hydrodynamic performance for high catch efficiency and selectivity and may be applied to commercial fishing operations.

Practical squat assessment for a ship manoeuvring in muddy environments

The condition of the bottom is practically never assessed in squat prediction formulae, but plays an important role in the magnitude of the squat prediction, as is shown in the present paper. Based on a comprehensive dataset of historical towing tank tests, a comprehensive 6 DOF manoeuvring model has been derived, which is used as a basis to derive a practical correction term which can be applied to common empirical squat prediction formulae to account for the presence of a muddy bottom. It seems that in normal sailing conditions, with small drift angles, a minor increase of the maximal sinkage prediction is needed when it is not desired to touch the mud layer with the ship.

Experimental and Numerical Study on the Characteristics of Motion and Load for a Floating Solar Power Farm under Regular Waves

Recently, the demand for floating solar power farms in lakes and coasts (rather than on land) has been increasing rapidly. It is important to develop a numerical analysis technique that considers environmental conditions to predict structural stability and accurate motion response while designing a floating solar power farm. In this study, we performed a comparison under conditions similar to those of the Inha University towing tank (IUTT) model test to verify the numerical analysis method. The results revealed that heave and pitch movements were dominant under head sea conditions. Relative behavior occurred because of the hinge connection of each unit, and complex motion characteristics appeared depending on the wave conditions. The numerical method was verified based on the motion response and load of the floating solar farm. The validity of the results was also confirmed.

Pressure reconstruction from PIV measurements in the bow region of a fast ship

The paper presents velocity measurements, using particle image velocimetry, as well as a reconstruction of hydrodynamic pressures for the analysis of fast ships. Stereoscopic PIV measurements with a towed underwater PIV system are conducted during towing tank tests to obtain the velocity field in the bow region of a fast ship at speeds up to Fr=0.8. While the model is kept at a fixed trim and sinkage, multi-plane PIV measurements with a total of 68 measurement planes are conducted to reconstruct a volumetric representation of the time-averaged velocity field in the bow region. The obtained velocity field is subsequently used for a volumetric description of the time-averaged hydrodynamic pressure field. In addition to these captive runs, forced oscillation tests are conducted. During these tests, the flow field is recorded in three successive planes to obtain a local phase-averaged description of the velocity and its gradients for the reconstruction of the phase-averaged hydrodynamic pressure field. The postprocessing procedure for the pressure reconstruction, including the solution of the Poisson equation, is implemented into the open-source CFD package OpenFOAM. For the detection of the free surface and the ship hull, an automated procedure is presented. Experimental results are finally compared to results from numerical simulations. Results show that the PIV method is capable of capturing the flow characteristics in the bow region of a fast ship. In addition, it can be used together with the pressure Poisson equation to obtain the hydrodynamic pressure field. However, large out-of-plane velocities require a large dynamic range, which limits the resolution of local effects close to the ship hull.

Ship Bow Wings with Application to Trim and Resistance Control in Calm Water and in Waves

Flapping foils for augmenting thrust production have drawn attention as a means of assisting ship propulsion in waves due to their high efficiency rate compared to traditional screw propellers. However, they can also offer substantial resistance reduction when used as stabilizers. In this work, the aim is to investigate the feasibility of a symbiotic concept combining the ship’s propeller with a foil arranged at the ship’s bow at a fixed position operating as a trim-pitch stabilizer. The work presents results obtained from experiments conducted in the towing tank of the Laboratory of Ship and Marine Hydrodynamics of the National Technical University of Athens (LMSH NTUA), as well as results from an in-house CFD solver. The test cases focused on the resistance and the dynamic behavior of the wing–vessel configuration in calm water conditions and in head waves. All results were compared against the performance of a bare hull (without foil). The findings of this work are based both on numerical and experimental data and indicate that a bow wing in static mode can be used for trim-control of a vessel by altering the angle of attack leading to a possible drop in wave resistance both in calm water and in waves. In the latter case, utilizing the wing in head waves results in a significant reduction in the pitching and heaving responses of the vessel, which may lead to substantial enhancement of the propulsion performance.

AUTONOMOUS SHIP CONTROL IN SHALLOW AND CONFINED WATER

The paper describes the definition and usage of autonomous ship control systems in shallow and confined water. The application of such control system is shown in a case study : a systematic analysis of ship transits in the approach channel to the Port of Antwerp (Western Scheldt) .The control algorithm not only is capable of correctly navigating the ship through the fairway, it also delivers insight in rudder and propeller usage in function of the tidal conditions. Such control systems are validated based on model tests carried out in towing tanks. Flanders Hydraulics Research (FHR) has recently inaugurated their second towing tank, the Towing Tank for Manoeuvres in Shallow Water. One of the functionalities of this facility is the role it can play as test basin for autonomous shipping. Next to the further validation and expansion of existing control scenarios, it will be used to model interaction events between multiple ships. A numerical effort to model interaction events is presented, which will be validated in the model test basin.

Numerical and experimental study on free-surface wave generation by a submerged moving body in a towing tank

Free surface waves were generated by a submerged moving body in a towing tank using a numerical and a physical towing tank. Unlike a circulating water channel, a towing tank allows easy to control of acceleration and deacceleration range of the body motion. Numerical and experimental analysis can make the same moving conditions of the submerged body. Using a Numerical Towing Tank (NTT), free surface elevation was calculated according to body velocities, submerged depths and body volumes. The measured experimental data agreed well with the results of a fully nonlinear simulation and the data was also compared with the Russell comprehensive error method. The nonlinearity of the free surface waves increased as the submerged depth of the body became shallower and the body velocity became faster. The increase in wave height according to the increase of the body velocity or the increase in the wave height according to the decrease in the submerged depth of the body generally showed a proportional relationship. Overall, the nonlinear free surface boundary condition needs to be applied when calculating the free surface wave due to the motion of a submerged body. The ratio of wave height difference according to body displacement was similar to the ratio (percentage) of increase in body displacement to NACA shape displacement as body velocity becomes faster.

Underwater Sound Source Localization Based on Passive Time-Reversal Mirror and Ray Theory.

This study investigates the performance of a passive time-reversal mirror (TRM) combined with acoustic ray theory in localizing underwater sound sources with high frequencies (3–7 kHz). The TRM was installed on a floating buoy and comprised four hydrophones. The ray-tracing code BELLHOP was used to determine the transfer function between a sound source and a field point. The transfer function in the frequency domain obtained from BELLHOP was transformed into the time domain. The pressure field was then obtained by taking the convolution of the transfer function in the time domain with the time-reversed signals that were received by the hydrophones in the TRM. The location with the maximum pressure value was designated as the location of the source. The performance of the proposed methodology for source localization was tested in a towing tank and in the ocean. The aforementioned tests revealed that even when the distances between a source and the TRM were up to 1600 m, the distance deviations between estimated and actual source locations were mostly less than 2 m. Errors originated mainly from inaccurate depth estimation, and the literature indicates that they can be reduced by increasing the number of TRM elements and their apertures.

Проектирование гребных винтов с предзакручивающим аппаратом

Object and purpose of research. This research was intended to check if a pre-swirl stator installed before an open propeller of moderate skew could be a viable solution for non-cavitation hydrodynamic noise mitigation. This solution was compared with a typical open skewed silent propeller. Materials and methods. Design and verification calculations have been performed in a given wake field of pre-swirl stator. Pre-swirl stator was designed considering the wake field turn at the skew angles of typical propeller. Wake field behind the pre-swirl stator and design calculation of open propeller considering the heterogeneity of inflow were determined using the calculation methods. The tests and calculations of vibroacoustic parameters have been performed for evaluation of propulsion, cavitation and acoustic parameters of propellers with pre-swirl stators in comparison with open propeller. Main results. This work was also intended to answer the question if a moderate-skew propeller with pre-swirl stator could be an alternative to a silent skewed open propeller. To this effect, a model of propulsion system (propeller + pre-swirl stator) was designed and manufactured. This model was further used for comparative cavitation, hydrodynamic and acoustic testing in the cavitation tunnel, as well as for self-propulsion tests in deep-water towing tank. Conclusion. The tests have shown that propellers with pre-swirl stators have lower noise emission at high frequencies than open propellers. Cavitation and propulsion performance remain the same for both options.