Towing Tank Research Articles

Preliminary performance assessment from towing tank testing of a horizontal-axis turbine

Following the Norwegian government’s goal to be a ‘low emission society’ by 2050, there is a significant push to develop offshore renewable energy; principally offshore wind, but also wave and tidal stream. The MarinLab towing tank at the Western Norway University of Applied Sciences, Norway is 50 m long with a 3.0×2.2 m section and was inaugurated in 2016 as an educational and research facility, supporting local industry to achieve a low-carbon transition.
 To advance knowledge of aero- or hydrodynamic interaction of future turbines for both wind and tidal stream energy, a model scale test-bed horizontal axis turbine has been developed. This turbine will enable testing of rotor diameters typically in the range of 500-600 mm. The turbine is instrumented with a torque-thrust sensor of 5 Nm/100 N capacity, custom-manufactured by Marin in the Netherlands. The turbine is speed regulated via a 4096 counts angular encoder connected to a 200 W Maxon EC-i brushless motor.
 Before testing new rotor geometries, a benchmarking study is being undertaken with an existing known geometry. The benchmark rotor has diameter, D=700 mm and employs the same NACA 63418 airfoil as Mycek et al. (2014), allowing comparison. Unlike Mycek et al. (2014), the nacelle has a nominal diameter of 90 mm and length 760 mm. The size of the rotor risks exceeding the capacity of the torque sensor, such that maximum tow-speed is limited to 0.8 m/s. The benchmark blades are machined from solid aluminium in a four-axis CNC milling machine, following a method similar to Payne et al. (2017). Due to machining limitations, the minimum trailing edge thickness is specified as 0.2 mm. The final surface finish is achieved by manual polishing, and the final dimensions, quantified using a Hexagon ROMER Absolute laser scanner, are found to be within ±0.2 mm accuracy. Future blade sets will typically have a smaller diameter, allowing for 3D printing and consistency of surface finish.
 A blade-element momentum model has been run with Prandtl tip and hub losses and 0° pitch (Fig. I). Lift and drag coefficients were calculated for a range of chord Reynolds numbers using XFOIL, for angles of attack between -5 to 16° and extrapolated to ±180° with Viterna’s method. Due to difficulties in estimating laminar-turbulent transition around critical Reynolds numbers, there is uncertainty in the XFOIL coefficients. At peak TSR, the chord-based Reynolds number is approximately 280k at 3/4 span. Curves for both Rec =250k (solid) and 500k (dashed) with Ncrit = 9 are presented, showing a significant drop in performance for lower Reynolds numbers, due to a collapse in Cl/Cd. Initial tests were conducted in near- zero ambient turbulence, though passive turbulence grids are available for future testing. Reasonable agreement on CT is observed when corrected for blockage according to Bahaj (2007), and further assessment will provide results on CP, tower loads and wake recovery.

Verification and validation of blade-resolved viscous-flow tidal turbine simulations

Tidal turbines are a renewable energy source on the rise. The exceptional predictability of tidal currents contributes to a high reliability of this technology, which represents a key advantage in the endeavor to become a major contributor to the energy mix. To foster the development and to support the design process of tidal turbines, reliable numerical modeling techniques are required. This paper presents verification and validation work performed within the framework of the Supergen ORE Tidal Turbine Benchmarking Study. Viscous-flow CFD code ReFRESCO is used to conduct blade-resolved simulations of the towing tank experiments. In a first approximation, a steady-state frozen-rotor approach is chosen. A transition model, γ-Reθ, is employed to predict the flow state transition on the turbine blades. In the process, the sensitivity to input turbulence quantities is highlighted. The numerical uncertainty is estimated based on mesh refinements. Finally, a conclusion is drawn to which accuracy the presented numerical models can predict the outcome of the experiments.

Tidal Turbine Benchmarking Project: Stage I - Steady Flow Blind Predictions

This paper presents the first blind prediction stage of the Tidal Turbine Benchmarking Project being conducted and funded by the UK's EPSRC and Supergen ORE Hub. In this first stage, only steady flow conditions, at low and elevated turbulence (3.1%) levels, were considered. Prior to the blind prediction stage, a large laboratory scale experiment was conducted in which a highly instrumented 1.6m diameter tidal rotor was towed through a large towing tank in well-defined flow conditions with and without an upstream turbulence grid. Details of the test campaign and rotor design were released as part of this community blind prediction exercise. Participants were invited to use a range of engineering modelling approaches to simulate the performance and loads of the turbine. 26 submissions were received from 12 groups from across academia and industry using solution techniques ranging from blade resolved computational fluid dynamics through actuator line, boundary integral element methods, vortex methods to engineering Blade Element Momentum methods. The comparisons between experiments and blind predictions were extremely positive helping to provide validation and uncertainty estimates for the models, but also validating the experimental tests themselves. The exercise demonstrated that the experimental turbine data provides a robust data set against which researchers and design engineers can test their models and implementations to ensure robustness in their processes, helping to reduce uncertainty and provide increased confidence in engineering processes. Furthermore, the data set provides the basis by which modellers can evaluate and refine approaches.

Accuracy of BEMT and CFD on the power and trust prediction of tidal turbines

We compute the loads on a model-scale tidal turbine with Blade Element Momentum (BEM) theory and Computational Fluid Dynamics (CFD) simulations, and we compare the results with towing tank tests. CFD simulations are wall-resolved, steady, Reynolds-averaged Navier-Stokes simulations with a k − ω SST turbulence model, where only a 120◦ wedge domain with a single blade is resolved in a non-inertial frame of reference. We undertake a detailed uncertainty analysis to identify the sources of error. BEM uncertainty is computed with a Monte-Carlo approach based on the differences in the predictions of CFD and Xfoil for the sectional lift and drag coefficients,while CFD uncertainty is based on the errors due to the finite number of iterations and spatial resolution. The maximum error of CFD (8.0%) with respect to the experimental data is about half of that of BEM (15.5%) for the power (CP) and the thrust (CT) coefficients and both errors are within 4.1% for CFD and within 7.2% for BEM around the optimal tip-speed ratio (λ = 6.03). The BEM error is within the uncertainty associated with the imprecise knowledge of the sectional lift and drag coefficients. The sectional forces from CFD and BEM disagree at both the tip and the root, resulting in a substantial BEM underprediction of CP at high λ values (up to 15.5%), yet CT is well predicted (within 2.3%) at every λ. The CFD uncertainty is markedly smaller than the error, which is thus mostly due to a modelling error such as the turbulence model, the neglected effect of the support structure, the free surface, and the imprecise knowledge of the input conditions. Overall these results suggest that CFD provides both a maximum error and uncertainty that are substantially smaller than that of BEM, but both methods suffer from modelling errors that require further investigation.

Laboratory Tests Assessment of a Mechanical Sensor-less MPPT Control Strategy for Tidal Turbines

The purpose of this study is to demonstrate through towing tank experiments the effectiveness of a novel sensor-less Maximum Power Point Tracking (MPPT) control for Tidal Stream Turbines (TST) under fluctuations of the onset flow. Ocean energies will play a crucial role in the renewable energy sector over the next decades. Instream turbines for tidal currents are a rapidly maturing technology to exploit a highly predictable energy source. However, short-term fluctuations on the inflow velocity caused by waves or turbulence determine fatigue loads that affect system reliability. Power control strategies to maximize the energy yield can be also used to mitigate the effects of transient loading on drivetrain components. Aim of this paper is to present a straightforward and robust MPPT control method based on the linear relationship between the current and voltage squared of the generator's DC outputs. The method requires pre-determined turbine characteristics to establish the control reference that is effective across operating conditions. The proposed MPPT model was derived mathematically through linearization and simplifications of the turbine power conversion system and validated by model tests carried out in the wave-towing tank facility of CNR-INM in Rome, Italy, using the 1.5 m diameter Tidal Turbine Testing (TTT) device developed at the Queen’s University Belfast (QUB). In the study, a conventional TSR control method was also considered in order to perform a comparative analysis of system response to inflow speed fluctuations with time scales comparable to turbine revolution periods. TSR control was tested using two control references: TSR = 5 (design point) and TSR = 6 (over-speed zone) to verify the operation of the turbine under different loading conditions. The tests were conducted in two scenarios: calm water (steady state) and unsteady inflow with a regular (sinusoidal or monochromatic) waveform, with amplitude chosen to simulate an extreme wave case. The power output was measured from the turbine during regular wave conditions and compared to results from steady flow to assess the impact of wave-induced velocity on turbine performance (Fig. 1). Test results showed that by using the proposed MPPT control strategy, the algorithms converged to the maximum power coefficient (Fig. 2), which validates the proposed methodology. Results also demonstrated the capability of the proposed MPPT to significantly reduce mechanical loads fluctuations as compared to the TSR control. In the full-length paper, the proposed MPPT control strategy is outlined, the test methodology, set-up and conditions are described, and main results are presented and discussed.

Modelling the effects of boundary proximity on a tidal rotor using the actuator line method

For a turbine in unconstrained flow, the maximum power coefficient (CP ) is limited to 16/27 of the undisturbedkinetic energy flux through the rotor area according to work attributed to Betz, Lanchester, and Joukowsky [1].This maximum, often referred to as the Betz limit, occurs when the rotor presents the optimum resistance tothe incoming flow, imparting enough force to generate power without overly choking the flow through the rotor.However, in the context of tidal stream energy, the flow is often constrained by the seabed and the free surfacechanging the balance of optimal rotor resistance. Thus, the limit for maximum power extraction is modifiedto the form first presented by Garrett and Cummins, CP = (16/27)(1 − B)−2 [2]. The factor B, known as theblockage ratio, represents the fraction of the channel cross-section occupied by the rotor swept area, and allowsrotors operating in confined conditions to theoretically exceed the Betz limit. Subsequent theoretical work byNishino and Willden extended this model to demonstrate that constructive interference effects between closelyspaced turbines in a co-planar fence can allow efficiency increases above the Betz limit, even in an infinitely widechannel where the global blockage ratio is negligible [3]. This phenomenon, attributed to the concept of localblockage, defined as the ratio of rotor swept area to the surrounding flow passage area, has been observed byseveral studies both experimentally, and numerically using actuator disk and blade element momentum methods[4, 5, 6]. However, neither the experimental nor the numerical studies provided detail on unsteady loading effectsstemming from azimuthal variations in the flow field caused by anisotropy in the local blockage, such as whenthe rotor is not centred in the flow passage. In this study, a single tidal rotor is simulated using the actuatorline model embedded in a Reynolds-Averaged Navier-Stokes solver with varying degrees of anisotropic blockageimposed by proximity to a non-deformable upper boundary. The investigation is carried out in the contextof the Supergen ORE Unsteady Tidal Turbine Benchmarking Project using a 1.6 m rotor in a computationaldomain equivalent to the towing tank dimensions at the Qinetiq Haslar facility in which the turbine was testedexperimentally [7]. The discrete blade representation and unsteady nature of the actuator line method allowsinvestigation of variations in loads and the importance of boundary proximity by extracting the spanwise loaddistributions and local flow parameters at different positions around the azimuth. The effects of two differenttip-loss correction models on the spanwise force distributions and overall loads are investigated. Analysis of theintegrated rotor loads shows potential for an increase in the maximum CP of ∼1% with changing proximity tothe upper boundary without any detriment to the power-to-thrust ratio. However, anisotropy in the local flowpassage can result in azimuthally varying blade forces, introducing an additional source of fatigue loading tothe rotor and drive train.

PIV wake survey of a drone propeller for amphibious applications

Aerial and underwater drones are one of the most attractive technologies to a large number of applications, extending from surveillance of natural reserves to the monitoring of structures. For these reasons, innovative and uncommon applications are constantly appearing. Within this context, drones capable to operate continuously and efficiently in air and water can extend the standard aerial capabilities allowing underwater inspections of harbour structures, pipelines or marine reserves. For this reason, an attractive characteristic for drones is the capability to manoeuvre in different fluids as air and water. This article presents an experimental campaign, carried out both in a towing tank and in a cavitation tunnel, aimed at study the fluid-dynamics of an aeronautic drone-propeller in off-design conditions. Dynamometric, PIV and Stereo-PIV tests are carried out to evaluate the aeronautic propeller functioning in water and optimize it for amphibious missions. In particular, the forces are measured for different advance ratios J provided by varying both rotational regime and free stream velocity, in open water conditions (Towing tank tests) and in confined water (Cavitation tunnel tests). PIV and Stereo-PIV fields, acquired in the Cavitation tunnel, allow us to investigate the behaviour of the propeller in the water in a number of selected, operative conditions.

Numerical analysis of influencing factors on wave load for an amphibious aircraft

Numerical method of wave load for an amphibious aircraft is carried out in this paper, and its effectiveness is verified by comparing with towing tank model test. It shows that at the same wavelength and speed, the aircraft’s wave load is positively correlated with the change of wave height, but with the increase of wavelength, the difference shows a gradually decrease. And wave load is positively correlated with the speed, the greater the speed, the greater the wave load is. The wave load decreases with the increase of the wavelength. The peak load generally occurs in the short wave state of 1.2 times the length of the fuselage. With the increase of the wavelength, the aircraft load gradually decreases.

Development of rapid and effective oil-spill response system integrated with oil collection, recovery and storage devices for small oil spills at initial stage: From lab-scale study to field-scale test

In the present study, through the laboratory-to-field scale experiments and trials, we report the development and evaluation of an integrated oil-spill response system capable of oil collection, recovery (separation), and storage, for a timely and effective response to the initial stage of oil-spill accidents. With the laboratory-scale experiments, first, we evaluate that the water-surface waves tend to abate the oil recovery rate below 80% (it is above 95% for the optimized configuration without the waves), which is overcome by installing the hydrophilic (and oleophobic) porous structures at the inlet and/or near the water outlet of the separator. In the follow-up meso-scale towing tank tests with a scaled-up prototype, (i) we optimize the maneuverability of the assembled system depending on the speed and existence of waves, and (ii) evaluate the oil recovery performance (more than 80% recovery for the olive oil and Bunker A fuel oil). Although more thorough investigations and improvements are needed, a recovery rate of over 50% can be achieved for the newly enforced marine fuel oil (low sulfur fuel oil, LSFO) that was not targeted at the time of development. Finally, we perform a series of field tests with a full-scale system, to evaluate the rapid deployment and operational stability in the real marine environment. The overall floating balance and coordination of each functional part are sustained to be stable during the straight and rotary maneuvers up to the speed of 5 knots. Also, the collection of the floating debris (mimicking the spilled oil) is demonstrated in the field test. The present system is now being tested by the Korea Coast Guard and we believe that it will be very powerful to prevent the environmental damage due to the oil spills.

NUMERICAL ANALYSIS OF MODIFIED ANGLE OF ENTRANCE AND DUCKTAIL ADDITION OF THE 1,500 GT RO-RO FERRY HULL VALIDATED WITH SHIP MODEL RESISTANCE TEST

The main problem of ferry ships is maintaining the speed due to ship resistance. There are many efforts to reduce the resistance by modifications of them. This study aims to reduce resistance due to modification of the hull form on the bow Angle of Entrance (AoE) and ducktail addition at the transom stern. The object of the research is a 1,500 GT ro-ro ferry. The AoE minimizes wave resistance and improves flow patterns, while the ducktail reduces the negative effects of the circulation zone on the wetted transom so that waves and wake are reduced. Computational Fluid Dynamics (CFD) was used to analyze these modifications. The final results of the numerical simulation are then verified by the ship model resistance test in the towing tank. Tests were conducted in calm water conditions at a draft of 3.30 meters with a speed variation of 10 - 18 knots. The results show that with the combination of AoE and Ducktail, at ship speeds of 13 to 18 knots, the reduction in resistance from CFD ranges from 13.13% - 16.69% while the experimental results range from 16.00% - 16.54%. While separately the AoE modification is between 8.89% – 12.19% and ducktail is 3.12% -3,62%.

Experimental and CFD Resistance Validation of Naval Combatant DTMB 5415-51 Model

The hull model of the surface combatant DTMB 5415 has been selected as a recommended benchmark naval ship for computational fluid dynamics (CFD) validation for resistance analysis. CFD is used to present a method for predicting surface combatant ship resistance. The resistance calculations of a 3 m DTMB 5415 hull model was compared using experimental and CFD techniques. The bare hull shape experiments were set up and carried out at the towing tank facilities at the Military Technical College's Hydrodynamic Laboratory in Egypt. A benchmark test will ensure that the equipment, procedures, and estimates of uncertainty are adequate. For the purpose of model validation, CFD calculations for a DTMB 5415-51 model are performed using three different mesh sizes for Froude numbers ranging from 0.10 to 0.40. For the same Froude numbers and free model conditions, results from towing tank experiments on the model's resistance, sinkage, and trim are presented. The numerical simulations are quantitatively consistent. The numerical results are compared in terms of wave field and resistance coefficients to determine the accuracy of the solution parameters. As evidenced by the resistance curves, the experimental investigation appears to provide very good agreement, indicating that the CFD model is capable of simulating the steady flow around a ship hull with acceptable accuracy and can thus be used as a supplement to laboratory model tests for ship design and ship hydrodynamic research.

Numerical simulation of planing motion and hydrodynamic performance of a seaplane in calm water and waves

ABSTRACTThe high-speed motion of a seaplane involves the coupled hydrodynamic and aerodynamic effects. The suction force, pressure, free surface and motion of the seaplane model were numerically investigated to understand the characteristics of the seaplane's planing motion. The study utilized the SST-DDES turbulence model to analyse the coupled hydrodynamic and aerodynamic effects. Overset mesh method and rigid body motion were employed to simulate the high-speed and substantial motion of seaplane. The volume of fluid method (VOF) was used to capture and sharpen the interface between water and air. First, verification and validation (V&V) were performed by comparing the results with those of the towing tank experiments. Second, the air-water entrainment in calm water and free surfaces were presented, and the pressure distribution on the seaplane was analysed and discussed. Numerical simulations were performed while considering the wave parameters of different velocities, wavelengths, and wave heights. The accelerations of the fore, aft, and centre of gravity of the seaplane demonstrated the presence of the suction effect. The evolution of the air-water entrainment at the bottom of the fuselage was observed. The investigation of suction characteristics revealed that the aerodynamic force in waves plays a substantial role in influencing motions of the seaplane.HighlightsThe study of hydrodynamics and aerodynamics of seaplanes is interdisciplinary. The numerical schemes, including a SST-DDES turbulent model, overset mesh method, and volume of fluid (VOF) method, have proven to be effective and accurate for simulating the motion of the seaplane and flow field characteristics.The greater the speed and wave height, the faster is the motion of the seaplane and the greater are peak and trough values of the pitch. As the wavelength increased, the peak value of the motion decreased gradually.The accelerations of the aft, fore, and centre of gravity of the seaplane in the waves exhibited significant periodicity. The peak value of the acceleration at the aft was the largest, whereas that at the centre of gravity was mild, and the peak value of the acceleration at the centre of gravity was greater than that at the fore. The acceleration indicates that suction at the aft of the seaplane hinders the takeoff during the planing motion.

A novel CFD approach for the prediction of ride control system response on wave-piercing catamaran in calm water

Ride Control Systems (RCS) on high-speed vessels help improve passenger comfort and mitigate dynamic structural loads. Incat Tasmania Wave-Piercing Catamarans (WPC) use RCS consisting of a central T-foil, and a stern tab on each deli-hull. Previous towing tank studies on a 2.5 m model of a 112 m WPC have demonstrated significant reductions in motions with the use of a T-foil and stern tabs. To extend this work, this study examines the use of Computational Fluid Dynamics (CFD) to predict the ship's response with RCS implemented. The model-scale WPC was simulated in calm water conditions, traveling at 2.89 m/s (Fr∼0.6), with step responses applied at the T-foil and stern tabs, to determine the trim and sinkage. The T-foil was implemented in CFD using two methods: 1) Overset mesh; 2) Forcing function. By replacing the geometric mesh with a lift force coefficient and forcing function, the setup difficulty and computational cost were reduced. Only about 7% difference was observed between CFD and experiments, but no significant difference was found between the methods of overset mesh and forcing function. This has proven the ability of CFD to predict vessel responses to RCS step changes in calm water, and the simplified forcing function method is recommended.

Design and Verification for Measurement and Control System of Large-scale Ship Model

The navigation performance of the ship model is generally studied by the hydrodynamic parameters of ship models with the help of test chamber towing tanks or external waters. Considering the difference among the conditions of the indoor field towing tank or the outer field water area and the actual sea state, to solve the problem of the large-scale ship model series test under the actual sea state in this article, a large-scale ship model measurement and the control system was designed, including heading control system, controller, propeller propulsion device, rudder actuator and power supply module and others. It is used to test the maneuvering performance of large-scale ship models in the sea. On this basis, the reliability and effectiveness of the system were verified by testing the speed and maneuverability of the model in open water.

Ship motions and unsteady pressures on ship hull in oblique waves estimated by potential-based solvers

The seakeeping performances in oblique wave conditions, such as ship motions and unsteady pressures on a ship hull, are getting more important, recently. These physical quantities are used for estimating loads induced by waves and evaluating the safety of a ship. Thanks to the evaluation of various incident wave angles, a rational ship design with sufficient safety and less construction cost can be realized. Therefore, the estimation by numerical schemes in those conditions is demanded for the ship design. However, schemes were usually validated through the comparison with experimental data in only heading wave conditions because few towing tanks can carry out the experiment for a ship advancing in oblique waves. The computed results in oblique waves should be validated well before the industrial usage of schemes. Although Computational Fluid Dynamics is useful to know the seakeeping performance in oblique waves thanks to the development of computer performance in these years, it takes much longer time than the schemes based on a potential theory. For this reason, three solvers based on the potential theory, the Salvesen-Tuck-Faltinsen method, the Green function method, and the Rankine panel method, are validated by comparing ship motions and unsteady pressure on the ship hull obtained from towing tank tests in oblique wave conditions.

The use of Ctrl+Z in ship design: removing a bulbous bow

A bulbous bow is a widely employed feature to enhance the hydrodynamic resistance of ships. More than 95% of the merchant fleet incorporates various types of bulbous bows to reduce wave-making resistance and wave breaking. Bulbous bow is also utilized in smaller vessels like fishing vessels. In certain cases, the initial ship design includes the addition of a bulbous bow by default. In this study, which focuses on a typical Argentinian trawler fishing vessel, we investigate the impact of the bulbous bow on resistance. Taking a reverse approach, we modify the fishing vessel by removing the bulbous bow and we evaluate its hydrodynamics. This research aims to assess the reduction in ship resistance achieved by the bulbous bow under different load conditions and speeds, comparing the vessel with and without the bulbous bow. The numerical analysis uses OpenFOAM, and the results are validated through towing tank experiments. The research demonstrates that the performance of the bulbous bow varies across different conditions, indicating that not always the vessel with a bulbous bow is better.

An experimental and numerical hydrodynamic study on the Argentinian fishing vessels

The purpose of this work is the validation of the hydrodynamic resistance for a small fishing vessel providing more details about the ship’s hydrodynamics. The selected ship is a traditional vessel that has been sailing in Argentina for more than 20 years without a previous numerical resistance evaluation. Because of the absence of previous numerical optimization, the ship might be a non-efficient vessel from a consumption, operating and polluting point of view. For this reason, in order to bring a more detailed evaluation of the flow around this hull, numerical studies are carried out with the open source OpenFOAM code and validated with experimental results obtained at the University of Buenos Aires towing tank. Reynolds Averaged Navier Stokes (RANS) method together with volume of fluid (VOF) are used for the numerical procedure. Once the resistance results are validated, the total resistance coefficient is split into viscous and wave coefficients in order to provide insight into ship hydrodynamics. Pressure distributions on the hull and wave patterns will be also presented for different ship loading conditions, focusing on the flow around the hull that might give information for a future hull optimization.

Experimental study of the influence of catamaran parameters on water resistance

The advantages of multihulls over monohulls are better running characteristics at high speeds and a larger deck area, which is critical for passenger ships. When designing, it is necessary to choose geometric parameters, which include the distance between the hulls. A brief description of the existing methods for calculating the towing resistance of multihull vessels is provided in the paper. New results of an experimental study of the effect of transverse clearance and the ratio of the one hull width to draft on the resistance of a displacement catamaran are also provided. The studies are carried out by testing a small non-self-propelled model of a multihull vessel in the towing tank. The model length is 1.84 m; the width of one hull is 0.15 m; the draft varies from 0.07 m to 0.11 m. The experiment is carried out in the towing tank of the Admiral Makarov State University of Maritime and Inland Shipping. The results are processed according to the Froude method without taking into account dynamic heel and they are presented as a dependence of the residual resistance coefficients on the Froude number. By approximating the experimental data, an expression allowing evaluating the influence of these parameters on the value of the residual resistance in the range of Froude numbers 0.30–0.70 is obtained. One hull is also tested at two draft values in the same speed range. These data are not approximated, but they are presented in the paper in the form of experimentally obtained points. The results of the work can be used at the initial stages of designing catamarans and at studying the influence of these factors on the residual resistance.

Experimental investigation on vortex-induced vibration of a flexible pipe in combined uniform and oscillatory flow

Vortex-induced vibrations (VIV) induced by ocean currents can cause severe fatigue damage to marine risers. The background current and oscillatory flow caused by the motion of the floating platform form a combined flow field, which complicates the VIV of risers. In this paper, a series of model tests were conducted in a towing tank to investigate the VIV characteristics of a flexible pipe in combined uniform and oscillatory flow. Effects of oscillating amplitude, oscillating period and uniform flow velocity on VIV of the pipe were investigated and discussed based on the experimental results. VIV fatigue damage of the pipe was also calculated. Intermittent vibration, hysteresis and mode transition were observed in combined flow. Large oscillating amplitude and short oscillating period tend to enlarge the fatigue damage. But when the uniform flow velocity is too high, small oscillating amplitude and long oscillating period tend to enlarge the fatigue damage. Critical short period and combined ratio threshold are discovered in the VIV of the pipe in combined flow. VIV characteristics in different types of flows are also compared. VIV in combined flow may cause severe fatigue damage, which should be given enough attention.

Hydrodynamic performance of high-speed displacement vessel with hull vane

The hydrodynamic performance of high-speed displacement vessels can be enhanced by installing basic stern fittings such as stern wedges, stern flaps, interceptors, and hull vanes. The hydrodynamic performance of a high-speed displacement vessel with hull vane is analysed and compared to the bare hull condition in this study. The hull vane is a hydrofoil wing attached transversely to the bottom of the transom that focuses largely on minimising the wave-making resistance of the vessel by altering the flow at the stern. It increases flow on its upper surface, which interferes favourably with the stern wave system, hence lowering wave-making resistance. This research investigates the effects of installing the hull vane on a high-speed displacement vessel at various locations in the X and Z directions from the transom edge. The tests were carried out on model ship with and without a hull vane to verify the accuracy of the computational findings. The experiments were carried out in the towing tank facility available at Indian Institute of Technology Madras (IIT Madras), Department of Ocean Engineering (DOE). Computational fluid dynamics (CFD) simulations were carried out for model vessel with and without hull vane for a range of Froude numbers from 0.17 to 0.42 using RANSE-based commercial software. The research examines the resistance, trim, pressure distribution, and stern flow development of the vessel at various speeds and in calm water conditions. The results indicate that installing a stern hull vane reduces resistance and trim of the vessel. The effect of a hull vane is enhanced when it is positioned away from the stern of the vessel.