Wake Fraction Research Articles

Effective wake estimation of KCS hull at full-scale by GEOSIM method based on CFD

In this study, the effective wake factors of a self-propelled container ship were investigated by Telfer's GEOSIM method based on CFD (Computational Fluid Dynamics). For this purpose, the self-propulsion characteristics of KCS (KRISO Container Ship) at three different model scales and full-scale were determined at Fr = 0.26 by unsteady RANS (Reynolds Averaged Navier-Stokes) solver. All scales corresponded to a wide range of Reynolds numbers. Turbulent flow around the KCS hull has been investigated by the k-ε turbulence model. Heave and pitch motions of KCS hulls have been modeled with the DFBI (Dynamic Fluid Body Interaction) method and overset mesh technique. The rotational motion of KP505 propeller has been simulated using the RBM (Rigid Body Motion) approach. The result obtained by the extrapolation of the effective wake fraction with the GEOSIM method was compared with the full-scale ones of CFD and ITTC methods. The results by the GEOSIM method are more compatible with those of full-scale CFD than the results of 1978 ITTC method. In the wake region, where viscous effects are predominant, the change of effective wake fraction with Reynolds number has been successfully represented by the GEOSIM method, thus more robust and reliable results have been found.

Power increase and propulsive characteristics in regular head waves of KVLCC2 using model tests

This paper employs experimental methods to predict speed-power-rpm relationship along with resistance and propulsion characteristics in regular head waves with varying wave lengths and steepness ratios. The objective ship is KVLCC2. The resistance and self-propulsion tests in calm water and in waves have been conducted. The self-propulsion point in waves changes to increase propeller revolution, thrust and torque with wave steepness. The added propeller revolution, thrust, torque and delivered power due to waves increase with wave steepness, and are not simply proportional to the square of the wave amplitude, which are similar tendencies to the added resistance. The thrust deduction and wake fraction, propeller advance coefficient, hull, propeller-open-water, relative rotative and propulsive efficiencies due to waves decrease with wave steepness. The effects of ship speed and delivered power in waves on the propulsive performances have been looked into. The wave conditions are the regular head waves of λ/LPP = 0.6 and 1.0 with three wave steepness ratios at three ship speeds of 13.5, 14.5 and 15.5 knots (design speed). In the case of λ/LPP = 0.6 at design speed, two more wave steepness ratios have been used.

Fouling effect on the resistance of different ship types

Considering that the ship hydrodynamic behaviours differ by the ship types and dimensions, the effect of biofouling on ship resistance can also vary with different ships. In this study, Unsteady Reynolds Averaged Navier-Stokes (URANS) based towed ship models were developed to simulate the roughness effect of biofouling on ship resistance. A container ship (KCS) and a tanker (KVLCC2), representing slender and a full hull forms, were modelled with various scale factors and speeds. The CFD simulations were conducted with several fouling conditions by embedding the roughness function of barnacles into the wall-function of the CFD model (i.e. modified wall-function approach). The fouling effects on the resistance components, form factors, wake fractions and the flow characteristics were investigated from the simulations. Significant differences were observed varying with the hull types, lengths (scales) and speeds of the ships and it was concluded that these differences are dominated by two parameters; relative roughness height and the roughness Reynolds number.

CFD Simulations of Self-Propulsion and Turning Circle Maneuver up to 90º of Ship in Waves

In the present work, a Reynolds-Averaged Navier-Stokes (RANS)-overset method is used to numerically investigate self-propulsion and turning circle maneuver in waves for a container ship. A computational fluid dynamics (CFD) solver naoe-FOAM-SJTU is used for the numerical computations of the fully appended Duisburg Test Case ship model. Overset grids are used to handle the motions of the ship hull appended with the propeller and the rudder. Open source toolbox waves2Foam is used to prevent wave reflection in the computational domain. The current numerical method is validated by comparing the ship speed in the self-propulsion case between CFD and Experimental Fluid Dynamics (EFD). Predicted ship 6-DOF motions, hydrodynamic forces, free surfaces, and inflow of the propeller are presented. The propulsion characteristic is mainly studied. Assuming the thrust identification method works even in unsteady conditions, the wake fraction and propulsion efficiency are discussed. The effect of orbital motion of water particle and ship motion on the propulsion performance are identified. In conclusion, the present RANS-overset method is a reliable approach to directly simulate self-propulsion and turning circle maneuver in waves.

Comparison of Unsteady Reynolds-Averaged Navier-Stokes Prediction of Self-Propelled Container Ship Squat Against Empirical Methods and Benchmark Data

The demand to increase port throughput has driven container ships to travel relatively fast in shallow water whilst avoiding grounding and hence, there is need for more accurate high-speed squat predictions. A study has been undertaken to determine the most suitable method to predict container ship squat when travelling at relatively high speeds (Frh ≥ 0.5) in finite water depth (1.1 ≤ h/T ≤ 1.3). The accuracy of two novel self-propelled URANS CFD squat model are compared with that of readily available empirical squat prediction formulae. Comparison of the CFD and empirical predictions with benchmark data demonstrates that for very low water depth (h/T < 1.14) and when Frh < 0.46; Barass II (1979), ICORELS (1980), and Millward’s (1992) formulae have the best correlation with benchmark data for all cases investigated. However, at relatively high speeds (Frh ≥ 0.5) which is achievable in deeper waters (h/T ≥ 1.14), most of the empirical formulae severely underestimated squat (7-49%) whereas the quasi-static CFD model presented has the best correlation. The changes in wave patterns and effective wake fraction with respect to h/T are also presented.

황천 시 최소추진출력 관련 IMO 잠정 가이드라인의 진행 현황과 적용 결과 검토

Literature survey was conducted on the 2013 Interim Guidelines for determining the minimum propulsion power to maintain the manoeuvrability of ships in adverse conditions. The International Maritime Organization (IMO) Marine Environment Protection Committee (MEPC) documents related with the Guidelines were reviewed. Compatibility of the present Guidelines can be checked by two different levels: (Level 1) minimum power lines assessment and (Level 2) simplified assessment. The IHS (Information Handling Services) sea-web data on the bulk carriers and the tankers, which were built after 2000, were used to examine the Level 1 assessment. KVLCC2 was used to examine the Level 2 assessment. Regarding the Level 2 assessment, effects of the adverse weather conditions, the added resistance due to waves, the wake fractions and the thrust deduction factors were discussed.

Experimental and Numerical Investigation of DARPA Suboff Submarine Propelled with INSEAN E1619 Propeller for Self-Propulsion

The Defense Advanced Research Projects Agency (DARPA) Suboff Submarine propelled by the Italian Ship Model Basin (INSEAN) E1619 propeller is extensively used in submarine validation studies. Although there are several numerical studies where the DARPA Suboff submarine is used in combination with E1619 propeller there are no experimental data available in open literature for the self-propulsion condition. In this article, the self-propulsion characteristics of the DARPA Suboff submarine model with INSEAN E1619 propeller obtained with experimental and numerical methods are presented and discussed by means of Taylor wake fraction, thrust deduction, hull efficiency, relative rotative efficiency, and propulsive efficiency. To experimentally investigate the submarine form, a self-propulsion experimental setup is designed and manufactured. Resistance and self-propulsion experiments are conducted in Istanbul Technical University Ata Nutku Ship Model Testing Laboratory. Resistance tests are carried out for three different speeds, and the results show good agreement with the published experimental results. Propulsion tests are conducted by using the load-varying self-propulsion test method for constant speed and seven different propeller rotation rates. Rotational speed, thrust, and torque forces at self-propulsion point are investigated. For the numerical computations a commercial Computational Fluid Dynamics (CFD) code is used. Propeller open water characteristics and nondimensional velocities behind the propeller are calculated. Self-propulsion point of the submarine and propeller assembly is also solved numerically and the results are compared with the results obtained from the experiments, and it is seen that especially the propeller rate of revolution and thrust force are predicted with very good approximation.

Numerical study on the influence of air layer for propeller performance of large ships

Based on the RANS equation and the VOF two-phase flow model, numerical simulations were conducted about the air layer on the lower surface of a large ship model with and without propeller. The influence of air injection on the propulsion performance of the propeller is investigated by comparing of thrust and torque of propeller. Numerical results indicate that the air does not enter the propeller working area directly for the design bottom cavity in this paper. The average of the axial speed at the propeller disk surface would increase and the average wake fraction decrease with air injection. Under the same inflow velocity and rotation of the propeller, the bottom air injection would make the propeller thrust and torque become smaller, so the KT and KQ decrease. The air injection reduces the resistance of the ship model and the ship's self-propelled point would change. Therefore, the studies of the effect of the air injection on the propeller propulsion performance require to be analyzed at the new self-propelled point.

The Improvement of the Powering Performance on the Japan Bulk Carrier (JBC) using Rudder Thrust Fin as a Post-Swirl Stator

Post Swirl Stator (PSS), also known as the 'Energy Saving Device (ESD) Behind the Propeller' operates within the slip stream of the propeller. The aim of this project is to investigate the powering performance of a JAPAN Bulk Carrier (JBC) upon the installation of PSS. The prediction of resistance and propulsive factors were conducted using CFD simulation using SHIPFLOW CFD code. In order to find the optimised design of the ESD, three parameters were laid out and studied. The parameters are the left side fin length, right side fin length and the orientation of the fins.The aim of this study are to improvise the thrust fin design by changing the parameters to get the optimum powering performance result, and to quantify the powering performance of the JBC upon the installation of PSS in model and full scale. There were 27 different post-swirl stator configurations. All the 27 different configurations were compared in terms of its performance in model and full scale. All the 27 different configurations were simulated using full RANSE. The free surface of the water were modelled using panel method. A grid dependence study was conducted to determine the best grid cell resolution and it was chosen at a total of 7 millions cells. The bare hull resistance of the JBC were validated with available published experiment results. All the thrust fins were modelled using appendage setting in SHIPFLOW. The best thrust fins design were selected in judging its minimum drag, minimum thrust deduction fraction, minimum wake fraction, and the delivered power. It was found that he thrust fins design for case study #20 by CFD is the most optimised configuration in terms of its performance criteria as mentioned earlier. The thrust fins design was able to reduce delivered power to 6.012%. Thrust deduction was also reduced to 2.927%. Total efficiency was increased to 6.709%

A Numerical and Experimental Study on the Hull-Propeller Interaction of A Long Range Autonomous Underwater Vehicle

Range is an important factor to the design of autonomous underwater vehicles (AUVs), while drag reduction efforts are pursued, the investigation of body-propeller interaction is another vital consideration. We present a numerical and experimental study of the hull-propeller interaction for deeply submerged underwater vehicles, using a proportional-integral- derivative (PID) controller method to estimate self-propulsion point in CFD environment. The hydrodynamic performance of hull and propeller at the balance state when the AUV sails at a fixed depth is investigated, using steady RANS solver of Star-CCM+. The proposed steady RANS solver takes only hours to reach a reasonable solution. It is more time efficient than unsteady simulations which takes days or weeks, as well as huge consumption of computing resources. Explorer 1000, a long range AUV developed by Shenyang Institute of Automation, Chinese Academy of Sciences, was studied as an object, and self-propulsion point, thrust deduction, wake fraction and hull efficiency were analyzed by using the proposed RANS method. Behind-hull performance of the selected propeller MAU4-40, as well as the hull-propeller interaction, was obtained from the computed hydrodynamic forces. The numerical results are in good qualitative and quantitative agreement with the experimental results obtained in the Qiandao Lake of Zhejiang province, China.

Investigation of the nominal and effective propeller inflow for a family of inland waterway vessels

Inland vessels operate in a wide range of waterway conditions and sometimes extremely shallow water, where the propeller inflow becomes critical. Knowledge of the wake field is of utmost importance for a proper and efficient design of the propulsion system. To gain a deeper understanding of the inflow conditions at different water depths, four representative inland vessels with the dimensions of a large Rhine vessel and different stern shapes were designed and built as scale models. For the detailed determination of the complex flow phenomena at the stern, a complementary approach of model tests, Stereo Particle Image Velocimetry (3C-PIV) and RANSE CFD calculations was chosen. While the measured resistance profiles differ negligibly between the designs, there are significant differences in the power demand. The integral wake fraction from the propulsion test does not adequately describe this behaviour. Finally, experimental and numerical flow fields could be compared for a deeper insight. • PIV and CFD studies on hull-propulsor interaction of inland ships in shallow water. • Small influence of hull shape in resistance, but distinct effects with propulsion. • Design optimization of inland ships for operating conditions of great importance.

Investigating the impact of wakes on single turbine availability

The current trend in wind energy leads to closely packed wind farms. These short distances result in a higher turbulence and consequently higher loads. In this paper an approach is proposed to evaluate single turbine availability losses due to the wake effect by modeling material fatigue behavior of selected turbine components. First, a characteristic wake scenario is defined: While one turbine is exposed to a wind field conforming to wind class IIB, another turbine experiences higher turbulence intensities according to Frandsen’s wake model. Using a dynamic multibody simulation, turbine loads are calculated which then are analyzed using the rainflow counting algorithm (giving load range and cycle counts) before being extrapolated to design lifetime of 20 years. Subsequently, artificial material curves are estimated representing the minimum load cycles to withstand during design lifetime. The slope of these curves is determined using the slope of a representative S-N-curve for each component. Position is then determined applying Miner’s rule with unity damage at the end of design lifetime. Lastly, failure behavior is approached by using a lognormal probability distribution. As a result increased failure probabilities for the rotor hub unit (main bearing and rotor hub) are found in the wake case. First results for an annual time in wake fraction of 20 % show an increase in total failure probability from 2.30 % to up to 41.85 % at a distance between the two turbines of 3D. Applying a mean down time per failure from literature (based on modern EU wind farms) finally leads to an overall technical availability decrease due to hub unit failure of 0. 035 % in the wake case compared to the free stream case.

Prediction of Self-Propulsion Performance of Ship Model with Double L-Type Podded Propulsors and Conversion Method for Full-Scale Ship

In this study, the self-propulsion performance of a ship model with double-L-type podded propulsors was predicted. Additionally, a conversion method for the performance of a full-scale ship was established based on the correction method published by the International Towing Tank Conference (ITTC) for the scaling effect of a single podded propeller and research reports on pod tests conducted by different ship research institutes. The thrust deduction and wake fraction of the ship were also analyzed. Furthermore, the self-propulsion performance of a full-scale ship with double L-type pods was determined, the full- and model-scale ships compared in terms of their flow fields and pressure charts, and the influence of the scaling effect analyzed. In addition, the calculation results were compared with the conversion results of a full-scale ship, and the reliability of the method adopted for the performance estimation of a full-scale ship with double podded propulsors was verified. The findings reported herein can provide statistics and technical support for the design of L-type podded propulsors and their application in full-scale ships, which are of theoretical significance and practical value in the engineering domain.

Numerical investigation of scale effect of nominal wake of four-screw ship

An experimental investigation is conducted to obtain the nominal wake of a four-screw ship in a towing tank using a particle image velocimetry (PIV) measurement system. A computational study on the viscous flow fields of the ship at different scales is carried out using the Reynolds-averaged Navier–Stokes (RANS) method, without considering the free surface effect. The scale effect for the axial nominal wake field of the inside and outside propeller discs is analyzed in detail. The results show that the CFD results and PIV data are in good agreement. Different circumferential distribution patterns of the axial nominal wake field exist between the inside and outside propeller discs, especially in the zone behind the brackets. The mean wake fraction of the inside propeller is generally larger than that of the outside propeller when different radii are considered. For both the inside and outside propellers, the mean axial wake fraction is relatively large in the inner radius area (0.3 ≦ r/R ≦ 0.6), whereas it is relatively small and almost coincident in the outer radius area (0.7 ≦ r/R) at different scales. ωx (Axial wake fraction) decreases nonlinearly with the increase in Reynolds number for both the inside and outside propeller discs, whereas Δωx (wake fraction difference) increases linearly with the increase in the Reynolds number. With the increase of the model scale, the harmonics components decreases gradually.

Comparative study of prediction methods of power increase and propulsive performances in regular head short waves of KVLCC2 using CFD

This paper employs computational tools to predict power increase (or speed loss) and propulsion performances in waves of KVLCC2. Two-phase unsteady Reynolds averaged Navier-Stokes equations have been solved using finite volume method; and a realizable k-ε model has been applied for the turbulent closure. The free-surface is obtained by solving a VOF equation. Sliding mesh method is applied to simulate the flow around an operating propeller. Towing and self-propulsion computations in calm water are carried out to obtain the towing force, propeller rotating speed, thrust and torque at the self-propulsion point. Towing computations in waves are performed to obtain the added resistance. The regular short head waves of λ/LPP = 0.6 with 4 wave steepness of H/λ = 0.007, 0.017, 0.023 and 0.033 are taken into account. Four methods to predict speed-power relationship in waves are discussed; Taylor expansion, direct powering, load variation, resistance and thrust identity methods. In the load variation method, the revised ITTC-78 method based on the ‘thrust identity’ is utilized to predict propulsive performances in full scale. The propulsion performances in waves including propeller rotating speed, thrust, torque, thrust deduction and wake fraction, propeller advance coefficient, hull, propeller open water, relative rotative and propulsive efficiencies, and delivered power are investigated.

Modeling the ship white water wake in the midwave infrared.

We have modeled the white water wake of a ship as a single layer of bubbles packed on the sea surface within the perimeter of the trailing turbulent wake. The size of the bubbles is considered greater than the midwave infrared wavelengths such that the optical geometrical approximation remains valid. The upper half bubble hemisphere is meshed into facets, and we calculate the probability density function of their slopes and constrain that distribution by the geometrical limits imposed by the position of the receiver through the shadowing of facets by other bubbles and of facets that are facing away from the receiver. For the facets that are visible, we compute the midwave infrared emitted and reflected radiance for the white water wake for atmospheric, solar, and sea conditions that prevailed during a ship wake measurement trial using a homegrown simulation code, the Sea Surface Radiance Simulator. The range of slopes that are visible to the receiver for the white water wake greatly exceeds those that are present in the turbulent wake and in the sea background. Consequently, the variability in the white water wake radiance is substantial. As a function of the downstream distance astern of the ship, we have ad hoc assumed that the white water wake fraction decays linearly or proportionally to the turbulent intensity in the wake. Comparing to measurements, we find an agreement in trend behavior of the midwave radiance contrast of the white water wake with downstream distance for a white water wake fraction that decays proportionally to the square of the turbulence intensity.

Self-propulsion estimations for a bulk carrier

ABSTRACTSelf-propulsion predictions are important to understand propeller–hull interactions and the communication between the main engine and the propulsion system. Experimental work encapsulates the core of self-propulsion estimations, but due to their high cost, initial calculations are made using empirical relations suggested by IMO. However, these empirical relations may, at times, fail predicting some of the self-propulsion parameters. In this study, a specific example was chosen in which IMO recommendations fail to predict the wake fraction. Numerical results utilizing RANSE are used to solve for the hull-propeller system to assess their validity in contrast with the experiments. RANSE is implemented in two different methods. The first one is by simulating directly the hull-propeller system, while the second is the hull-virtual disk system, in which the propeller is only modelled as a circular disk. It was found out that numerical results are more robust with respect to empirical relations advised by IMO.

Method for estimation of icebreaker propulsion performance in ice

Classical methods of ship propulsion estimations based on the traditionally applied system of hull and propeller interaction coefficients are not suitable for prediction of icebreaker performance in ice. This is because the wake fraction tends to assume negative values in operating conditions typical of navigation in ice. This paper suggests an alternative system of hull/propeller interaction coefficients to overcome the problem. The alternative system comprises the thrust deduction factor t and the factors allowing for hull effect on thrust iTB and hull effect on torque iQB. An alternative method of ship propulsion performance estimations is developed based on this system of coefficients to take into account all characteristics of icebreaker sailing in ice. The method was used to analyze the full-scale data obtained from sea trials of the icebreakers Vladivostok and Novorosiisk. The initial data inputs for the analysis were ship speed in ice and consumed power measured during the sea trials. The number of propeller revolutions and ice resistance were calculated. The estimated propeller revolutions were compared with the full-scale data gathered during the sea trials in ice. The ice resistance was compared with the predictions based on model tests in ice basin. It is shown that at slow speeds in ice, ranging from 0.5 to 5.0 knots, the predicted number of propeller revolutions is in good agreement with the data measured during the sea trials. Propulsion performance of the Vladivostok icebreaker in 1.5-m thick ice was estimated

Оценка коэффициента попутного потока по результатам модельных испытаний водометного движителя

Оценка коэффициента попутного потока по результатам модельных испытаний водометного движителя

Energy balance analysis of a propeller in open water

This paper proposes a methodology based on control volume analysis of energy, applied on Computational Fluid Dynamics (CFD) results, for analyzing ship propulsion interaction effects as a complement to the well-established terminology, including thrust deduction, wake fraction and propulsive efficiency. The method, titled Energy Balance Analysis, is demonstrated on a propeller operating in open water. Through consideration of a complete energy balance, including kinetic energy flux, turbulent kinetic energy flux, internal energy flux (originating from dissipation) and pressure work, all possible hydrodynamic losses are included in the analysis, implying that it should be possible to avoid sub-optimized solutions. The results for different control volumes and grid refinements are compared. The deviation of the power obtained from the proposed energy balance analysis relative to the power based on integrated forces on the propeller is less than 1%. The method is considered promising for analyzing and understanding propulsor hull interaction for conventional, as well as novel propulsion configurations. The energy balance analysis is conducted as a post-processing step and could be used in automated optimization procedures.