Micro-bubble Drag Reduction on a High Speed Vessel Model

Ship hull form of the underwater area strongly influences the resistance of the ship. The major factor in ship resistance is skin friction resistance. Bulbous bows, polymer paint, water repellent paint （highly water-repellent wall）, air injection, and specific roughness have been used by researchers as an attempt to obtain the resistance reduction and operation efficiency of ships. Micro-bubble injection is a promising technique for lowering frictional resistance. The injected air bubbles are supposed to somehow modify the energy inside the turbulent boundary layer and thereby lower the skin friction. The purpose of this study was to identify the effect of injected micro bubbles on a navy fast patrol boat （FPB） 57 m type model with the following main dimensions： L=2 450 ram, B=400 mm, and T=190 mm. The influence of the location of micro bubble injection and bubble velocity was also investigated. The ship model was pulled by an electric motor whose speed could be varied and adjusted. The ship model resistance was precisely measured by a load cell transducer. Comparison of ship resistance with and without micro-bubble injection was shown on a graph as a function of the drag coefficient and Froude number. It was shown that micro bubble injection behind the mid-ship is the best location to achieve the most effective drag reduction, and the drag reduction caused by the micro-bubbles can reach 6%-9%.

Resistance Analysis of Unsymmetrical Trimaran Model with Outboard Sidehulls Configuration

The application of multi-hull ship or trimaran vessel as a mode of transports in both river and sea environments have grown rapidly in recent years.Trimaran vessels are currently of interest for many new high speed ship projects due to the high levels of hydrodynamic efficiency that can be achieved,compared to the mono-hull and catamaran hull forms.The purpose of this study is to identify the possible effects of using an unsymmetrical trimaran ship model with configuration（S/L） 0.1-0.3 and R/L=0.1-0.2.Unsymmetrical trimaran ship model with main dimensions： L=2000mm,B=200 mm and T=45 mm.Experimental methods（towing tank） were performed in the study using speed variations at Froude number 0.1-0.6.The ship model was pulled by an electric motor whose speed could be varied and adjusted.The ship model resistance was measured precisely by using a load cell transducer.The comparison of ship resistance for each configuration with mono-hull was shown on the graph as a function of the total resistance coefficient and Froude number.The test results found that the effective drag reduction could be achieved up to 17% at Fr=0.35 with configuration S/L=0.1.

Ship Resistance of Quadramaran With Various Hull Position Configurations

Multihull ships are widely used for sea transportation, and those with four hulls are known as quadramarans. Hull position configurations of a quadramaran include the diamond, tetra, and slice. In general, multihull vessels traveling at high speeds have better hydrodynamic efficiency than monohull ships. This study aims to identify possible effects of various quadramaran hull position configurations on ship resistance for hull dimensions of 2 m length, 0.21 m breadth, and 0.045 m thickness. We conducted a towing test in which we varied the hull spacing and speed at Fr values between 0.08 and 0.62 and measured the total resistance using a load cell transducer. The experimental results reveal that the lowest total resistance was achieved with a diamond quadramaran configuration at Fr = 0.1-0.6 and an effective interference factor of up to 0.35 with S/L = 3/10 and R/L = 1/2 at Fr = 0.62.

On the generation of drift flows in wall-bounded flows transiting to turbulence

Despite recent progress, laminar-turbulent coexistence in transitional planar wall-bounded shear flows is still not well understood. Contrasting with the processes by which chaotic flow inside turbulent patches is sustained at the local(minimal flow unit) scale, the mechanisms controlling the obliqueness of laminar-turbulent interfaces typically observed all along the coexistence range are still mysterious. An extension of Waleffe's approach [Waleffe, 1997] is used to show that,already at the local scale, drift flows breaking the problem's spanwise symmetry are generated just by slightly detuning the modes involved in the self-sustainment process. This opens perspectives for theorizing the formation of laminar-turbulent patterns.

Scale effects on bow wave breaking of KCS ship model:Insights from DDES investigations

Ship bow wave breaking is a common phenomenon during navigation,involving complex multi-scale flow interactions.However,the understanding of this intense free surface flow issue is not sufficiently deep,especially regarding the lack of research on the impact of scale effects on bow wave breaking.This paper focuses on the benchmark ship model KCS and conducts numerical simulations and comparative analyses of bow wave breaking for three model scales under the condition of Fr=0.35.The numerical calculations were performed using the in-house computational fluid dynamics(CFD)solver naoe-FOAM-SJTU,which is developed on the open source platform OpenFOAM.Delayed detached eddy simulation(DDES)method is utilized to calculate the viscous flow field around the ship hull.The present method was validated through measurement data of wave profiles and wake flows obtained from model tests.Flow field results for three different scales,including bow wave profiles,vorticity at various sections,and wake distribution,were presented and analyzed.The results indicate that there is small difference in the bow wave overturning and breaking for the first two occurrences across different scales.However,considerable effects of scale are observed on the temporal and spatial variations of the free surface breaking pattern after the second overturning.The findings of this study can serve as valuable data references for the analysis of scale effects in ship bow wave breaking phenomena.

Boundary element method of hydrodynamic characteristics of lubricant in three-leaf dislocated floating-ring bearing

The problem of hydrodynamics of the three-leaf dislocated floating-ring bearing was studied by means of boundary element method.The law including the distribution of pressure on boundary surface(axial,bearing and floating-ring)and its friction loss in different eccentricities was obtained.The results show that the inner friction of three-leaf dislocated bearing increases from 390.875to 1 091.65,and the inner friction of three-leaf dislocated floating-ring bearing increases from 94.2523to 114.5069with eccentricity varying from 0to 0.075in nondimensional.So changing the pressure and flow field of bearing by adding floating-ring is more stability and less wasted work of friction than three-leaf dislocated bearing.

Research of Interaction Force Coefficients Based on Model Test in Loading Condition

It is very difficult,for the component-type ship mathematical model,to estimate the interaction force coefficients among the hull,propeller and rudder. Some coefficients such as wake fraction and flow straightening coefficient were studied from the model tests in diffierent loading conditions and the normal force of rudder was tested in captive model tests to obtain the coefficients. From these results of the tests,the flow straightening coefficients increase with the increase of trims or drafts. Similarly,wake fraction coefficients are larger for the large drafts,however,become small as the trims increase. The resistance is obviously different in fully loaded condition with the trims by stern,however ,the difference is not evident when the draft decreases and the bulbous bow is above the water surface.

Regularization Method and Immune Genetic Algorithm for Inverse Problems of Ship Maneuvering

Ship maneuverability, in the field of ship engineering, is often predicted by maneuvering motion group （MMG） mathematical model. Then it is necessary to determine hydrodynamic coefficients and interaction force coefficients of the model. Based on the data of free running model test, the problem for obtaining these coefficients is called inverse one. For the inverse problem, ill-posedness is inherent, nonlinearity and great computation happen, and the computation is also insensitive, unstable and time-consuming. In the paper, a regularization method is introduced to solve ill-posed problem and genetic algorithm is used for nonlinear motion of ship maneuvering. In addition, the immunity is applied to solve the prematurity, to promote the global searching ability and to increase the converging speed. The combination of regularization method and immune genetic algorithm（RIGA） applied in MMG mathematical model, showed rapid converging speed and good stability.