Museok Song

Unsteady flow around a two-dimensional section of a vertical axis turbine for tidal stream energy conversion

The two-dimensional unsteady flow around a vertical axis turbine for tidal stream energy conversion was investigated using a computational fluid dynamics tool solving the Reynolds-Averaged Navier-Stokes equations. The geometry of the turbine blade section was NACA653-018 airfoil. The computational analysis was done at several different angles of attack and the results were compared with the corresponding experimental data for validation and calibration. Simulations were then carried out for the two-dimensional cross section of a vertical axis turbine. The simulation results demonstrated the usefulness of the method for the typical unsteady flows around vertical axis turbines. The optimum turbine efficiency was achieved for carefully selected combinations of the number of blades and tip speed ratios.

Parametric Numerical Study on the Performance of Helical Tidal Stream Turbines

Abstract − The characteristics of a helical turbine to be used for tidal stream energy conversion have beennumerically studied with varying a few design parameters. The helical turbines were proposed aiming atmitgating the well known poor cut-in characteristics and the structural vibration caused by the fluctuatingtorque, and the basic concept is introducing some twisting angle of the vertical blade along the rotation axisof the turbine. Among many potential controling parameters, we focused, in this paper, on the twisting angleand the height to diameter ratio of the turbine, and, based on the numerical experiment, We tried to proposea configuration of such turbine for which better performance can be expected. The three-dimensional unsteadyRANS equations were solved by using the commercial CFD software, FLUENT with k- ω SST turbulencemodel, and the grid was generated by GAMBIT. It is shown that there are a range of the twisting angleproducing better efficiency with less vibration and the minimum height to diameter ratio above which the efficiencydoes not improve considerably.Keywords: Vertical Axis Tidal Stream Turbine(수직축 조류발전 터빈), Computational Fluid Dynamics(전산유체역학), Twisting Angle(비틀림각), Height to Diameter Ratio(길이 직경 비), Tip-Speed Ratio(날개속도비), Helical Turbine(헬리컬 터빈)

Experimental Study on the Drag Reduction by Injection of Microbubble and Polymer Solution in a Water Tunnel for Water-Borne Vehicles

Experiments on the friction drag reduction by injecting microbubbles or polymer solutions have been carried out for both a flat wall and an axisymmetric body. In the first setup, the drag reduction on the flat ceiling of a water channel was measured for a fully developed turbulent channel flow by using a newly developed skin friction sensor of floating element type. The effects of key controlling parameters were investigated for higher drag reduction with varying the concentration and the injection rate of the additives. The integrated frictional drag has been reduced up to 25% with the microbubble injection and 50% with the polymer solution injection. The drag reduction for a slender axisymmetric body was also investigated in a cavitation tunnel based on the parameters obtained from the channel experiments.Copyright

A PRACTICAL APPLICATION OF AIR LUBRICATION ON A SMALL HIGH SPEED BOAT

This paper discusses how three geometrically similar models are made to investigate the scale law governing the air lubrication on a small high speed boat. Air is supplied behind a step placed on the bottom of the model and relations between flow rates of air and the shapes and areas of the air cavity as well as the resistance reduction are also observed. The reduction of resistance with lubrication of an air cavity and the similarity relations involved are investigated with a series of towing tank tests on geometrically similar models. Three similar models of different size are tested in the towing tank and the results indicate that an introduction of air cavity by artificially supplying air beneath the bottom of a model ship with a backward-facing step is effective for reduction of model resistance. The areas of air cavity and the required flow rates of air, both of which are directly related to the effective wetted surface area and the overall energy saving are found do be dominated by the Froude number scaling. Both the traditional two-dimensional method and Telfers three-dimensional method seem to be applicable to the extension of the model resistance in the tested range if corrections are made to account for the changes in the frictional resistance caused by the changes in the effective wetted surface area. Based on these finings, a small test boat of practical size is constructed and the effect of air cavity on the reduction of ship resistance is studied. The results confirm the results found in the model tests and assure practical applicability of the air cavity for the resistance reduction of a real ship.