Jong-Soo Seo

Application of signal processing techniques to the detection of tip vortex cavitation noise in marine propeller

The tip vortex cavitation and its relevant noise has been the subject of extensive researches up to now. In most cases of experimental approaches, the accurate and objective decision of cavitation inception is primary, which is the main topic of this paper. Although the conventional power spectrum is normally adopted as a signal processing tool for the analysis of cavitation noise, a faithful exploration cannot be made especially for the cavitation inception. Alternatively, the periodic occurrence of bursting noise induced from tip vortex cavitation gives a diagnostic proof that the repeating frequency of the bursting contents can be exploited as an indication of the inception. This study, hence, employed the Short-Time Fourier Transform (STFT) analysis and the Detection of Envelope Modulation On Noise (DEMON) spectrum analysis, both which are appropriate for finding such a repeating frequency. Through the acoustical measurement in a water tunnel, the two signal processing techniques show a satisfactory result in detecting the inception of tip vortex cavitation.

Experimental Estimation of a Scaling Exponent for Tip Vortex Cavitation Via Its Inception Test in Full-and Model-Ship

Tip vortex cavitation noise of marine propeller became primary concerns to reduce hazardous environmental impacts from commercial ship or to keep the underwater surveillance of naval ships. The investigations of the tip vortex and its induced noise are normally conducted through the model test in a water cavitation tunnel. However the Reynolds number of model-test is much smaller than that of the full-scale, which subsequently results in the difference of tip vortex cavitation inception. Hence, the scaling law between model- and full-scales needs to be identified prior to the prediction and assessment of propeller noise in full scale. From previous researches, it is generally known that the incipient caivtation number of tip vortex can be represented as a power of the Reynolds number. However, the power exponent for scaling, which is the main focus of this research, has not been clearly studied yet. This paper deals with the estimation of scaling exponent based on tip vortex cavitation inception test in both full- and model-scale ships. Acoustical measurements as well as several kind of signal processing technique for an inception criterion suggest the scaling exponent as 0.30. The scaling value proposed in this study shows slight difference to the one of most recent research. Besides, extrapolation of model-ship noise measurement using the proposed one predicts the full-scale noise measurement with an acceptable discrepancy.

Experimental Study on the Cavitation Noise of a Hydrofoil

In order to investigate the noise characteristics of the different caviation, noise measurements were carried out in a large cavitation tunnel of the Samsuug Ship Model Basin(SSMB). The noise measurements for a 3-dimensional hydrofoil were carried out at the angle of attack of and according to the decrease in cavitation number. It is exhibited that sound pressure level(SPL) increased sharply with cavitation inception. The frequency of the noise induced by sheet cavitation was higher than that of tip vortex cavitation in the phase of cavitation inception. Within the range of the high frequency, in the case of fully developed cavitation, sheet cavitation noise was significantly increased in sound pressure level compared with tip vortex cavitation noise. In this study, the noise characteristics of the different cavitation types were considered experimentally and would be utilized as a basis for the analysis of propeller cavitation noise.

Development and Application of Trim Optimization and Parametric Study Using an Evaluation System (SoLuTion) Based on the RANS for Improvement of EEOI

In general, the speed power performance of ships is optimized for design speed and draught in accordance with the contract condition. But, the contract condition may not be always the same as the actual operating condition. Therefore, in order to reduce the fuel consumption practically, it is necessary to optimize the performance under various conditions considering the actual voyage. This is the reason that the trim optimization covering various operating profiles becomes the main issue in reducing fuel oil consumption.In this paper, a numerical study is carried out to optimize trim conditions through the computational evaluation system called SoLuTion developed by Samsung Heavy Industries Co. (SHI) with variation in draught, ship speed and voyage trim. In order to get more accurate results in resistance and self-propulsion performance, the Reynolds-Averaged Navier-Stokes (RANS) equations including Volume of the Fluid (VOF) method by Hirt and Nichols (1981) for free-surface boundary condition and moving mesh technique for propeller rotation effect are employed as governing equations. Reynolds Stresses Model (RSM) with the wall function is applied as a turbulence model for turbulent flow computation.Through this study, the followings are the main three factors to be considered in evaluation of the quantitatively accurate speed performance by varying the draught, speed and voyage trim. First, the distance from hull surface to the first grid point should be adjusted to eliminate the effects of frictional resistance. Second, the grid system should be generated to avoid the grid dependency on variation of draught. Finally, the running trim and sinkage of the voyage conditions should be considered. The results of the trim optimization performed numerically are well-matched with the towing test results conducted at Samsung Ship Model Basin(SSMB). As the results of this study, it is confirmed that SoLuTion is a useful and efficient tool for trim optimization and the provided optimum trim will be able to contribute to fuel savings under the operating conditions. From trim optimization study, it is found that there is certain level of effects due to the bulb and transom immersion to find the optimum trim condition. Also SoLuTion is applicable not only to trim optimization but also to parametric studies in optimum shape of the ships considering the actual operating profile.Copyright

Improvements of model-test method for cavitation-induced pressure fluctuation in marine propeller

Although the prediction of propeller cavitation-induced pressure fluctuation strongly depends on the model-scale measurement in a cavitation tunnel, there is still a lack of correlation with full-scale data. This paper deals with the enhancement of such a correlation deficiency by improving the conventional model-test technique, two majors of which are in the following. One is to take into account the boundary layer effect of wooden fairing plate at the ceiling of water cavitation tunnel. The other is to avoid the resonance frequency range of model-ship via adjusting the revolution speed of model propeller. Through a case study, for which both model and full-scale test data are available, the improved method in this study shows its validness, and furthermore a close correlation with full scale measurement.

Numerical Study on the Effects of Combination of Blade Number for Shaft Forces and Moments of Contra-Rotating Propeller

The effects of the combination of blade number for forward and after propeller on the propeller shaft forces of a contra-rotating propeller (CRP) system are presented in the paper. The research is performed through the numerical simulations based on the Reynolds-Averaged Navier-Stokes equations (RANS). The simulation results of the present method in open water condition are validated comparing with the experimental data as well as the other numerical simulation results based on the potential method for 4-0-4 CRP (3686＋3687A) and 4-0-5 CRP (3686＋3849) of DTNSRDC. Two sets of CRP are designed and simulated to study the effect of the combination of blade number in behind-hull condition. One set consists of 3-blade and 4-blade, while the other is 4-blade and 4-blade. A full hull body submerged under the free surface is modeled in the computational domain to simulate directly the wake field of the ship at the propeller plane. From the simulation results, the fluctuations of axial force and moment are dominant in the case of same blade numbers for forward and after propellers, whereas the fluctuations of horizontal and vertical forces and moments are very large in the case of different blade numbers.

The Investigation of Non-Linear Effect for a Circular Cylinder Shaped FPSO

Nowadays, a circular cylinder shaped FPSO appears as a new type FPSO because it does not need the expensive turret system. A circular cylinder shaped FPSO has an excellent motion performance because pitch and roll natural periods are longer compared to traditional FPSO and heave natural period is also longer due to its small water plane area. Samsung heavy industries has developed a circular cylinder shaped FPSO called as S-Line. The feature of S-line has a shape cutting a groove around draft to reduce the water plane area. Through reducing water plane area, the heave natural frequency moves to lower frequency which is far from ocean waves. According to the results of a linear calculation by WAMIT, S-Line showed reduced vertical motion of heave, roll and pitch compared to conventional FPSO. But, S-Line showed unsuspected large pitch response in 1st model test which was carried out with horizontal mooring system by spring in SSMB (Samsung Ship Model Basin). It is conjectured that this large pitch response is caused by 2nd nonlinearity of the wave and/or parametric pitch phenomena. To verify the mooring load and improve the 2nd order pitch motion, model tests were carried out in SSMB and offshore basin of MOERI (Maritime & Ocean Engineering Research Institute) with two kinds of bilge box. This paper describes the nonlinear effect of 2nd order for pitch motion of a circular cylinder shaped FPSO based on model test and numerical simulation. As a result, S-Line developed by Samsung was also verified being able to use SCR (Steel Catenary Riser) due to its improved heave motion performance.Copyright