PERFORMANCE PREDICTION OF A 112M WAVE-PIERCING CATAMARAN

The prediction of power required to propel a high-speed catamaran involves the hydrodynamic interactions between the hull surface and the surrounding fluid that may be difficult to compute numerically. In this study model-scale experiments are used as a basis for comparison to full-scale sea trials data measured on a 112m Incat wave-piercing catamaran to predict the full-scale powering requirements from model-scale testing. By completing water jet shaft power measurements on an Incat vessel during sea trials, comparison of these results was made to model-scale test results to provide good correlation. The work demonstrates that the International Towing Tank Conference (ITTC) extrapolation techniques used provide a good basis for extrapolating the data from model-scale to full-scale to predict the power requirements for the full-scale catamaran vessel operating at high Froude Number with water jet propulsion. This provides a useful tool for future designers and researchers for determining the power requirements of a catamaran vessel through model tests.

THE EXPERIMENTAL METHOD AND CFD STUDY OF THE BARE HULL FORM OF UNDERWATER VEHICLE AT NEAR SURFACE CONDITION

Hull form selection, resistance and powering are important in designing underwater vehicle. An underwater vehicle bare hull form is based on the five parameters due to the interaction between the propeller and the hull. When they are running on the surface condition, there will be problems likely as surface vessel, but the main hull of the underwater vehicle is below the waterline with low freeboard. The underwater vehicles are operating with high speed at a high Froude Number. Therefore, the wave making component becomes important in surface resistance. The wave making resistance of the underwater vehicle model at surface condition is analyzed by using CFD tools. Friction component of resistance is calculated by using ITTC’57 correlation line. The flow around the ship’s hull is complicated, so that model experiments are still the most reliable data source on ship resistance determination. The bare hull form of underwater vehicle resistance is based on the model experiments and CFD results. The towing mechanism arrangement should be considered at model. Therefore, towing mechanism is designed for model testing. This paper discusses the towing method and result between model test and CFD. This paper also makes comparison of wave formation Towing tank test and CFD at various speeds. The model was tested with bare hull form of underwater vehicle in the Ship Model Towing Tank at Marine Hydrodynamics Centre, Myanmar Maritime University. CFD analysis is also carried out and the results are compared for surface condition. The three software packages XFLOW, XMESH and XPAN are used for CFD simulations. The comparison of results shows that the coefficient differences are less staggered based on the speed.

High Froude Number Experimental Investigation of the 2 DOF Behavior of a Multihull Float in Head Waves

Dynamic Velocity Prediction Programs are taking an increasingly prominent role in high performance yacht design, as they allow to deal with seakeeping abilities and stability issues. Their validation is however often neglected for lack of time and data. This paper presents an experimental campaign carried out in the towing tank of the Ecole Centrale de Nantes, France, to validate the hull modeling in use in a previously presented Dynamic Velocity Prediction Program. Even though with foils, hulls are less frequently immersed, a reliable hull modeling is necessary to properly simulate the critical transient phases such as touchdowns and takeoffs. The model is a multihull float with a waterline length of 2.5 m. Measurements were made in head waves in both captive and semi-captive conditions (free to heave and pitch), with the model towed at constant yaw and speed. To get as close as possible to real sailing conditions, experiments were made at both zero and non-zero leeway angles, sweeping a wide range of speed values, with Froude numbers up to 1.2. Both linear and nonlinear wave conditions were studied in order to test the limits of the modeling approach, with wave steepness reaching up to 7% in captive conditions and 3.5% in semi-captive ones. The paper presents the design and methodology of the experiments, as well as comparisons of measured loads and motions with simulations. Loads are shown to be consistent, with a good representation of the sustained non-linearities. Pitch and heave motions depict an encouraging correlation which confirms that the modeling approach is valid.

Dispersive and dispersive-like bores in channels with sloping banks

In this paper a detailed analysis of undular bore dynamics in channels of variable cross-section is presented. Two undular bore regimes, low Froude number (LFN) and high Froude number (HFN), are simulated with a Serre–Green–Naghdi model, and the results are compared with the experiments by Treske (1994). We show that contrary to Favre waves and HFN bores, which are controlled by dispersive non-hydrostatic mechanisms, LFN bores correspond to a hydrostatic phenomenon. The dispersive-like properties of the LFN bores is related to wave refraction on the banks in a way similar to that of edge waves in the near shore. A fully hydrostatic asymptotic model for these dispersive-like bores is derived and compared to the observations, confirming our claim.

Performance Prediction of a 112m Wave-Piercing Catamaran

The prediction of power required to propel a high-speed catamaran involves the hydrodynamic interactions between the hull surface and the surrounding fluid that may be difficult to compute numerically. In this study model-scale experiments are used as a basis for comparison to full-scale sea trials data measured on a 112m Incat wave-piercing catamaran to predict the full-scale powering requirements from model-scale testing. By completing water jet shaft power measurements on an Incat vessel during sea trials, comparison of these results was made to model-scale test results to provide good correlation. The work demonstrates that the International Towing Tank Conference (ITTC) extrapolation techniques used provide a good basis for extrapolating the data from model-scale to full-scale to predict the power requirements for the full-scale catamaran vessel operating at high Froude Number with water jet propulsion. This provides a useful tool for future designers and researchers for determining the power requirements of a catamaran vessel through model tests.

Cloud Cavitating Flow That Surrounds a Vertical Hydrofoil Near the Free Surface

Unstable cavitation presents an important speed barrier for underwater vehicles such as hydrofoil craft. In this paper, the authors concern about the physical problem about the cloud cavitating flow that surrounds an underwater-launched hydrofoil near the free surface at relatively high-Froude number, which has not been discussed in the previous research. A water tank experiment and computational fluid dynamics (CFD) simulation are conducted in this paper. The results agree well with each other. The cavity evolution process in the experiment involves three stages, namely, cavity growth, shedding, and collapse. Numerical methods adopt large eddy simulation (LES) with Cartesian cut-cell mesh. Given that the speed of the model changes during the experiment, this paper examines cases with varying constant speeds. The free surface effects on the cavity, re-entry jet location, and vortex structures are analyzed based on the numerical results.

CFD and EFD Based Studies of Hull Wetness of Fast Mono-WPC

Fast mono-wave-piercing craft (Fast Mono-WPC) use a tumblehome design and run at high speeds. Efforts have been made to study hull wetness problems because of the special hull shape and running state of Fast Mono-WPC. Numerical and experimental methods were adopted to study hull wetness problems of Fast Mono-WPC both in calm water and waves. Numerical methods were based on RANS. Dynamic mesh methods were used to simulate model motions. Influence of velocity, spray strips and hull motions on hull wetness of Fast Mono-WPC in calm water was studied. Results of CFD and EFD in calm water show that hull wetness of Fast Mono-WPC is sensitive to hull motions such as heave and pitch, and spray strips on the bow can reduce hull wetness. Model tests in regular and irregular waves and numerical simulations in regular waves of a free Fast Mono-WPC model were conducted at a high Froude number, Fn = 0.815. Numerical results show that wetness is slight when Fast Mono-WPC runs in 1L-long and 4L-long waves, and water climbs up the freeboard to the deck when Fast Mono-WPC cuts through 2L-long waves. Serious hull wetness was also experimentally observed both in regular and irregular waves.