A numerical study of resistance components of high-speed catamarans and the scale effects on form factor

Resistance and powering of ships is one of the most important aspects for Naval Architects and it still heavily relies on scale model testing. The catamaran is a particular case that is treated slightly different from the monohull due to the interference effects between demihulls. There are various works from previous researches dealing with this challenge. Since catamarans have been introduced, resistance components and form factor have been evaluated by focusing on the influence of many aspects such as hull form, speed, and hull separation. The methods by which the resistance is evaluated, either experimentally or numerically are mostly driven by the size of physical test facilities or computational power available. This thesis investigates the resistance components of high-speed catamarans and the effect of scale on form factor using a commercial CFD code. The numerical investigation into resistance components and form factor of the high-speed catamarans focuses on the hull geometry, separation to length ratio (S/L) and scale. The hull geometries include the Wigley III and NPL round bilge 5b catamarans. The CFD results are compared against Insel’s experimental series. The investigation into the effect of scale on form factor (1+k) are made for the Wigley III and NPL 5b catamarans S/L = 0.3. Three different models of those hull configurations are created, which are 1L, 2L for both hulls, 4L for Wigley III catamaran and 10L for NPL 5b catamaran respectively. The numerical domain to ship dimension ratios are constant for the Wigley III catamarans. The influence of domain size, recreating two towing tanks and the unbounded condition, are investigated. Wave elevations, resistance components and form factors are evaluated and compared. The CFD results show good agreement with the original experimental results, and allow deeper understanding into the resistance components, form factor and free surface wave elevations. The CFD simulation domain size has an important influence on the resistance. To some extent the ITTC recommended blockage corrections can correct for this, but the wave system is significantly different and cannot be corrected. The CFD investigation into the influence of scale on form factor shows that form factor is both speed and model length dependent. So, it is noted that a larger model produces more accurate predictions of form factor and that the form factor should be evaluated at the design speed rather than determined by either low speed or high-speed runs

Numerical study of resistance and form factor of high-speed catamarans

Since the prediction of resistance of the full-scale ship mainly relies on extrapolation of form factor of the model, it is important to determine the form factor precisely. Nowadays, the computer performance has been developed, commercial CFD code with Reynolds-averaged Navier-Stokes Equations (RANS), which is widely accepted and used by many researchers is capable of determining resistance components. This paper presents the development and procedures for the prediction resistance components and form factor of displacement catamarans by using commercial CFD code, STAR CCM+, with SST k-ω turbulence model. The Wigley catamarans with three hull configurations including S/L = 0.2, 0.3 and 0.4 are investigated at Froude number between 0.2 and 0.8. Resistance components, which are total (CT), skin friction (CF), viscous (CV), residual (CR) and wave (CW) resistance, form factor (1+k), form resistance interference factor (β), wave resistance interference factor (τ) and wave elevation along the hull are estimated and validated against experiment retrieved from Insel (1992). The results show that CFD code with RANS equations is capable of estimating resistance components and demonstrates that form factor increases with speed (Fn)

Blockage effects on resistance prediction of high-speed catamarans

Model experiment is the most reliable method to estimate ship resistance; however, price and complexities are the most disadvantage factors. Another aspect that affects the precision of the results are blockage effects due to tank dimension and model size. Some correction procedures introduced by ITTC are widely used. However only resistance components that can be corrected while flow field and wave elevation cannot be justified using the blockage correction procedures. To demonstrate how the blockage affects the flow around catamaran, this study focuses on resistance prediction and flow characteristics around high-speed catamarans using CFD code with RANS equations. Two Southampton University towing tanks – Solent and Boldrewood – are compared. The NPL 5b model is used as it is widely investigated and used in the commercial applications. Four Froude numbers (Fn = 0.273, 0.433, 0.70 and 0.90) are investigated. CFD results show that resistance components measured from the higher blockage towing tank are higher than the bigger tank. Wave contour plots and wave cut show that the higher blockage towing tank causes the higher wave elevation due to the reflection of the wall which results in differences for the wave resistance prediction between the two towing tanks