Numerical Investigation into the Pressure and Flow Velocity Distributions of a Slender-Body Catamaran Due to Viscous Interference Effects

Abstract

A computational fluid dynamics investigation was carried out on a slender body catamaran to determine the effect of pressure and flow velocity changes for varied hull separations. The investigation was conducted using an NPL 4a model with a slenderness (length to breadth) ratio of about 11 together with the use of a commercial code (CFX) with hull separations of S/L = 0.3 and 0.4 along with a variation in Reynolds numbers of 2.86×105, 3.43×105, 4.01×105, and 4.44×105. Pressure and flow velocity around the hull were measured to obtain a fluid effect attributed to the influence of catamaran hull interference. A computational fluid dynamics investigation was carried out with the same configurations as those in the experimental tests. The overall results were in good agreement, with the order of discrepancy at about 1.76%; the computational fluid dynamics results were consistently lower than the experimental ones. Both tests demonstrated a viscous interaction between the hulls and, thus, the form factors for the demihull and catamaran were properly derived: the form factor for the demihull [1+k] was 1.254 and for the catamaran (1+ßk) was 1.420, indicating interaction effects of about 13.2%. The form factor for the catamaran was consistently higher than the demihull, suggesting some viscous interference between the hulls. The effect of catamaran hull interference variation can be recognized through the velocity augmentation ratio (a), pressure change ratio [ϕ], and the viscous interference factor [ß]. In addition, the ß value is very helpful for finding out the interference of the hull on a catamaran when sophisticated experimental and numerical tools are not available.