I Ketut Aria Pria Utama, Wasis Dwi Aryawan, Ahmad Nasirudin, Sutiyo, Yanuar

Corresponding email: kutama@na.its.ac.id

Corresponding email: kutama@na.its.ac.id

**Published at : ** 25 Jan 2021

**Volume :** **IJtech**
Vol 12, No 1 (2021)

**DOI :** https://doi.org/10.14716/ijtech.v12i1.4269

Utama, I.K.A.P., Aryawan, W.D., Nasirudin, A., Sutiyo, Yanuar, 2021. Numerical Investigation into the Pressure and Flow Velocity Distributions of a Slender-Body Catamaran Due to Viscous Interference Effects.

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I Ketut Aria Pria Utama | 1. Department of Naval Architecture, Institute of Technology Sepuluh Nopember, Surabaya 60111, Indonesia 2. Research Center for Marine-Earth Science and Technology, Institute of Technology Sepuluh No |

Wasis Dwi Aryawan | Department of Naval Architecture, Institute of Technology Sepuluh Nopember, Surabaya 60111, Indonesia |

Ahmad Nasirudin | 1. Department of Naval Architecture, Institute of Technology Sepuluh Nopember, Surabaya 60111, Indonesia 2. Department of Systems and Naval Mechatronic Engineering, National Cheng Kung University, Ta |

Sutiyo | Department of Naval Architecture, University of Hang Tuah, Surabaya 60111, Indonesia |

Yanuar | Department of Mechanical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, Depok 16424, Indonesia |

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+?k) 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 (?),
pressure change ratio (?), and the viscous interference factor (?). In
addition, the ? value is very helpful for finding out the interference of the
hull on a catamaran when sophisticated experimental and numerical tools are not
available.

Catamaran; CFD; Flow velocity; Pressure distribution; Viscous interference factor

Introduction

Multihull ships have progressively received considerable attention. One of the most popular is the catamaran (Utama, 1999). Catamarans have a unique hydrodynamics phenomenon known as viscous and wave interactions which occur between the demihull of the catamaran. The technique to conduct and analyze the viscous resistance of a catamaran can be done using the computational fluid dynamics (CFD) method on a reflex model. In this case, the free surface is treated, allowing the isolation of the viscous resistance by omitting any influences from surface waves. The use of reflex models in a CFD simulation, therefore, provides an approximate means of directly measuring the total viscous resistance of the model without wave resistance.

In the last 50 years,
the development of catamaran theory has been proposed by many researchers to
explain the resistance of catamarans. The reflex model was a technique
pioneered by Joubert and Matheson (1970), where the resistance
of the hull was measured in a wind tunnel. Utama
(1999) conducted a detailed
experimental investigation in a low-speed wind tunnel on a single ellipsoid (as
a reflex model) and a pair of ellipsoids nearby representing a catamaran.
Theoretical, numerical, and experimental investigations have been carried out
on multihull vessels and further research has been conducted by Zaghi et
al. (2011).

An increasing number
of researchers are calculating ship resistance using CFD. Broglia et al. (2014) completed a study on
catamarans with Froude numbers between 0.3 and 0.5 which showed that the
configuration of the narrow hull distance between catamarans has a more
significant interference effect. Numerical computation to illustrate the
hydrodynamic factors that influence ship resistance using a FLUENT code has
been investigated by Deng et al. (2011). Jamaluddin et al. (2013) conducted experimental and numerical
investigations to analyze the components of resistance and interactions between
hulls in catamarans, and Samuel et al. (2015) studied the selection of optimal catamaran
hulls on traditional fishing vessels.

Previous studies have discussed a lot of catamaran hull interference, but not many have conducted detailed research related to interference due to viscous form factors. Broglia et al. (2019) have been conducting research to improve the capabilities of state-of-the-art CFD tools in the prediction of the flow-field around a multihull catamaran. Viscous resistance represents an integral part of the total resistance of a catamaran in which intermediate Froude Number value interference effects are dominant (Farkas et al., 2017). A potential-flow method was carried out to determine the lift force of single-deadrise hulls and catamaran configurations in which hydrodynamics pressure was more pronounced between two catamaran hulls (Bari and Matveev, 2017). Iqbal and Samuel, (2017) have conducted research catamaran hull form show that the fluid form that surrounds the ship hull influences ship resistance. Mittendorf and Papanikolaou (2020) investigated catamaran resistance and also found an increase in total resistance due to viscous interference. Therefore, the CFD technique could be used to optimize the hull of a catamaran (Miao et al., 2020; Yongxing and Kim, 2020).

The study objectives were to determine the viscous interference due to pressure and flow velocity changes between the catamaran hulls by using a reflex model and to derive viscous form factors (?) in the catamaran models using CFD. The results were validated with an experimental investigation which was carried out in a wind tunnel on a symmetrical catamaran using a reflex model of NPL4a by Jamaluddin et al. (2013).

Conclusion

The effects of two variations of hull separations
for a catamaran, which were investigated by CFD analysis, were compared with a
wind tunnel experiment to validate the results. Numerical simulation and
experiment (wind tunnel) results show a relatively small difference in the
value of flow and pressure components, which is 1.76% on average. This result
shows consistency and is entirely accurate. The difference in viscous
resistance is about 13.2%, where the catamaran resistance is greater than that
of the demihull and is attributed to the interaction between the hulls of the
catamaran. The numerical simulation clearly illustrates the change in flow at
the inner hulls which causes the result that viscous resistance at S/L = 0.3 is
higher than that at S/L = 0.4. Also, it applies to the pressure acting on the
model catamaran; the viscous pressure increases as the S/L decreases. The
effect of the catamaran hull interference variation can be recognized through
the velocity augmentation ratio (?), pressure change ratio (?), and viscous
interference factor (?). The influence of
interference resistance between two ship hulls causes the symmetrical flow of
water around the demihull to be asymmetrical due to high pressure (which
relates to s) and flow velocity
(associated with f) which occurs around the hull and is relatively
unequal to the hull centerline. In addition, the ? value is
very helpful for finding out the hull interference on a catamaran when
sophisticated experimental and numerical tools are not available.

Acknowledgement

The authors wish to thank the Ministry of Research,
Technology, and Higher Education (Kemristekdikti) and the Institut Teknologi
Sepuluh Nopember (ITS) for funding the current work under a scheme called the
World-Class Professor (WCP) Program with the contract number
T/42/D2.3/KK.04.05/2019.

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