Investigation of Hydrodynamic Performance of a Twin-Screw Trawler Using CFD
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Previously experimental methods or empirical knowledge laid the foundation for
ship design. Performing a model test in a towing tank is time consuming and
expensive. In addition to this, studying detailed effects in a laboratory proves to
be difficult.
The exponential growth in computer power allows engineers to use computers and
software to solve Navier-Stokes equations using computational fluid dynamics
(CFD). This method allows the engineers to test different models, in a time and
cost-efficient manner.
In this master thesis, a study of CFD codes to perform a resistance prediction on
two different hull design for the trawler, Roaldnes, have been made. The first
design being the current design from Seacon, Hull 1, the second being a design
based on findings from project report (Svoren, 2014), Hull 2. The difference
between these hulls can be found at the aft shoulder where Season’s hull is
designed with chines while the second design is without. Special emphasis was
made on the inflow conditions for the propeller. This was done in order to compare
and observe the effect of the changes made to the hull. STAR CCM+ from CDadapco
was chosen as CFD software, due to its user-friendly interface and
powerful built in post processing tools.
A detailed and thorough convergence study was performed to find the optimal
simulation setup. This study resulted in a mesh with approx. 2 mill cells, time
step of 0.01s, and a domain which stretched 4 lwl aft and forth, 5 lwl to the side,
1.7 lwl up, and 3.5 lwl down. Due to symmetry, half of the model is simulated.
A total of eight simulations on both hulls, at trawl and transit speed, and with
and without nozzle and virtual disk, were performed. Hull 1 displayed a lowpressure
ridge over the sharp edge created by the chines. This lead to increased
vorticities in this area compared to Hull 2. These vorticities propagated onto the
propeller plane, where Hull 2 showed a slightly better wake field at trawl speed,
and similar wake field at transit speed. A plot of the streamlines showed a lowvelocity
swirl of water on the inside of each skeg at both velocities. These were
however almost gone when introducing suction from the virtual disk simulating
the propeller.
VIII
Calculation of thrust reduction on Hull 2 showed an improvement of 3.75%
compared to Hull 1. Full-scale calculations showed an improvement in total hull
resistance of 6.7% at 4 knots, and 2.2% at 10 knots in favor of Hull 2.
Validation of results were made to known experimental data for a similar vessel,
R/V Gunnerus. This investigation showed that the results obtained in this report
was within an acceptable range from the reference vessel.
As CDF is a numerical tool to replicate reality, error sources, and uncertainty
will always be of presence. Nevertheless, it can be used to show trends. Results
obtained in this report suggests that Hull 2 might produce a more uniform inflow,
and a better wake field for the propellers, than what can be seen from Hull 1. It
also indicates that Hull 2 might produce lower thrust reduction, as well as an
overall lower hull resistance at both trawling and transit.

University: 
Norwegian University of Science and Technology
Date: 
Monday, June 1, 2015
Author Name: 
Nikolas Øksdal Svoren
Pages: 
165
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