The prediction of the resistance of a ship is, together with the propeller performance
prediction, part of the key aspects during the design process of a ship, as it partly ensures the
quality of the power-prediction. Body fitted structured grids for ship simulations can be rather
challenging and time consuming to build, especially when dealing with appended ship geometries. For
this reason, unstructured hexahedral trimmed grids are more and more used. Such grids can be build
by various CFD package such as CD-Adapcos Star CCM+, NUMECAs Hexpress grid generator or OpenFOAMSs
SnappyHexMesh. Although their use is increasing or even already adopted, the numerical uncertainty
of these simulations seems to be a well-kept secret.
In the study presented, an attempt at quantifying the numerical uncertainty of the resistance for
the combination of the RANS Solver ReFRESCO [1] with grids generated using the com-
mercial package Hexpress is made. The studied case is the flow around the bare-hull KVLCC2 at
model scale Reynolds number. Extensive verification and validation on the same test
case has already been published for the combination of ReFRESCO and structured grids by Pereira et
al. [2].
The method to generate grids as geometrically similar as possible is presented, and
the uncertainty analysis by L. Ec¸a and M. Hoekstra [3] is performed on the integral r√esults
obtained.
The simulations are performed using the k − ω SST , k − ω TNT and the k − kL turbulence
models. The velocity fields calculated in the propeller plane are compared to the measured ones
and to the results obtained by Pereira et al. [2] on structured grids.
The results show that the differences with the experimental results are in the same range as the
differences obtained with structured grids. The numerical uncertainties are, however, higher. They
are also strongly dependent on the turbulence model used, like for structured grids, and are spread
between 1.3% and 12%.
Concerning the wake flow details, not all features present in the experimental results
are obtained and, compared to structured grids, the flow features are smoothed. The wake flow is
also influenced by the turbulence modelling and needs to be adressed in more detail.
