Practical Ship Hydrodynamics

90 Practical Ship Hydrodynamics
Panel methods are still the most important CFD instrument for form
improvement of ships. Worldwide research aims at faster methods, wider
applications, and higher accuracy:
ž Faster methods
Cluster or multigrid techniques could make existing methods faster by
one order of magnitude (Söding (1996)). The price is a more complicated
program code. At present, the actual computation in practice accounts for
less than 10% of the total response time (from receiving the hull description
to delivering the CFD report) and even less of the cost. So the incentive to
introduce these techniques is low in practice.
ž Wider applications
Non-Linear solutions are limited today to moderate non-linearities. Ships
with strong section flare close to the waterline and fast ships still defy
most attempts to obtain non-linear solutions. Planing boats feature complex
physics including spray. Panel methods as described here will not be applied
successfully to these ships for some years, although first attempts in this
direction appeared in the early 1990s. State of the art computations for
planing boats are based on special non-CFD methods, e.g. slender-body
approaches.
ž Higher accuracy
The absolute accuracy of the predicted resistance is unsatisfactory. Patch
methods as proposed by Söding and discussed briefly in section 6.5.2,
Chapter 6, may overcome this problem to some extent. But the intersection
between water surface and ship will remain a problem zone, because the
problem is ill-posed here within a potential flow model. The immediate
vicinity of the bow of a ship always features to some extent breaking waves
and spray not included by the currently used methods. The exact simulation
of plunging waves is impossible for panel methods. Ad-hoc solutions are
subject to research, but no convincing solution has been published yet. One
approach of overcoming these limitations lies in methods discretizing the
fluid volume rather than boundary element methods. Such methods can
simulate flows with complicated free surface geometries (breaking waves,
splashes) allowing the analyses of problems beyond the realm of BEM
applications.
3.5.2 Viscous flow computations
RANSE solvers are state of the art for viscous ship flows. A computational
prediction of the total calm-water resistance using RANSE solvers to replace
model tests would be desirable, but so far the accuracy of the RANSE predictions
is largely perceived as still insufficient. Nevertheless, RANSE solvers
are widely applied to analyse:
ž the flow around aftbodies of ships
ž the flow around appendages
The first research applications for RANSE solutions with wavemaking for ships
appeared in the late 1980s. By the late 1990s various research groups also
presented results for ships free to trim and sink. However, most computations
for actual ship design projects in practice still neglected all free-surface effects