Page 464 - Practical Design Ships and Floating Structures

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To improve the design, a pram-with-gondola-type afterbody was suggested by MARIN. Displacement
volume was brought to the sides resulting in a significant reduction of the buttock slope. To
accommodate the engine, a rather wide gondola was needed. Special attention was paid to the
orientation of the knuckle line at the location where pram and gondola merge. A compromise was
necessary in view of the variety of operating conditions: loaded, empty, unrestricted and restricted
water depth. The aft body shape of the proposed hull form is shown in Figure 1.

Figure 1 : Aft body design; fiame lines (left)
and ship in construction dock (right)

To check the design, model tests would normally have been the next step. Instead, a set of numerical
flow simulations was carried out with PARNASSOS. It was considered by the yard to be the only
reasonable option that the circumstances permitted.

4 VISCOUS FLOW SIMULATIONS
PARNASSOS is a numerical tool developed at MARIN to simulate the steady viscous flow around a
ship, under the assumption that the wave-making of the ship can be neglected. The underlying
mathematical model is the combination of the full Reynolds-averaged Navier-Stokes (RANS)
equations and a one-equation transport equation for the eddy viscosity. If required, the action of the
propeller can be included by representing the propeller as an actuator disk. The results of PARNASSOS
include the complete velocity field and the pressure distribution around the stem of the ship as well as the
viscous resistance of the hull. This allows streamlines to be traced and possible separation zones to be
detected. Details on the method can be found in e.g. Hoekstra (1999), Hoekstra & @a (1998). Another
example of its application to practical design problems can be found in Valkhof et ul. (1 998).

All calculations to be presented here are based on the use of a single block grid of H-0 topology,
containing between 620,000 and 930,000 nodes, wrapped around the afterbody. The ship has an
immersed transom; in order to avoid the complication of the recirculating flow zone aft of the transom in
the numerical simulations, the hull was smoothly extended. Moreover, the rudder and the headbox above
it were removed.