Feature 1 | CFD AND HYDRODYNAMICS
widely investigated small container ship. Figure 3 shows the comparison between computed and measured velocity distributions at (x-xp)/D=-0.201 behind the propeller and indicates a very good agreement. Today, the numerical propulsion test is applied to customer projects on a regular basis.
Figure 3: Hamburg Test Case: Axial Velocity behind propeller (computations (left) / measurements).
intensity when compared with the experiment (right).
Numerical propulsion test Propulsive efficiency is a key element in ship and propeller design as it largely determines fuel consumption and hence cost as well as the amount of emissions caused by a ship. Although the resistance of the hull is the largest overall contribution to the forces acting on a ship’s hull during operation and hence the accurate prediction of the resistance is of great importance, a final assessment of the quality of a new hull design can only be made once the propulsion performance is known. FreSCo+
offers an advanced concept
to perform a numerical propulsion test in that the RANS code is coupled with HSVA’s in-house vortex lattice code QCM
Figure 4.1: FreSCo+
[2] (Quasi Continuous Method) for propellers in an iterative fashion [3, 4]. At the start of the predictions, the (nominal) wake field is predicted and fed into the vortex lattice code where propeller thrust and torque are computed. The turning rate is adjusted until a propeller thrust to overcome the ship resistance is obtained. The hydrodynamic forces of the propeller are converted in the form of body forces, which are assigned to cells within the swept volume resulting from rotation of the propeller blades and fed back into the a new RANS calculation. Tis procedure runs iteratively until and equilibrium between the resistance of the ship under self-propulsion condition and the propeller thrust is reached. Te method has been validated using
several test cases, the first one being the self propelled “Hamburg Test Case” (HTC), a
has been used to compute the wave pattern
around the ship, wave elevation on the hull and behind the transom stern as well as the total resistance force.
Free surface – wave resistance predictions Determining the wave resistance of a new hullform is – still – among the most important tasks of a model basin. Although early CFD developments based on potential flow theory, e.g. HSVA’s panel code n-SHALLO have shown very good computational results already, the methods often fail to predict accurate results for blunt ships or hullforms sporting a wetted transom. FreSCo+
has
been applied to such cases. In a comprehensive computational exercise, FreSCo+
results have been
validated against experimental data obtained from both, the VIRTUE and the ABSS project where detailed wave measurements beside the ship and behind the transom have been performed, the latter using a novel laser cut technology. One of the vessels investigated in this project has been the so called VIRTUE container ship, a contemporary design for a 3000TEU vessel. The results obtained from these
predictions demonstrate that the new code allows the user to accurately predict the wave pattern around a ship hull, capturing
Figure 4.2: Predicted wave pattern for the VIRTUE container ship at FN = 0.25.
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The Naval Architect July/August 2010
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