Figure 2: Self-propulsion simulation. Propeller grid, velocity contours in a transverse plane aft of propeller plane.
generated and exported to SHIPFLOW that creates volume grids around the appendages. Shape and position can be varied without losing the automatic grid generation. Te full potential of the CFD solution is
illustrated with an example of an Energy Saving Device (ESD). SHIPFLOW has been successfully applied to various projects involving the optimisation of ESD’s in the past few years. A case study based on a well known VLCC tanker appended with a generic ducted three bladed pre-swirl stator is outlined below, though due to strict confidentiality rules we cannot name the vessel. The computational configuration
includes a background grid and several overlapping
component grids. The
background grid constituting the main computational domain discretises the volume surrounding the bare hull surface. Te non-axisymmetric converging duct has both varying chord and local angle of attack and it’s component grid is based on a surface mesh created with the Framework (see Figure 1), and the hyperbolic grid generator available in SHIPFLOW’s RANS solver. The fins are generated with a wing component (rudder object) which is built into the solver. All parts can be parametised which greatly simplifies optimisation tasks. Te size, shape and positioning of the device, therefore, can be controlled by the optimiser using design variables. Due to an extreme complexity of
the flow propeller efficiency, the ship performance cannot be evaluated easily even by a very experienced designer based on the wake field and resistance components. This is why many of our users utilise self-propulsion numerical simulations using SHIPFLOW, ranking alternatives with the overall performance expressed as the delivered power for the specified ship speed. Te computations are, therefore, performed with a working propeller modelled with a lifting line method inbuilt in the CFD code or an external propeller model linked to
The Naval Architect January 2012
Figure 3: Self-propulsion simulation. Propeller grid, velocity contours in a longitudinal plane, dynamic pressure coefficient on the duct surface.
SHIPFLOW. The propeller forces are transferred to the computational domain via an additional embedded cylindrical grid, Figure 2. The propeller model rpm is adjusted during the computations to balance the total ship resistance. The flow field is illustrated in Figures 2 and 3 where the axial velocity contours are shown at a transverse plane behind the propeller and a longitudinal plane close to the ship’s centre plane. The pre-swirl stator (PSS) creates more rotational flow especially to the port side by directing the flow downwards equalising the tangential velocities components at the
propeller plane. As a result the rotational energy loss is reduced increasing the propulsion efficiency. A well optimised PSS shows gains
of more than 5% in performance, reducing both emissions and fuel costs. SHIPFLOW validation to towing tank results agree with measurements proving SHIPFLOWs potential
in
optimising complex appendages in self-propulsion mode. An additional advantage of the system is that many designs and unusual configurations can be evaluated with ease with no risk of costly model tests. NA
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