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Feature 3 | CFD & HYDRODYNAMICS


Examples of flow field data from the ReFRESCO propeller simulations are presented in Figure 3. Figure 3 (top) shows the development of


tip and blade-root vortices of the different blades for the E779A propeller. Te vortex is moved downstream and diffusion of the vortex core is visible. Notice that the tip vortex continues until about ¾ of a revolution, aſter which the vortex region disappears. Tis is due to insufficient grid resolution in the downstream part of the domain. Refinement of the grid in the vortex area would be required to follow the vortex more downstream. Te blade-root vortex can be followed almost to the end of the domain due to higher (boundary-layer) resolution close to the hub. Separation, flow re-attachment and


the location of vortex formation can be visualised with limiting streamlines. Te limiting streamlines, tangent lines to the surface shear stress vector, show (re-) attachment and detachment patterns. In Figure 3 (bottom left) the limiting streamlines are presented for the skewed propeller subjected to a high loading. Separation can be observed on a large part of the leading edge reattaching further downstream. Close to the blade-root at the trailing edge another area of separation is observed. Near the tip detachment lines are visible that show the location where the tip vortex detaches from the blade. Te pressure distribution on the blades


and hub for the ducted propeller, shown in Figure 3 (bottom right), provides information about the loading distribution on the blades as well as the location of low pressure areas where cavitation might occur. Clearly, the detailed visualisations of CFD results can provide important clues for further design improvements.


Full-scale simulations Experimental open-water characteristics are measured for model-scale Reynolds numbers. Usually, these results are afterwards extrapolated to full scale, in order to correct for the different scale of the propeller. Most extrapolation methods only consider that the drag at full scale is lower than for model scale, which results in a slightly higher thrust and lower torque at full scale. Especially for complex propulsors, such as ducted propellers where interaction


54 The Naval Architect July/August 2011


between the different propulsor components plays a large role, this correction is not adequate and viscous CFD methods can provide a more detailed analysis of the difference between model-scale and full-scale flow. The change of Reynolds number


between model and full-scale has several effects on the flow around a ducted propeller: 1) variation of


the lift and


drag of the propeller/duct sections due to different boundary layer thickness and separation locations; 2) variation of the induced velocities upstream of the propulsor, consequently changing the local angle of attack of the propeller/duct sections; 3) changes on the interaction of


the duct boundary layer with the


tip-vortex which consequently alters the loading of the propeller. A numerical comparison between


model-scale and full-scale open-water results for the ducted propeller show that for full scale all open-water coefficients increase: propeller thrust, duct


thrust, propeller


torque and total open-water efficiency. Only for highly advanced ratios, the duct thrust is lower than for model-scale. Since the increase in thrust is larger than for torque the efficiency increases for full scale. Te gains in efficiency are between 4 and 10%, the highest values occurring for the lowest loadings. Even if these large values have to


be considered with some caution, the trend is clear: the complete propulsor unit is more efficient for full-scale than for model-scale with a significant gain.


Conclusions Te capability of the viscous CFD method ReFRESCO for propeller open-water flows has been demonstrated using three different propellers. For all propellers the open-water simulations were compared with experimental and with propeller potential-flow results. From this study is concluded that: • The thrust, torque and open-water efficiency is well captured. Thrust and torque values for all propellers show differences with the experiments in the order of 2-3% around design condition, and within 5% for off-design conditions.


• A verification and validation analysis procedure for the E779A propeller showed that the numerical uncertainty for thrust and open-water efficiency is in the order of 4% and 3% for the torque, for the propeller design condition.


• For thrust and torque predictions around design condition, PROCAL (BEM) potential-flow results showed a good comparison with the measurements. The added value of RANS method for propellers is the more accurate prediction at higher propeller loading where viscous


Figure 4: Comparison between model-scale (left) and full-scale (right) vorticity. Notice the difference in pitch of the tip vortex and interaction with the boundary layer between model and full scale.


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