Feature 3 | CAD/CAM
Figure 3. Momentum source propeller model
larger diameter. As the propeller diameter increases, the clearance between the propeller tips and the hull, as well as the submergence of the wheel often suffer. Tese trade-off’s and decisions are always present when the hull characteristics remain constant and the need for more power is desired.
The momentum source disk model The next phase of the modelling used a momentum source to simulate a rotating propeller. Te momentum source is based on fan laws, and thus the performance curve of a four-blade propeller was input to the model. As flow moves into the vicinity of the momentum source, the flow is accelerated as if it were being “sucked” and “pushed” by rotating blades. Te four blade propeller was used as it is a common selection for vessels of the type and size under consideration. Although we noted that the 90degs blade orientation might create problematic interactions with the flanking rudder and shaſt struts, ACMA felt that the large BAR of the wheel might mitigate those issues. Based on real-time monitoring of the
analysis, 130 seconds of simulation time was found sufficient to quantify the steady state flow phenomena. A time-step of 0.05 seconds was used for the simulation, based on the blade passing frequency. Based on the physics models used and the density of the volume mesh, each time-step represented roughly 64 million simultaneous equations being solved. Te resulting flow field revealed flow lines
at the top of the propeller moving much slower than the flow velocity at the bottom of the propeller. Tis type of asymmetrical loading tends to accentuate any vibration
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Figure 4. Rotating propeller model
problems such as those already noted due to the large transverse flow. What was also noted was that a force coupling developed with the impact loads on the hull at the blade passing frequency. Tis coupling was due in part to the proximity of the propeller blades to the hull.
The five-blade rotating propeller model A number of options to mitigate the adverse effects that were seen with the four-blade propeller were proposed, including a five blade propeller. Tis model also stepped to a further level of complexity by including a 3-D rotating model of the five blade propeller. In this simulation, the solid representing the propeller was rotated in a fluid domain embedded within the larger fluid domain. The software mapped the physics effects between the two domains such that the flow was accelerated into the propeller and thrust aſt. The results were promising. The flow
irregularities were somewhat tempered owing to the increased passing frequency of the blades. The change in propeller geometry lead to de-coupling of the impulse loading on the hull observed in the four blade analysis. Although it did not result in the complete elimination of potential flow-induced vibration sources, it was a step in the right direction. In closing, the technology needed
for accurate simulation of complex hydrodynamics through CFD soſtware has come of age. The computational power and soſtware is affordable for even small consulting firms. CFD can be used to give real-time recommendations to vessel owners and designers facing difficult flow related issues.
Major changes in a vessel design, such
as the propeller, can be easily simulated with the results available in a few days. Tis has already shown itself to be a big cost saver to the owner and/or shipyard since it is cheaper to run soſtware than to replace equipment in a trial and error process. In this case, if we had run the analysis before the client purchased the problematic four blade propeller, the cost savings would have been realised in the purchase of only the five blade propellers, and the elimination of the associated down time required to dry-dock the vessel for propeller replacement. In addition to this exercise, ACMA has
had very good experiences in utilising the soſtware as a virtual tow tank, in which the impact of physical vessel modifications, such as hull form, and operational changes, such as trim, can be quickly modelled to estimate impact on powering requirements and fuel efficiency. ACMA has found a good correlation between the CFD-predicted values and real-world performance through direct in-house comparisons. CFD also allows for full scale simulation, eliminating issues of scaling that can be difficult to capture and handle in traditional model testing. ACMA expects CFD analysis to continue to expand its role as a key engineering tool in the future.
Authors’ Biographies Scott McClure, P.E., President of Alan C. McClure Associates (ACMA), has 29 years of experience in the offshore and marine industry. Donald Burris, P.E., Chief Hydrodynamicist at ACMA, has 10 years of experience in the offshore and marine industry. NA
The Naval Architect October 2012
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