Feature 3 | CAD/CAM Predicting the flow
For years hydrodynamicists have been trying to mathematically simulate the three-dimensional (3-D) flow that a vessel experiences underway. Scott C. McClure, president and Donald D. Burris, chief hydrodynamicist, Alan C. McClure Associates explain the challenges
I
nitially plagued by the high cost of computational power, as well as crude soſtware, it appeared this goal
would only be accomplished in the distant future. However, with the recent dramatic advances in high-speed computers, today’s computational
fluid dynamics (CFD)
software packages enable credible and accurate results on a cost-effective scale. In late 2008, Alan C. McClure Associates,
Inc. (ACMA) decided to make a significant investment in computer resources and in the STAR-CCM+ software developed by CD-Adapco. This software allows ACMA to perform CFD analysis that breaks a 3-D volume of space into a number of volumetric cells. Mathematical equations based on the various physics models are then calculated for each of the cells in the volume, with the solution of one cell setting the boundary conditions of adjacent cells. Changes to the solution of the physics equations over time are then calculated to simulate real-world effects. ACMA recently performed a propulsion
study to simulate the hydrodynamic effects that a vessel would experience while underway. The study was done in three phases: a base flow model, momentum source model, and a fully modelled rotating propeller. The base model provided an understanding of the simple flow around the hull and appendages - hull, rudders (steering and flanking), propeller shaſt and support struts. Te base model was then modified to include a momentum source disk to simulate a four-blade propeller. A final model phase utilised a 3-D model of a rotating five-blade propeller.
Basic flow model The basic flow model was completed in order to observe flow phenomena around the hull in the absence of the propeller. This model was invaluable in attempts to identify areas of potential separation of flow from the hull, as well as areas of turbulent flow that would be exaggerated by
50 The Naval Architect October 2012 Figure 1. Overall geometry of study vessel
a propeller. It was observed that there was a significant transverse flow that initiated from the chine into the outboard flanking rudders, imparting flow disturbances into the propeller. Tis transverse flow would tend to increase the likelihood of the vessel developing vibration issues that could lead to premature steel component and equipment failures, as well as making a noisy, uncomfortable ride for the crew. Although there was a lot of transverse flow as the water came off the chine, there was
not any separation of the flow as it moved up the buttocks longitudinally. Separation of the flow due to steepness of the buttock lines is frequently a concern since owners/naval architects want more power in a compact hull form leading to steeper buttock lines to accommodate propulsion equipment. As the power requirement goes up,
the ability of the propeller to absorb this additional power requirement means that the blade area ratio (BAR) needs to increase and/or the wheel needs to have a
Figure 2. Basic flow model, flow on outboard flanking rudder
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