research & development The state of
Current Affairs Model tests, semi-empirical
tools or CFD calculations? In the “Current Affairs” Joint Industry Project (JIP), model tests, semi-empirical tool predic- tions and CFD computations are employed to estimate current loads on semi-submersibles and mono-hulls. Here is the latest bulletin.
Guilherme Vaz & Olaf Waals
G.Vaz@
marin.nl
Current Affairs JIP meeting in Bilbao: shwaring CFD experience in the industry
This JIP aims to develop tools and guidelines to assist engineers in the assessment of current effects in the different design stages. In the initial design, semi-empirical methods can be used, while in further stages, CFD and model tests are suitable. During the project, a semi-empirical tool has been developed which estimates the current loads on any structure by taking into account experimental results and a shielding model. Several CFD commercial codes, (CFX and STAR-CCM+), and the in-house code FreSCo, were used to compute the current forces. Model tests validated both computational methods for semi-submersibles and this is being done for mono-hulls.
Figure 1: time history of drag and lift coefficients. Comparison between numerical results and experimental data
Results encouraging A number of semi-submer- sible configurations were investigated varying from a one-column configuration to complex arrangements. A configuration with two rounded corner columns and one pontoon for an inflow angle of 0 deg. is analysed [1]. Figure 1 shows a comparison of the time history of drag (Cx) and lift (Cy) force coefficients. The semi-empirical estimate is fair in terms of speed/ complexity ratio but errors - in fact 40% - are observed for the average Cx load. The CFD compu- tations showed an 8% error for the average drag and 25% for the maximum lift. The experiments also revealed large variations on the loads with time, and should be repeated several times to get a more reliable analysis of the signal (not considering some outsider peaks in the experiments the errors of the best CFD calculations drop to 4% for the average drag and to 10% for the maximum lift). Using CFD, the flow field is easily visualised and the interaction with the components can be analysed. Figure (2) shows the vorticity field predicted by FreSCo.
The CFD results are encouraging but additional studies are being carried out. It seems that two factors play a role: laminar-turbulent transition and numerical errors. After the studies aspects such as free surface and scale effects can be considered with some reliability.
[1] Vaz, G., Waals, O., Ottens, H., Fathi, F., Le Suoef, T., Kiu, K. “Current Affairs: Model Tests, Semi-Empirical Predictions and CFD Computations for Current Coefficients of Semi-Submersibles“. OMAE2009, Honolulu, Hawaii, USA. June, 2009.
Figure 2: Total dimensionless vorticity for y=0 and z=0 planes. FreSCo results 22 report
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