By Dr Gregory J. Seil, Practice Leader – CFD
Sinclair Knight Merz, Sydney, Australia
Under its Corporate Enabling Research Program (CERP) initiative for future undersea warfare, the Australian Defence Science & Technology Organisation (DSTO) is investigating submarine concept designs. Sinclair Knight Merze (SKM) was commissioned by DSTO to undertake meshing and computational fluid dynamics (CFD) modelling of the DSTO “Joubert” generic submarine geometry shown in Figure 1. This study was subsequently expanded to investigate the effect of sail design on submarine hydrodynamic resistance and wake under straight-ahead and maneuvering conditions.
Software from Pointwise was used to produce a high quality, fully structured mesh of the modeled flow domain in order to accurately resolve the flow around the submarine. Accurate resolution of boundary layer development over the hull and wake propagation downstream of the sail is crucial to achieving high quality results. The geometry of the submarine was imported into the software via IGES format. The software's ability to handle imperfections in CAD geometry, such as gaps and overlapping surfaces, made surface meshing straightforward. Elliptic smoothing was used to improve mesh quality on relevant domains and blocks. The use of elliptic smoothing also helped reduce overall topological complexity that otherwise would have been necessary to produce high mesh quality in terms of minimum cell skewness. The mesh of the half-model contained 9,872,475 cells. Mirroring of the mesh blocks for the half-hull was used to produce the mesh for the complete model.
CFD calculations were performed using ANSYS FLUENT with the SST k-ω turbulence model and QUICK convective differencing. Apart from the baseline fin geometry shown in Figure 1, a number of different fins were studied, including the geometry shown in Figure 2. The impact of increased fin volume on resistance and the distortion of the wake at the nominal location of a propulsor were investigated. The maneuvering performance of these geometries also was studied for a range of maneuvering conditions. All calculations were performed at model-scale Reynolds numbers of ~107. The study easily can be extended to full-scale Reynolds numbers.
Changes to the mesh to integrate new sail geometry were implemented by importing the new sail geometry in IGES format, deleting mesh blocks in the vicinity of the sail affected by geometry and mesh topology changes and then remeshing locally. The mesh away from the sail remained unchanged, leading to more consistent CFD results.
The study showed that with careful design, it is possible to double the volume of the fin without increasing the resistance of the submarine and without a significant increase in wake distortion at the propulsor.
It is the author's experience that Pointwise software is well suited to maritime platform system applications due to its inherent stability and its capability to handle poor CAD geometry, create arbitrarily shaped connectors and its powerful elliptic smoothing capability for structured meshing.
Note that in the above figures, the color contours on the surface of the submarine show a representative pressure distribution. The velocity of the flow around the hull is shown by the colored contours away from the surface. With both sets of contours, red and blue contours represent high and low values, respectively.
Reference: Seil, G. J. and Anderson, B (2012), “A Comparison of Submarine Fin Geometry on the Performance of a Generic Submarine”, Proc. Pacific 2012 International Maritime Conference, 31 January-2 February, 2012, Sydney.